The sensitivity of some nonhelical proteins to structural modification by sodium dodecyl sulfate and its homologues: Circular dichroism studies on Bence-Jones protein, concanavalin A, soybean trypsin inhibitor and trypsin

Abstract

Effect of sodium octyl, decyl, and dodecyl sulfates on the circular dichroism (CD) of Bence-Jones protein, concanavalin A, soybean trypsin inhibitor, and trypsin was investigated. The concentrations of the detergents were varied and the minimal amounts were determined that affect the conformation. It was found that: 1. 1.|The soybean trypsin inhibitor was most sensitive to the conformational change of its polypeptide chain backbone and Bence-Jones protein and concanavalin A were less sensitive in that order. 2. 2.|The side chain chromophores could be perturbed by lower mass ratios of detergent to protein than the polypeptide chain backbone. 3. 3.|The detergents reduced the CD bands in the near ultraviolet spectral zone and induced the formation of negative bands at 218–222 nm and 205–210 nm and a positive band at 190–192 nm; thus the interactions resulted in an increase of the α-helix content. 4. 4.|The effect of high alkyl sulfate concentrations on the side chain chromophores resulted in similar CD curves for all four proteins having several weak negative bands at 260–295 nm indicating removal of vicinal effects on these chromophores. 5. 5.|The effectiveness of the alkyl sulfates increased strongly with the increasing length of the hydrocarbon chain, e.g. one molecule of sodium dodecyl sulfate produced a similar effect as 10–30 molecules of decyl sulfate. 6. 6.|It was concluded that in nonhelical proteins the transition to α-helical conformation occurs chiefly with the bends and loops and other irregular segments of the polypeptide chains which in the native macromolecules were stabilized chiefly by hydrophobic interactions.