EXCITED TRIPLET-STATE OF N-(9-METHYLPURIN-6-YL)PYRIDINIUM CATION AS AN EFFICIENT PHOTOSENSITIZER IN THE OXIDATION OF SULFUR-CONTAINING AMINO-ACIDS - LASER FLASH AND STEADY-STATE PHOTOLYSIS STUDIES

Sulfur-containing amino acids were oxidized via photosensitization by the N-(9-methylpurin-6-yl)pyridinium cation (Pyr(+)) in neutral aqueou s solutions. The mechanism of this reaction was investigated by laser flash and steady-state photolysis. Rate constants were determined for the quenching of the pyridinium cation triplet state by five sulfur-co ntaining amino acids. For comparison, analogous rate constants were me asured for the quenchers 2,2'-thiodiethanoic acid, alanine (a non-sulf ur amino acid), and ammonium acetate. The rate constants for these rea ctions were found to be in the range of 2.1 x 10(8) to 7.8 x 10(9) M(- 1) s(-1) for all sulfur-containing compounds but were found to be 3-4 orders of magnitude lower for ammonium acetate and alanine. Time-resol ved transient absorption spectra accompanying the quenching events wer e;assigned to the excited triplet state of the pyridinium cation, (3)P yr(+) (lambda = 550 nm), the (S therefore S)(+) radical cations of som e of the amino acids (lambda = 480 nm), and the N-(9-methylpurin-6-yl) pyridinyl radical, Pyr(.) (lambda = 610 nm). The presence of the latte r species was separately confirmed in complementary pulse radiolysis e xperiments. The formation of the Pyr(.) radical occurred for some amin o acids (methionine and thiaproline) in two temporally distinct proces ses. A fast component, that occurred on a nanosecond time scale, was a scribed to electron transfer from the sulfur atom to the triplet state of the pyridinium cation followed by diffusion apart of the CT comple x. A slower formation of Pyr(.), that occurred on a microsecond time s cale, was characterized by a pseudo-first-order Irate constant that de pended linearly on the pyridinium cation ground-state concentration (k = 2.9 x 10(9) M(-1) s(-1)). This dark reaction was assigned to the on e-electron reduction of the Pyr(+) ground state by alpha-aminoalkyl ra dicals. These alpha-aminoalkyl radicals were formed from the radical c ations of the amino acids as a result of an intramolecular electron tr ansfer from the carboxyl group to the sulfur-centered radical cation, followed by a decarboxylation. Steady-state photolysis studies led to the identification of the water-insoluble 4,4'-dimeric, reduced pyridi ne compound formed by the dimerization of pyridinyl radicals. Quantum yields of Pyr(.) formation in the flash photolysis experiments were de termined and compared with the quantum yields of CO2 formation and of Pyr(+) disappearance measured in the steady-state photolysis. A detail ed mechanism for the pyridinium cation-sensitized photooxidation of su lfur-containing amino acids is proposed and discussed.