Measurement and characterization of singlet oxygen production in copper ion-catalyzed aerobic oxidation of ascorbic acid

Production of singlet molecular oxygen (1O2) in the aerobic oxidation of ascorbic acid catalyzed by copper ion was measured and characterized using [4′-(9-anthryl)-2,2′:6′,2″-terpyridine-6,6″-diyl]bis(methylenenitrilo)tetrakis(acetate)-Eu3+ (ATTA-Eu3+) as a highly sensitive and selective time-resolved luminescence probe for 1O2. The 1O2 produced in the reaction was further characterized and confirmed by (i) chemical trapping of 1O2 with 9,10-diphenylanthracene (DPA), the corresponding endoperoxide was detected by HPLC and (ii) spin trapping of 1O2 with 2,2,6,6-tetramethyl-4-piperidinol (TMP-OH), the corresponding free radical of TMP-OH oxide (TMPO) was detected by electron spin resonance (ESR) spectroscopy. The effects of deuterium oxide, sodium azide and histidine on the 1O2 signal were investigated. The mechanism investigation of 1O2 production implied that the ascorbic acid–Cu(I) complex formed in the reaction could be an important intermediate for the 1O2 production. The reaction of ascorbic acid with copper ion monitored by 1H NMR and absorption spectroscopy demonstrated the formation of a copper ion–ascorbic acid complex. Except for Cu2+ and Cu+–ascorbic acid systems, no detectable 1O2 was produced in other transition metal cation–ascorbic acid systems in the studied range.