A single-stranded amphipathic alpha-helix in aqueous solution: design, structural characterization, and its application for determining alpha-helical propensities of amino acids.

In order to investigate the positional effect of alpha-helical propensities of amino acids in an amphipathic alpha-helix, an amphipathic alpha-helical model peptide (Ac-Glu-Ala-Glu-Lys-Ala-Ala-Lys-Glu-Ala-Glu-Lys-Ala-Ala-Lys-Glu-Ala-Glu-Lys- amide) was designed and characterized by circular dichroism and 2D-NMR spectroscopies. This peptide contains 65% alpha-helical structure in solution, and its monomeric molecular weight in aqueous solution was determined by size-exclusion chromatography. The independence of alpha-helical structure and stability on peptide concentration demonstrates that helix formation of this peptide is a monomolecular process. To compare the effect of substitutions in the hydrophobic and hydrophilic face of the helix on structure and stability, a leucine, alanine, or glycine was individually substituted in the hydrophobic face (position 9) or hydrophilic face (position 7) of the model peptide. The change in helix content and stability upon substitution was measured by circular dichroism studies in the absence and presence of TFE or urea. The results indicate that each amino acid has a different helix propensity when it is located in the hydrophobic face versus hydrophilic face and the effect of substitution is more significant in the hydrophobic face. This single-stranded amphipathic alpha-helical peptide provides an appropriate model system to determine helix propensities of amino acids on both hydrophobic and hydrophilic faces.