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We would like to thank Dr Grosser et al1 for their continued interest in our work. While we may not agree with their arguments or their opinions of our study,2 we welcome debate in this extremely important area. We take this opportunity to address the points that they raise. First, although urine may well be a convenient compartment within which to measure markers of prostacyclin and thromboxane A2, numerous observations,3–10 of which ours is only the latest,2 indicate that they poorly reflect production within the circulation and the reactivities of endothelial cells and platelets. Second, the literature that Grosser et al1 cite is somewhat selective and in places inaccurate. For example, reference 3,11 which the authors cite to substantiate the statement “Most insights into the in vivo biology and pharmacology of the prostaglandin pathway have derived from the …

An electrospun liquid microjet has been developed that delivers protein microcrystal suspensions at flow rates of 0.14–3.1 µl min−1to perform serial femtosecond crystallography (SFX) studies with X-ray lasers. Thermolysin microcrystals flowed at 0.17 µl min−1and diffracted to beyond 4 Å resolution, producing 14 000 indexable diffraction patterns, or four per second, from 140 µg of protein. Nanoflow electrospinning extends SFX to biological samples that necessitate minimal sample consumption.

Ab initio calculations were performed on 18 fluorinated and unfluorinated alcohols at the B3LYP and HF levels with the 6-311G∗∗ basis set. Molar volumes of the alcohols were computed at each level and averaged to produce a scale of relative size. From this, various isosteric replacements of potential use in drug design were suggested: ethyl by FCH2CH2 or HCF2CH2, propyl by CF3CH2, isopropyl by CF3(CH3)CH or (FCH2)2CH, isobutyl or t-butyl by (CF3)2CH, and 3-methyl-2-butyl by CF3(CH3)2C. Calculation of the charge on oxygen and the Wiberg index of the CO bond allowed an electronegativity scale to be constructed for the fluoroalkyl groups. Electronegativity decreased in the order: (CF3)3C>(CF3)2CH>C2F5CH2>CF3CH2>CH3(CF3)2C>HCF2CH2>CF3(CH3)CH>(FCH2)2CH>FCH2CH2>CF3(CH3)2C. This ranking agreed with literature acid dissociation data for the alcohols studied.

The reactions of thiirane with ammonia, a series of primary amines and aniline have been studied by means of ab initio Hartree-Fock calculations at the 6-31G* level. The transition state geometries were characterized and used to determine the relative rates of reaction. In all cases, anti attack by the nucleophile was favored. Relative rates for the aliphatic amines varied by a factor of less than twenty, with ethylamine being most reactive. Ammonia reacted more than twenty and aniline almost 8 x 10 4 times slower than tert-butylamine. The results can be rationalized on the basis of relative nucleophilicity and nonbonding interactions.

Collision-induced dissociation of metal-cationized N-CBZ-Gly-Pro-Gly-Pro-Ala was studied by Fourier transform mass spectrometry. Lithium-, sodium-, potassium- and rubidium-cationized peptide species were generated by matrix-assisted laser desorption/ionization (MALDI) using 2,5-dihydroxybenzoic acid as matrix, together with appropriate metal salts. The experimental mass spectrometric results were interpreted with the aid of Monte Carlo conformational searches using the Amber * force field, together with ab initio molecular orbital calculations with Gaussian-94 for the singly lithium- and potassium-cationized peptides. It is concluded that metal coordination plays a key role in guiding the gas-phase fragmentation of the cationized peptide. In contrast to lithium and sodium, potassium and rubidium apparently do not coordinate to the C-terminal carbonyl. When the peptide is cationized with the two smaller alkali metals, losses corresponding to alanine and CBZ are observed, while the coordination of potassium and rubidium results in only CBZ loss upon dissociation.

The hydrolysis of phosphorus fluoridates was studied by semiempirical and ab initio methods. The reaction proceeds through a metastable intermediate separated from the reactants and products by transition structures. Both methods gave similar qualitative results; however, the first transition (formation of the trigonal bipyrimid intermediate) occurred earlier and the second (loss of F-) later with semiempirical methods. Including solvation in the calculations is critical. The AMSOL program generated reasonable energies especially when the limitations of the semiempirical Hamiltonian are considered. The Onsanger model was not a significant improvement over calculations on isolated molecules.

A neutral gas phase reaction pathway for the reaction of methanol with dimethylphosphinic fluoride is presented. This reaction pathway occurs via a proton transfer from methanol to the fluorine leaving group through the phosphinyl oxygen. Gas phase ab initio level calculations at the RHF/6-31G(d), RHF/6-31G(d,p), B3LYP/6-31+G(2d), and B3LYP/6-311++G(2d,p) level of theories were used to follow the complete pathway for this reaction. A 41.2 and a 30.9 kcal/mol gas phase energy barrier was determined for both the RHF/6-31G(d,p) and B3LYP/6-311++G(2d,p) levels of theory, respectively.

One Protein, Two Probes A central challenge in the use of x-ray diffraction to characterize macromolecular structure is the propensity of the high-energy radiation to damage the sample during data collection. Recently, a powerful accelerator-based, ultrafast x-ray laser source has been used to determine the geometric structures of small protein crystals too fragile for conventional diffraction techniques. Kern et al. (p. 491 , published online 14 February) now pair this method with concurrent x-ray emission spectroscopy to probe electronic structure, as well as geometry, and were able to characterize the metal oxidation states in the oxygen-evolving complex within photosystem II crystals, while simultaneously verifying the surrounding protein structure.

Nucleobases absorb strongly in the ultraviolet region, leading to molecular excitation into reactive states. The molecules avoid the photoreactions by funnelling the electronic energy into less reactive states on an ultrafast timescale via non-Born-Oppenheimer dynamics. Current theory on the nucleobase thymine discusses two conflicting pathways for the photoprotective dynamics. We present our first results of our free electron laser based UV-pump soft x-ray-probe study of the photoprotection mechanism of thymine. We use the high spatial sensitivity of the Auger electrons emitted after the soft x-ray pulse induced core ionization. Our transient spetra show two timescales on the order of 200 fs and 5 ps, in agreement with previous (all UV) ultrafast experiments. The timescales appear at different Auger kinetic energies which will help us to decipher the molecular dynamics.

A PM3 and SM3-PM3 semiempirical molecular orbital study of the 2,2′-dichlorodiethyl sulfide conventional S N 2 and neighboring group hydrolysis reaction mechanisms in the gas phase and in aqueous solution is described. The calculations predict substantially faster reactions in aqueous solution, with the neighboring group mechanism always being preferred. Detailed consideration is given to the geometries, relative energies, and partial atomic charges of all species involved in the reaction mechanisms considered and the extent to which aqueous solvation impacts these quantities. The results are consistent with expectation and with reported calculations concerning the intramolecular S N 2 reaction of 2-chloroethyl methyl sulfide. We also present the lowest energy mustard chlorohydrin structures according to PM3 and AM1 conformational analysis.

The fully optimized structures and relative energies of all possible methoxy-1,3,5-cycloheptatriene (MCHT) isomers have been determined by semiempirical, ab initio, and density functional theory (DFT) molecular orbital calculations. All methods identify the boat conformation of 1-methoxy-1,3,5-cycloheptatriene as the most stable species in this group of compounds. In order to evaluate boat interconversion barriers, optimizations of the planar isomers were also performed. For comparison purposes, we applied the same computational methodologies to boat and planar conformations of 1,3,5-cycloheptatriene (CHT). Among the semiempirical methods, the SAM1 approximation was found to best reproduce the ab initio and DFT results. Examination of rotational energy profiles allowed for identification of the factors controlling the preferred orientations of the methoxy group in these compounds. The calculations predict that methoxy substitution has little influence on the preferred conformation of the seven-membered ri...

We report measurements of laser absorption for high-contrast ultrashort pulses on a variety of solid targets over an intensity range of ${10}^{13}$ to ${10}^{18}$ W/ ${\mathrm{cm}}^{2}$. These data give an experimental determination of the target energy content and an indirect measure of dense plasma electrical conductivity. Our calculations accurately reproduce the behavior of aluminum targets, while the other materials show signs of additional absorption mechanisms. At high intensity all target materials reach a ``universal plasma mirror'' state and reflect about 90% of the incident light.

Most of the dioxygen on earth is generated by the oxidation of water by photosystem II (PS II) using light from the sun. This light-driven, four-photon reaction is catalyzed by the Mn 4 CaO 5 cluster located at the lumenal side of PS II. Various X-ray studies have been carried out at cryogenic temperatures to understand the intermediate steps involved in the water oxidation mechanism. However, the necessity for collecting data at room temperature, especially for studying the transient steps during the O–O bond formation, requires the development of new methodologies. In this paper we report room temperature X-ray diffraction data of PS II microcrystals obtained using ultrashort (4 CaO 5 cluster in PS II at room temperature. We show that these data are comparable to those obtained in synchrotron radiation studies as seen by the similarities in the overall structure of the helices, the protein subunits and the location of the various cofactors. This work is, therefore, an important step toward future studies for resolving the structure of the Mn 4 CaO 5 cluster without any damage at room temperature, and of the reaction intermediates of PS II during O–O bond formation.

We present our first results of a UV-pump X-ray-probe study of the photoprotection mechanism of thymine. The experiment used element specific Auger spectroscopy and was carried out at the LCLS.

The K -shell emission from porous aluminum targets is used to infer the density and temperature of plasmas created with 800 and 400 nm, 140 fsec laser light. The laser beam is focused to a minimum spot size of 5 μm with 800 nm light and 3 μm with 400 nm light, producing a normal incidence peak intensity of 10 18 W/cm 2 . The effects of design, laser characteristics, and diagnostic needs is discussed.

Computational chemistry was used to elucidate the reaction paths, transition structures, and energies of activation for the hydrolysis of a series of phosphinate, phosphonate, and phosphate esters. Calculations were performed at the Hartree-Fock level of theory with the density functional theory and 2nd order Moller-Plesset level using the 6-311G(2d.2p) basis set. The SCI-PCM continuum solvation model was also used to determine the roll that solvation plays in stabilizing the various transition structures. Transition structures containing one and/or two water molecules had lower energies than those with no water because the water served as a bridge for transporting the proton from the nucleophile to the leaving group on the other side of the molecule.

Quantum chemical methods (ab initio, semiempirical, and Hartree-Fock) were used to calculate the energy of several phosphite-phosphonate tautomers, and thereby determine the position of equilibrium in the gas phase. Unless the phosphorus moiety contains significant electron withdrawing groups, the equilibrium lies far toward the phosphonate. None of the methods consistently produced thermodynamic values that agreed within 5 kcal/mole. In the most stable conformation of trimethyl phosphite, two of the methoxy ligands were oriented upward (with respect to the lone pair) in a pseudocistoid or gauch arrangement and the other pointed downward in a transoid orientation. Dimethylmethyl phosphonate (DMMP) had a similar structure. The methyl group assumed the Trans position, and the two methoxy were in the gauch. The two long alkyl chains in BIS were oriented in a V fashion with the ends on the molecule about 14 angstroms apart. The vibrational spectra for trimethyl phosphite, DMMP, and bis(2-ethylhexyl) phosphonate were calculated at the HF/6-3 11 G** level and compared to experimentally measured frequencies. Graphic representations of the calculated vibrations and correlation with traditional group frequencies were used to make the individual assignments.

The relative rates of reaction of thiirane and thiirane derivatives with NH3, a series of secondary amines including aziridine, and trimethylamine were determined in the gas phase by means of B3LYP/6-31+G(d)//HF/6-31+G(d) computations and transition state theory. Convergence of the results was selectively tested using the 6-311++G(d,p) basis set. Comparison with MP2/6-31 + G(d)//MP2/6-31G(d) computations was made in model cases. These results are significant in that they supplement the only reported gas-phase experimental study of this type of reaction. The reaction rates of thiirane with secondary amines can best be rationalized by means of an interplay of steric and polarizability effects. While beta-halo substituents retard S(N)2 reactions in solution, both 2-fluorothiirane and its acyclic model react more than l0(6) times faster with NH3 than the unsubstituted compounds in the gas phase. 2-Fluorothiirane was calculated to react with NH3 at C2 by a factor of 0.142 with respect to thiirane itself; attack at C3 was found to be 3.42 x 10(6) times faster than the parent compound. 2-Methylthirane reacts with NH3 at 0. 230 the rate of thiirane with a 12.8-fold regioselectivity for C3. In the reaction of 2,2-dimethylthirane and NH3, this preference for C3 increases to a factor of 124. Ground-state destabilization of cis-2,3-dimethylthiirane is sufficient to account for its calculated rate acceleration with respect to the trans isomer.

Catalytic antibodies are proteins that catalyze chemical reactions by binding to the transition state and lowering the energy of activation. Transition state analogs are stable compounds that mimic the geometry and electronic structure of the transition structure. These haptens are conjugated to proteins and used to elicit the desired immunoglobulins. Catalytic antibodies have been used to accelerate hydrolysis and ammonolysis reactions, cis-trans isomerizations, Diels-Alder reactions, and to alter the product ratios for cyclization reactions.

We present recent results of x-ray laser amplification of spontaneous emission in Ne-like and Ni-like transient collisional excitation schemes. The plasma formation, ionization and collisional excitation can be optimized using two laser pulses of 1 ns and 1 ps duration at table-top energies of 5 J in each beam. High gain of 35 cm{sup -1} has been measured on the 147 {Angstrom} 4d{r_arrow}4p J=0{r_arrow}1 transition of Ni-like Pd and is a direct consequence of the nonstationary population inversion produced by the high intensity picosecond pulse. We report the dependence of the x-ray laser line intensity on the laser plasma conditions and compare the experimental measurements with hydrodynamic and atomic kinetics simulations for Ne-like and Ni-like lasing.

The net absorption for a 100 femtosecond, 400 nm laser pulse's interaction with a wide range of solid density target materials is determined by measuring the total reflected and scattered light signals from the target. Absorption is determined over the intensity range of 1013 to 1018 W/cm2, a range is sufficiently broad to observe the transition in absorption from a low intensity region dominated by the cold material's electronic band structure, to an absorption at the highest intensities marked by a convergence to a nearly universal, material independent value.

The ultra-fast ionization of argon, krypton and xenon by intense ultra-short laser pulses is studied. Intensities in the range 10 14 to 10 16 W/cm 2 at a wavelength of 616 nm are provided by a colliding pulse mode-locked dye laser system. To date, ion yields have been monitored using a 1 meter time.of flight spectrometer. Intensity dependent photoelectron signals are deduced from ion data. In an ultra-fast strong field environment, ionization times can be less than an optical period, affording a better distinction between a field ionization picture and a multiphoton ionization picture. For most cases that we study, Keldysh parameters are below 1, indicating that we are in a field ionization regime. As a laser system under development progresses towards the design goal of 1 joule per 100 femtosecond pulse, we will extend these investigations to peak intensities of order 10 18 to 10 19 Watts/cm 2 .

The hypothiocyanite ion (OSCN - ) is a normal component of human saliva. It is a highly reactive oxidizing agent, and at concentrations above the values normally found in human saliva, it inhibits the growth and metabolism of oral bacteria. This finding has led to the suggestion that antibacterial properties of human saliva might be enhanced in vivo by appropriate supplements which elevate OSCN - concentrations. Since DNA is sensitive to oxidizing agents (hydrogen peroxide attacks nucleosides), high concentrations of OSCN - in human saliva might damage DNA and produce deleterious effects on the oral mucosa. In the present study, the effect of high OSCN - concentrations on several mutagen-sensitive Salmonella typhimurium strains was determined. These strains are used to detect base-pair substitutions and frameshift mutations. We also studied the effects of OSCN - on a Saccharomyces cerevisiae (yeast) strain commonly employed as a test cell for evaluating the potential of a compound to produce gene conversion, mitotic crossing-over, or reverse mutation. By recording the UV spectra of mixtures of calf thymus DNA and OSCN - , we explored the possible in vitro reactions of this oxidizing agent with eucaryotic genetic material. Our results show that, at concentrations above 10 μM, OSCN - is toxic for the tested Salmonella typhimurium strains. The mutant strains with defects in cell wall lipopolysaccharides are killed more readily by OSCN - than is the strain lacking these defects. However, OSCN - was not mutagenic for any of the tested strains. Saccharomyces cerevisiae was not affected by OSCN - even at concentrations above 800 μM. Calf thymus DNA was not oxidized by OSCN - . We conclude that the elevated concentrations of OSCN - required to produce antibacterial effects in the human mouth pose no threat to the genetic material of host tissues.

When photolyzed in situ for as little as 15 s 2-azido-9-fluorenone oxime causes Type II and Type III transformation in C3H 10T 1/2 CL8 cells. In the yeast strain Saccharomyces cerevisiae D-7, simple reversions and gene conversions occurred and also mitotic crossing over, to a lesser extent, but no mitochondril "petite" mutants occurred. No mutations or transformations were induced in the dark or by the light itself.

Mutations were induced by photolabeling 2-azido-9-fluorenone oxime in Salmonella typhimurium tester strain TA1538. Since the critical adducts were formed directly by the photogenerated nitrene, metabolic activation was bypassed. The bacteria themselves possessed the deep rough cell wall and uvr B − mutations, thereby minimizing comutagenic effects resulting from transport or DNA repair. This technique thus permitted detection of comutagenicity resulting from altneration in the binding of the mutagen to its critical receptor. No compound tested increased mutagenicity. Cholic acid had no effect at low dose, but inhibited mutagenicity at 10 −4 M, while trans -retinol had no effect at any concentration. Harman, norharman and ethidium reduced the number of mutants only slightly at concentrations from 10 −6 to 10 −5 M.

Summary Ethidium bromide (EB) is known as a highly efficient mutagen of mitochondrial DNA in yeast. The key step to mutagenesis is thought to be a covalent EB attachment to mitochondrial DNA. A light sensitive ethidium azide has been used to produce covalent attachment of ethidium to mitochondrial DNA under conditions where EB mutagenesis does not occur, i.e. at high glucose concentrations, and in the petite negative yeast K . fragilis . A bypass attachment of ethidium through photolysis of the azide was effective in inducing mutagenesis.

1. The concentration of phosphate in the external medium of a suspension of unfertilized Strongylocentrotus eggs remains constant, or increases, while in a suspension of fertilized eggs, the concentration of phosphate in the external medium decreases.2. Fertilized Strongylocentrotus eggs absorb P32 and phosphate from sea water at identical rates, revealing that the exchange of phosphate between the cell interior and the external medium is inappreciable.3. The rate at which phosphate is removed from sea water by fertilized Strongylocentrotus eggs is relatively independent of the external concentration as long as this exceeds 15 to 20 micrograms P per liter.4. When unfertilized and fertilized sea urchin eggs are continuously exposed to sea water containing P32 and more than 20 micrograms P per liter, 95.9 to 96.4 per cent of the P32 which enters the eggs is found in the trichloroacetic acid-soluble fraction, with 3.6 to 4.1 per cent of the P32 being recovered in the acid-insoluble fraction. The distribution...

2-Azidoisopthalic acid and 5-azidoisopthalic acid were prepared from the corresponding amines through diazonium salt intermediates. Kinetic studies indicate that they are competitive inhibitors of glutamate dehydrogenase. Photolysis of the enzyme-inhibitor complex with light > 300 nm generates a nitrene which reacts irreversibly with the enzyme and exhibits fluorescence emission at 430 nm when excited by 350 nm radiation.

N-Acetylglycyl adenylate anhydride has been shown to be readily converted in high yield to N-acetylglycyl imidazolide in the presence of excess imidazole at pH 7. The aminoacyl group can then be transferred from the imidazolide to become esters of mono- or polynucleotides. These observations suggest that histidine may be in the active site of the aminoacyl-tRNA synthetases, catalyzing the transfer of aminoacyl groups from the adenylate to tRNA.

Thymine-requiring mutants of Escherichia coli and Bacillus subtilis will grow in the presence of sulfur-containing analogs of thymine and thymidine as the sole thymine-like source. Both 4-thiothymine and 4-thiothymidine promote exponential growth similar to that of their parent compounds, thymine and thymidine. Although the parent compound 5-bromouracil promotes growth, its sulfur-containing analog, 4-thio-5-bromouracil, does not. Additionally, the thymine-resembling compounds, 5-ethyluracil and 4-thio-5-ethyluracil, neither promote nor inhibit bacterial growth. 4( O -methyl) thymine oxime, a thymine analog with an oxime ether in place of oxygen at C-4, is both non-conducive and non-inhibitive to bacterial growth, and hence is not used by the cell as a thymine substitute. Although 4-thiothymine and 4-thiothymidine support bacterial growth, determination of the sulfur content of bacteria grown in the presence of these analogs shows that less than 1 per cent of the DNA still contains sulfur, thereby indicating that the sulfur atom is removed after entering the cell and probably prior to incorporation into DNA.

The contemporary genetic code and the process of protein biosynthesis most assuredly evolved from a simpler code and process. We believe that there was obligatory coevolution of the two and that the earlier code and process must have involved a more direct linkage between the amino acids and the informational macromolecule. We propose that an early form of translating existed in which amino acids were attached directly to the ‘messenger’ RNA along the backbone as 2’OH aminoacyl esters. These esters then condensed with each other on the RNA backbone yielding a peptide covalently attached to the RNA, without the use of tRNAs and ribosomes. This presentation is concerned with experimental data which indicate that such a simple translation system is possible and must have involved the following steps: (1) formation of the aminoacyl adenylate anhydride, (2) transfer of the amino acid from the adenylate to imidazole, (3) transfer of the amino acid from imidazole to 2’OH groups along the backbone of RNAs, (4) condensation of the amino acids to yield peptides. Steps (1)–(3) have been confirmed in chemical systems. Our preliminary evidence indicates step (4) is also possible. The aminoacylation of polyribonucleotides and the subsequent formation of peptides is a dynamic and experimentally accessible system for studying genetic coding specifities and our present studies are now concentrated on step (4), looking for such specifities.

2-Azidofluorene was synthesized to serve as a specific photoaffinity label in chemical carcinogenesis studies. The drug exhibited little toxicity, and was non-transforming in the C3H 10T 1 2 CL8 cells. When photolysed at 360 nm in water to generate N-hydroxy-2-aminofluorene, then added to the cells, the drug again exhibited little toxicity but was very weakly transforming. When it was irradiated inside the cells cytotoxicity was enhanced greatly. Both Types II and III transformed foci were observed in transformation experiments and were comparable to those obtained with N-acetoxy-2-acetylaminofluorene. Irradiation alone was neither toxic nor transforming. These results suggest that photoaffinity labeling is a promising tool in studying chemical carcinogenesis by bypassing metabolic activation.

Frameshift mutations have been produced in specific repair-negative Salmonella tester strains by photoaffinity labeling technique using ethidium azide. Reversions requiring a +1 addition or a -2 deletion were specially sensitive. Mutagenesis was reduced by the simultaneous addition of non-mutagenic ethidium bromide, and was prevented by photolysis of the azide prior to culture addition. Identical tester strains active in DNA excision repaire were not mutagenized by the azide. These results are consistent with the interpretation that photolysis of the bound ethidium analog converts the drug from its noncovalent mode of binding (presumably intercalation) to a covalent complex with consequent production of frameshift mutations. Such photoaffinity labeling by drugs which bind to DNA not only confirms the importance of covalent drug attachment for frameshift mutagenesis, but also provides powerful techniques for studying the molecular deatils of a variety of genetic mechanisms.

A photosensitive azide derivative of ethidium bromide, 3(8)-amino-8(3)-azido-5-ethyl-6-phenyl phenanthridinium chloride, was prepared which binds covalently to DNA via a light-generated nitrene intermediate. When exposed to light in aqueous solvent the azide loses free nitrogen to yield a reactive nitrene, which reacts with water to form the hydroxylamine derivative. When the drug is photolyzed while tightly bound to DNA, rather than free in solution, a covalent complex is generated in situ due to the attack of the nitrene to form a covalent bond with the nucleic acid. A selectivity for cytoplasmic DNA reported previously for ethidium bromide was also shown for the azido analog by enhanced production of petite mutants in Saccharomyces following the photolysis of the azide in vivo . Isolated mitochondrial DNA following binding of the ethidium analog in vivo also appeared to retain a greater number of drug molecules per nucleotide than nuclear DNA. In vitro binding experiments did not suggest the same extent of preferential binding. This technique of photoaffinity labeling provides a means of studying the precise model of binding nucleic acid active drugs in relation to their biological effects.

Publisher Summary Photoaffinity labeling is a valuable approach for studying nucleic acids in vitro and in vivo. In contrast to most protein binding, the specific ligands that bind nucleic acids do so with high affinity (K C a 10 -6 to 10 -7 are not unusual). The reagents of interest are of sufficient size and complexity to offer many opportunities for attachment of reactive groups, and many are strong chromophores in the visible and near-UV range where cells are sufficiently transparent to permit photolysis of drug nucleic acid complexes in situ. This chapter concentrates on concentrated on nitrene labeling. Since the carcinogens are attached to DNA by eleetrophilic addition, there is no hampering of severely by the less reactive nitrenes seeking a more reactive site. Nitrenes generated by photolysis of aryl azides undergo few rearrangements. The aryl azide is relatively small, consisting of an additional two nitrogen atoms if substitution has been for an amine. The azide is conjugated with the aryl ring(s), which absorb at long wavelength, depending on substitution, so that irradiation and nitrene generation can be affected without exposing the DNA, cell, or animal to shortwavelength UV light. The reaction of the unbound nitrene with water during in vivo experiments may present problems. The chapter explains Preparation of 2-Azidofluorene, Preparation of 4-Azido-7-chloroquinoline, 3-Azidoacridine, 9-Azidoacridine, Ethidium Azides, Propidium Azide, and Stability and Photolysis Conditions.

Several 2-azidofluorenes have been synthesized for use as photoaffinity labels inside bacteria. In the dark they were not mutagenic for any Salmonella typhimurium tested. When photolyzed inside the bacteria, all were mutagenic for strain TA1538 to varying degrees, and were considerably less mutagenic in the corresponding repair positive TA1978. None were mutagenic for strain TA1535 or TA1537, although most compounds were toxic for those strains when photolyzed.

This work reports the transfer of the N-acetylglycine from the adenylate anhydride to the 2′OH groups along the backbone of homopolyribonucleotides. This transfer involves an N-acetylglycylimidazole intermediate; no transfer was observed in the absence of imidazole, and the rate of transfer was different for the various polynucleotides: poly U > poly A > poly C = poly G = 0. These results suggest that catalysis is necessary for transfer of aminoacyl from adenylates to polyribonucleotides and the data are consistent with a model involving a histidine residue in the active site of aminoacyl-tRNA synthetases. They are also consistent with a model for primordial protein formation involving polymerization of amino acids which are attached at the 2′OH groups along the polyribonucleotide backbone.

We present a spectrogram-based timing technique for x-ray free electron lasers (XFELs) that reports x-ray/optical delay below 1 fs RMS error to correct for timing jitter.

We will present first results of a UV-pump X-ray-probe study of the photoprotection mechanism of thymine. The experiment used element specific Auger spectroscopy and was carried out at the LCLS.

Scott(1) has shown that the rat's thyroid takes up 50 to 200% more radioiodine under the influence of hesperidine methyl chalcone (H.M.C.). It was therefore decided to try this drug in human beings in order to determine its effect upon thyroidal accumulation of radioiodine. This drug is known to suppress the output of I131 through the kidney; probably I131 stays in the body in circulation a long enough time to favor increased uptake by the gland.Method. Nineteen patients having the diagnoses shown in Table I were studied. All of these were considered to be euthyroid except K.E. and O.C. who were judged to be hyper-and hypothyroid respectively . Two periods of tests were done on each patient. These were the first or the patient's control period before the administration of H.M.C. and the second or his testing period following the administration of H.M.C. At the start of the control period, 20 μc I131 was given orally. Thyroid gland uptake measurements were done at 24-hour intervals for 96 hours. After a re...