Gas-phase compaction of helical polymers

To complement the ion mobility mass spectrometry data from the gas-phase experiments on polymers and proteins we have investigated conformations of helical polypeptides by molecular dynamic simulations. Due to strong residue attraction and hydrogen bonding, not mitigated by solvent molecules, a large content of helices, turns and bends was found to persist in long alanine polypeptides (Ala)n even at very high temperatures. As a result, the unsolvated chain dimensions are significantly contracted relative to the random-coil state and the compact structures such as crumpled coils, melted and nematic globules arise at enhanced temperatures. The compactness exponents determined from the power-law plots of the radius of gyration vs lengths suggest that the scaling theory underestimates the polypeptide compaction in the gas phase. At room temperature the polyalanine chains are organized into a variety of helical bundles of antiparallel helical fragments linked by short coils.