Secondary structure changes of disulfide bridge-cleaved bovine serum albumin in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate

Secondary structure of bovine serum albumin, disulfide bridges of which were reduced and blocked by iodoacetamide, was examined in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate. The profile of structural changes in the reduced albumin was compared with that in the unreduced protein with the intact disulfide bridges. The relative proportions of α-helix, β-structure, and random coil were estimated by simulating a mixed circular dichroism (CD) spectrum of reference spectra of the corresponding structures to the experimentally obtained spectrum of the protein. The helical proportion was 25% for the reduced albumina dn the proportion increased up to 50% in a dodecyl sulfate solution. On the other hand, the helical proportion is 66% for the unreduced protein and it decreases down to 50% by the addition of the identical surfactant [K. Takeda et al., J. Colloid Interface Sci. 117, 120 (1987)]. The helical proportions of both the reduced and the unreduced proteins decreased in the presence of urea and guanidine. the unreduced albumin resisted the unfolding of the helix below 4 M urea and a 1.5 M guanidine. In contrast, the helical proportion in the reduced protein began to decrease even in lower concentrations of the two denaturants.