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Description, An RNA that catalyzes the formation of GTP was developed with an emulsion-based in vitro selection system., Synthesis of RNA in early life forms required chemically activated nucleotides, perhaps in the same form of nucleoside 5′-triphosphates (NTPs) as in the contemporary biosphere. We show the development of a catalytic RNA (ribozyme) that generates the nucleoside triphosphate guanosine 5′-triphosphate (GTP) from the nucleoside guanosine and the prebiotically plausible cyclic trimetaphosphate. Ribozymes were selected from 1.6 × 1014 different randomized sequences by metabolically coupling 6-thio GTP synthesis to primer extension by an RNA polymerase ribozyme within 1016 emulsion droplets. Several functional RNAs were identified, one of which was characterized in more detail. Under optimized reaction conditions, this ribozyme produced GTP at a rate 18,000-fold higher than the uncatalyzed rate, with a turnover of 1.7-fold, and supported the incorporation of GTP into RNA oligomers in tandem with an RNA polymerase ribozyme. These results are discussed in the context of early life forms.

Colloidal suspensions of 3-aminopropylmethyl(tetraphenyl)silole nanoparticles can be used as selective chemosensors for carcinogenic chromium(VI) analyte. Methylhydrosilole is functionalized by hydrosilation of allylamine, and the colloid is prepared by the rapid addition of water to a THF solution of the silole. The method of detection is through electron-transfer quenching of the fluorescence of the silole colloid (lambda(em) = 485 nm at 360 nm excitation) by the analytes, with hundred parts per billion detection limits. Stern-Volmer plots are linear up to 10 ppm in the case of chromium, but exhibit saturation behavior near 5-10 ppm for arsenic. Dynamic light scattering experiments and AFM measurements show the particle sizes to be around 100 nm in diameter and dependent on solvent composition, with a particle size dispersity of +/-25%. The fluorescence lifetimes of the silole in solution and colloid are approximately 31 ps and approximately 4.3 ns, respectively, while the silole has a lifetime of 6 ns in the bulk solid. A minimum volume fraction of 80% water is necessary to precipitate the colloid from THF, and the luminescence continues to rise with higher water fractions. Colloids in a pH 7 phosphate-buffered suspension show both higher sensitivity and greater selectivity (100-fold) for CrO4(2-) detection than for other oxoanion interferents, NO3-, NO2-, SO4(2-), and ClO4-.

Despite early claims that nitric oxide does not react with cobalamin under any circumstances, it is now accepted that NO has a high affinity for cobalamin in the 2+ oxidation state [Cbl(II)]. However, it is still the consensus that NO does not react with Cbl(III). We confirmed that NO coordinates to Cbl(II) at all pH values and that Cbl(III) does not react with NO at neutral pH. At low pH, however, Cbl(III) does react with NO by way of a two-step process that also reduces Cbl(III) to Cbl(II). To account for the pH dependence, and because of its intrinsic interest, we also studied reactions of NO with cobinamide [Cbi] in the 2+ and 3+ oxidation states. Both Cbi(II) and Cbi(III) react readily with NO at all pH values. Again, Cbi(III) is reduced during the process of coordinating NO. Compared to cobalamin, cobinamide lacks the tethered 5,6-dimethylbenzamidazolyl moiety bound to the cobalt ion. It may, therefore, be considered a "base-off" form of Cbl. To explain the reaction of Cbl(III) at low pH, we infer that the base-off form of Cbl(III) exists in trace amounts that are rapidly reduced to Cbl(II), which then binds NO efficiently. Base dissociation, we postulate, is the rate-limiting step. Interestingly, Cbi(II) has 100 times greater affinity for NO than does Cbl(II), proving that there is a strong trans effect due to the tethered base in nitrosyl derivatives of both Cbl(II) and Cbl(III). The affinity of Cbi(II) for NO is so high that it is a very efficient NO trap and, consequently, may have important biomedical uses.

The synthesis, spectroscopic characterization, and fluorescence quenching efficiency of polymers and copolymers containing tetraphenylsilole or tetraphenylgermole with Si-Si, Ge-Ge, and Si-Ge backbones are reported. Poly(tetraphenyl)germole, 2, was synthesized from the reduction of dichloro(tetraphenyl)germole with 2 equivs of Li. Silole-germole alternating copolymer 3 was synthesized by coupling dilithium salts of tetraphenylsilole dianion with dichloro(tetraphenyl)germole. Other tetraphenylmetallole-silane copolymers, 4-12, were synthesized through the Wurtz-type coupling of the dilithium salts of the tetraphenylmetallole dianion and corresponding dichloro(dialkyl)silanes. The molecular weights (M(w)) of these metallole-silane copolymers are in the range of 4000 approximately 6000. Detection of nitroaromatic molecules, such as nitrobenzene (NB), 2,4-dinitrotoluene (DNT), 2,4,6-trinitrotoluene (TNT), and picric acid (PA), has been explored. A linear Stern-Volmer relationship was observed for the first three analytes, but not for picric acid. Fluorescence spectra of polymetalloles or metallole-silane copolymers obtained in either toluene solutions or thin polymer films displayed no shift in the maximum of the emission wavelength. This suggests that the polymetalloles or metallole-silanes exhibit neither pi-stacking of polymer chains nor excimer formation. Fluorescence lifetimes of polymetalloles and metallole-silanes were measured both in the presence and absence of TNT, and tau(o)/tau is invariant. This requires that photoluminescence quenching occurs by a static mechanism.

Fluorescence lifetimes of five representative xanthene dye species-the rhodamine B zwitterion (RB'), the rhodamine B cation (RB+), the rhodamine 66 cation (R6G+), the rhodamine 101 zwitterion (R101') and the fluorescein dianion (F2-)-were measured in HzO, D20 and in a series of alcohol solvents ranging from methanol to octanol. The lietimes of both RB* and RB+ increased markedly as the solvent was varied from water to actanol. In contrast, the lifetimes of R6G+ and R101' decreased slightly over the alcohol series and that of F2- increased only slightly in the same series. For all the dyes studied the fluorescence lifetimes observed in DzO were slightly longer than those in H20. Possible causes for the variations observed are discussed.

Kinetics of NO dissociation were characterized for three five-coordinate systems, heme-NO, HSA-heme-NO (human serum albumin), GC-NO (soluble guanylate cyclase), and for the six-coordinate system, Im-heme-NO. Nitrosyl myoglobin was redetermined for comparison. Previously known, six-coordinate R and T state nitrosyl hemoglobins are also included in the comparison. The data indicate that NO dissociates more than 1000 times faster from five-coordinate model heme than it does from the six-coordinate analog. Such a negative trans-effect between NO and a proximal base is in sharp contrast to carboxy heme derivatives, in which ligand dissociation rates are greatly slowed in when a trans base is present. As a result of opposite trans-effects, six-coordinate carboxy and nitrosyl derivatives have comparable dissociation rates, even though the five-coordinate species are very different. In proteins, five- and six-coordinate forms do not show a large difference in dissociation rates. Part of the reason may be due to different probabilities for geminate recombination in the different proteins, but this cannot explain all the facts. There must also be influences of the protein structure on bond-breaking rate constants themselves. With the exception of hemoglobin in the T state, nitrosyl guanylate cyclase shows the highest NO dissociation rate constant, k(obs) = 6 x 10(-4) s(-1). This would yield a half-life of about 2 min at 37 degrees C for dissociation of NO from GC-NO, a number that has implications for the mechanism of regulation of the activity of this key heme enzyme.

Interaction of CO with hemeproteins has physiological importance. This is especially true for nitric-oxide synthases (NOS), heme/flavoenzymes that produce .NO and citrulline from L-arginine (Arg) and are inhibited by CO in vitro. The kinetics of CO ligation with both neuronal NOS and its heme domain module were determined in the presence and absence of tetrahydrobiopterin and Arg to allow comparison with other hemeproteins. Geminate recombination in the nanosecond time domain is followed by bimolecular association in the millisecond time domain. Complex association kinetics imply considerable heterogeneity but can be approximated with two forms, one fast (2-3 x 10(6) M-1 s-1) and another slow (2-4 x 10(4) M-1 s-1). The relative proportions of the two forms vary with conditions. For the heme domain, fast forms dominate except in the presence of both tetrahydrobiopterin and Arg. In the holoenzyme, slow forms dominate except when both reagents are absent. Geminate recombination is substantial, approximately 50%, only when fast forms predominate. Stopped-flow mixing found dissociation constants near 0.3 s-1. These data imply an equilibrium constant such that very little CO should bind at physiological conditions unless large CO concentrations are present locally.

The emission of the merocyanines derived from the photochromic 1, 3, 3-trimethylspiro[indoline-2,3′[3H]naphth[2,1-b][1, 4]oxazine] 1, the 4, 5-dimethyl-substituted compound 2 and the 6′-piperidino-substituted derivative 3, in solution with acetonitrile or ethanol, was studied by means of a time-correlated single photon counting system with ca 10 ps time resolution. The signal decays were described by a sum of two or three exponential components with τ1=10–60 ps, τ2= 0.5–1ns, τ3=1–2 ns, corresponding to the contribution of different fluorescing merocyanine isomers. The most intense emission, the shortest lived one, is characterized by extremely low quantum yields (

In either sperm whale or horse heart myoglobin, binding of NO and lowering of solution pH work together to weaken, and ultimately break, the bond between iron and the proximal histidine. This is reminiscent of the reaction observed at neutral pH in the case of guanylate cyclase, the heme enzyme that catalyzes the conversion of GTP to cGMP. Bond breaking is characterized by a spectral change from a nine-line to a three-line ESR signal and accompanied by a shift from 420 to 387 nm in the UV-vis spectrum of the Soret band maximum. Analysis of the pH-dependent spectral changes shows that they are reversible, at least within a few hours, that the transition is cooperative, involving six protons during pH lowering but only two as it is raised, and that the pK is about 4.7. Different proteins exhibit different pK values, which are generally lower than that for "chelated" protoheme. The pK differences reflect the extra bond stability afforded by the protein structure. Investigations of thermal and photochemical NO displacement by CO suggest that the local pocket around the ligand, although significantly altered (according to circular dichroism investigations), nonetheless still imposes a barrier against the outward diffusion of ligand into the solvent. Nanosecond and picosecond flash photolysis shows that in proteins at low pH there is an extremely efficient geminate recombination of the ligand with the four-coordinated species through a single-exponential process. This occurs to a significantly larger extent than for the case of NO-"chelated" protoheme (where no distal barrier for ligand is present). At neutral pH, when the proximal histidine bond is intact, the geminate recombination for NO takes longer and displays multiexponential kinetics. Altogether, these results suggest that, even though distal effects probably also play a role, proximal effects make an important contribution in modulating ligand-iron bond formation.

The quantum yields for photolysis of imidazole-iron(II) porphyrin-NO complexes have been determined in the following solvents: toluene, aqueous 1-methylimidazole, Nujol, and glycerol. The corresponding yields are 0.2, 0.077 (30% 1-methylimidazole in water), 0.064, and 0.007. Steric effects on NO photolysis were also investigated. The increasingly hindered porphyrins (mesoheme, anthracene-7,7-cyclophane heme, adamantane-6,6-cyclophane heme, and pyridine-5,5-cyclophane heme) had, in toluene, quantum yields of 0.21, 0.21, 0.29, and 0.49, respectively. Several conclusions are drawn from these results. (1) Polar solvents decrease NO quantum yields

Two different ΔS=2 intersystem crossing processes, the 1 MLCT→ 5 T 2 and 5 T 2 → 1 A 1 conversions which follow the photoexcitation of low-spin Fe II complexes, are examined on the nanosecond, picosecond, and subpicosecond time scales

The kinetics of transient absorption changes for a number of protoheme-ligand systems after subpicosecond photolysis have been investigated. When the photolyzed ligand is tert-butyl, pentachlorophenyl, pentafluorophenyl, 5α-cholestan-3α-yl, or 5α-cholestan-3β-yl isocyanide or 1-methylimidazole, a concentration-independent relaxation is observed. Its decay is accuratel exponential. Therefore the geminate pair created by photolysis disappears in a clear first-order process and does not follow the power law kinetics reported for similar systems with other ligands in high-viscosity solvents or in glasses at low temperatures

The picosecond photolysis of protoheme dimethyl ester bis(tert-butyl isocyanide) in pure tert-butyl isocyani in toluene results in loss of one isocyanide to produce an electronically excited five-coordinated species. The excicited state decays to ground-state five-coordinated heme within 40 ps. In contrast to chelated protoheme-tert-butyl isocyanide or imidazole-protoheme dimethyl ester-tert-butyl isocyanide, no cage return is seen.

A remarkable correlation has been discovered between fluorescence lifetimes of bound NADPH and rates of hydride transfer among mutants of dihydrofolate reductase (DHFR) from Escherichia coli. Rates of hydride transfer from NADPH to dihydrofolate change by a factor of 1,000 for the series of mutant enzymes. Since binding constants for the initial complex between coenzyme and DHFR change by only a factor of 10, the major portion of the change in hydride transfer must be attributed to losses in transition-state stabilization. The time course of fluorescence decay for NADPH bound to DHFR is biphasic. Lifetimes ranging from 0.3 to 0.5 ns are attributed to a solvent-exposed dihydronicotinamide conformation of bound coenzyme which is presumably not active in catalysis, while decay times (tau 2) in the range of 1.3 to 2.3 ns are assigned to a more tightly bound species of NADPH in which dihydronicotinamide is sequestered from solvent. It is this slower component that is of interest. Ternary complexes with three different inhibitors, methotrexate, 5-deazafolate, and trimethoprim, were investigated, along with the holoenzyme complex; 3-acetylNADPH was also investigated. Fluorescence polarization decay, excitation polarization spectra, the temperature variation of fluorescence lifetimes, fluorescence amplitudes, and wavelength of absorbance maxima were measured. We suggest that dynamic quenching or internal conversion promotes decay of the excited state in NADPH-DHFR. When rates of hydride transfer are plotted against the fluorescence lifetime (tau 2) of tightly bound NADPH, an unusual correlation is observed. The fluorescence lifetime becomes longer as the rate of catalysis decreases for most mutants studied. However, the fluorescence lifetime is unchanged for those mutations that principally alter the binding of dihydrofolate while leaving most dihydronicotinamide interactions relatively undisturbed. The data are interpreted in terms of possible dynamic motions of a flexible loop region in DHFR which closes over both substrate and coenzyme binding sites. These motions could lead to faster rates of fluorescence decay in holoenzyme complexes and, when correlated over time, may be involved in other motions which give rise to enhanced rates of catalysis in DHFR.

The effect of pressure on the recombination kinetics of small ligands binding to sperm whale myoglobin, protoheme dimethyl ester, and monochelated protoheme was studied with use of laser flash photolysis. The volumes of activation observed indicate that in both the protein and the models bond formation is the rate-determining step only for carbon monoxide, while for oxygen, isocyanides, and 1-methylimidazole almost no bond formation occurs in the transition state of the observed reaction. The effect of pressure on the escape of carbon monoxide, oxygen, and methyl isocyanide from the heme pocket of sperm whale myoglobin was also investigated

Steric effects on the binding of isonitrile ligands to iron(II) porphyrins were investigated by picosecond flash photolysis. Two different types of steric effects were distinguished and characterized: (1) steric restrictions to porphyrin planarity and (2) blocking of the pathway for ligand approach. Heme planarity was restricted by coordinating 1,2-dimethylimidazole trans to the ligand binding site being investigated. Blocking of the binding site was explored by using adamantane heme 6,6-cyclophane, in which the adamantane moiety forms a cap over the binding site. Results of picosecond kinetic measurements demonstrate that the first effect, heme nonplanarity or trans strain, influences the bond-making step, whereas the second effect, ligand blocking, involves a conformational preequilibrium prior to bond making. Relevance of these findings for contact pair recombination, in general, and for heme protein ligation, in particular, are discussed.

The kinetics of geminate recombination were studied for the methylisocyanide derivative of carp hemoglobin. Carp hemoglobin is of interest because it has been established that the fully liganded form switches between a high affinity R state at pH 9 and a low affinity T state at pH 6 in the presence of IHP. Geminate recombination was observed on both the picosecond and the nanosecond time scales under all conditions; however, only a small variation is observed in the rates and the yields of geminate recombination as the protein switches from the R to the T state. Taken together with overall "on" and "off" rates, the data indicate that the change from the R to the T configuration affects bond breaking most, but also influences subsequent escape from the protein as well as both entry into the protein and bond formation. There is some reason to postulate tertiary conformational change in the T state on the microsecond time scale following ligand escape from the protein.

A family of heteroleptic RuII coordination complexes containing substituted 1,10-phenanthroline (phen) ligands with extended conjugation was found to exhibit two simultaneously emissive excited states at room temperature in fluid solution. These systems demonstrate a breakdown of the standard non-radiative decay pathways that normally lead to a single, dominant, lowest energy emissive excited state in RuII complexes and most other chromophores. The structural requirements for dual emission were explored through the synthesis and characterization of isomeric systems. Two features were found to be primarily responsible for resolvable dual emission. Extended conjugation at the 4-position of the 1,10-phenanthroline ligand was identified as an essential feature, and asymmetry in the phenanthroline ligand substitutions appears to greatly facilitate the production of these two non-equilibrated emissive states. Additional complexes were studied which displayed “tunable” emissive characteristics for the two excited states as a function of covalent and non-covalent modification.

Dual emission is observed from a family of simple acetylene-linked dinuclear RuII complexes, where two MLCT excited states coexist at room temperature and in fluid solution. This unique behavior is attributed to a specific substitution pattern on the bridging ligand and provides the opportunity to investigate the structural and electronic features that result in decoupling of standard nonradiative decay pathways.

Oxygenase and reductase domains in nitric oxide synthase are linked by a peptide region that binds calmodulin. Here we study the effects of modifying the length of the interdomain linker in a deletion mutant lacking 15 amino acids (residues 503-517) in bovine eNOS. The kinetics of CO ligation with the mutant were determined in the presence and absence of tetrahydrobiopterin and arginine and compared with the CO binding kinetics of wild-type eNOS and the eNOS oxygenase domain. In the mutant, electron flow is interrupted. The association kinetics of CO with both mutant and wild-type eNOS can be approximated with two kinetic phases, but the relative proportions change in the mutant. Both the abrogation of electron flow in the mutant and the differences in CO binding may be explained by an alteration in the docking of the FMN domain to the heme domain. We propose that the calmodulin binding residues form a helix that is critical for the proper alignment of the adjacent reductase and oxygenase domains within the active eNOS dimer in achieving proper electron transfer between them.

Absolute fluorescence quantum yields are reported for the rhodamine 6G cation and the fluorescein dianion dyes in nine solvents. This information is combined with previously reported fluorescence lifetimes to deduce radiative and nonradiative decay rates. Along the alcohol series from methanol to octanol, rhodamine 6G displays an increasing radiative rate, in parallel with the square of the refractive index increase, and a slightly decreasing nonradiative rate. Fluorescein is different: the apparent radiative rate actually decreases, suggesting that the emissive species is perturbed in some fashion. For both dyes, fluorescence yields are enhanced in D2O, rising to 0.98, in parallel with a corresponding increase in lifetimes. Protonated solvents invariably give shorter lifetimes and lower quantum yields, contrary to some previous speculation. From this work and an analysis of existing literature values, more precise values have been obtained for two previously proposed absolute quantum yield standards. The yield of fluorescein in 0.1 N NaOH(aq) is 0.925+/-0.015, and for rhodamine 6G in ethanol, it is 0.950+/-0.015. In both cases, the solutions are assumed to be in the limit of low concentration, excited close to their long-wave absorption band and at room temperature but may be either air-saturated or free of oxygen.

Nitric-oxide synthases (NOS) catalyze the conversion of l-arginine to NO, which then stimulates many physiological processes. In the active form, each NOS is a dimer; each strand has both a heme-binding oxygenase domain and a reductase domain. In neuronal NOS (nNOS), there is a conserved cysteine motif (CX(4)C) that participates in a ZnS(4) center, which stabilizes the dimer interface and/or the flavoprotein-heme domain interface. Previously, the Cys(331) --Ala mutant was produced, and it proved to be inactive in catalysis and to have structural defects that disrupt the binding of l-Arg and tetrahydrobiopterin (BH(4)). Because binding l-Arg and BH(4) to wild type nNOS profoundly affects CO binding with little effect on NO binding, ligand binding to the mutant was characterized as follows. 1) The mutant initially has behavior different from native protein but reminiscent of isolated heme domain subchains. 2) Adding l-Arg and BH(4) has little effect immediately but substantial effect after extended incubation. 3) Incubation for 12 h restores behavior similar but not quite identical to that of wild type nNOS. Such incubation was shown previously to restore most but not all catalytic activity. These kinetic studies substantiate the hypothesis that zinc content is related to a structural rather than a catalytic role in maintaining active nNOS.

Previous work has proved that the enzyme-soluble guanylate cyclase, GC, is activated several 100-fold by the combination of carbon monoxide plus a benzylindazole derivative called YC-1. That is about the same as activation by nitric oxide, which has a well-established role both in vivo and in vitro. This report addresses several spectroscopic, equilibrium, and kinetic effects wrought by YC-1 on carboxyl guanylate cyclase, including the following: a shift in the Soret absorption band by 4 nm to shorter wavelength; an increase in CO affinity by an order of magnitude; a dramatic change in the kinetics of CO association. After photolytic dissociation of CO, the majority, but not all, of bimolecular ligand recombination occurs with a time constant about 1000-fold faster than in the absence of YC-1, while a smaller fraction recombines almost, but not quite, the same as usual. This is reminiscent of the kinetics of NO association with GC, which also shows two prominent phases. The results just listed pertain in the presence of GTP/cGMP, which would be present during enzyme catalysis. Qualitatively similar, but smaller, effects occur in the absence of GTP/cGMP. Measurements are reported to characterize other changes in buffer conditions. The results are consistent with a mechanistic model that attributes a crucial role to the proximal bond that connects the heme iron to a histidine side chain in GC but also requires protein control of the distal environment.

Nitric-oxide synthase (NOS) catalyzes conversion of L-arginine to nitric oxide, which subsequently stimulates a host of physiological processes. Prior work suggests that NOS is inhibited by NO, providing opportunities for autoregulation. This contribution reports that NO reacts rapidly (ka congruent with 2 x 10(7) M-1 s-1) with neuronal NOS in both its ferric and ferrous oxidation states. Association kinetics are almost unaffected by L-arginine or the cofactor tetrahydrobiopterin. There is no evidence for the distinct two phases previously reported for association kinetics of CO. Small amounts of geminate recombination of NO trapped in a protein pocket can be observed over nanoseconds, and a much larger amount is inferred to take place at picosecond time scales. Dissociation rates are also very fast from the ferric form, in the neighborhood of 50 s-1, when measured by extrapolating association rates to the zero NO concentration limit. Scavenging experiments give dissociation rate constants more than an order of magnitude slower: still quite fast. For the ferrous species, extrapolation is not distinguishable from zero, while scavenging experiments give a dissociation rate constant near 10(-4) s-1. Implications of these results for interactions near the heme binding site are discussed.

Recently it has been reported that in the presence of YC-1, a benzyl indazole derivative, carbon monoxide activates soluble guanylate cyclase, GC, to about the same extent as its best known activator, nitric oxide. Kinetic studies utilizing flash photolysis of GC complexed with CO in the presence and absence of YC-1 show, in contrast to another recent report of a mixing experiment, that YC-1 has a profound effect on bimolecular association kinetics and a smaller, but significant, effect on ligand affinity. Most prominent is the appearance of a major, new phase in the bimolecular recombination kinetics in the presence of 200 microM YC-1: This major fraction rebinds CO approximately 1000-fold more rapidly than in the absence of YC-1. Another portion, considerably less than half, exhibits kinetics that are almost exactly the same as in the absence of YC-1. It is now clear that both YC-1 and CO have a strong synergistic effect on enzyme activity and also a dramatic effect on ligand binding behavior. It is, therefore, a reasonable inference that ligand binding at the heme iron atom is intimately connected with enzyme activation, a hypothesis that would have been difficult to maintain if the earlier report, that YC-1 has no effect on CO binding, were correct. Possible reasons for the discrepancy between the two measurements are suggested.

Changes in mean arterial pressure were monitored in rats following 50% isovolemic exchange transfusion with solutions of chemically modified hemoglobins. Blood pressure responses fall into three categories: 1) an immediate and sustained increase, 2) an immediate yet transient increase, or 3) no significant change either during or subsequent to exchange transfusion. The reactivities of these hemoglobins with nitric monoxide (⋅NO) were measured to test the hypothesis that different blood pressure responses to these solutions result from differences in ⋅NO scavenging reactions. All hemoglobins studied exhibited a value of 30 μm −1 s−1 for both⋅NO bimolecular association rate constants and the rate constants for ⋅NO-induced oxidation in vitro. Only the ⋅NO dissociation rate constants and, thus, the equilibrium dissociation constants varied. Values of equilibrium dissociation constants ranged from 2 to 14 pm and varied inversely with vasopressor response. Hemoglobin solutions that exhibited either transient or no significant increase in blood pressure showed tighter⋅NO binding affinities than hemoglobin solutions that exhibited sustained increases. These results suggest that blood pressure increases observed upon exchange transfusion with cell-free hemoglobin solutions can not be the result of ⋅NO scavenging reactions at the heme, but rather must be due to alternative physiologic mechanisms.

Kinetic studies of soluble guanylate cyclase complexed with nitric oxide prove that NO dissociation in the presence of the substrate GTP and Mg2+ is as much as 50 times faster than in their absence. In the presence of those two reagents the dissociation rate constant is k(obs) = 0.04 +/- 0.01 s-1 at 20 degrees C, which is by far the fastest NO dissociation rate constant ever reported for a ferrous heme protein. Extrapolated to 37 degrees C, this corresponds to a half life of about 5 s for NO dissociation from soluble guanylate cyclase at physiological conditions, which is presumably fast enough to account for deactivation of the enzyme in biological systems. Dissociation rate constants are also reported for a variety of other reagent conditions.

The kinetics of tert-butyl isocyanide binding to the heme protein horseradish peroxidase (HRP) at 22 degrees C was examined on all time scales, from minutes to picoseconds, in aqueous borate buffer at pH 9.08. Unlike myoglobin (Mb) or hemoglobin, HRP shows two bimolecular ligand binding processes. For comparison, binding of the same ligand with Mb was measured under identical conditions. Ligand entry into the protein from the solvent in a mixing experiment is extremely slow in HRP: the bimolecular association constant is 0.04 M-1 s-1, while in Mb it is 4 x 10(3) M-1 s-1. Surprisingly, in view of that difference, picosecond and nanosecond photolyses reveal that once the ligand has reached the iron(II) site there is no difference in cage return or escape from the protein. The rate for the fastest cage return (from the contact pair) is close to 6 x 10(10) s-1 in both proteins. The rates of escape from the contact pair to form a secondary protein-caged pair are also similar: for Mb, 10 x 10(10) s-1, and for HRP, 8.5 x 10(10) s-1. The rate of rebinding from the protein-separated cage is near 4 x 10(6) s-1 in both proteins, and the rate of escape from protein to solvent is close to 3.7 x 10(6) s-1 in both. The difference between the two proteins lies in the low-millisecond time domain. After flash photolysis of HRP, there is a concentration-dependent recombination not seen in mixing experiments. This bimolecular rate constant varies slightly for different HRP preparations, being 2.6 x 10(4) or 4.0 x 10(4) M-1 s-1 in two cases, both of which are much faster than is observed in mixing experiments, namely, 0.04 M-1 s-1. In Mb, photolysis and mixing experiments consistently give the same combination rate, which is somewhat slower than the faster part of the HRP recombination. Similar measurements for the smaller ligand methyl isocyanide revealed no anomalous behavior. The interpretation proposed involves tertiary relaxation after ligand escape, which is significant in blocking the return of the large t-BuNC, but has no apparent effect on smaller ligands. Thus, HRP-t-BuNC reveals in dramatic fashion a phenomenon merely hinted at in earlier work involving the T-state binding kinetics of hemoglobin.

The kinetics of geminate recombination of horse heart myoglobin with the diatomic ligands carbon monoxide, dioxygen, and nitric oxide have been reexamined. The new measurements are distinguished from previous studies by (1) consideration of the complete time range longer than 1 ps, (2) inclusion of the effect of temperature changes near ambient, (3) attention to the relation between recombination kinetics and the yield of dissociated partners on the millisecond time scale, and (4) use of singular value decomposition in the analysis. These picosecond results, together with earlier nanosecond data, for O2 prove that models incorporating one, two, or even three discrete intermediates are not sufficient to account for all features of geminate recombination kinetics. Instead, a continuous evolution of the geminate pair distribution is preferred.

Ligand photolysis and subsequent recombination in cytochromes b5 and c have been studied with picosecond resolution. In both proteins, an iron-histidine bond is broken after excitation with 314-nm light, and recombination occurs with a rate constant of about 1.4 x 10(11) s-1. Photolysis and reformation of the iron-histidine bond may be surprising as these hemoproteins do not reversibly bind ligands in nature. The findings are explained using results both from experiments on model hemes and from computer investigations with atomic resolution on the three-dimensional structure of the protein. After photolysis, the formation and recombination of the geminate contact pair are attributed to simple low amplitude ligand bond rotations, a result that can be applied to geminate processes in other hemoproteins and model heme compounds as well.

The geminate recombination of CO with Hb following dissociation by a 10-ns laser pulse has been studied as a function of pH (9.2 and 7.0 without inositol hexaphosphate and 6.0 with inositol hexaphosphate) and temperature (5-35 degrees C). The hemoglobins studied included adult, Rothschild, rabbit, opossum, and carp. Despite significant differences in their structural and functional properties, the first four of these hemoglobins show similar trends in the yields, rates, and activation energies of the geminate recombination. The nature of the "cage recombination" in hemoglobin is discussed in the light of such findings. Neither a slow diffusion model nor a model based upon a specific non-heme binding site accounts for the observations.

Rabbit hemoglobin shows significantly lower affinity for CO than does human hemoglobin (Hb A). The overall ligand combination and dissociation rate constants reveal, however, only small differences between Hb A rabbit Hb; this is mainly due to the fact that beta chains in rabbit hemoglobin determine the kinetics of ligand dissociation and combination. The heme environment in these chains is probably not very different in rabbit Hb and human Hb A. Rabbit hemoglobin alpha chains, on the other hand, exhibit greatly reduced CO and O2 combination rates in the R state and are primarily responsible for the overall low CO affinity of rabbit Hb. We postulate that the low ligand affinity of alpha chains in rabbit Hb is due to the substitution of larger residues at positions B10(Leu leads to Val), Cd6(Leu leads to Phe), and CD7(Ser leads to Thr). The possible implication of such substitutions for the kinetic and equilibrium properties of rabbit Hb are discussed.

Laser-excited luminescence lifetimes of lanthanide ions bound to bacteriorhodopsin have been measured in deionized membranes. The luminescence titration curve, as well as the binding curve of apomembrane (retinal-free) with Eu3+, has shown that the removal of the retinal does not significantly affect the affinity of Eu3+ for the two high affinity sites of bacteriorhodopsin. The D2O effects on decay rate constants indicate that Eu3+ bound to the high affinity sites of native membrane or apomembrane is coordinated by about six ligands in the first coordination sphere. Tb3+ is shown to be coordinated by four ligands. The data indicate that metal ions bind to the protein with a specific geometry. From intermetal energy transfer experiments using Eu3+-Pr3+, Tb3+-Ho3+, and Tb3+-Er3+, the distance between the two high affinity sites is estimated to be 7-8 A.

We describe a method for determining chemical kinetic constants and diffusion coefficients by measuring the rates of decay of spontaneous concentration fluctuations. The equilibrium of the system is not disturbed during the measurement. We measure the number of molecules of a specified type in a defined open volume as a function of time and compute the time course of the deviations from the thermodynamic mean concentration. The method is based on the principle that the rates of decay of spontaneous microscopic fluctuations are determined by the same phenomenological rate coefficients as those of macroscopic departures from equilibrium which result from external perturbations. Hence, an analysis of fluctuations yields the same chemical rate constants and diffusion coefficients as are measured by conventional procedures. In practice the number of the specified molecules is measured by a property such as absorbance or fluorescence which is specific and sensitive to chemical change. The sample volume is defined by a light beam which traverses the cell. As the molecules appear in or disappear from the light beam, either due to diffusion or chemical reaction, their concentration fluctuations give rise to corresponding fluctuations of the intensity of absorbed or emitted light. This paper presents the theory needed to derive chemical rate constants and diffusion coefficients from these fluctuations in light intensity. The theory is applied to three examples of general interest: pure diffusion in the absence of chemical reaction; the binding of a small rapidly diffusing ligand to a larger slowly diffusing macromolecule; and a unimolecular isomerization. The method should be especially useful in studying highly cooperative systems, relatively noncooperative systems with intermediate states closely spaced in free energy, small systems, and systems not readily subject to perturbations of state.

— Time-resolved fluorescence decay profiles following picosecond excitation of plant chloro-plasts or algae are now being measured with high precision and reproducibility. A number of suggestions have been put forward to explain the complex decay kinetics which are observed. In this paper three models are defined explicitly, solved rigorously, and compared in detail with experimental data. The complete description of energy transport in chloroplasts is no doubt very complicated, and only such quantitative comparisons will make it possible to decide which models are consistent with experiments. Of the three models examined, only the heterogeneous bipartite model has solutions which agree with commonly agreed upon features of the data. It offers, therefore, one starting point for more elaborate models and a guide to the design of further experiments.

— Fluorescence lifetimes of spinach chloroplasts were measured with a modelocked dye laser and time-correlated single photon counting. Information about energy transport and functional organization of the chloroplasts is revealed by such time-resolved fluorescence studies. Quenching experiments using treatment with UV light or the chemical agent dibromothymoquinone are consistent with the notion that there is heterogeneity associated with PS II units and that such heterogeneity is reflected over the entire time range of fluorescence decay, not just in a single component. Phosphorylation experiments were also carried out which permit us to relate these kinetic studies to previous steady state observations.

synopsis This paper describes the first experimental application of fluorescence correlation spectroscopy, a new method for determining chemical kinetic constants and diffusion coefficients. These quantities are measured by observing the time behavior of the tiny concentration fluctuations which occur spontaneously in the reaction system even when it is in equilibrium. The equilibrium of the system is not disturbed during the experiment. The diffusion coefficients and chemical rate constants which determine the average time behavior of these spontaneous fluctuations are the same as those sought by more conventional methods including temperature-jump or other perturbation tecliniques. The experiment consists essentially in measuring the variation with time of the number of molecules of specified reactants in a defined open volume of solution. The concentration of a reactant is measured by its fluorescence; the sample volume is defined by a focused laser beam which excites the fluorescence. The fluorescent emission fluctuates in proportion with the changes in the number of fluorescent molecules as they diffuse into and out of the sample volume and as they are created or eliminated by the chemical reactions. The number of these reactant molecules must be small to permit detection of the concentration fluctuations. Hence the sample volume is small (10-8 rnl) and the concentration of the solutes is low (-10-9 M). We have applied this technique to the study of two prototype systems: the simple example of pure diffusion of a single fluorescent species, rhodamine 6G, and the more interesting but more challenging example of the reaction of macromolecular DNA with the drug ethidium bromide to form a fluorescent complex. The increase of the fluorescence of the ethidium bromide upon formation of the complex permits the observation of the decay of concentration fluctuations via the chemical reaction and consequently the determination of chemical rate constants.