Solvent Mediation of Peptide Conformations: Polyproline Structures in Water, Methanol, Ethanol, and 1-Propanol as Determined by Ion Mobility Spectrometry-Mass Spectrometry

Ion mobility spectrometry and circular dichroism spectroscopy are used to examine the populations of the small model peptide, polyproline-13 in water, methanol, ethanol, and 1-propanol over a range of solution temperatures (from 288 to 318 K). At low temperatures, the less-polar solvents (1-propanol and ethanol) favor the all- cis polyproline I helix (PPI); as the temperature is increased, the trans-configured polyproline II helix (PPII) is formed. In polar solvents (methanol and water), PPII is favored at all temperatures. From the experimental data, we determine the relative stabilities of the eight structures in methanol, ethanol, and 1-propanol, as well as four in water, all with respect to PPII. Although these conformers show relatively small differences in free energies, substantial variability is observed in the enthalpies and entropies across the structures and solvents. This requires that enthalpies and entropies be highly correlated: in 1-propanol, cis-configured PPI conformations are energetically favorable but entropically disfavored. In more polar solvents, PPI is enthalpically less favorable and entropy favors trans-configured forms. While either Δ H ⁰ or Δ S ⁰ can favor different structures, no conformation in any solvent is simultaneously energetically and entropically stabilized. These data present a rare opportunity to examine the origin of conformational stability.