PHOTOLYSIS AND RADIOLYSIS OF PHENYL-ALANYLGLYCINE

T he P hotolysis of glycine and alanine containing peptides in oxygenated solution has been shown to result in the formation of glyoxylic and pyruvic acids, through oxidative cleavage of the peptide bond[1]. The same products were formed in the radiolysis of such peptides in the presence of oxygen[2]. In a peptide containing aromatic amino-acids the absorption of light results in the excitation of the aromatic ring and it has been postulated[3] that energy is transferred to the peptide bond, resulting in the formation of a radical on the carbon atom adjacent to the peptide bond. Radiolysis, however, involves the attack of hydroxyl radicals and other reactive species formed in the absorption of radiation by water[2]. This study compares the behavior of phenylalanylglycine to photolysis and radiolysis in aerated aqueous solution in relation to a possible energy transfer in photolysis.     

PHOTOLYSIS AND RADIOLYSIS OF THE BENZENE-FUMARONITRILE SYSTEM

Abstract— The photolytic and radiolytic product formation of the complex benzene-fumaronitrile are compared. The equilibrium constants for the complexes of benzene-fumaronitrile and benzene-maleonitrile determined by UV and NMR spectroscopy are given. The products which were found in both photolysis and radiolysis of the system benzene-fumaronitrile were biphenyl. phenylsuccinodinitrile and maleonitrile. The production of a 2:1 adduct of fumaronitrile and benzene in very small yields could only be confirmed in the case of photolysis. Additional products in radiolysis were phenylcyclohexadienes and benzonitrile. Experiments using filtered light proved that isomerisation of fumaronitrile to maleonitrile and production of phenylsuccinodinitrile proceed via the excited charge-transfer complex. There are indications that the excited charge transfer complex is likewise an intermediate in the radiolytic formation of these nitriles. The addition of fumaronitrile to benzene reduces the yield of biphenyl in both photolysis and radiolysis.     

PRIMARY PROCESSES IN THE LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS OF l-EPHEDRINE

Abstract— Transient absorption spectra produced by laser flash-photolysis of an aqueous solution of ephedrine have been measured under a variety of conditions. Ephedrine was found to photoionise via a biphotonic process. The apparent yield of photoionisation increases with lowering of pH, a value of 8.8 being found for the p K _a associated with this change. The cation radical absorption spectrum has been determined using the techniques of both pulse radiolysis and laser flash photolysis. The extinction coefficient of the cation at 295 nm was determined to be 1.37 × 10⁴ dm³ mol^-1 cm^-1 and 1.2 × 10⁴ dm³ mol-¹cm-¹ by the two techniques, respectively, at pH 11. It is also shown that the rate constant for electron abstraction by the azide radical to form the ephedrine cation is controlled by protonation of the amine group in the side chain. The ephedrine anion radical spectrum and its extinction coefficient at 305 nm were also determined. The excited states responsible for photoionisation and photodegradation are discussed.     

A LASER FLASH PHOTOLYSIS AND PULSE RADIOLYSIS STUDY OF AMILORIDE IN AQUEOUS AND ALCOHOLIC SOLUTION

Abstract Laser flash photolysis and pulse radiolysis have been carried out on the diuretic drug amiloride. The primary photochemical processes in aqueous solution were found to be photoionisation and triplet formation. Photoionisation was found to occur by a biphotonic process for 265 nm excitation but by a monophotonic process for excitation at 353 nm. The spectral properties of the resulting cation radical of amiloride were determined by pulse radiolysis using one electron oxidation by the radical anion Br₂·^¯ Photoexcitation of amiloride in isopropanol did not result in photoionisation but instead a semireduced radical of amiloride was observed. The spectral properties of the semireduced radical of amiloride were determined using one electron reduction by the CO₂·^¯ radical.     

PRIMARY PROCESSES IN THE PHOTOCHEMISTRY OF AQUEOUS SULPHACETAMIDE: A LASER FLASH PHOTOLYSIS and PULSE RADIOLYSIS STUDY

Abstract— The spectra have been measured of the transient species formed in the nanosecond flash photolysis of aqueous solutions of sulphacetamide under a variety of conditions. In addition to the excited triplet state, the cation radical and the solvated electron were observed. The ionisation of aqueous sulphacetamide was found to occur by a biphotonic process. The extinction coefficient of the cation radical of sulphacetamide was determined by both laser flash photolysis and pulse radiolysis techniques, a value of 1.9 times 10₃ dm₃mol_-1cm_-1 being obtained. The rate of electron reaction with sulphacetamide and the anion radical spectrum were also determined by the two techniques, good agreement being obtained. The spectrum of the product of the reaction of the superoxide anion radical and the corresponding rate constant have also been determined. A possible mechanism of photosensitized skin reaction due to sulphacetamide is discussed.     

X-RAY AND VIBRATIONAL STUDIES OF 1:1 ADDUCT OF NITROMETHANE AND TITANIUM TETRACHLORIDE

Abstract

The crystal structure of Ti₂ Cl₈ (CH₃ NO₂)₂ has been determined from three dimensional X-ray data. Crystals are monoclinic (space group P 2₁/c) with four TiCl₄ (CH₃ NO₂) units per unit cell. Lattice constants are:

a=9.55 (1) Å b=5.93 (1) Å c=16.38 (2) Å β=119.8 (1)° Atomic parameters were refined by full matrix least-squares to a final R equal to 0.055 using 1038 non-zero independent reflections. The compound is dimeric with a double chlorine bridge between titanium atoms. Titanium is octahedrally surrounded by five chlorine atoms and one oxygen atom of nitromethane which behaves as a monodentate ligand. IR and Raman spectra are discussed on the basis of the actual structure and assignments are proposed.     

NANOSECOND LASER PHOTOLYSIS OF RHODOPSIN AND ISORHODOPSIN

Kinetic and spectral measurements have been carried out on the primary intermediate in the photolysis of rhodopsin and isorhodopsin, initiated by a 457 nm, 6 ns (FWHM) laser pulse. In rhodopsin the kinetic decay of bathorhodopsin was found to be 140 ± 15 ns at 20°C. The decay of bathorhodopsin to lumirhodopsin has an activation energy of 51 ± 4 kJ/mol (12.2 ± 1 kcal/mol). The decay kinetics of bathorhodopsin were found to be the same for rhodopsin in membrane and detergent solubilized suspensions. The kinetic decay of the batho product in the photolysis of isorhodopsin was found to be the same as rhodopsin.
The corrected transient spectrum 50 ns following excitation in rhodopsin has two peaks near 560 and 440 nm. A peak was also observed in isorhodopsin near 550 nm at 50 ns following excitation but no transient was observed in the blue. The 550 nm peak in isorhodopsin has an intensity similar to that in rhodopsin indicating that the quantum yields for the formation of batho products of rhodopsin and isorhodopsin are similar under the irradiation conditions used here. Transient spectra for rhodopsin and isorhodopsin 1 μs following excitation are also different. In isorhodopsin the corrected transient spectrum has a peak at 500 nm, similar to low temperature steady state irradiation spectra. The 1 μs transient spectrum in rhodopsin is more intense than in isorhodopsin and shows a peak at 475 nm.     

PHOTOLYSIS AND HYDROLYSIS OF ADENOSINE 5'-PHOSPHATES

Abstract— Mild acid hydrolysis of adenosine 5'-di-, tri- and tetra-phosphate (ADP, ATP and ADTP) occurs predominantly in the side chain, yielding orthophosphate and adenosine 5'-mono-phosphate (AMP). Photolysis results in breaking of the 5'-C—O—P bond, yielding orthophosphate for AMP and mainly condensed phosphates for ADP, ATP and ADTP, together with decomposition products of adenosine. The photolytic breaking of the 5'-C—O—P bond is not enhanced by excitation of the strongly absorbing adenine ring followed by energy transfer but results from direct excitation of the very weakly absorbing ribosephosphate- moiety of the molecule. The arguments for this are: (i) cyclic adenosine 3'5'-mono-phosphate, which is sterically hindered for direct energy transfer from the adenine ring to the phosphate-ester bond, is photolyzed at approximately the same rate as AMP, and (ii) the rate of disappearance of the strong absorption band due to the adenine ring is higher than the rate of liberation of orthophosphate.     

取代苯基重氮盐的直接光解和敏化光解的研究

本工作合成了一系列带取代基的苯基重氨盐化合物。对它们的直接光解研究表明:带推电子基的重氮盐化合物具有较高的光解反应速度,但在敏化光解研究中发现:带有拉电子基的重氟盐化合物,不论是它的敏化光解或是猝灭敏化剂荧光的能力都较带推电子基的重氮盐为强,这清晰地表明,此敏化过程是通过电子转移反应而实现的。工作中还发现,在基态条件下,重氮盐和N,N-二甲基苯胺间可生成电荷转移络合物(CTC),经Benesi-Hildebrand公式处理表明:可形成1:1的CTC。     

INTERMEDIATES IN THE ROOM TEMPERATURE FLASH PHOTOLYSIS AND LOW TEMPERATURE PHOTOLYSIS OF PURINE SOLUTIONS

The flash photolysis of purine in acetonitrile and in water at different pH was studied. The transients produced on flash excitation of degassed aqueous solutions have been identified as the triplet excited state, the hydrated electron, a purine radical cation and radical anion on the basis of quenching experiments and comparison to transients observed in low temperature photolysis.     

LASER FLASH PHOTOLYSIS AND PHOTOINACTIVATION OF SUBTILISIN BPN'

Abstract— Laser flash photolysis of subtilisin BPN'at 265 nm has shown that photoionization of tryptophanyl (Trp) and tyrosinyl (Tyr) residues are the principal initial photochemical reactions. The initial products are the corresponding oxidized radicals. Trp and Tyr, and hydrated electrons (e_aq) which react with the enzyme at: k (e_aq+ subt. BPN') = 2.1 × 10¹⁰ M⁻¹ s⁻¹. The photoionization quantum yield was 0.032 ± 0.005 at 265 nm, which was enhanced 3.5-fold by simultaneous excitation at 265 and 530 nm. The photoionization yields were unchanged by 3 M bromide ion and 8 M urea. which did affect the enzyme fluorescence excited at 265 and 295 nm. A similar lack of correlation between the effects of perturbants on the photionization yields and fluorescence yields was found for subtilisin Carlsherg. The results indicate that the monophotonic and biphotonic ionization of the Trp residues does not involve the thermally-equilibrated. lowest excited singlet state and that singlet energy transfer from Tyr to Trp does not contribute to Trp photoionization. The photoinactivation quantum yield was 0.014 for 265 nm laser excitation. which was not changed by simultaneous 530 nm excitation. The corresponding quantum yield was 0.009 for low intensity 254 nm radiation, indicative of a biphotonic contribution to photoinactivation. The results are explained by postulating that photolysis of Trp-113 leads to disruption of hydrogen bonding to Asn-117 and a shift in the primary chain sequence associated with the aromatic substrate binding sites. The photoionization quantum yields in subtilisin BPN'and subtilisin Carlsberg agree with a model based on the assumption that exposed Trp and Tyr residues contribute independently at intrinsic photoionization efficiencies characteristic of the chromophores.     

新型含硫化合物的荧光光谱及其光解机理

本工作合成了硫代α-萘甲酸-s-苯酯和硫代苯甲酸-s-苯并噻唑酯,对其光化学和光物理行为进行了研究。由于萘基和苯并噻唑基的引入,有效地增加了该类化合物的最大吸收光谱范围,并在受光激发后,可观察到有荧光发射。工作中发现,硫代α-萘甲酸-s-苯酯荧光发射强度会随光照时间的增加而加强,这是因该化合物光解时生成了有高荧光量子产率的α,α'-联萘;但在硫代苯甲酸-s-苯并噻唑酯的光解中观察不到上述现象,文章对所得结果进行了讨论。     

硝基甲烷光重排反应机理的从头算和组态相互作用的研究

硝基甲烷是最简单的硝基化合物,同时也是一种高能物质,在工业上有着重要的作用。对于硝基甲烷形成亚硝酸甲酯的光反应,不同的实验研究得到了不同的结论。     

THE PHOTOLYSIS RATES OF SOME DI- AND TRIPEPTIDES OF TRYPTOPHAN

We have measured the relative rates of photolysis of free tryptophan (trp), the dipeptides Gly-Trp, Trp-Gly, Leu-Trp, and Trp-Leu, and the tripeptides Gly-Trp-Gly and Leu-Trp-Leu. The photolyses were performed in neutral 0.1 mM aqueous solutions at 25°C using monochromatic 290 nm Xe arc radiation. Tryptophan loss was monitored by absorption, fluorescence and phosphorescence spectroscopy. The rate of tryptophan fluorescence loss was found to be different in the di-and tripeptides than in tryptophan monomer. These rate differences depended on both the identity of the neighboring amino acid (gly or leu) and on the nature of the linkage, e.g., the rate of Gly-Trp photolysis was more than 10 times greater than the rate of Trp-Gly photolysis. Degassing was found to markedly reduce (factor of 8) the photolysis rates of Trp, Trp-Gly, and Trp-Leu, but degassing only slightly reduced (less than a factor of 2) the photolysis rates of the other di-and tri-peptides. Photochemical product structures were not determined, but absorption and fluorescence spectra were obtained and products could be inferred in some cases by comparison with data of previous workers. The products appeared to differ greatly among the various peptides studied; Trp, Trp-Gly, and Trp-Leu gave oxidation products, while Gly-Trp and Leu-Trp apparently gave ring closure products, not requiring oxygen.     

三(对-甲苯磺酰氧甲基)硝基甲烷的还原和N-三(对-甲苯磺酰氧甲基)甲基羟氨的空气氧化

三(对-甲苯磺酰氧甲基)硝基甲烷(1)于苯中,以锌粉和醋酸在加或不加醋酸酐下可以被还原,并分別形成相应的羟氨(2)或其醋酸酯(3)。值得注意的是,当2和3以1M甲醇钠在氯仿中处理时,受空气氧化后又形成硝基化合物(1),但在排除空气时,不发生这一氧化反应。同样,三(苯甲酰氧甲基)硝基甲烷当用锌粉-醋酸还原时可生成相应的胺。     

PHOTOLYSIS MECHANISM OF AQUEOUS TYROSINE AND TYROSYL PEPTIDES*

Abstract— Laser flash photolysis at 265 nm has been employed for measuring the initial hydrated electron (e^-_aq) and p-alanylphenoxyl radical (Tyr) in aqueous Tyr, small Tyr peptides and R Nase A. The results indicate that monophotonic photolysis not involving the fluorescent or triplet states is the principal initial process. Equivalent yields of e and Tyr were found in all cases except Tyr, where the Tyr yield was 60% higher than e^-_aq attributed to splitting of the phenolic bond. Computer analysis of e^-_aq and Tyr decays for Tyr indicates the importance of electron-radical recombination in competition with electron scavenging and bimolecular radical-radical reactions. Evidence for intramolecular electron migration has been obtained in cystinyl-bis-Tyr.     

GAMMA AND PULSE RADIOLYSIS STUDIES OF THE REACTION BETWEEN SUPEROXIDE IONS AND OXYHEMOGLOBIN-METHEMOGLOBIN SYSTEM

Abstract— The radiolytic studies of oxyhemoglobin or methemoglobin in neutral aerated aqueous solutions with formate ions, lead to three conclusions:
The oxidation of oxyhemoglobin by O^-₂ is not important. The observed low oxidation yield is probably due to the slow reaction with hydrogen peroxide produced by O^-₂ disproportionation.
The reduction of methemoglobin in γ radiolysis reaches a plateau which could be explained by structural considerations.
The reduction of methemoglobin by O^-₂ ions, if it occurs, is relatively slow: k = 1.4 × 10³ M ^-1 s^-1. But a problem remains concerning the spectral characteristics of the product.