we have discovered an acylated heptapeptide
amide comprising six Ca-tetrasubstituted amino acids and
one histidine groupthat adopts two helical conformations
according to the polarity of the solvent: a a or 310 helix in
higher or lower polarity solvents, respectively. We were able
to correlate the position of the a-/310-helix equilibrium with
the empirical solvent polarity parameter ENT
and also able to
show that hydrophobic interactions between the side arms of
the amino acids provide a major driving force for the
stabilization of the a helix in aqueous solution. As shown by
the molecular calculations, switching from one conformation
to the other elongates (310 helix) or shortens (a helix) the
peptide, thus it behaves like a solvent-driven molecular
spring.
Since, in converting the peptide from the 310- into a ahelical
conformation, we trade an intramolecular hydrogen
bond for a discrete decrease in the solvent-exposed hydrophobic
surface, these data indicate that, by keeping the
hydrophobic contribution of each individual amino acid in the
folded oligopeptides constant, longer sequenches will prevailing
adopt a a-helical conformation in water while shorter ones
will favor 310 helices. This finding is in full accord with results
found by other research groups.
