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The thermally-induced helix–coil transition in polyamino acids is a good model for determining the helix-forming propensities of amino acids but not for the two-state folding/unfolding transition in globular proteins. The equilibrium and kinetic treatments of the helix–coil transition are summarized here together with a description of applications to various types of homopolymers and copolymers. Attention is then focused on the helix–coil transition in poly-l-alanine as an example of a non-polar polyamino acid. To render such a non-polar polymer water soluble, it is necessary to introduce polar amino acids such as lysines, but care must be taken as to the location of such polar residues. If they are attached as end groups, as in a triblock copolymer, they do not perturb the helix-forming tendency of the central poly-l-alanine block significantly, but if they are introduced within the sequence of alanine residues, then the …