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

Authors:

• Bronisław Marciniak,
• Jaroslaw Rozwadowski,
• Gordon L. Hug,
• Krzysztof Bobrowski

Abstract

Sulfur-containing amino acids were oxidized via photosensitization by the N-(9-methylpurin-6-yl)pyridinium cation (Pyr⁺) in neutral aqueous 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-containing amino acids. For comparison, analogous rate constants were measured for the quenchers 2,2'-thiodiethanoic acid, alanine (a non-sulfur amino acid), and ammonium acetate. The rate constants for these reactions were found to be in the range of 2.1 × 10⁸ to 7.8 × 10⁹ 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-resolved transient absorption spectra accompanying the quenching events were assigned to the excited triplet state of the pyridinium cation, ³Pyr⁺ (λ = 550 mn), the (S∴S)⁺ radical cations of some of the amino acids (λ= 480 nm), and the N-(9-methylpurin-6-yl)pyridinyl radical, Pyr· (λ = 610 nm). The presence of the latter species was separately confirmed in complementary pulse radiolysis experiments. The formation of the Pyr· radical occurred for some amino acids (methionine and thiaproline) in two temporally distinct processes. A fast component, that occurred on a nanosecond time scale, was ascribed to electron transfer from the sulfur atom to the triplet state of the pyridinium cation followed by diffusion apart of the CT complex. A slower formation of Pyr·, that occurred on a microsecond time scale, was characterized by a pseudo-first-order rate constant that depended linearly on the pyridinium cation ground-state concentration (k = 2.9 × 10⁹ M^-1s^-1). This dark reaction was assigned to the one-electron reduction of the Pyr⁺ ground state by α-aminoalkyl radicals. These α-aminoalkyl radicals were formed from the radical cations of the amino acids as a result of an intramolecular electron transfer 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 pyridine compound formed by the dimerization of pyridinyl radicals. Quantum yields of Pyr· formation in the flash photolysis experiments were determined and compared with the quantum yields of CO₂ formation and of Pyr⁺ disappearance measured in the steady-state photolysis. A detailed mechanism for the pyridinium cationsensitized photooxidation of sulfur-containing amino acids is proposed and discussed. © 1995, American Chemical Society. All rights reserved.