Partial denaturation of macromolecules by chemical agents

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Abstract
The denaturation of a macromolecule such as a polypeptide is considered in the case where the total number of noncovalent internal bonds broken in the binding process of the chemical agent is only a fraction, αs, of the total number of noncovalent internal bonds involved in its helical native conformation. Starting from a two‐parameter (s, σ) model of the helix–random coil transition of polypeptides, the transition temperature is derived as a function of the fraction αs, and of the concentration and the binding constant of the chemical agent. The lower limit of the transition temperature and the corresponding slope of the transition curve are shown to depend on αs. As an illustration, existing data on the partial acid denaturation of the enzymes ribonuclease A and muromidase are analyzed. The resulting average enthalpies of a noncovalent bond in the native ribonuclease A and muromidase are found to be 1.81 kcal/mole and 1.54 kcal/mole, respectively; the corresponding entropies are 5.4 and 4.4 cal/deg./mole, respectively. As a further example, existing data on a partially methylated enzyme ribonuclease A are also considered.