Stability of secondary structural elements in a solvent-free environment: the alpha helix

The stability of the alpha helix as an element of secondary structure is examined in the absence of solvation, in the gas phase. Mass-analyzed ion kinetic energy (MIKE) spectrometry was applied to measure intercharge repulsion and intercharge distance in multiply protonated melittin, a polypeptide known to possess a stable helical structure in a number of different environments. The experimental results, interpreted in combination with molecular mechanics calculations, suggest that triply charged melittin retains its secondary structure in the gas phase. The stability if the alpha-helical conformation of the polypeptide in the absence of solvent molecules reflects the fact that a network of intrinsic helical hydrogen bonds is energetically more favorable than unfolded conformations.