PEPTIDE chain turns are those parts of a globular protein where the backbone is folded back upon itself. We show here that the number of turns is a linear function of the number of amino acid residues in the protein, and we compare the two contrasting models for turn formation. In a sequence-dependent model, turns are a linear function of the molecular weight, while in a shape-dependent model, they are a function of the two-thirds power of the molecular weight. But, a shape-dependent model also behaves, to a good approximation, as a linear function within the domain of interest. We suggest some consequences of this work for protein folding.
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