Vibrational modes of flavin bound to riboflavin binding protein from egg white: resonance raman spectra of lumiflavin and 8-substituted riboflavin

The resonance Raman (RR) spectra of 8-halogenated-riboflavin, 8-demethyl-riboflavin(8-H-RF), 8-amino-riboflavin(8-NH2-RF), 8-methoxy-riboflavin(8-OCH3-RF), lumiflavin, and 3-methyl-lumiflavin were observed. The Raman lines with the highest frequency are at 1624, 1620, and 1615 cm-1 for 8-chloro-riboflavin, 8-bromo-riboflavin, and 8-iodo-riboflavin, respectively. This systematic shift confirms that the 1631 cm-1 line of riboflavin is derived from the benzene part of isoalloxazine. Substitution at the 8-position by an amino or methoxy group, which has a large influence on the electronic structure of isoalloxazine, changes the RR spectrum markedly in comparison with that of 8-halogenated riboflavin. The 1583 cm-1 line of riboflavin, which involves the vibrational displacement of N(5) and C(4a) atoms of isoalloxazine, is shifted to the low frequency side by substitution at the 8-position with an amino or methoxy group. The corresponding line of 8-H-RF, on the contrary, shifts to the high frequency side. The RR spectrum of lumiflavin is very different from that of riboflavin in the range from 1200 to 1300 cm-1. Although the pi-electronic structure is little affected by the substitution at the 10-position, the Raman spectrum of lumiflavin in this region is very sensitive.