Formation of [M-nH+mNa]((m-n)+) and [M-nH+mK]((m-n)+) ions in electrospraymass spectrometry of peptides and proteins

The [M - nH + mNa]((m-n)+) and [M - nH + mK]((m-n)+) ions are common in the electrospray mass spectra of proteins and peptides. The feasibility of for ming these ions in the gas phase via collision activation and/or ion-molecu le reaction is investigated. Sodium and potassium affinities of the N-methy lacetamide anion, the acetate anion, and the 1-propanamide anion have been calculated using density functional theory at the B3LYP/6-311++G(d,p) level of theory. These anions were chosen as models for the functional groups on a protein or peptide. These affinity values are then used to calculate rea ction enthalpies of alkali hydroxides, chlorides, and hydrates with N-methy lacetamide, acetic acid, the acetate anion, and 1-propanamine, model reacti ons that may lead to formation of the [M - nH + mNa]((m-n)+) and [M - nH mK]((m-n)+) ions. It is found that a number of these reactions are exotherm ic or slightly endothermic (DeltaH degrees < + 20 kcal/mol) and are accessi ble after collision activation in the lens region. The potential energy hyp ersurfaces of model reactions between NaOH and formamide as well as NaCl an d formamide show relatively flat surfaces devoid of significant barriers. ( C) 2000 American Society for Mass Spectrometry.