Photochemical Behavior of 6-Methylpterin in Aqueous Solutions: Generation of Reactive Oxygen Species

FRANCO M. CABRERIZO, CAROLINA LORENTE, MARIANA VIGNONI, ROMINA CABRERIZO, ANDRÉS H. THOMAS AND ALBERTO L. CAPPARELLI

PHOTOCHEMISTRY AND PHOTOBIOLOGY

American Society for Photobiology

Año: 2005 vol. 81 p. 793 - 801

0031-8655

Pterins are a family of heterocyclic compounds present in a wide range of living systems that participate in relevant biological functions and are involved in different photobiological processes. 6-Methylpterin (MPT) was investigated for its efficiency of singlet-oxygen (1O2) production and quenching in aqueous solution. The quantum yields of 1O2 production (FD) was determined by measurements of the 1O2 luminescence in the near-IR upon continuous excitation of the sensitizer. Values of FD were found to be 0.10 ¡Óƒn0.02 and 0.14¡Ó0.02 in acidic and alkaline media, respectively. Studies of the photooxidation of MPT in acidic (pH= 5.0-6.0) and alkaline (pH=10.2-10.8) aqueous solutions at 350 nm and room temperature have been performed. The photochemical reactions were followed by UV-visible spectrophotometry, HPLC and an enzymatic method for H2O2 determination. MPT is not light-sensitive in the absence of oxygen, but it undergoes a photooxidation reaction in the presence of oxygen, yielding several non-pteridinic products. The quantum yields of MPT disappearance were determined and values of 2.4(¡Ó0.5)x10-4 and 8.1(¡Ó0.8)x10-4 were obtained in acidic and alkaline media, respectively. H2O2 was detected and quantified in irradiated solutions of MPT. The rate constant of the chemical reaction between 1O2 and MPT (kr) was determined to be 4.9x106 M-1 s-1 in alkaline medium and the role of 1O2 in the photooxidation of MPT is discussed.