On the helix-coil transition in alanine based polypeptides in gas phase

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 $α$ 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.