Raman spectroscopic studies of the anhydrous complexes of avidin and streptavidin with biotin

FT Raman spectra of avidin, streptavidin, and of the two anhydrous biotin complexes were obtained by excitation in near infrared (NIR). As far as avidin and the avidin-biotin complex are concerned, the excitation in NIR did not change the main features of the spectra compared with those previously obtained by visible excitation. However, the intensity of Trp bands did not change due to the formation of the complex when excited by NIR excitation, whereas an intensity increase was observed by 514 nm excitation. The percentages of secondary structure of the two proteins and the biotin complexes were obtained from the Raman spectra. The vibrational results indicate that as a consequence of the interaction with biotin, the percentages of β-sheet conformation of the proteins decrease whereas the percentages of α-helix conformation increase. The observed changes of the conformation of streptavidin induced by biotin interaction are comparable with those obtained with avidin; this fact confirms that the type of the binding should be similar for the two molecules. Moreover, as a result of the biotin binding, the hydrophobicity of the environment of the Trp residues of streptavidin slightly increases according to the increase in intensity of the 1360 cm−1 component. This result could suggest that the streptavidin-biotin complex is stabilized mainly by hydrophobic interactions.