SPIN-TRAPPING and ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES and POLYPEPTIDES IN AQUEOUS SOLUTIONS—III. TRYPTOPHAN and RELATED COMPOUNDS

Abstract— The UV photolysis of tryptophan (Trp) and Trp-containing peptides in aerated aqueous solutions has been studied by ESR and spin-trapping techniques using f-nitrosobutane as the spin-trap. The photolysis of Trp alone at 290 nm gave rise to the addition of the spin-trap to carbon 3 of the indole ring. A large ESR signal from the hydronitroxide spin-adduct was also observed revealing the formation of hydrated electrons. Generally, the photolysis of Trp-containing dipeptides generated the deamination radical of the N-terminal amino acid followed by addition to the spin-trap. In the case of lysyl-Trp, a deamination radical from the side chain of lysine was proposed. A sensitization experiment with Trp as sensitizer and glycine (Gly) as substrate led to the generation of the deamination radical of Gly. Most of the observed free radicals resulting from the photolysis of Trp-containing peptides can be explained in terms of hydrated electrons reacting with the carbonyl group followed by deamination of the N-terminus.     

SPIN-TRAPPING AND ESR STUDIES OF THE DIRECT PHOTOLYSIS OF AROMATIC AMINO ACIDS, DIPEPTIDES, TRIPEPTIDES AND POLYPEPTIDES IN AQUEOUS SOLUTIONS—I. PHENYLALANINE AND RELATED COMPOUNDS

Abstract— The direct UV photolysis of l -Phe and peptides containing l -Phe in aqueous solutions has been investigated at room temperature. The short-lived free radicals formed during photolysis were spin-trapped by t -nitrosobutane and identified by electron spin resonance. During the photolysis of l -Phe the decarboxylation and the deamination radicals were spin-trapped. For N-formyl and N-acetyl- l -Phe the decarboxylation radicals were observed. For dipeptides containing Phe the decarboxylation radicals were observed and in some cases the deamination radicals from the N-terminal residue were found. For the tripeptides Gly- l -Phe- l -Ala and Gly-Gly- l -Phe, the C-terminal decarboxylation radical was spin trapped; for l -Phe-Gly-Gly only the deamination radical of the N-terminal residue could be detected. However, for Gly- l -Phe-Gly, five different radicals were identified. The results of the spin-trapping experiments of the 260 nm photolysis of RNase-S-peptide, containing 20 amino acid residues, was interpreted in terms of a chain scission between the alpha carbon of the Phe residue and the adjacent carbonyl group.     

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.     

LASER FLASH PHOTOLYSIS OF INDOLE IN THE PRESENCE OF AMINO ACIDS

The laser flash photolysis of indole at 265 nm in the presence of glycine, proline and hydroxy proline was studied. The relative yields of c _aq, triplet state, and indole cation radical were determined in the absence and in the presence of the amino acids. The yields were determined as a function of laser intensity and the values at very low intensity were compared with the fluorescence quenching results. It was concluded that in these conditions the photoionization of indole occurs via the fluorescent state. From the curves of triplet yield vs laser intensity, the triplet quantum yield extrapolated at low laser intensity was obtained, φr = 0.55 φ 0.05, relative to the literature value of 0.15 for φ_eag. This gives φ_F+φ_eaq= 1.0 ± 0.1 at room temperature. When proline and hydroxy proline were used as singlet quenchers, the yield of In was greater than the yield of c_aq. This was considered as evidence that a fraction of the quenching processes leads to complete electron transfer from indole to the amino acids.     

PHOTOCHEMISTRY OF BIOLOGICAL MOLECULES – II. PHOTOLYSIS OF DIPEPTIDES IN THE SOLID STATE

Abstract— Photolysis with light of 2537 Å of a series of aliphatic dipeptides in the solid state has been shown to lead to bond rupture and free radical formation. The structures of the radicals have been determined by ESR techniques and in general, the free electron has been shown to reside on the carbon atom attached to the nitrogen atom of the peptide bond. Dipeptides containing phenylalanine residues show ESR spectra typical of the free amino-acid in the terminal position.     

ESR STUDIES OF THE LOW TEMPERATURE IRRADIATION OF KERATIN AND ITS COMPONENT AMINO ACIDS

Abstract— The photoejection of electrons from the aromatic amino acids at 77°K by 290–340 nm radiation was found to involve both monophotonic and biphotonic processes. However, only monophotonic processes are involved when electrons are photoejected from cystine or cysteine. These electrons prefer to attach themselves to adjacent sulphur groups rather than the dielectric traps in the matrix, so that both anionic and cationic radicals are formed.
In keratin at 77°K, the disulphide anion radicals are produced by both monophotonic and biphotonic mechanisms, indicating that the electrons captured by the disulphides are photo-ejected from the aromatic amino acids. At room temperature the thiyl and aromatic radicals are produced only by monophotonic reactions.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—I. SPIN TRAPPING OF PHOTOLYSIS PRODUCTS FROM SULFANILAMIDE, 4-AMINOBENZOIC ACID AND RELATED COMPOUNDS

The photodecomposition of sulfanilamide, 4-aminobenzoic acid and other related analogs has been studied with the aid of the spin trap 2-methyl-2-nitrosopropane. UV photolysis of an aqueous solution of sulfanilamide yielded the following radicals C˙₆H₄SO₂NH₂, C₆H₅SO_2˙, S˙O₂NH₂ and SO^-_#˙ (or SO^-_2dot;). Under the same conditions 4-aminobenzoic acid gave C˙₆H₄COOH. In addition both sulfanilamide and 4-aminobenzoic acid, but not 4-dimethylaminobenzoic acid, generated e^-_aq or hydrogen atoms during UV irradiation. The C₆H₄SO₂NH₂ radical was also produced by photolysis of 4-iodobenzenesul-fonamide and 4-nitrobenzenesulfonamide. The C₆H₄COOH radical was generated by photolysis of 4-nitrobenzoic acid and 4-iodobenzoic acid. Finally the C₆H₄NO₂ radical was formed during the irradiation of 4-nitroaniline, 1,4-dinitrobenzene and 4-iodonitrobenzene. The free radicals generated by sulfanilamide and 4-aminobenzoic acid may play an important role in the phototoxic and photoallergic responses elicited by these drugs in certain individuals.     

PHOTOLYSIS AND PHOTO-OXIDATION OF AMINO ACIDS AND PEPTIDES—VI. A STUDY OF THE INITIATION OF DISULPHIDE INTERCHANGE BY LIGHT IRRADIATION

Abstract— Investigations concerning the occurrence of disulphide interchange for disulphides related to cystine are made with either a single unsymmetrical disulphide or an equimolar mixture of two symmetrical disulphides. It is found that in the pH range 1–6 where spontaneous interchange can usually be disregarded, interchange can be initiated by the addition of a small amount of a thiol or by irradiation with ultraviolet light from either of two artificial sources, interchange being more rapid at pH 4–6 than at pH 1–3. Direct sunlight and, to a lesser extent, visible (filtered tungsten) light are also effective. Disulphide interchange in the solid state can also be promoted by ultraviolet light. A comparison is made between the roles' of S-S and C-S bond fission in the photochemistry of disulphides, with respect not only to the promotion of disulphide interchange, but to other phenomena such as the dismutation of disulphides to thioethers and trisulphides.     

PHOTOLYSIS OF NITRATE ESTERS—II. SOLUTION PHOTOLYSIS OF ARALKYL NITRATE ESTERS: KINETICS, ESR SPECTRA, AND PHOTOPRODUCTS

Abstract— Irradiation of benzyl nitrate, meso- and dl-hydrobenzoin dinitrates, and cis- and trans -1,2-acenaphthenediol dinitrates in benzene solutions 0.02M in nitrato groups at 265–313 mp decomposed the esters with a quantum yield of 0.4±0.2 in reasonable agreement with earlier work on the gas-phase photolysis of ethyl nitrate. An important primary process was ArRCHONO₂+h v ArRCHO +NO₂, although ArRCHONO₂+h v ArRCHONO + O was not excluded. In secondary reactions solvent benzene was oxidized and nitrated, aldehydes were formed from the nitrate esters, and nitric oxide was evolved. The higher quantum yields of the photodecomposition in ethanol (0.8±0.3) and ether (3±1) solutions were interpreted in terms of a charge-transfer state similar to that found with nitroalkanes in these solvents. Acetaldehyde was produced in a rapid secondary reaction in alcohol solutions at 25° and at 77° K, NO₂ was identified as an intermediate from the ESR spectrum. No stereospecificity was detected in a comparison of rates, photoproducts, and ESR spectra for the stereoisomers. The overall results indicated inter- rather than intramolecular migration of oxy-nitrogen groups in secondary reactions. An apparently new, long-lived, oxynitrogen radical was detected in benzene solutions of nitric oxide and nitrate esters irradiated at 25°.     

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.     

SPECTROSCOPIC STUDIES OF CUTANEOUS PHOTOSENSITIZING AGENTS—X. A SPIN-TRAPPING AND DIRECT ELECTRON SPIN RESONANCE STUDY OF THE PHOTOCHEMICAL PATHWAYS OF DAUNOMYCIN AND ADRIAMYCIN

Abstract— Irradiation of daunomycin (or adriamycin) and the spin trap 5,5-dimethyl-l-pyrroline-1-oxide (DMPO) at 490 nm in the presence or in the absence of air generated the hydroxyl radical adduct (DMPO-OH). The observed DMPO-OH signal was not affected by the addition of hydroxyl radical scavengers (ethanol, formate), suggesting that direct trapping of the hydroxyl radical was not involved. The DMPO-OH signal was insensitive to superoxide dismutase and catalase, which ruled out the possibility of superoxide or H₂O₂ involvement. These findings demonstrate that daunomycin (or adriamycin) does not generate hydroxyl radicals or superoxide radical anions when subjected to 490-nm excitation. However, when daunomycin (or adriamycin) was irradiated at 310 nm DMPO adducts derived from two carbon-centered radicals, superoxide and the hydroxyl radical were detected. The superoxide adduct of DMPO was abolished by the addition of SOD, providing unequivocal evidence for the generation of the superoxide anion radical. The daunomycin semiquinone radical, observed upon 310-nm irradiation of daunomycin in the absence of DMPO, appears to be the precursor of the superoxide radical anion. One of the carbon-centered radicals trapped by DMPO exhibited a unique set of hyperfine parameters and was identified as an acyl radical. This suggests that the known photochemical deacylation of daunomycin occurs via a homolytic cleavage mechanism. The free radicals generated photolytically from adriamycin and daunomycin may be involved in the etiology of the skin ulceration and inflammation caused by these drugs. A knowledge of the dependence of these photogenerated radicals on the wavelength of excitation may be important in the development of adriamycin and daunomycin for photodynamic therapy.     

THE U.V. PHOTOLYSIS (Λ= 185nm) OF ETHYLENE GLYCOL IN AQUEOUS SOLUTION*

Abstract— –The u.v. photolysis (Λ= 185 nm) of 2 M aqueous solutions of ethylene glycol was studied at 22°C. Products (quantum yields) are hydrogen (0.20₄) formaldehyde (0.19₄), glycolaldehyde (0.08), methanol (0.07₄), glycerol (0.06), erythritol (0.03), acetaldehyde (0.02), 3,4-dihydroxybutanal (0.01) and succinaldehyde (0.001). With increasing temperature the yields of all products remain essentially unchanged except that of acetaldehyde (φ= 0.32 at 90°C) which is formed in a chain reaction. The photolysis of (CD₂OH)₂ yields 91% HD, indicating that the most important primary process is the homolytic splitting of the O-H bond. The resulting oxy radicals fragmentate largely into formaldehyde and CH₂OH radicals. Molecular fragmentation processes yielding hydrogen and glycolaldehyde, as well as formaldehyde and methanol, are discussed in the proposed decomposition scheme.     

STUDIES ON ORGANOPHOSPHORUS COMPOUNDS PART IX.1 THE ACTION OF Di-, TRIALKYL PHOSPHITES AND THIOL ACIDS ON SOME ENAMINOLACTONES

Abstract

Di-, and trimethylphosphites react with enaminolactone, 4, only in methanol to give N-benzoyl-3-phenyl-3-phosphonoalanine-trimethylester (7) in good yields. Diphenylphosphinodithioic acid also reacts with 4 to give N-benzoyl-3-phenylcysteine-methyl ester-diphenylphosphinodithioate (9). A mechanism is proposed to account for the formation of 7. Compatible analytical and spectroscopic results were obtained for the new compounds.