α-Helical Peptides Are Not Protonated at the N-Terminus in the Gas Phase

摘要

Amino acids and small peptides are zwitterionic under most conditions. Consequently, biochemists often draw peptide and protein structures in their respective zwitterionic structures. We have recently shown that formamide chains show a very substantial preference for protonation on the terminal C=O. Protonation of these chains at the NH_2 results in rupture of the H-bond between the N-protonated and the other formamides. This result suggested that peptides that contain H-bonding chains that begin at the N-terminus may not be zwitterionic. However, such a conclusion is not completely established, as the nitrogens protonated in the formamide chains are amido, while those in peptides are amino, which are generally more basic. In solution, the probable effect of solvation must also be considered, as the zwitterionic structures of amino acids and small peptides are known to be better solvated than their "normal" counterparts.