Metal-flavin complexation. A resonance Raman investigation.

Several groups have recently shown that high quality resonance Raman spectra can be obtained for flavin species in spite of their intense fluorescence. We are interested in obtaining the resonance Raman spectra of flavins in various chemical environments in order to determine whether the spectra are useful in probing the chemical interaction between flavins and protein in flavoenzymes. We have obtained the resonance Raman spectrum of a nonfluorescent Ag+ complex of FMN. Several large changes occur in the FMN resonance Raman spectrum upon Ag+ complexation; among these are changes in the 1580 cm-1 region of the FMN spectrum (assigned to nu C=N at N-5 and C-4a), the 1410 cm-1 region and the 1260 cm-1 region (associated with a vibration having some delta N-N-H character at N-3). Similar changes are observed in the same region of a Ru2+-FMN complex. Since these spectral changes occur in two metal flavin complexes with very different electronic spectra, they would seem to be due to vibrational changes induced by metal complexation at N-5 and the oxygen at C-4 of flavin rather than the details of the vibronic interactions which give rise to the resonance enhancement of the spectrum. A structure for the Ag+-FMN complex is suggested. This study has potential physiological significance, because it illustrates the possible role of resonance Raman spectroscopy as a tool for the determination of direct flavin metal interaction in dilute aqueous solution of metalloflavoproteins.