Structural elucidation studies of erythromycins by electrospray tandem mass spectrometry II

Erythromycin A (EryA), sec-butyl erythromycin B (SEryB), oleandomycin (Olea n) and a synthetic derivative, roxithromycin (Rox), were used to investigat e the fragmentation of polyketide macrolide antibiotics by collision induce d dissociation (CID) tandem mass spectrometry (MS/MS). Analyses were perfor med with two commercially available mass spectrometers: a Q-TOF hybrid quad rupole time-of-flight instrument and a BioApex II (4.7 Tesla) Fourier trans form ion cyclotron resonance (FTICR) instrument both equipped with electros pray ionisation (ESI) sources. One of the first fragmentation processes is the loss of an H2O molecule from the [M+H](+) ion, EryA has three hydroxyl groups on the polyketide ring and loses three H2O molecules during CID, Thi s study indicates that these facts are not necessarily related, Deuterium e xchange experiments were carried out in order to isotopically label free hy droxyl groups,O-18. exchange experiments were also carried out in order to label the carbonyl group at the 9-position. In EryA and its analogue the fi rst H2O loss shifts in mass from loss of 18 Da to loss of 20 Da in deuterat ed solvents, For both molecules the loss also shifts in mass from loss of 1 8 Da to loss of 20 Da during the O-18-exchange experiments. This suggests t hat the first loss of H2O is from the 9-position carbonyl group, indicating that this, and not the nitrogen of the amino sugar, is the site of protona tion of the activated MH+ ions. For Rox the initial loss of H2O is replaced by loss of the 9-position oxime group, the rest of the fragmentation seque nce being the same as for EryA, For Olean, there is no H2O loss from the pa rent ion. The results have allowed the proposal of a mechanism for the firs t loss of H2O in the EryA h MS/MS fragmentation. Copyright (C) 1999 John Wh ey & Sons, Ltd.