Raman spectroscopy of avidin: secondary structure, disulfide conformation, and the environment of tyrosine

An analysis of the amide I region of Raman spectra indicates that avidin has 10 +/- 5% and 55 +/- 4% of its residues in helical and beta-strand conformations, respectively. Predictions of secondary structure on the basis of the sequence of avidin are consistent with the high percentage of residues in the beta conformation. We observe no differences between the spectra of avidin in solution and in crystals nor is there a significant difference between the secondary structures of avidin and the complex of avidin with biotin. In addition, the ratio of the intensities of the tyrosine doublet at 826 and 855 cm-1 indicates the lone tyrosine side chain of an avidin subunit is in a strong hydrogen bond as a proton acceptor. The Raman data also indicate the single disulfide of an avidin subunit has dihedral angles of 0-50 degrees for each of its two C beta-S bonds and a dihedral angle of 85 +/- 20 degrees for its disulfide bond. We discuss the significance of these results in relation to findings of earlier work on avidin.