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Short peptides that contain significant alpha-helical structure in aqueous solution allow the investigation of the role of amino acid side chains in stabilizing or destabilizing alpha-helix structure. A host-guest system of soluble synthetic peptides was designed that consisted of chains with the block sequence TyrSerGlu4Lys4X3Glu4Lys4, denoted EXK, in which X represents any "guest" amino acid residue. Circular dichroism spectroscopy indicates that the extent of helicity of these peptides follows the order Ala greater than Leu greater than Met greater than Gln greater than Ile greater than Val greater than Ser greater than Thr greater than Asn greater than Gly. This order differs from both host-guest copolymer values (Met greater than Ile greater than Leu greater than Ala greater than Gln greater than Val greater than Thr greater than Asn greater than Ser greater than Gly) and the tendencies of these amino acids to occur in helices in globular proteins (Ala greater than Met greater than Leu greater than Gln greater than Ile greater than Val greater than Asn, Thr greater than Ser greater than Gly), but matches the order found in a series of synthetic coiled-coil alpha helices, except for Ser. Proton nuclear magnetic resonance analysis of several EXK peptides indicates that these peptides are partially helical, with the helical residues favoring the amino terminus.
Lyu et al. (Fri,) studied this question.