Quantitative determination of helical propensities from trifluoroethanol titration curves

The formation of local secondary structure is an essential step in the folding of a polypeptide from a random coil to a well-defined native conformation. Detection of hidden structural propensities in amino acid sequences may provide important insight into how this is accomplished. 2,2,2-Trifluoroethanol (TFE) has been shown to induce helical structure in polypeptides, and TFE titration has been used as a qualitative probe for helical tendency. We have investigated the propensity of five synthetic peptides to adopt helical structure in TFE. The free energy of helix formation exhibits linear dependence on the mole ratio of TFE to water, and the constant of proportionality (m-value) can be perturbed systematically by altering the peptide length and unsystematically by altering the temperature. Three peptides with closely related sequences but different N-cap residues show different titration behavior from 5 to 75 degrees C, suggesting that TFE acts only within the context of a preexisting helix-coil equilibrium. These observations can be reconciled with a model for TFE/H2O exchange at peptide binding sites. Our results support the viability of TFE titration as a tool for extrapolation of quantitative helix-coil equilibrium constants for peptides with little or no apparent helical content in aqueous solution.