Helix-Forming Tendencies of Amino Acids in Short (Hydroxybutyl)-L-glutamine Peptides: An Evaluation of the Contradictory Results from Host-Guest Studies and Short Alanine-Based Peptides

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Abstract
The helix propensities ("s-values") of amino acids measured using short alanine-based peptides are different, in both magnitude and rank order, from those found using random sequence copolymers of a "guest" amino acid and a (hydroxyalkyl)-L-glutamine "host". The origin of these differences is investigated here. In short alanine-based peptides containing 1-5 (hydroxybutyl)-, (hydroxypropyl)-, or (hydroxyethyl)-L-glutamines (HBQ, HPQ, and HEQ, respectively), we find the rank order of helix propensities to be Ala > HBQ > HPQ > HEQ, which is consistent with earlier results for HBQ, HPQ, and HEQ homopolymers and is attributed to helix-stabilizing hydrophobic interactions [Lotan, N., Yaron, A., & Berger, A. (1966) Biopolymers 4, 365-368]. The apparent s-values of nonpolar amino acids in a 17-residue, HBQ-based peptide cluster around 1, as they do in the host-guest studies, but in contrast to results with alanine-based peptides. The differences between the host-guest results and results obtained using alanine-based peptides may be rationalized in terms of side-chain interactions involving the hydroxyalkyl moiety.
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