Secondary structural changes of non-reduced and reduced ribonuclease A in solutions of urea, guanidine hydrochloride and sodium dodecyl sulfate

Abstract

The secondary structures of ribonuclease A (RNAase A) before and after reduction of the disulfide bridges and blockage of the thiol groups with iodoacetamide were examined in solutions of urea, guanidine hydrochloride, and sodium dodecyl sulfate (SDS). The relative proportions of α-helix, β-structure, and disordered structure were estimated by the curve-fitting method of circular dichroism (Chen, Y.H., Yang, J.T. and Chau, K.H. (1974) Biochemistry 13, 3350–3359). The native RNAase A, with the disulfide bridges intact, contained 19% helix and 38% β-structure. Reduction of its disulfide bridges led to a decrease in the proportion of these structures to 9% for the α-helix and 17% for the β-structure. The non-reduced RNAase A resisted unfolding in low concentrations of urea and guanidine hydrochloride. The β-structure which remained after reduction appeared to be stable even in solutions of 6 M guanidine and 9 M urea. A considerable amount of the β-structure in both the non-reduced and the reduced RNAase A remained unaffected by high concentrations of SDS.