Helix Propensities of Amino Acid Residues via Thioester Exchange

We describe the use of thioester exchange equilibria to measure the propensities of amino acid residues to participate in helical secondary structure at room temperature in the absence of denaturants. Thermally or chemically induced unfolding has previously been employed to measure α-helix propensities among proteinogenic α-amino acid residues, and quantitative comparison with precedents indicates that the thioester exchange system is reliable for residues that lack side chain charge. This system allows the measurement of α-helix propensities for d-α-amino acid residues and propensities of residues with nonproteinogenic backbones, such as those derived from a β-amino acid, to participate in an α-helix-like secondary structure.