Resonance inverse Raman spectroscopy was used to examine the interactions between the flavin and surounding protein in L ‐lactate oxidase (from Mycobacterium smegmatis ) and Old Yellow Enzyme (from brewer's bottom yeast). Spectra were taken of the enzymes both free and bound to various ligands. For L ‐lactate oxidase, the ligands consisted of substrate analogs (acetate, propionate) and inorganic anions (phosphate, sulfate and nitrate). For the inorganic anions, the expected attenuation with detuning was not observed for several bands which are associated with flavin rings II and III. This effect appears to be due to a disruption of ring stacking between the uracil–pyrazine end of the flavin moiety and an aromatic amino acid. A shift in the 1232 cm −1 band and changes in the bandwidths of bands in the 1450–1600 cm −1 region indicate minor reorganization of the hydrogen bonding structure around the isoalloxazine on ligand binding. For Old Yellow Enzyme, the ligand was chloride. Chloride caused a slight change to the Raman spectrum, indicative of decreased hydrogen bonding to the flavin
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