Thermodynamic parameters of helix-coil transitions in polypeptide chains.

The aim of this paper is to outline new possibilities of more detailed thermodynamic analysis of helix coil transition parameters, particularly with respect to their splitting into energy and entropy terms. Because of difficulties with the ternary systems, water—organic solvent polypeptide, or, two organic solvents polypeptide, caused particularly by the phenomenon of specilic sorption of one solvent component, the discussion is restricted to water soluble synthetic polypeptides whose limited range can fairly be extended by copolymerization. It is shown how the combination of data from different experimental sources (optical rotatory dispersion, potentiometric titration curves, intrinsic viscosity. calorimetry) can yield an average number of monomer units in the helical region in the middle of the transition. The parameters characterizing the initiation and elongation of helical regions determined for a number of polypeptides differing only in their side groups give us valuable information about the intramolecular interactions in polypeptide chains. There is also a significant correspondence between the helix• coil equilibrium constants of natural amino-acids in aqueous media and the distribution of these amino-acids among helical and non-helical regions in globular proteins. This correspondence, together with a treatment of intramolecular interactions, can serve to predict the secondary structure of globular proteins from the primary one.