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Theoretical determination of helix-coil parameter σ from a model of partly helical polypeptide chain

par Englert, Anne ;Mandel, Michel ;Van Nypelseer, Anne
Référence Macromolecules, 8, 5, page (655-660)
Publication Publié, 1975

Article révisé par les pairs

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Résumé :

The Zimm and Bragg parameter σ is calculated numerically for poly(L-alanine), polyglycine, and the copolymers of L-alanine and glycine using the molecular theory of s and σ as developed by Go, Go, and Scheraga in a modified formulation. In this formulation, σ is obtained from the partition function of the whole chain in the helix-coil transition region and represents therefore the contributions from the ends of helical and coil sequences and from the interactions between atoms in a coil sequence with those in the neighboring helical sequence. When the parameter σ is calculated numerically from a hard-sphere potential, it appears that steric interactions between atoms in the coil sequence with atoms in the neighboring helical sequence, which have been neglected in previous calculations, contribute significantly to the value of σ. Owing to these interactions the entropy of the coil sequence as well as σ decrease, but the decrease of σ is larger in poly(L-alanine) than in polyglycine, because of the higher flexibility of the monomer in polyglycine. The numerical value of σ for polyglycine compared with that of poly(L-alanine) might be overestimated however by the model presented here due to approximations inherent in the hardsphere treatment and because only regular helical sequences are considered.