Helix-enhancing propensity of fluoro and alkyl alcohols: influence of pH, temperature and cosolvent concentration on the helical conformation of peptides
1999
We have analyzed the effects of trifluoroethanol (TFE) and three other alcohols(1-propanol, 2-propanol and
hexafluoro-2-propanol) on S-peptide (residues 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-enhancing propensity of fluoro and alkyl alcohols under different
environmental conditions of cosolvent concentration, pH and temperature by circular dichroism (CD). The dependence of cosolvent
concentration on helix-induction showed a plateauing effect in all cases. 1-Propanol and 2-propanol were as effective as TFE in all the
three peptides. Hexafluoro-2-propanol (HFIP) was a better helix enhancer in all cases however, the relative effectiveness varied with
the peptide sequence. The alcohol transitions were analyzed assuming a two-state transition. The free energy decreased linearly in the
cosolvent concentration range of 0-5 m for all the three peptides. The m-value (constant of proportionality) varied between peptides
but was similar for any given peptide for TFE, 1-propanol or 2-propanol. The m-values of HFIP for all three peptides was much
higher compared to other cosolvents. The isothermal cosolvent helix-induction curves for the three peptides exhibited similar 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 p H -dependence of helix formation was observed in both TFE
and 1-propanol solutions for S-peptide and TC-peptide, respectively, while in Sa-peptide, which was designed to perturb the
p H -effect, helix formation was unaffected. The overall results provide some insight into the mechanism of
cosolvent-mediated helix-enhancement in protein segments and are likely to facilitate optimization of conditions for cosolvent usage in
chemistry and biology.
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