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 spectrome try of doubly protonated angiotensin II is demonstrated by the use of mass-analyzed ion kinetic energy spectrometry (MIKES) and kinetic ener gy release distributions (KERDs). Linked scans at a constant B/E sever ely discriminate against product ions formed by charge-separation reac tions. 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 determ ined from the KERDs, and these values are compared to the KERs determi ned 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 t o calculate a most probable intercharge distance, which is compared to results from molecular dynamics calculations. The results provide evi dence with regard to the mechanisms of fragmentation of multiply charg ed ions and the location of the charge site in relation to the decompo sition reactions.