Rj. Anderegg et al., THE MASS-SPECTROMETRY OF HELICAL UNFOLDING IN PEPTIDES, Journal of the American Society for Mass Spectrometry, 5(5), 1994, pp. 425-433
Two model peptides, melittin and a growth hormone releasing factor (GR
F) analog, have been studied by mass spectrometry and tandem mass spec
trometry during the course of their deuterium exchange. Both peptides
are known from previous work to form alpha-helices in solution. When t
he peptides are exposed to deuterated solvents, their masses increase
as deuterium atoms replace protons in the exchangeable sites of the pe
ptides. The mass spectrometry results clearly indicate multiple popula
tions of exchangeable protons: Some exchange very fast, and are presum
ably on the surface and not involved in hydrogen bonding; others excha
nge much more slowly, indicating that they are probably participating
in hydrogen bonding. Tandem mass spectrometric experiments were conduc
ted, and the masses of the product (fragment) ions were used to determ
ine where in the peptide the deuterium atoms were incorporated. The re
sults agree very well with NMR studies of the same peptides. Melittin
appears as two helical segments with a kink around Pro-14. The GRF ana
log 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.