Helix-enhancing propensity of fluoro and alkyl alcohols: influence of pH, temperature and cosolvent concentration on the helical conformation of peptides

We have analyzed the effects of trifluoroethanol (TFE) and three other alco hols(l-propanol, 2-propanol and hexafluoro-2-propanol) on S-peptide (residu es 1-20) of ribonuclease A, an analog of S-peptide (QHM-->AAA, Sa-peptide) and TC-peptide (residues 295-316) of thermolysin to assess the helix-enhanc ing propensity of fluoro and alkyl alcohols under different environmental c onditions of cosolvent concentration, pH and temperature by circular dichro ism (CD). The dependence of cosolvent concentration on helix-induction show ed a plateauing effect in all cases. 1-Propanol and 2-propanol were as effe ctive as TFE in all the three peptides. Hexafluoro-2-propanol (HFIP) was a better helix enhancer in all cases however, the relative effectiveness vari ed with the peptide sequence. The alcohol transitions were analyzed assumin g a two-state transition. The free energy decreased linearly in the cosolve nt concentration range of 0-5 m for all the three peptides. The m-value (co nstant of proportionality) varied between peptides but was similar for any given peptide for TFE, l-propanol or 2-propanol. The m-values of HFIP for a ll three peptides was much higher compared to other cosolvents. The isother mal cosolvent helix-induction curves for the three peptides exhibited simil ar features of shape and character for 1-propanol, 2-propanol and TFE. The additivity of cosolvent-induced helix formation was observed for different blends of alkyl and/or fluoro cosolvents. The pH-dependence of helix format ion was observed in both TFE and l-propanol solutions for S-peptide and TC- peptide, respectively, while in Sa-peptide, which was designed to perturb t he pH-effect, helix formation was unaffected. The overall results provide s ome insight into the mechanism of cosolvent-mediated helix-enhancement in p rotein segments and are likely to facilitate optimization of conditions for cosolvent usage in chemistry and biology.