Environmentally induced spectral shifts of oxidized lumiflavin: charge transfer and Electrochromic effects

Abstract

Spectroscopic INDO calculations have been performed on molecular complexes of lumiflavin and lumiflavin in the presence of point charges. In complexes with appropriate “ligands”, both model flavins and certain flavoenzymes exhibit red-shifted absorption spectra. In addition, redshifted flavoprotein spectra may arise from the presence of nearby charged amino acid residues. Optical spectra have been calculated for stacked charge transfer complexes of lumiflavin and anthranilic acid based on crystal coordinates (H.-S. Shieh et al., Biochemistry, 20 (1981) 4766). Calculations on 1:1 complexes red shifted the lowest energy π→π * transition relative to lumiflavin by ca. 1400 cm − and added an out-of-plane (2) component to its transition moment vector. The red shift arises from a destabilization of the highest occupied molecular orbital (HOMO) which, in the complex, contains about 50% contribution from the anthranilic acid. The lowest unoccupied molecular orbital (LUMO) remains > 98% lumiflavin; its energy is unaffected by the presence of the anthranilic acid. These results demonstrate that the lowest energy transition of the 1:1 complex reflects a transfer of net charge from the anthranilic acid donor (HOMO) to the lumiflavin acceptor (LUMO). The addition of an “extra” lumiflavin to the stack dramatically decreased the calculated red shift because of lumiflavin-lumiflavin exciton interactions that favored the enhancement of a higher energy component. These interactions were found to be very dependent on the relative lateral displacement of the lumiflavin subunits. The electrostatic shifts induced by positive and negative point charges placed both 3.5 Å above the lumiflavin and in-plane 2 Å outside the van der Waals surface were also calculated. Red shifts of various magnitudes were obtained with both positive and negative charges at all positions, except within a plane that lies roughly normal to the transition dipole direction of the lowest energy π→π ∗ transition and bisects the flavin. These results suggest that charged amino acid groups within flavoproteins will induce mostly red shifts.