Helix breakers in block copolypeptides

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Abstract
The ability of residues of type Y to disrupt the helix formed by residues of type X has been studied in X‐Y‐X‐block copolypeptides. The degree of polymerization of each X‐block is so large that it can be considered to be infinite. Matrix methods are used to obtain a general expression for the helix content of the central residue in the Y‐block. The resulting expression is specialized to the case where the statistical‐weight matrix is of dimensions 2 × 2, with elements, 1, 1, σs, and s. The behavior is evaluated for physically realistic values of σ and s. Two useful generalizations emerge: (1) The ability of the Y‐block to disrupt the helix is determined primarily by the value of s for residues of type Y. Helix disruption does not correlate well with the helix content of a Y‐homopolymer of infinite degree of polymerization. (2) In contrast, the ability of the X‐block to resist the influence of the Y‐block is determined primarily by the helix content of the X‐block. It does not correlate well with s for residues of type X. A random‐coil region in a Y‐block that continues into the following X‐block is more stable if residues of type X have a large value of σ because there is helix initiation in the X‐block. It is this influence of σ that prevents a correlation between s and the ability of the X‐block to resist the disruptive influence of the Y‐block.