A new method for the accurate determination of the isotopic state of single amide hydrogens within peptides using Fourier transform ion cyclotron resonance mass spectrometry

A new method is presented to accurately determine the probability of having a deuterium or hydrogen atom on a specific amide position within a peptide after deuterium/hydrogen (D/H) exchange in solution. Amide hydrogen exchan ge has been proven to be a sensitive probe for studying protein structures and structural dynamics. At the same time, mass spectrometry in combination with physical fragmentation methods is commonly used to sequence proteins based on an amino acid residue specific mass analysis. In the present study it is demonstrated that the isotopic patterns of a series of peptide fragm ent ions obtained with capillary-skimmer dissociation, as observed with a 9 .4 T Fourier transform ion cyclotron resonance (FTICR) mass spectrometer, c an be used to calculate the isotopic state of specific amide hydrogens. Thi s calculation is based on the experimentally observed isotopic patterns of two consecutive fragments and on the isotopic binomial distributions of the atoms in the residue constituting the difference between these two consecu tive fragments. The applicability of the method is demonstrated by followin g the sequence-specific D/H exchange rate in solution of single amide hydro gens within some peptides, Copyright (C) 2000 John Wiley & Sons, Ltd.