Difference Spectra of Bovine κ-Casein with Urea

Abstract

Bovine κ-casein showed a typical CD spectrum for an aperiodic conformation in the far UV region. The comparison of the near UV absorption spectrum of native κ-casein with that of a model compound mixture (Ac-tyr-OEt+Ac-trp-OEt, molar ratio=9:1) showed a red shift of the former by 2nm to the longer wavelength. The difference spectra produced by the addition of urea to κ-casein solution showed three peaks at 280, 287, and 292 nm, of which the sign was negative (denaturation blue shift). The magnitude of the blue shift of the tryptophyl group was found to be -2, 200. It is concluded from these results that some tyrosyl groups are exposed to the solvent, and the other tyrosyl groups and a tryptophyl group are buried in the hydrophobic regions which is very susceptible to the action of a denaturing agent, urea. All the chromophores in κ-casein was exposed to the solvent in the presence of 4M urea. κ-Casein in the native state was proved to have an aperiodic structure but not a flexible random coil.