期刊名称：Proceedings of the National Academy of Sciences
印刷版ISSN：0027-8424
电子版ISSN：1091-6490
出版年度：1977
卷号：74
期号：8
页码：3340-3344
DOI：10.1073/pnas.74.8.3340
语种：English
出版社：The National Academy of Sciences of the United States of America
摘要：In water, glucagon exists in an equilibrium between a trimer in which more than half of the peptide groups are in an alpha-helical configuration and a monomer which has a random coil configuration with few alpha-helical residues. The thermodynamics of this self-association have been evaluated by studying the temperature- and concentration-dependence of the mean residue ellipticity at 220 nm. The enthalpy and entropy changes of association were negative at all temperatures between 5 degrees and 50 degrees and had large negative temperature dependencies. Usually an association that involves nonpolar groups is considered to be driven by a positive entropy term. Such an explanation is not tenable in the case of glucagon. However, if the effects of nonpolar groups on the coil-to-helix transition of a polypeptide are included into the thermodynamic considerations of hydrophobic interactions, then the negative parameters observed for glucagon association can be readily understood. The hydrophobic interaction is therefore not necessarily controlled by the entropy change because, if there are significant conformational changes, the reaction may be controlled by the enthalpy change. Consequently, the more important parameter characteristic of all hydrophobic reactions is the heat capacity change.
