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Effect of side‐chain hydrophobic bonding on the stability of homopolyamino acid α‐helices: Conformational studies of poly‐L‐leucine in water

The conformational properties of block copolymers of poly‐L‐leucine in water have been examined. The degree of polymerization of the poly‐L‐leucine block was 11 and 21, respectively, for samples prepared by the Merrifield procedure, and 56 for a sample prepared by the polymerization of leucine N‐carboxyanhydride. The optical rotatory dispersion parameter b0 was used to obtain the helix content θh at various temperatures. Application of the Lifson‐Roig theory gave the following parameters for the transition of a residue from a coil to a helical state: v = 0.05–0.011, ΔH = +100 cal/mole, ΔS = +0.70–1.00 e. u. These parameters, as well as those for other polyamino acids, are accounted for by hydrophobic bonds involving the nonpolar side chains in the helical and randomly coiled forms. From the data for poly‐L‐alanine and theoretical values of the thermodynamic parameters for hydrophobic bond formation, the parameters for formation of a polyglycine helix are computed. By separating the contributions of the backbone, it is possible to obtain a set of thermodynamic parameters for the side‐chain contributions of a number of polyamino acids. Increased size of the nonpolar side chain (with a larger contribution from hydrophobic bonding) makes a larger contribution to the stability of the α‐helix which is reflected, among other ways, in a higher helix content at given temperature.