Optical rotatory dispersion circular dichroism and far-ultraviolet spectra of avidin and streptavidin

摘要

1. The optical-rotatory-dispersion and circular-dichroism curves of avidin showed positive Cotton effects centred at 228mμ and 280mμ, close to the ultraviolet-absorption bands of tryptophan. These effects disappeared when avidin was dissociated into sub-units in guanidine hydrochloride. 2. Binding of biotin had only a small effect on the optical-rotatory-dispersion curve of avidin. 3. The absence of negative circular dichroism at wavelengths above 216mμ showed that there was little or no α-helix present in avidin. This interpretation was confirmed by Moffitt–Yang plots of the partial rotation due to the peptide bonds in the visible region of the spectrum. The calculated dispersion constants were remarkably similar to those of γ-globulin and suggested the presence of peptide conformations other than α-helix and random coil. 4. The far-ultraviolet spectrum was also similar to that of γ-globulin, the mean extinction coefficient of the peptide chromophore being much lower than the experimental value for a random-coil structure. 5. Streptavidin resembled avidin in showing two positive Cotton effects, but the negative dichroism below 220mμ suggested the presence of more α-helix than was found in avidin. Formation of the complex with biotin was accompanied by changes in rotation that were rather larger than those observed with avidin.