Search

W(CNAr)₆ (CNAr = arylisocyanide) photoreductants catalyze base-promoted homolytic aromatic substitution (BHAS) of 1-(2-iodobenzyl)-pyrrole in deuterated benzene. Moderate to high efficiencies correlate with W(CNAr)₆ excited-state reduction potentials upon one-photon 445 nm excitation, with 10 mol % loading of the most powerful photoreductants W(CNDipp)₆ (CNDipp = 2,6-diisopropylphenylisocyanide) and W(CNDippPh^(OMe3))6 (CNDippPh^(OMe3) = 4-(3,4,5-trimethoxyphenyl)-2,6-diisopropylphenylisocyanide) affording nearly complete conversion. Stern–Volmer quenching experiments indicated that catalysis is triggered by substrate reductive dehalogenation. Taking advantage of the large two-photon absorption (TPA) cross sections of W(CNAr)₆ complexes, we found that photocatalysis can be driven with femtosecond-pulsed 810 nm excitation. For both one- and two-photon excitation, photocatalysis was terminated by the formation of seven-coordinate WII-diiodo [WI₂(CNAr)₅] complexes. Notably, we discovered that W(CNDipp)₆ can be regenerated by chemical reduction of WI2(CNDipp)5 with excess ligand present in solution.

Modular syntheses of oligoarylisocyanide ligands that are derivatives of 2,6-diisopropylphenyl isocyanide (CNdipp) have been developed; tungsten complexes incorporating these oligoarylisocyanide ligands exhibit intense metal-to-ligand charge-transfer visible absorptions that are red-shifted and more intense than those of the parent W(CNdipp)_6 complex. Additionally, these W(CNAr)_6 complexes have enhanced excited-state properties, including longer lifetimes and very high quantum yields. The decay kinetics of electronically excited W(CNAr)_6 complexes (*W(CNAr)_6) show solvent dependences; faster decay is observed in higher dielectric solvents. *W(CNAr)_6 lifetimes are temperature dependent, suggestive of a strong coupling nonradiative decay mechanism that promotes repopulation of the ground state. Notably, *W(CNAr)_6 complexes are exceptionally strong reductants: [W(CNAr)_6]+/*W(CNAr)_6 potentials are more negative than −2.7 V vs [Cp_2Fe]^+/Cp_2Fe.

We report a new metallolabeled blue copper protein, Re126W122Cu(I) Pseudomonas aeruginosa azurin, which has three redox sites at well-defined distances in the protein fold: Re(I)(CO)3(4,7-dimethyl-1,10-phenanthroline) covalently bound at H126, a Cu center, and an indole side chain W122 situated between the Re and Cu sites (Re-W122(indole) = 13.1 Å, dmp-W122(indole) = 10.0 Å, Re-Cu = 25.6 Å). Near-UV excitation of the Re chromophore leads to prompt Cu(I) oxidation (50 ns), followed by slow back ET to regenerate Cu(I) and ground-state Re(I) with biexponential kinetics, 220 ns and 6 μs. From spectroscopic measurements of kinetics and relative ET yields at different concentrations, it is likely that the photoinduced ET reactions occur in protein dimers, (Re126W122Cu(I))2 and that the forward ET is accelerated by intermolecular electron hopping through the interfacial tryptophan: *Re//←W122←Cu(I), where // denotes a protein-protein interface. Solution mass spectrometry confirms a broad oligomer distribution with prevalent monomers and dimers, and the crystal structure of the Cu(II) form shows two Re126W122Cu(II) molecules oriented such that redox cofactors Re(dmp) and W122-indole on different protein molecules are located at the interface at much shorter intermolecular distances (Re-W122(indole) = 6.9 Å, dmp-W122(indole) = 3.5 Å, and Re-Cu = 14.0 Å) than within single protein folds. Whereas forward ET is accelerated by hopping through W122, BET is retarded by a space jump at the interface that lacks specific interactions or water molecules. These findings on interfacial electron hopping in (Re126W122Cu(I))2 shed new light on optimal redox-unit placements required for functional long-range charge separation in protein complexes.

The homoleptic arylisocyanide tungsten complexes, W(CNXy)_6 and W(CNIph)_6 (Xy = 2,6-dimethylphenyl, Iph = 2,6-diisopropylphenyl), display intense metal to ligand charge transfer (MLCT) absorptions in the visible region (400–550 nm). MLCT emission (λ_max ≈ 580 nm) in tetrahydrofuran (THF) solution at rt is observed for W(CNXy)6 and W(CNIph)_6 with lifetimes of 17 and 73 ns, respectively. Diffusion-controlled energy transfer from electronically excited W(CNIph)_6 (*W) to the lowest energy triplet excited state of anthracene (anth) is the dominant quenching pathway in THF solution. Introduction of tetrabutylammonium hexafluorophosphate, [Bun4N][PF_6], to the THF solution promotes formation of electron transfer (ET) quenching products, [W(CNIph)6]+ and [anth]^•–. ET from *W to benzophenone and cobalticenium also is observed in [Bun4N][PF6]/THF solutions. The estimated reduction potential for the [W(CNIph)6]^(+)/*W couple is −2.8 V vs Cp_(2)Fe^(+/0), establishing W(CNIph)_6 as one of the most powerful photoreductants that has been generated with visible light.

Co-diglyoxime complexes catalyze H(2) evolution from protic solutions at modest overpotentials. Upon reduction to Co(I), a Co(III)-hydride is formed by reaction with a proton donor. Two pathways for H(2) production are analyzed: one is a heterolytic route involving protonation of the hydride to release H(2) and generate Co(III); the other is a homoytic pathway requiring association of two Co(III)-hydrides. Rate constants and reorganization parameters were estimated from analyses of laser flash-quench kinetics experiments (Co(III)-Co(II) self-exchange k = 9.5 x 10(-8) - 2.6 x 10(-5) M(-1) s(-1); lambda = 3.9 (+/-0.3) eV: Co(II)-Co(I) self-exchange k = 1.2 (+/-0.5) x 10(5) M(-1) s(-1); lambda = 1.4 (+/-0.05) eV). Examination of both the barriers and driving forces associated with the two pathways indicates that the homolytic reaction (Co(III)H + Co(III)H --> 2 Co(II) + H(2)) is favored over the route that goes through a Co(III) intermediate (Co(III)H + H(+) --> Co(III) + H(2)).

In a continuing effort to unravel mechanistic questions associated with metalloenzymes, we are developing methods for rapid delivery of electrons to deeply buried active sites. Herein, we report picosecond reduction of the heme active site of inducible nitric oxide synthase bound to a series of rhenium−diimine electron-tunneling wires, [Re(CO)_3LL‘]^+, where L is 4,7-dimethylphenanthroline and L‘ is a perfluorinated biphenyl bridge connecting a rhenium-ligated imidazole or aminopropylimidazole to a distal imidazole (F_8bp-im (1) and C_3-F_8bp-im (2)) or F (F_9bp (3) and C_3-F_9bp (4)). All four wires bind tightly (K_d in the micromolar to nanomolar range) to the tetrahydrobiopterin-free oxidase domain of inducible nitric oxide synthase (iNOSoxy). The two fluorine-terminated wires displace water from the active site, and the two imidazole-terminated wires ligate the heme iron. Upon 355-nm excitation of iNOSoxy conjugates with 1 and 2, the active site Fe(III) is reduced to Fe(II) within 300 ps, almost 10 orders of magnitude faster than the naturally occurring reduction.

We report the synthesis and characterization of Ru-diimine complexes designed to bind to cytochrome p450cam (CYP101). The sensitizer core has the structure [Ru(L(2))L'](2+), where L' is a perfluorinated biphenyl bridge (F(8)bp) connecting 4,4'-dimethylbipyridine to an enzyme substrate (adamantane, F(8)bp-Ad), a heme ligand (imidazole, F(8)bp-Im), or F (F(9)bp). The electron-transfer (ET) driving force (-deltaG degrees ) is varied by replacing the ancillary 2,2'-bipyridine ligands with 4,4',5,5'-tetramethylbipyridine (tmRu). The four complexes all bind p450cam tightly: Ru-F(8)bp-Ad (1, K(d) = 0.077 microM); Ru-F(8)bp-Im (2, K(d) = 3.7 microM); tmRu-F(9)bp (3, K(d) = 2.1 microM); and tmRu-F(8)bp-Im (4, K(d) = 0.48 microM). Binding is predominantly driven by hydrophobic interactions between the Ru-diimine wires and the substrate access channel. With Ru-F(8)bp wires, redox reactions can be triggered on the nanosecond time scale. Ru-wire 2, which ligates the heme iron, shows a small amount of transient heme photoreduction (ca. 30%), whereas the transient photoreduction yield for 4 is 76%. Forward ET with 4 occurs in roughly 40 ns (k(f) = 2.8 x 10(7) s(-)(1)), and back ET (Fe(II) --Ru(III), k(b) approximately 1.7 x 10(8) s(-)(1)) is near the coupling-limited rate (k(max)). Direct photoreduction was not observed for 1 or 3. The large variation in ET rates among the Ru-diimine:p450 conjugates strongly supports a through-bond model of Ru-heme electronic coupling.

Rates of reduction of Os(III), Ru(III), and Re(I) by Cu(I) in His83-modified Pseudomonas aeruginosa azurins (M-Cu distance approximately 17 A) have been measured in single crystals, where protein conformation and surface solvation are precisely defined by high-resolution X-ray structure determinations: 1.7(8) x 10(6) s(-1) (298 K), 1.8(8) x 10(6) s(-1) (140 K), [Ru(bpy)2(im)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(bpy)(3+)-]; 3.0(15) x 10(6) s(-1) (298 K), [Ru(tpy)(phen)(3+)-]; 9.0(50) x 10(2) s(-1) (298 K), [Os(bpy)2(im)(3+)-]; 4.4(20) x 10(6) s(-1) (298 K), [Re(CO)3(phen)(+)] (bpy = 2,2'-bipyridine; im = imidazole; tpy = 2,2':6',2' '-terpyridine; phen = 1,10-phenanthroline). The time constants for electron tunneling in crystals are roughly the same as those measured in solution, indicating very similar protein structures in the two states. High-resolution structures of the oxidized (1.5 A) and reduced (1.4 A) states of Ru(II)(tpy)(phen)(His83)Az establish that very small changes in copper coordination accompany reduction but reveal a shorter axial interaction between copper and the Gly45 peptide carbonyl oxygen [2.6 A for Cu(II)] than had been recognized previously. Although Ru(bpy)2(im)(His83)Az is less solvated in the crystal, the reorganization energy for Cu(I) --Ru(III) electron transfer falls in the range (0.6-0.8 eV) determined experimentally for the reaction in solution. Our work suggests that outer-sphere protein reorganization is the dominant activation component required for electron tunneling.

The luminescence lifetime of Ru(tpy)_2^(2+) (tpy = 2,2‘:6,2‘ ‘-terpyridine) is 8.0 μs in H_2SO_4/H_2O and HSO_3F/H_2O glasses (25% v/v) at 77 K, and 10.2 μs in D_2SO_4/D_2O (25% v/v). Addition of moderate concentrations of the powerfully oxidizing Fe(OH_2)_6^(3+) ion to the glasses leads to accelerated and highly nonexponential *Ru(tpy)_2^(2+) decay kinetics. The quenching is attributed to electron transfer from *Ru(tpy)_2^(2+) to randomly dispersed Fe(OH_2)_6^(3+) complexes. The luminescence decay kinetics and quantum yields in the three aqueous glasses indicate that the electron-transfer rate constants decrease from ∼10^(13) s^(-1) at van der Waals contact with an exponential distance decay constant of 1.68 ± 0.07 Å^(-1).

Ferrocytochrome c (Fe(II)cyt c) is ∼10 kcal/mol (410 meV) more stable toward unfolding than Fe(III)cyt c, owing mainly to the stabilization of the ferroheme by hydrophobic encapsulation and enhanced iron−methionine bonding. To determine the magnitudes of these two components, we have measured the binding constants of N-acetylmethionine (AcMet) and imidazole to ferric and ferrous N-acetylmicroperoxidase-8 (AcMP8), a heme-containing proteolytic fragment of cyt c. Our results show that the AcMet affinity of the heme significantly increases upon reduction, as confirmed by electrochemical measurements, and this increase accounts for 130 meV of the 410-meV stability difference between Fe(II)- and Fe(III)cyt c. A 240-mV upshift in reduction potential in the folded protein is attributed mainly to water exclusion from the heme environment, based on an analysis of the potentials of eight structurally characterized c-type cytochromes. Our analysis shows that the potentials of heme proteins can be tuned by roughly 50...

Photoexcited [Ru(bpy)3]2+ (bpy = 2,2‘-bipyridine) can be quenched with [Co(NH3)5Cl]2+ to give [Ru(bpy)3]3+; this photogenerated oxidant (E0 = 1.25 V vs NHE) reacts with horseradish peroxidase isoenzyme c (HRPc) to produce oxidized protein species. Spectra and kinetics measured by laser-flash transient spectroscopy show that oxidation of the trivalent resting state, [PFeIII]+ (P = porphyrin dianion), to ferryl compound II, [PFeIVO], is preceded by generation of a π-cation porphyrin radical intermediate, [PFeIII]•2+. In the interval 7.8 < pH < 9.8, the rate-limiting step for the transformation of the radical intermediate to compound II is the binding of a water molecule to the five-coordinate heme iron, kobsd = (4.1 ± 0.9) s-1; this step is followed by fast proton and electron transfer to give the ferryl species. There is a burst in compound II formation in the pH region (10.3 < pH < 10.8) in which the heme iron changes from a five-coordinate, high-spin species to a six-coordinate, low-spin complex (pKa = 1...

The electron-transfer reactivity of microperoxidase-8 (MP8), the heme octapeptide derived from enzymatic cleavage of cytochrome c, has been studied by nanosecond flash photolysis methods. Ferric MP8 is rapidly oxidized by photogenerated Ru(bpy)33+ in acidic solutions to a ferric cation-radical porphyrin (k ∼ 5.6 × 109 M-1 s-1); the oxidation product in alkaline solutions is ferryl MP8 (k ∼ 2.2 × 109 M-1 s-1). Numerical simulations of the kinetics for the direct oxidation of ferric to ferryl MP8 predict a marked pH effect on the rate of reaction in alkaline solutions; however, only a very weak pH dependence is observed in the range 7−8.5, indicating that the ferryl species is generated by intramolecular electron transfer within a ferric cation-radical porphyrin. Transient spectra taken between pH 6 and 8.5 show increasing ferryl absorption as the pH is increased, demonstrating a pH-dependent equilibrium between the two oxidized forms of MP8.

The rates of Ru(His33)cytochrome c electron-transfer (ET) reactions have been measured over a driving-force range of 0.59 to 1.89 eV. The driving-force dependence of Fe2+ → Ru3+ ET in RuL2(im)(His33)cyt c [L = 2,2‘-bipyridine (bpy), 4,4‘,5,5‘-tetramethyl-2,2‘-bipyridine (4,4‘,5,5‘-(CH3)4-bpy), 4,4‘-dimethyl-2,2‘-bipyridine (4,4‘-(CH3)2-bpy), 4,4‘-bis(N-ethylcarbamoyl)-2,2‘-bipyridine (4,4‘-(CONH(C2H5))2-bpy), 1,10-phenanthroline (phen); im = imidazole] is well described by semiclassical ET theory with kmax = 2.7 × 106 s-1 (HAB = 0.095 cm-1) and λ = 0.74 eV. As predicted by theory, the rate of an exergonic (−ΔG° = 1.3 eV) heme reduction reaction, *Ru2+(bpy)2(im)(His) → Fe3+, falls in the inverted region (k = 2.0 × 105 s-1). In contrast, the rates of three highly exergonic heme reductions, *Ru2+(phen)2(CN)(His) → Fe3+ (2.0 × 105 s-1; 1.40 eV), Ru+(4,4‘-(CONH(C2H5))2-bpy)2(im)(His) → Fe3+ (2.3 × 105 s-1; 1.44 eV), and Ru+(phen)2(CN)(His) → Fe3+ (4.5 × 105 s-1; 1.89 eV), are much higher than expected for reac...

Fe(II) complexes containing the fluorinated ligand 1,2-bis(perfluorophenyl)ethane-1,2-dionedioxime (dAr(F)gH(2); H = dissociable proton) exhibit relatively positive Fe(II/I) reduction potentials. The air-stable difluoroborated species [(dAr(F)gBF(2))(2)Fe(py)(2)] (2) electrocatalyzes H(2) generation at -0.9 V vs SCE with i(cat)/i(p) ≈ 4, corresponding to a turnover frequency (TOF) of ∼20 s(-1) [Faradaic yield (FY) = 82 ± 13%]. The corresponding monofluoroborated, proton-bridged complex [(dAr(F)g(2)H-BF(2))Fe(py)(2)] (3) exhibits an improved TOF of ∼200 s(-1) (i(cat)/i(p) ≈ 8; FY = 68 ± 14%) at -0.8 V with an overpotential of 300 mV. Simulations of the electrocatalytic cyclic voltammograms of 2 suggest rate-limiting protonation of an Fe("0") intermediate (k(RLS) ≈ 200 M(-1) s(-1)) that undergoes hydride protonation to form H(2). Complex 3 likely reacts via protonation of an Fe(I) intermediate that subsequently forms H(2) via a bimetallic mechanism (k(RLS) ≈ 2000 M(-1) s(-1)). 3 catalyzes production at relatively positive potentials compared with other iron complexes.

Picosecond transient kinetics and difference spectra have been recorded for the singlet and triplet (dsigma*psigma) excited states in the d/sup 8/-d/sup 8/ dimers Rh/sub 2/(TMB)/sub 4//sup 2 +/ (TMB = 2,5-dimethyl-2,5-diisocyanohexane) and (Ir(..mu..-pz)(COD))/sub 2/ (pz = pyrazolyl, COD = 1,5-cyclooctadiene). The singlet excited state in the rhodium dimer (tau=- 820 ps) displays a strong transient absorption feature maximizing at 440 nm that is not present in the spectrum of the triplet excited state. This intense absorption feature, characteristic of a /sup 1/(dsigma-psigma) electronic configuration, is assigned to a /sup 1/(dsigma*psigma) ..-->.. /sup 1/(psigma/sup 2/) excitation. This singlet excited state lifetime of the iridium dimer in cyclohexane is less than 20 ps. Though the solvent 1,2-dichloroethane (DCE) quenches luminescence from both singlet and triplet excited states in (Ir(..mu..-pz)(COD))/sub 2/ and oxidatively adds to the dimer upon steady-state illumination, picosecond spectroscopy finds no evidence for any chemical reactivity of the very short-lived singlet excited state. The quenching of (Ir(..mu..-pz)(COD))/sub 2/ singlet luminescence in DCE appears to result from enhanced singlet ..-->.. triplet intersystem crossing in DCE relative to that in cyclohexane. Also, the invariance of triplet yield in these two solvents indicates that the efficiency of intersystem crossing is near unity.

We have investigated the kinetics of NO escape from Geobacillus stearothermophilus nitric oxide synthase (gsNOS). Previous work indicated that NO release was gated at position 223 in mammalian enzymes; our kinetics experiments include mutants at that position along with measurements on the wild type enzyme. Employing stopped-flow UV–vis methods, reactions were triggered by mixing a reduced enzyme/N-hydroxy-l-arginine complex with an aerated buffer solution. NO release kinetics were obtained for wt NOS and three mutants (H134S, I223V, H134S/I223V). We have confirmed that wt gsNOS has the lowest NO release rate of known NOS enzymes, whether bacterial or mammalian. We also have found that steric clashes at positions 223 and 134 hinder NO escape, as judged by enhanced rates in the single mutants. The empirical rate of NO release from the gsNOS double mutant (H134/I223V) is nearly as rapid as that of the fastest mammalian enzymes, demonstrating that both positions 223 and 134 function as gates for escape of the product diatomic molecule.

The photophysical properties of Ir(III) corroles differ from those of phosphorescent porphyrin complexes, cyclometalated and polyimine Ir(III) compounds, and other luminescent metallocorroles. Ir(III) corrole phosphorescence is observed at ambient temperature at wavelengths much longer (800 nm) than those of most Ir(III) phosphors. The solvatochromic behavior of Ir(III)-corrole Soret and Q absorption bands suggests that the lowest singlet excited states (S(2) and S(1)) are substantially more polar than the ground state.

To explore tertiary contact formation in alpha-synuclein, a natively unfolded protein implicated in Parkinson's disease, we have measured the rates of reaction between a powerful electron donor, the tryptophan (W) triplet excited state, and an acceptor, 3-nitro-tyrosine (Y(NO2)) in six different variants, probing loop sizes between 15 and 132 residues. Electron transfer rates decrease with loop size with the fastest contact time of 140 ns for the N-terminal pair and the slowest of 1.2 mus for the N- to C-terminal pair. Diffusion coefficients ranging from approximately 2 x 10-6 to approximately 10-5 cm2 s-1 were extracted from simultaneous fits of the W to Y(NO2) electron (triplet excited state) and energy transfer (singlet excited state) kinetics.

Ru(II)- and Re(I)-diimine wires bind to the oxygenase domain of inducible nitric oxide synthase (iNOSoxy). In the ruthenium wires, [Ru(L)2L']2+, L' is a perfluorinated biphenyl bridge connecting 4,4'-dimethylbipyridine to a bulky hydrophobic group (adamantane, 1), a heme ligand (imidazole, 2), or F (3). 2 binds in the active site of the murine iNOSoxy truncation mutants Delta65 and Delta114, as demonstrated by a shift in the heme Soret from 422 to 426 nm. 1 and 3 also bind Delta65 and Delta114, as evidenced by biphasic luminescence decay kinetics. However, the heme absorption spectrum is not altered in the presence of 1 or 3, and Ru-wire binding is not affected by the presence of tetrahydrobiopterin or arginine. These data suggest that 1 and 3 may instead bind to the distal side of the enzyme at the hydrophobic surface patch thought to interact with the NOS reductase module. Complexes with properties similar to those of the Ru-diimine wires may provide an effective means of NOS inhibition by preventing electron transfer from the reductase module to the oxygenase domain. Rhenium-diimine wires, [Re(CO)3L1L1']+, where L1 is 4,7-dimethylphenanthroline and L1' is a perfluorinated biphenyl bridge connecting a rhenium-ligated imidazole to a distal imidazole (F8bp-im) (4) or F (F9bp) (5), also form complexes with Delta114. Binding of 4 shifts the Delta114 heme Soret to 426 nm, demonstrating that the terminal imidazole ligates the heme iron. Steady-state luminescence measurements establish that the 4:Delta114 dissociation constant is 100 +/- 80 nM. Re-wire 5 binds Delta114 with a K(d) of 5 +/- 2 microM, causing partial displacement of water from the heme iron. Our finding that both 4 and 5 bind in the NOS active site suggests novel designs for NOS inhibitors. Importantly, we have demonstrated the power of time-resolved FET measurements in the characterization of small molecule:protein interactions that otherwise would be difficult to observe.

Inner-sphere electron-transfer reorganization energies of Zn(protoporphyrin IX) and Zn(octaethylporphyrin) are determined from band-shape analyses of the first ionization obtained by gas-phase valence photoelectron spectroscopy. The experimentally determined total inner-sphere reorganization energies for self-exchange (120−140 meV) indicate that structural changes upon oxidation are largely confined to the porphyrin ring, and substituents on the ring or solvent and other environmental factors make smaller contributions. Computational estimates by different models vary over a wide range and are sensitive to numerical precision factors for these low reorganization energies. Of current computational models that are widely available and practical for molecules of this size, functionals that contain a mixture of Hartree−Fock exchange and DFT exchange-correlation appear to be the most applicable.

Diethylaniline-terminated oligo(phenyl-ethynyl)-thiol (DEA-OPE-SH) wires on Au-bead electrodes facilitate electron tunneling to and from the deeply buried topaquinone (TPQ) cofactor in Arthrobacter globiformis amine oxidase (AGAO). Reversible cyclic voltammograms were observed when AGAO was adsorbed onto this DEA-OPE-SAu surface: the 2e^-/2H^+ reduction potential is −140 mV versus SCE.

High-potential iron−sulfur proteins (HiPIPs) are found in photosynthetic purple nonsulfur bacteria. The three-dimensional structure of Chromatium vinosum HiPIP features two short segments of α-helix, three strands of antiparallel β-pleated sheet, and a small helix near the N-terminus. The cubane [Fe_4S_4] cluster is attached covalently to the polypeptide matrix through Fe−S^γ bonds to cysteines 43, 46, 63, and 77. The side chains of Tyr19, Phe48, Trp60, Phe66, Trp76, Trp80, and other nonpolar residues encapsulate the cluster in a hydrophobic cavity that is inaccessible to solvent. Tyr19, which contacts the [Fe_4S_4] core, has been suggested to play a particularly important structural role. In both oxidation states, the cysteinyl and core inorganic sulfur atoms are involved in H-bonding interactions with peptide NH protons.

Variations in tryptophan fluorescence intensities confirm that copper(II) interacts with alpha-synuclein, a protein implicated in Parkinson's disease. Trp4 fluorescence decay kinetics measured for the F4W protein show that Cu(II) binds tightly (Kd 100 nM) near the N-terminus at pH 7. Work on a F4W/H50S mutant indicates that a histidine imidazole is not a ligand in this high-affinity site.

Experiments in several laboratories have shown that electron transfer (ET) can take place at appreciable rates over long distances (> 10 {angstrom}) in organic and inorganic molecules and in proteins. There is a bewildering array of potential ET pathways in proteins; interestingly, the through-peptide routes (if there are any) generally involve so many bonds that they cannot possibly account for the observed rates. In searching for good pathways through cytochrome c, the authors discovered a relatively short route from His-39 to the heme in the Candida krusei (C.k.) protein. Along this route, the crucial shortcut is a hydrogen bond that bridges Gly-41 and the heme. Since experimental information relevant to protein pathway models is lacking, they have extracted donor-acceptor electronic coupling constants from an analysis of the driving-force dependence of ET rates in Ru(His-39)-modified C.k. zinc cytochrome c.

Aromatic amino acid radicals are key intermediates in nucleic acid biosynthesis, DNA repair, dioxygen reduction by cytochrome oxidase, water oxidation by PSII, as well as other biological procesess. In our work on electron tunneling in proteins, we have developed laser flash/quench methods that potentially could facilitate the study of such highly reactive radicals. To test our methods, we are investigating two structurally characterized proteins, [Re(CO)_3(L)(H83)]^+AzM^(2+) and [Re(CO)_3(L)(H107)]^+AzM^(2+) (L ) 1,10-phenanthroline (phen) or 4,7-Me_2phen; Az ) Pseudomonas aeruginosa azurin; M ) Cu or Zn). Of special interest is that calculations and experiments on the H107 protein show that Cu^+ oxidation via electron transfer (ET) through an intervening tyrosine (Cu^+ → Y108^(./) → Re(2+)) is over 2 orders of magnitude faster than optimized (Cu^+ → Re^(2+)) electron tunneling.

Small-molecule analogs of the active sites of heme proteins are under investigation in many laboratories. Covalent attachment of ligands to the porphyrins is often desirable as it reduces complications due to axial ligand dissociation and scrambling in cases involving mixed axial ligation. The preparation of ligand-linked (tailed) porphyrin systems, however, often involves complex multistep syntheses. We have found that trypsin-catalyzed reverse proteolysis provides a simple preparative route to a novel class of water-soluble tailed porphyrins based on the well-characterized microperoxidase (MP) framework. Using reverse proteolysis reactions, we have obtained mutant microperoxidase decapeptides (MP10s) with C-terminal histidine (H23MP10), tyrosine (Y23MP10), and methionine (M23MP10) residues. In addition, we have investigated the electronic absorption and resonance Raman spectra of these MP10 mutants.

Two new Ru-modified, Zn-substituted derivatives of horse heart cytochrome c have been prepared, Rua{sub 4}L(histidine-33)-Zn-cyt c (a = NH{sub 3}; L = pyridine, isonicotinamide). Molecular mechanics modeling indicates that the 11.7 {angstrom} edge-to-edge separation between the redox centers is virtually identical with that reported for the Ru-pentaammine derivative. Rates of photoinduced charge separation and recombination in Rua{sub 4}L (His-33)-Zn-cyt c lie in the range of 2.0 {times} 10{sup 5}-3.3 {times} 10{sup 6} s{sup {minus}1} (22{degree}C). These kinetics, along with those already reported, provide a total of eight intramolecular electron-transfer reactions that have been measured in Ru-M-cyt c (M = Fe, Zn) at driving forces (-{Delta}G{degree}) ranging from 0.18 to 1.05 eV. The variation of the rate with driving force is in general agreement with the semiclassical theory of electron-transfer reactions. Fitting the Ru-Zn-cyt c charge-separation data yields a reorganization energy ({lambda}) of 1.15 (5) eV and an electronic coupling matrix element (H{sub AB}) of 0.13 (1) cm{sup {minus}1}. The charge-recombination data are fit with the parameters {lambda} = 1.24 (5) eV and H{sub AB} = 0.10 (1) cm{sup {minus}1}, and the Ru-Fe cyt c electron-transfer rate can be described with {lambda} = 1.2 eV and H{sub AB} = 0.03 cm{supmore » {minus}1}.« less

Excited-state difference spectra and lifetimes are reported for Ru(tpy)/sub 2//sup 2 +/ (tpy = 2,2',2''-terpyridine) and a series of pentaammineruthenium(II) complexes. The initially formed excited states (generated by approx. 25-ps excitation with 532-nm light) are well modeled by a Ru/sup III/(L/sup .-/) electronic configuration (L is an aromatic, N-heterocyclic ligand) since an excellent correspondence is found between their absorption features and those of (L/sup .-)H/sup +/ radicals. The transients are, therefore, assigned as MLCT states. The Ru(tpy)/sub 2//sup 2 +/ excited-state lifetime is 250 ps. The Ru(NH/sub 3/)/sub 5/L/sup 2 +/ excited-state lifetimes vary from less than or equal to 20 (L = protonated pyrazine, protonated 4,4'-bipyridine, and 4-acetylpyridine) to approx. 200 (L = pyrazine, isonicotinamide, 4,4'-bipyridine) ps in water at approx. 22/sup 0/C. Ammine aquation products, Ru(NH/sub 3/)/sub 4/(H/sub 2/O)L/sup 2 +/, are also observed and are shown to result from multiphoton absorption of the ps-excitation pulses. The MLCT states of Ru(NH/sub 3/)/sub 5/L/sup 2 +/ (L = pz or 4,4'-bpy) undergo protonation by solvent water and deactivation to Ru(NH/sub 3/)/sub 5/LH/sup 3 +/; rate constants for proton loss from the protonated species (Ru(NH/sub 3/)/sub 5/LH/sup 3 +/ + H/sub 2/O ..-->.. Ru(NH/sub 3/)/sub 5/L/sup 2 +/ + H/submore » 3/O/sup +/) are (2-3) x 10/sup 7/ s/sup -1/ and 2 x 10/sup 5/ s/sup -1/ for L = pz and 4,4'-bpy, respectively. The Ru(NH/sub 3/)/sub 5/L/sup 2 +/ MLCT-excited-state lifetimes are discussed in terms of the lowest excited state tuning model of Ford and co-workers.« less

This paper reports transient absorption kinetics studies in the 10/sup -11/-10/sup -3/s time range for (Re/sub 2/Cl/sub 8/)/sup 2 -/ (I), (Mo/sub 2/Cl/sub 8/)/sup 4 -/ (II), Mo/sub 2/Cl/sub 4/(CH/sub 3/CN)/sub 4/ (III), and Mo/sub 2/Cl/sub 4/(PBu/sub 3/)/sub 4/ (IV) in solution at room temperature. A detailed description of the optical layout and equipment used to measure the kinetics in the 10/sup -8/-1 s time range following picosecond excitation is also given. Complexes I-III have small steric barriers to rotation about the M-M bond and upon photoexcitation appear to form /sup 1/(sigmasigma*)/sub staggered/ metal-metal charge-transfer (MMCT) states in

The kinetics of the bimolecular electron transfers from triplet-excited zinc-substituted horse heart cytochrome c (Zn-cyt c*) to Ru(NH/sub 3/)/sub 5/L/sup 3 +/ have been measured (L = NH/sub 3/, k(298 K) = 1.4 x 10/sup 7/ M/sup -1/ s/sup -1/; L = histidine, k(298 K) = 2.4 x 10/sup 7/ M/sup -1/ s/sup -1/), along with those of the corresponding thermal back-reactions (L = NH/sub 3/, k(298 K) = 1.5 x 10/sup 8/ M/sup -1/ s/sup -1/; L = histidine, k(298 K) = 3.6 x 10/sup 8/ M/sup -1/ s/sup -1/). The derivatized metalloprotein ruthenium pentaammine histidine-33-zinc-substituted cytochrome c (Ru-Zn-cyt c) has been prepared and characterized by atomic absorption spectroscopy and HPLC analysis of its tryptic digestion fragments. The rate of intraprotein electron transfer from the triplet-excited Zn-porphyrin moiety to the 11.8-A distant Ru(NH/sub 3/)/sub 5/(His-33)/sup 3 +/ residue (k(298 K) = 7.7 x 10/sup 5/ s/sup -1/), as well as that of the thermal back-reaction (k(298 K) = 1.6 x 10/sup 6/ s/sup -1/), has been measured by transient spectroscopy. Intraprotein electron transfer from Ru(II) quenches Zn-cyt c* with a rate constant of 2.4 x 10/sup 2/ s/sup -1/. These kinetics are discussed in terms of the semiclassical theorymore » of electron-transfer reactions.« less

X-ray absorption edge and extended x-ray absorption fine structures (EXAFS) spectra have been measured for acidic-aqueous solutions containing Mo(II), Mo(III), Mo(IV), and Mo(V). The edges increase in energy according to Mo(II) << Mo(III) < Mo(IV) < Mo(V). Bond distances calculated for the binuclear species are as follows: Mo(II)/1 M CF/sub 3/SO/sub 3/H (aqueous), Mo-Mo = 2.12 A; Mo-O(OH/sub 2/) = 2.14 A; Mo(III)/3 M CH/sub 3/SO/sub 3/H (aqueous), Mo-Mo = 2.54 A; Mo-O (bridging OH) = 2.06 A; Mo-O(OH/sub 2/) = 2.20 A; Mo(V)/3 M HCl (aqueous) (Mo/sub 2/O/sub 4//sup 2 +/ core), Mo-Mo = 2.56 A; Mo-O (terminal oxo) = 1.68 A; Mo-O (bridging oxo) = 1.93 A. The EXAFS data for Mo(IV) in 4 M methanesulfonic acid are consistent with the accepted Mo/sub 3/O/sub 4//sup 4 +/ (aq) (Mo-Mo = 2.49 A; Mo-O(..mu../sub 2/) = 1.88 A) formulation for the red ion. Estimates of the Mo-Mo Debye-Waller factors were obtained by comparison with sigma = 0.0377 A calculated for Mo/sub 2//sup 4 +/ (aq). The stiffest Mo-Mo bond is in Mo/sub 3/O/sub 4//sup 4 +/ (aq) (sigma = 0.031 A); sigma values for Mo/sub 2/(..mu..-OH)/sub 2//sup 4 +/ and Mo/sub 2/O/sub 4//sup 2 +/ (aq) are 0.044 andmore » 0.037 A, respectively.« less

In the ideal photochemical energy storage system a photoinduced charge separation must be generated rapidly with minimum energy loss and be maintained by virtue of inefficient electron-hole recombination. These rigorous requirements necessitate a thorough understanding of the diverse parameters that determine the barriers to electron-transfer reactions: free energy changes; donor-acceptor separation, orientation, and electron coupling; inner-sphere reorganization energy; and solvent reorganization energy and dynamics. Clearly, in order to elucidate the contributions of these parameters to electron-transfer barriers, model electron-transfer systems must be prepared which can be selectively modified to vary one or more of these parameters. An important requirement for such systems is the covalent coupling of donor and acceptor into one molecule to surpass diffusional limits and permit study of extremely fast intramolecular electron-transfer reactions. They report here results from a prototypal synthetic donor-acceptor system which exhibits photoinduced electron-transfer rate constants in the vicinity of 10/sup 12/ s/sup -1/ at driving forces of ca. 1 eV and charge-recombination rates slower than 2 x 10/sup 10/ s/sup -1/.

In contrast to Ru(bpy)/sub 3//sup 2 +/ and Os(bpy)/sub 3//sup 2 +/ (bpy = 2,2'-bipyridine) the lowest excited state of Fe(bpy)/sub 3//sup 2 +/ is LF (ligand field) rather than MLCT (metal-to-ligand charge transfer) in character. Replacement of a bpy by two stronger field CN/sup -/ ligands to give Fe(bpy)/sub 2/(CN)/sub 2/ does not alter this ordering. Here the authors report observations that show that in water this ordering is retained for Fe(bpy)(CN)/sub 4//sup 2 -/ even with its stronger ligand field but that the extraordinary solvent sensitivity of this complex can be used to access the MLCT state: the MLCT state is observed in the weak acceptor solvent acetone.