Quantitative Correlation of Solvent Polarity with the ?Ñ-/310-Helix Equilibrium: A Heptapeptide Behaves as a Solvent-Driven Molecular Spring

The paper describes compelling experimental evidence of the reversible interconversion of an heptapeptide between two helical conformations and provides a rationale for the phenomenon, i.e. the polarity of the solvent. It also demonstrates that the dependence from this latter parameter is not just qualitative but quantitative. The reversibility of the process indicates that the heptapeptide behaves like a molecular spring when it is dissolved in solvents of different polarity: in fact its lengths varies from 15 to 17 Å as it switches from the alpha- to the 310-helix conformation. Several molecular calculations had suggested that the experiments that we illustrate for the first time here should indeed happen. However, the difficulty in finding a short peptide highly folded with an helical conformation had hampered the possibility to verify those computational indications. These results have relevant consequences for the interpretation of structural phenomena occurring in the active/recognition sites of enzymes