An unusual rearrangement of Zofenopril,a new ACE inhibitor drug: mass spectrometric and conformational studies

Zofenopril (1) is a new ACE inhibitor, used in therapy for hypertension and post-myocardial infarction. The protonated quasi-molecular ion (m/z 430) of 1, obtained under positive electrospray ionization conditions, loses a benzoic acid molecule (m/z 308), which in turn decomposes via loss of CO (m/z 280) when low-energy collisional-induced dissociation (CID) and in-source experiments are performed. This rearrangement is the main fragmentation process and can be observed both in-source and in the product ion tandem mass spectra, using either an ion trap or a triple quadrupole instrument. Other known diastereoisomers of 1, an impurity with an acetyl in the place of the benzoyl group (2) and an impurity with two propanoyl chains in series (3), give the same rearrangement. On the other hand, the mass spectra of the methyl ester (4) and an impurity with two proline moieties (5) do not show this unusual fragmentation. Time-resolved CID spectra of 1 show that the rearrangement occurs after about 2 ms, a time scale comparable to those of the other non-rearrangement cleavages. These experiments suggest a conformation in the gas phase for 1 in which the benzoyl group is close to the hydroxyl of the carboxylic acid group, from which the rearrangement could readily occur. Since compounds 4 and 5 do not show the same behaviour, the presence of a carboxylic acid in the proline ring seems to play a crucial role in the rearrangement, probably due to an intramolecular hydrogen bond. To confirm this hypothesis, deuterium exchanges in mass spectrometric experiments and a conformational analysis via computational methods were performed.