Kearney G C, Gates P J, Leadlay P F, Staunton J, Jones R
Department of Chemistry, University of Cambridge, Lensfield Road, Cambridge CB2 1EW, UK.
Rapid Commun Mass Spectrom. 1999;13(16):1650-6. doi: 10.1002/(SICI)1097-0231(19990830)13:16<1650::AID-RCM693>3.0.CO;2-8.
Erythromycin A (EryA), sec-butyl erythromycin B (SEryB), oleandomycin (Olean) and a synthetic derivative, roxithromycin (Rox), were used to investigate the fragmentation of polyketide macrolide antibiotics by collision induced dissociation (CID) tandem mass spectrometry (MS/MS). Analyses were performed with two commercially available mass spectrometers: a Q-TOF hybrid quadrupole time-of-flight instrument and a BioApex II (4.7 Tesla) Fourier transform ion cyclotron resonance (FTICR) instrument both equipped with electrospray ionisation (ESI) sources. One of the first fragmentation processes is the loss of an H(2)O molecule from the M+H ion. EryA has three hydroxyl groups on the polyketide ring and loses three H(2)O molecules during CID. This study indicates that these facts are not necessarily related. Deuterium exchange experiments were carried out in order to isotopically label free hydroxyl groups. (18)O-exchange experiments were also carried out in order to label the carbonyl group at the 9-position. In EryA and its analogue the first H(2)O loss shifts in mass from loss of 18 Da to loss of 20 Da in deuterated solvents. For both molecules the loss also shifts in mass from loss of 18 Da to loss of 20 Da during the (18)O-exchange experiments. This suggests that the first loss of H(2)O is from the 9-position carbonyl group, indicating that this, and not the nitrogen of the amino sugar, is the site of protonation of the activated MH(+) ions. For Rox the initial loss of H(2)O is replaced by loss of the 9-position oxime group, the rest of the fragmentation sequence being the same as for EryA. For Olean, there is no H(2)O loss from the parent ion. The results have allowed the proposal of a mechanism for the first loss of H(2)O in the EryA MS/MS fragmentation.
红霉素A（EryA）、仲丁基红霉素B（SEryB）、竹桃霉素（Olean）及其合成衍生物罗红霉素（Rox），被用于通过碰撞诱导解离（CID）串联质谱（MS/MS）研究聚酮类大环内酯抗生素的碎片化过程。分析使用了两台市售质谱仪进行：一台Q-TOF混合四极杆飞行时间仪器和一台BioApex II（4.7特斯拉）傅里叶变换离子回旋共振（FTICR）仪器，两者均配备电喷雾电离（ESI）源。最初的碎片化过程之一是M + H离子失去一个H₂O分子。EryA在聚酮环上有三个羟基，在CID过程中会失去三个H₂O分子。本研究表明这些事实不一定相关。进行了氘交换实验以对游离羟基进行同位素标记。还进行了¹⁸O交换实验以标记9位的羰基。在EryA及其类似物中，在氘代溶剂中，第一次H₂O损失的质量从损失18 Da变为损失20 Da。对于这两种分子，在¹⁸O交换实验期间，损失的质量也从损失18 Da变为损失20 Da。这表明第一次H₂O损失是来自9位羰基，这表明这是活化的MH⁺离子的质子化位点，而不是氨基糖的氮。对于Rox，最初的H₂O损失被9位肟基的损失所取代，其余的碎片化序列与EryA相同。对于Olean，母离子没有H₂O损失。这些结果使得能够提出EryA MS/MS碎片化中第一次H₂O损失的机制。
