MECHANISM OF HELIX INDUCTION BY TRIFLUOROETHANOL - A FRAMEWORK FOR EXTRAPOLATING THE HELIX-FORMING PROPERTIES OF PEPTIDES FROM TRIFLUOROETHANOL WATER MIXTURES BACK TO WATER/

TO establish a framework for extrapolating the helix-forming propertie s of peptides from TFE/H2O mixtures (TFE = 2,2,2-trifluoroethanol) bac k to water, the thermal unfolding curves have been measured by circula r dichroism for four repeating-sequence peptides, with chain lengths f rom 7 to 22 residues. The unfolding curves were measured between 0 and 50 volume percent TFE and were fitted to the modified Lifson-Roig the ory. A single set of helix-coil parameters fits the results for the fo ur peptides at each TFE concentration; only two of the basic helix-coi l parameters, <w>, the mean helix propagation parameter of residues in the sequence repeat, and Delta H, the enthalpy change per residue on unfolding the helix, are allowed to vary with TFE molarity. The succes s in fitting these curves over a wide range of experimental variables shows that helix formation is basically the same in TFE/H2O mixtures a s in water. Moreover, a simple model based on a linear dependence of I n <w> and Delta H on TFE molarity can be used to extrapolate the resul ts from 25% TFE (approximately 4 M) back to water. The results also gi ve curves of helix formation induced by TFE at constant temperature, a nd the properties of these helix induction curves explain some of the puzzling results shown by other peptides in the literature. The averag e helix propensity increases regularly from 0 to 25% TFE but levels of f at higher TFE concentrations, which explains why the extent of helix formation levels off in this range. The change in the apparent cooper ativity of thermal unfolding curves in concentrated TFE solutions resu lts from the decrease of the enthalpy change for helix unfolding at hi gher TFE concentrations. The rapid decrease in the plateau values of a pparent helix content with increasing temperature results mainly from the strong temperature dependence of the ellipticity of the complete h elix. To determine whether the helix-stabilizing effect of TFE arises from strengthening the hydrogen bonds in the helix backbone, the stren gth of the hydrogen bond in a model compound, salicylic acid, has been measured in TFE/H2O mixtures from the pK(a) difference between salicy lic acid and a similar compound which cannot form the hydrogen bond. T he curve of hydrogen bond strength versus increasing TFE concentration matches both in shape and magnitude the increase in average helix pro pensity in TFE/H2O mixtures.