EXCITED-STATE DYNAMICS OF FULLY REDUCED FLAVINS AND FLAVOENZYMES STUDIED AT SUBPICOSECOND TIME RESOLUTION

The photophysics of the fully reduced states of a number of flavins (f lavin mononucleotide, flavin adenine dinucleotide and 3-N-methyllumifl avin) and flavoenzymes (glucose oxidase from Aspergillus niger and the flavodehydrogenase component isolated from flavocytochrome b(2)) was studied using subpicosecond laser excitation at lambda = 312 mm. The p rompt transient absorption spectra (measured from 400 to 850 nm) were all closely similar in the ease of the free flavins in aqueous solutio n. The decay of the transient absorbance obeyed biexponential kinetics with a fast component of lifetime ranging from 4 to 130 ps and a slow er phase with a lifetime above 1 ns. The spectral structure changed ap preciably during the rapid decay phase. In contrast, in the case of th e enzymes only a very slight decay was apparent over the probed time i nterval (1 us) and the shape of the spectrum remained unchanged. It is proposed that the two transient spectra appearing in the free flavins correspond to two conformations differing by their degree of nonplana rity, whereas in the flavoenzymes only one conformation is possible.