  Using multicanonical simulations, the authors study the effect of charged end
groups on helix formation in alanine based polypeptides. They confirm earlier
reports that neutral polyalanine exhibits a pronounced helix-coil transition in
gas phase simulations. Introducing a charged Lys+ at the C terminal stabilizes
the helix and leads to a higher transition temperature. On the other hand,
adding the Lys+ at the N terminal inhibits helix formation. Instead, a more
globular structure was found. These results are in agreement with recent
experiments on alanine based polypeptides in gas phase. They indicate that
present force fields describe accurately the intramolecular interactions in
proteins.
