Redox reaction between m-thiocresol and riboflavin glycosides with 2:1 complex formation; regulation by the steric effect of sugar in the side chain

Abstract

We investigated the reduction of riboflavin-2′, 3′, 4′, 5′-tetra-acetate (AcB2), riboflavin-1′-glucoside-2″, 3″, 4″, 6″, 2′, 3′, 4′, 5′-hepta-acetate (AcB2gl) and lumiflavin using m-thiocresol (mTc) in the presence of tetrabutylammonium hydroxide. The series of rate constants for AcB2 and AcB2gl reductions indicated that modified Lineweaver–Burk plots were best fit by assuming a 1:2 complex formation. The complex formation in the reaction was supported by the 2-D nuclear Overhauser enhancement spectroscopy and circular dichroism spectra. The modified Michaelis–Menten constants (K m) for AcB2 and AcB2gl with mTc were 1.32 and 0.86 × 10− 3 M2, respectively, and the maximum rate constant k 2 were 4.45 and 4.35 × 10− 2 M− 1 s− 1, respectively. The E 1/2 values of AcB2 and AcB2gl were − 331 and − 341 mV, respectively, which indicated that their reduction activities were almost the same. It was established that the redox function depended on the formation of the complex and was regulated by the steric effect of the sugar in the side chain.