FT-ICR MS of electrosprayed extra heavy crude oil resolves and identifies elemental compositions of several thousand heteroatomic species from a single mass spectrum. Separately, we have generated a list of isobaric peptide pairs differing in exact mass due to 1, 2, or 3 mutations or post-translational modifications. Although the number of such isobars is high (>500 for 2 mutations/modifications), many have the same mass difference (e.g., CH4 vs. O). Virtually all can be resolved by ESI FT-ICR MS for peptides of 1,000-2,500 Da. We also show that the number of carbons in a biomolecule may be determined from the nominal mass difference between molecules expressed from natural abundance (1% 13C) and enriched (99% 13C) media. Once the number of carbons is known, the number of possible elemental compositions at a given mass measurement accuracy drops dramatically. Combined with accurate-mass MS/MS, it becomes possible to establish a unique elemental compositions for a phospholipid of ~900 Da.
We demonstrate experimentally the linear relation between ion charge and mass resolving power, for electrosprayed ions whose isotopic distributions are resolved to <1 Da. Thus, charge reduction becomes unnecessary and is in fact undesirable. As long as the targeted mass spectral peaks can be resolved, there is no need to reduce the number of peaks, and charge reduction actually degrades resolution.
We demonstrate experimentally the resolution of complex mixtures, ranging from arson accelerants (petroleum distillates, with hundreds of volatile components) to protein tryptic digests.
H/D exchange will be shown to provide a way to map the protein segments at the contact interface in binary and ternary protein complexes for which NMR and x-ray data is unavailable.
Work supported by NSF (CHE-99-09502), NIH (GM-31683), Florida State U., and the National High Magnetic Field Lab in Tallahassee, FL.
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