Conformation of Doubly Protonated Peptides Studied by Charge-separation Reactions in Mass Spectrometry

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Charge-separation metastable processes were investigated by mass spectrometry and kinetic energy release (KER) distributions were determined from the measured metastable peak profiles. Benzene, p-N,N-dimethylaminoaniline and pipecuronium bromide (Arduan) were investigated as reference compounds to confirm that there is a well defined relationship between the KER distribution and the distance of the two charged centers. For these ‘rigid’ systems relatively narrow KER distributions were found, and the average values were in good agreement with the intercharge distances determined by a semi-empirical (MNDO) molecular orbital calculation. Two metastable processes of bradykinin, a nonapeptide and one of trilysine were studied. The distribution of KER suggests that the conformation of bradykinin is fairly rigid, whereas that of trilysine is flexible. The observed KER values clearly indicate far smaller intercharge distances than those corresponding to extended geometries. This is clear experimental evidence that medium-sized doubly charged peptides have a folded and not an extended geometry in the gas phase. © 1997 by John Wiley & Sons, Ltd.