The mass spectrometry of helical unfolding in peptides

Two model peptides, melittin and a growth hormone releasing factor (GRF) analog, have been studied by mass spectrometry and tandem mass spectrometry during the course of their deuterium exchange. Both peptides are known from previous work to form α-helices in solution. When the peptides are exposed to deuterated solvents, their masses increase as deuterium atoms replace protons in the exchangeable sites of the peptides. The mass spectrometry results clearly indicate multiple populations of exchangeable protons: Some exchange very fast, and are presumably on the surface and not involved in hydrogen bonding; others exchange much more slowly, indicating that they are probably participating in hydrogen bonding.Tandem mass spectrometric experiments were conducted, and the masses of the product (fragment) ions were used to determine where in the peptide the deuterium atoms were incorporated. The results agree very well with NMR studies of the same peptides. Melittin appears as two helical segments with a kink around Pro-14. The GRF analog contains a single long helix, spanning almost the entire length of the peptide. The dynamics of the unfolding of the helices can also be explored by observing how the exchange progresses with time.