Hydrophobicity profile of amino acid residues: A differential scanning calorimetry and circular dichroism study of leucine and isoleucine co-polypeptides of the protein-based polymers of elastin

The inverse temperature transition of hydrophobic folding and assembly of a "host-guest" model system based on the elastin derived polypentapeptide, poly(VPGVG), is employed to determine the relative hydrophobicity of amino acid residues of proteins and polypeptides. In this report the Ile- and Leu-containing co-polypentapeptides, described in terms of the model system as poly[f_v(VPGVG), f_x(VPGXG)] or poly[f_v(VPGVG),f_x(XPGVG)] where X=L(Leu), I(Ile) with f_v + f_x = 1, are prepared by solution peptide syntheses. The critical temperature and the heat of the inverse temperature transition of thesc polypeptides are measured by differential scanning calorimetry (DSC) and used as indices of the relative hydrophobicity of the guest residue. By both of the indices, temperature and heat, leucine and isoleucine are more hydrophobic than valine while less hydrophobic than the three amino acid residues with aromatic side chains. In the terms of endothermic heat of transition, ΔH, for the aromatic residues, Phe, Tyr and Trp, ΔH is approximately 5 kcal/mol: for Val, ΔH is about 1.1 kcal/mol, where as for both Ile and Leu, ΔH is 2.6±0. I kcal/mol. Whether based on the transition temperature, T_b, or the endothermic heat, ΔH, Ile and Leu are less hydrophobic than the aromatic residues and more hydrophobic than the valine residue. Using plots of either the temperature, T_b, or the heat, ΔH, vs. the molar fraction of the guest residue, f_x, leucine and isoleucine show similar hydrophobicity, whereas they are quite distinct from the more hydrophobic aromatic residues and the less hydrophobic valine.