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The occurrence of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II is demonstrated by the use of mass-analyzed ion kinetic energy spectrometry (MIKES) and kinetic energy release distributions (KERDs). Linked scans at a constant B/E severely discriminate against product ions formed by charge-separation reactions. Although the products are significantly more abundant in MIKES experiments, instrumental discrimination still makes quantitation of relative product ion abundances highly inaccurate. The most probable KERs (T m. p.) and the average KERs (T ave.) of the reactions are determined from the KERDs, and these values are compared to the KERs determined from the peak widths at half-height (T 0. 5). The measurement of T 0. 5 is a poor approximation to T m. p. and T ave.. The T m. p. is used to calculate a most probable intercharge distance, which is compared to results from molecular dynamics calculations. The results provide evidence with regard to the mechanisms of fragmentation of multiply charged ions and the location of the charge site in relation to the decomposition reactions.

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The gas-phase chemistry of anionic [ M + CatZ+ -3H] -complexes between CaZ+-specific peptides and the alkaline earth metal ions MgZ+, Caz+ and BaZ+ is reported. The metal ion complexes were studied using fast atom bombardment, collision-induced decomposition (CID) and molecular mechanical calculations. The CID reactions and molecular mechanical calculations revealed that the Caz +-peptide complexes are bound differently to the MgZ + -and Ba*+-peptide complexes and that the intrinsic (gas-phase) chemistry is reflected by known aqueousphase chemistry. AADVPA w c. Strain Energy = 99.1 kJ mol-' d. Strain Energy 80.7 kJ mol-' Figure 3. Lowest energy structures calculated for the (a. b) [M + Ca2+ -3H], (c) [ M + M g 2 + -3H] and (d) [ M + Ba2+ -3H]complexes of Ala-Ala-Asp-Val-Pro-Ala