Formation and spectra of flavin radicals in the presence of β-mercaptoethanol at pH 9 and 10

Spectral changes and rates of reaction of flavins and several radical species have been investigated at pH 7, 9, and 10 in the presence and absence of β-mercaptoethanol. The radicals •CO 2 − , e aq − , and [Formula: see text] reacted with FAD at pH 10 to give a spectrum of FAD •Fl − with rate constants of 7 ± 1 × 10 8 and 4 ± 1 × 10 8  M −1  s −1 for •CO 2 − and [Formula: see text] respectively. At pH 7 only •FlH was observed and at pH 9 a mixture of •FIH and •Fl − .Interactions between flavin radicals and sulphydryl at 10 −4  M concentration did not cause perturbations in the uv–visible spectra until either the radical and/or the sulphydryl were ionized. With FAD at pH 9 or 10 and LFl at pH 10 the 370 nm peak of •Fl − was enhanced by about 15% and a second larger growth occurred near 450 nm in the presence of 10 −4 to 10 −2  M sulphydryl. We attribute this to the formation of labile intermediate RSHFl• − , which must also be involved in the reduction of Fl by [Formula: see text] at pH 9 or 10.The second order rate constant k 13 for reaction of [Formula: see text] with FAD at pH 9 and 10 was found to be 4.2 ± 0.5 × 10 8  M −1  s −1 and 2.0 ± 0.4 × 10 8  M −1  s −1 respectively. The rate constant for the reaction between [Formula: see text] and LFl at pH 10 was slightly faster, 7 ± 1 × 10 8  M −1  s −1 , probably reflecting the fact that LFl lacks the bulky negatively charged adenine dinucleotide group of FAD.