Effect of sodium dodecyl sulfate on circular dichroism of some nonhelical proteins

Abstract

Circular dichroism measurements on solutions of histones F1 and F2a, of oxidized ribonuclease, and of reduced and S-carboxymethylated ribonuclease, trypsin, and soybean trypsin inhibitor indicated that the unordered polypeptide chains of these proteins were ordered by sodium dodecyl sulfate to a considerable degree. Resolution of the ellipticity curves into Gaussian peaks showed that sodium dodecyl sulfate incited the formation of negative bands at 203–209 nm and at 218–225 nm and positive bands at 188–194 nm. These transitions were promoted by low pH, especially in soybean trypsin inhibitor and the ribonuclease derivatives. Intact ribonuclease was affected by sodium dodecyl sulfate relatively little and the effects depended on pH. At 240–300 nm, the circular dichroism bands were weakened by the detergent in all cases. The detergent binding apparently resulted in shielding of the polypeptide helices and in the promotion of rotational freedom of the aromatic amino acid side chains.