FLUENCE-RATE-DEPENDENT PHOTOSENSITIZED OXIDATION OF NADH

The photosensitizing activity of dimethoxyhaematoporphyrin, excited by a laser pulse at 532 nm (YAG-Nd3+), was investigated using reduced ni cotinamide adenine dinucleotide (NADH) as a substrate. The photo-oxida tive modification of NADH was monitored by measuring the absorbance at 340 nm. The use of nanosecond pulses (15 and 0.5 ns) resulted in phot osensitized NADH oxidation which depended on the fluence but not on th e fluence rate up to a peak fluence rate of 10(7) W cm-2. At higher fl uence rates a decrease in NADH photo-oxidation was observed, as well a s on irradiation with picosecond pulses (35 ps). Stern-Volmer assay of the quenching by sodium azide revealed a decrease in quenching effici ency with increasing peak fluence rate. Oxidation of NADH was not supp ressed by the addition of 20 mM sodium azide at peak fluence rates abo ve 6 x 10(9) W cm-2. This observation, as well as the significant blea ching of dye absorption, indicates excitation of the photosensitizer i nto higher lying excited singlet states and the involvement of process es other than photodynamic action.