The importance of charge-separation reactions in tandem mass spectrometry of doubly protonated angiotensin II formed by electrospray ionization: Experimental considerations and structural implications

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, instru- mental discrimination still makes quantitation of relative product ion abundances highly inaccurate. The most probable KERs (Tm.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.