Structure of Lysozyme

Abstract

The thermal stability of lysozyme in various solutions has been studied by optical rotatory dispersion measurements. The molar effectiveness in increasing the value of [DELTA][M]436 (or [DELTA]ao) (the difference between the mean residue rotation (or the value of ao in the Moffitt equation) at high and low temperature ends) increases in the order tetramethylurea < dimethyl-urea, dimethylformamide, dimethylacetamide < acetamide < formamide <urea <guanidine hydrochloride. The value of [DELTA]bo (the difference between the value of bo at high and low temperature ends) decreases with the concentration of methyl-substituted ureas and amides. Furthermore, the ability of these compounds to lower the transition temperature is increased with increasing alkyl-substitution of NH2 groups. The same situation was also found for the thermal transition in aqueous solutions of methanol, ethanol, ethylene glycol, propylene glycol, dimethyl suf-oxide, and dioxane. At higher concentrations of dimethylformamide, methanol, ethanol, or ethylene glycol, the value of bo does not change with temperature while the value of -ao increases steeply within a narrow temperature range. These facts suggest that as more methyl groups are substituted in the solvent molecule, only the internal fold stabilized by hydrophobic interactions is disrupted by raising the temperature. Furthermore, it should be emphasized that the change with temperature of the specific rotation of proteins does not entirely reflect the helix-coil transition as generally believed but involves the contribution of other conformational change.