Structural Effects on the Reaction of Singlet Oxygen with Tertiary Amines in Aqueous Solution

Photosensitized excitation of molecular oxygen generates singlet oxygen, a reactive oxygen species that has been studied in biological systems, synthetic methods and in aquatic ecosystems. The reaction of singlet oxygen with tertiary amines is important because they are widely used as electron donors in photochemical reactions. Herein we studied the reaction of singlet oxygen with multiple tertiary amines including ethylenediamine tetraacetic acid (EDTA), triethanolamine (TEOA) and triethylamine (TEA). Singlet oxygen was generated using the photosensitizers methylene blue or chlorin e6 and red light with output at 660 nm. TEOA and TEA generated more hydrogen peroxide (H2O2), the stable end product, than EDTA at all pH values tested and regardless of the photosensitizer used. Both histidine and imidazole scavenged singlet oxygen and decreased H2O2 yield. The extent of histidine scavenging was pH-dependent for the combination of methylene blue and EDTA but not for TEOA or TEA. The combination of chlorin e6 and EDTA generated less H2O2 because both contain multiple negative charges that limit their interaction. Multiple tertiary amines that are used as biochemical buffers produced similar quantities of H2O2 as EDTA, TEOA and TEA. However, these sulfonic acid-containing tertiary amines did not function as electron donors in a benzoquinone photoreduction assay.