Short peptides that contain significant α-helical structure in aqueous solution allow the investigation of the role of amino acid side chains in stabilizing or destabilizing α-helix structure. A host-guest system of soluble synthetic peptides was designed that consisted of chains with the block sequence TyrSerGlu4Lys4X3Glu 4Lys4, 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 > Leu > Met > Gln > Ile > Val > Ser > Thr > Asn > Gly. This order differs from both host-guest copolymer values (Met > Ile > Leu > Ala > Gln > Val > Thr > Asn > Ser > Gly) and the tendencies of these amino acids to occur in helices in globular proteins (Ala > Met > Leu > Gln > Ile > Val > Asn, Thr > Ser > Gly), but matches the order found in a series of synthetic coiled-coil α 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.
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