Effect of proline on the production of singlet oxygen

Molecular oxygen in electronic singlet state is a very powerful oxidant. Its damaging action in a variety of biological processes has been well recognized. Here we report the singlet oxygen quenching action of proline. Singlet oxygen (1O2) was produced photochemically by irradiating a solution of sensitiser and detected by following the formation of stable nitroxide radical yielded in the reaction of 1O2 with the sterically hindered amine (2,2,6,6-tetramethylpiperidine, TEMP). Illumination of a sensitiser, toluidine blue led to a time dependent increase in singlet oxygen production as detected by the formation of 2,2,6,6-tetramethylpiperidine-1-oxyl (TEMPO) by EPR spectrometry. Interestingly, the production of TEMPO was completely abolished by the presence of proline at concentration as low as 20mM. These results show that proline is a very effective singlet oxygen quencher. Other singlet oxygen generating photosensitizer like hematopophyrin and fluorescein also produced identical results with proline. Since proline is one of the important solutes which accumulate in many organisms when they are exposed to environmental stresses, it is likely that proline accumulation is related to the protection of these organisms against singlet oxygen production during stress conditions. A possible mechanism of singlet oxygen quenching by proline is discussed.