Peptide .alpha.-helicity in aqueous trifluoroethanol: correlations with predicted .alpha.-helicity and the secondary structure of the corresponding regions of bovine growth hormone

The relationship between trifluoroethanol (TFE) enhancement of peptide alpha-helicity and protein secondary structure has been studied for a series of 11 peptides which span the complete primary sequence of bovine growth hormone (bGH). Ten of these peptides become increasingly alpha-helical as the solution concentration of TFE is increased. The amount of alpha-helicity developed by these peptides plateaus above 10 mol % TFE and ranges from 0 to 71%. The increased alpha-helicity, as determined by CD, closely correlates with the amount of alpha-helix predicted for eight of the eleven peptides analyzed (r = 0.9). Therefore, for this group of peptides, it appears that this technique can be used as a measure of alpha-helical propensity. Inclusion of the remaining three peptides in this analysis significantly lowers the correlation (r = 0.6). The reduced correspondence between TFE-enhanced and predicted alpha-helicity in this latter subset of peptides may be due to their relatively high hydrophobicity. In addition, the relevance of TFE-enhanced peptide alpha-helicity and the secondary structure of the corresponding protein regions was explored. Although the three peptides which form the largest amount of alpha-helicity in the presence of 10 mol % TFE correspond to alpha-helical regions of the protein, the overall correlation is significantly lower than is observed for the TFE-enhanced and predicted alpha-helicity. These findings suggest that the propensity of specific amino acid sequences for alpha-helix formation influences the amount of alpha-helicity which forms in corresponding protein sequences, but that other factors can modify this structure.