THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF THE NEAR-UV ABSORBING AMINO ACIDS–I. TRYPTOPHAN AND ITS SIMPLE DERIVATIVES

Abstract— The photophysics and photochemistry of tryptophan and its simple derivatives is comprehensively reviewed with special emphasis on excitation by near-UV radiation. Topics explicitly discussed include the origins of large Stokes shifts in the fluorescence spectra, photoionization, the puzzle of multiple tryptophan fluorescence decay time, photochemical reactions in the presence and absence of oxygen, and the possible mechanisms of these reactions. A separate section reviews the photosensitizing properties of N-formylkynurenine, an important photooxidation product of tryptophan.     

THE PHOTOPHYSICS AND PHOTOCHEMISTRY OF THE NEAR-UV ABSORBING AMINO ACIDS–II. TYROSINE AND ITS SIMPLE DERIVATIVES

Abstract— The photophysics and photochemistry of tyrosine and some of its simple analogues and derivatives are comprehensively reviewed. Topics discussed include emission spectroscopy, formation of photoproducts in direct and sensitized reactions, photooxidations, and mechanistics studies using steady state kinetic methods, flash photolysis, and electron spin resonance. Energy and electron transfer processes involving tyrosine and other molecules of biological interest are briefly summarized.     

PHOTOPHYSICS AND PHOTOCHEMISTRY OF PHOTOREACTIVATION

Abstract— Photoreactivating enzyme (PRE) monomerizes cyclobutyl pyrimidine dimers formed in DNA by UV light ( Λ < 300 nm). The enzyme requires near UV and visible wavelengths (300 < Λ < 600 nm) for activity. Possible mechanisms of action of the PRE are suggested by non-enzymatic processes in which pyrimidine dimers are monomerized by UV and visible light. Two such non-enzymatic processes are (a) photolysis of dimers resulting from direct absorption of UV, and (b) sensitized monomerization involving charge transfer complexes. Several lines of evidence suggest that the mechanism of action of the PRE more closely resembles (b) than (a). Recent experiments on the PRE from E. coli reveal the presence of new long wavelength absorption which may indicate the presence of a ground state complex. The known ability of PRE to monomerize dimers of thymine, cytosine and uracil suggests that the carbonyl groups at 2 position of the pyrimidine ring may be important in the interaction between enzyme and dimer.     

PHOTOPHYSICS AND PHOTOCHEMISTRY OF 4-DIHYDROPYRIDINONES

Dihydropyridinones (DHP) react photochemically with olefins to form cycloaddition products. The reactions proceed through the lowest excited state T ₁ of the DHPs, with rate constants which depend on the olefin and can approach the diffusional limit in the most favourable cases. Intra-and inter-molecular sensitization and quenching have been investigated, as well as spectroscopic properties such as absorption and luminescence spectra, and in particular electron energy loss spectra which have established the energy of the reactive T₁, state as 295 kj mol^-1 (3.07 eV).     

PHOTOCHEMISTRY AND PHOTOPHYSICS OF GUANINE-CONTAINING DINUCLEOSIDES

Abstract— Yields and action spectra are reported for photochemistry. fluorescence. and total lumincscence at 405 nm due to UV excitation (240–300 nm) or dilutc (-0.1 mM ) solutions or guanosine 5–monophosphate (GMP) and the dinucleosides linking guanine with adenine (ApG and GpA). cytosine (CpG) and uracil (GpU) in neutral ethylene glycol-water (7:3) glasses at 140–165 K. Phosphorescence lifetimes were determined at 140 K. Less complete data are presented for GpC, UpG and dpGpT. Quantum yields for all three processes were usually found to increase as the excitation wavelength increases. Although intramolecular exciplex formation was not dominant under these conditions interactions were strong enough to frustrate attempts at interpretation of results within the Forster very weak coupling framework. There is evidence that the GMP photochemistry proceeds from the triplet state. Surprisingly, this photochemistry is not quenched in ApG, GpA, and dpGpT at 163 K although the adenine (A) and thymine (T) moieties are known to have lower triplet states. At 140 K the phosphorescence from ApG and GpA was entirely characteristic of A but both G and T components were observed from dpGpT.     

ACRYLAMIDE QUENCHING OF TRYPTOPHAN PHOTOCHEMISTRY AND PHOTOPHYSICS

Studies of acrylamide quenching of tryptophan (Trp) fluorescence, photochemistry, and photoionization have been conducted. Quenching of Trp fluorescence in aqueous solution by addition of acrylamide in the concentration range 0.0-0.5 M was measured and resulted in a Stern-Volmer quenching constant of KSV = 21 +/- 3 M-1. Photolysis experiments were performed in which Trp was photolyzed at 295 nm in the presence of varying concentrations of acrylamide. The loss of Trp was monitored using reverse-phase high performance liquid chromatography (RP-HPLC) and was observed to follow first order kinetics. Production of N-formylkynurenine (NFK) was observed by RP-HPLC in irradiated Trp samples both in the presence and absence of added acrylamide. In addition, no new photochemical product was detected. This was taken as evidence that acrylamide did not alter the photochemical pathway but just reduced the reaction rate as expected for a physical quenching mechanism. Plotting the reciprocal of photolysis rate constant versus acrylamide concentration produced a Stern-Volmer constant for quenching of Trp photochemistry of KSV = 6 +/- 2 M-1. The KSV values for both fluorescence quenching and photolysis quenching were thus large, implying efficient quenching of both processes by acrylamide. Assuming an excited singlet state lifetime of 2.8 ns, the calculated second-order quenching rate constants for fluorescence and photolysis were kq = 7.5 x 10(9) and 2.1 x 10(9) M-1 s-1 respectively. The possible involvement of photoionization in the photolysis mechanism was investigated by studies of acrylamide quenching of voltage transients produced by xenon flash lamp excitation of Trp at aqueous/teflon or aqueous/mica interfaces.(ABSTRACT TRUNCATED AT 250 WORDS)     

杂环光化学——Ⅲ. 核酸碱基衍生物的电子光谱研究

测定了脲嘧啶、胸腺嘧啶、6-氮杂脲嘧啶、6-氮杂胸腺嘧啶和它们的相应的二甲基衍生物以及鸟嘌呤、8-氮杂鸟嘌呤等十个化合物的吸收光谱、荧光光谱和磷光光谱。讨论了这类化合物的分子结构与光物理过程及激发态之间的关系。     

CF3Br的红外光化学和光物理

We present the optothermal and optoacoustic spectra of CF_3Br in TEA CO_2 laser 00°1—02°0 R branch. Three absorption peaks has been observed at R(14) line (1074.6 cm~(-1)), R(18) line (1077.3cm~(-1)), and R(22) line (1079.9cm~(-1)) respectively. The peak at R(22) coincides with the position of P branch of CF_3Br linear IR spectrum, but two peaks at R(14) and R(18) can be interpreted as the hot bands.We also studied the multiphoton dissociation of CF_3Br and carbon isotope seperation.     

氨基酸及其衍生物的手性识别研究进展

本文综述了近年来开拓的多种人工受体模型对氨基酸及其衍生物的对映选择性识别性能，并做了相互比较，从中总结出手性识别的几个基本原则，预期了其应用前景。     

PHOTOLYSIS OF CYSTINE IN THE PRESENCE OF AROMATIC AMINO ACIDS

Se dan evidencias para explicar los distintos rendimientos cuánticos de fotólisis con lux u.v. para distintos residuos de cisteina en lisozima e insulina. En sistemas modelos de mezclas de aminoácidos, el aminoácido triptofano muestra un marcado aumento en el rendimiento cuántico en la fotólisis de la cisteina. La fotosensibilidad de los residuos de cisteina en la lisozima se interpretan usando la conformación terciaria de la enzima la cual provee una relación espacial entre los residuos de triptofano y cisteina.     

RESOLUTION OF OVERLAPPING BANDS IN THE NEAR-UV ABSORPTION SPECTRUM OF INDOLE DERIVATIVES

Abstract— The second derivative spectra of tryptophan in water and in ethylene glycol at 22°C have been integrated in order to obtain the corresponding primitive functions. The integration was carried out by making use of Tchebychev polynomials. The results show that the integrated primitive functions do not correspond to the original absorption spectra of tryptophan in various solvents, but they reflect only the contributions of the ¹L_b bands of the indolic chromophore. The identification of the electronic component, which generates the second derivative spectrum, was based on the solvent insensitivity of the derivative peaks. The comparison between the absorption spectra reported in this paper and those calculated for the ¹L_b←¹A electronic transition of indole confirmed the assumption that the derivation process eliminates the broad, although more intense, contributions coming from the ¹L_a←¹A electronic transition.     

SOLID STATE PHOTOCHEMISTRY OF THYMINE, ITS N-METHYLATED DERIVATIVES AND OROTIC ACIDS IN KBr MATRICES

Abstract— Photodimerization of a number of 2,4-diketopyrimidines including thymine, 1 -methylthymine, 3-methylthymine, 1,3-dimethylthymine, orotic and 5-methylorotic acids was studied in the solid state in KBr matrices. A technique developed for quantitative measurements of monomer and dimer concentrations under these conditions by means of u.v. and i.r. spectrophotometry is described. The absorption spectra of crystalline diketopyrimidines exhibit, in relation to the solution spectra in water, marked hypochromicity (33–69 per cent) and red shift of the long wavelength edge of the absorption (2000–3000 cm^-1) indicative of resonance intermolecular interaction and mixing of different electronic states. In most cases dimerization appeared the only photochemical reaction and the stereochemical form of main dimer corresponded with the predicted one on the basis of known crystal structure of the monomer. Initial quantum yields of monomer disappearance varied from solid to solid in the range of 0.1-0.9 M/Einstein being, apparently, to a degree dependent on the separation between nearest-neighbour C=C bonds. Reaction mechanism is discussed in terms of involvement of singlet excimer states.     

PHOTOCHEMISTRY, PHOTOPHYSICS, AND MECHANISM OF PYRIMIDINE DIMER REPAIR BY DNA PHOTOLYASE

Abstract— DNA photolyases photorepair pyrimidine dimers (PyroPyr) in DNA as well as RNA and thus reverse the harmful effects of UV-A (320–400 nm) and UV-B (280–320 nm) radiations. Photolyases from various organisms have been found to contain two noncovalently bound cofactors; one is a fully reduced flavin adenine dinucleotide (FADH^-) and the other, commonly known as second chromophore, is either methenyltetrahydrofolate (MTHF) or 8-hydroxydeazaflavin (8-HDF). The second chromophore in photolyase is a light-harvesting molecule that absorbs mostly in the near-UV and visible wavelengths (300–500 nm) with its high extinction coefficient. The second chromophore then transfers its excitation energy to the FADH^-. Subsequently, the photoexcited FADH^- transfers an electron to the Pyr<>Pyr generating a dimer radical anion (Pyr<>Pyr^-) and a neutral flavin radical (FADH^-). The Pyr<>Pyr^- is very unstable and undergoes spontaneous splitting followed by a back electron transfer to the FADH^-. In addition to the main catalytic cofactor FADH^-, a Trp (Trp277 in Escherichia coli ) in apophotolyase, independent of other chromophores, also functions as a sensitizer to repair Pyr <> Pyr by direct electron transfer.     

PHOTOCHEMICAL REDUCTION OF 5-BROMOURACIL BY CYSTEINE DERIVATIVES AND COUPLING OF 5-BROMOURACIL TO CYSTINE DERIVATIVES

Irradiation of pH 7, aqueous solutions of 5-bromouracil (BU) in the presence of cysteine peptide-like derivatives at 308 nm using a XeCl excimer laser yielded initial formation of only uracil (U) and the corresponding cystine derivative. Continued irradiation yielded an S-uracilylcysteinyl adduct as well as additional U and cystine derivative. Similar irradiation of a solution of BU and a cystine derivative yielded initial formation of U and the S-uracilylcysteinyl adduct. Formation of these products as well as secondary products of uracil photochemistry was observed upon irradiation of the respective solutions with 254 nm light. With 308 nm laser excitation, U-Cys adduct formation and reduction of BU to U are proposed to occur via initial electron transfer from the disulfide of the cystine derivative to triplet BU. The quantum yield of BU destruction with 308 nm excitation in the presence of cystine derivative is 1.1 X 10(-3). Reaction of triplet BU with the cysteine derivative does not yield U-Cys adduct but U and cystine derivative. A possible byproduct of reduction of triplet BU to U by a cysteinyl residue in a protein BU-DNA complex is a sulphenyl bromide which might yield a protein-DNA crosslink via nucleophilic substitution on sulfur by a nucleophilic site in DNA.     

四苯基卟啉及其衍生物的表面光电压谱

酞菁和卟啉染料是一类典型的分子半导体材料,由于其在生命过程、癌的治疗等生物学上的探索及其化学传感、光电特性在固体电子学器件研究中占有很重要的地位。尤以这类染料的光电性质深受人们的重视,这与太阳能光电池、光MOS晶体管等的应用前景是分     

9-甲基蒽单体、二聚体和夹心面包对在硅胶上的光物理和光化学性质研究

研究了在硅胶表面上的9-甲基蒽和它的二聚体及其低温光解产物(夹心面包对)的荧光光谱,激发光谱和荧光寿命。发现了一个由二聚9-甲基蒽、三氯甲烷和真空下700℃活化硅胶形成的复合物。该复合物具有与单体和二聚体不同的特征激发光谱和发射光谱,低温下光照容易分解。当分解产物缓缓加热到室温,则复合物几乎完全可以再现。二聚体低温光解产物,夹心面包对,当其缓缓加热到室温,则完全变为单体蒽。