Effect of proline on the production of singlet oxygen

Molecular oxygen in electronic singlet state is a very powerful oxidant. It s damaging action in a variety of biological processes has been well recogn ized. Here we report the singlet oxygen quenching action of proline. Single t oxygen (O-1(2)) was produced photochemically by irradiating a solution of sensitiser and detected by following the formation of stable nitroxide rad ical yielded in the reaction of O-1(2) with the sterically hindered amine ( 2,2,6,6-tetramethylpiperidine, TEMP). Illumination of a sensitiser, toluidi ne blue led to a time dependent increase in singlet oxygen production as de tected by the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR spectrometry. Interestingly, the production of TEMPO was completely abo lished by the presence of proline at concentration as low as 20 mM. These r esults show that proline is a very effective singlet oxygen quencher. Other singlet oxygen generating photosensitizer like hematopophyrin and fluoresc ein also produced identical results with proline. Since proline is one of t he important solutes which accumulate in many organisms when they are expos ed to environmental stresses, it is likely that proline accumulation is rel ated to the protection of these organisms against singlet oxygen production during stress conditions. A possible mechanism of singlet oxygen quenching by proline is discussed.