Jobst Karl J, Terlouw Johan K, Luider Theo M, Burgers Peter C
Department of Chemistry, McMaster University, 1280 Main St. W., Hamilton, ON L8S 4M1, Canada.
Anal Chim Acta. 2008 Oct 3;627(1):136-47. doi: 10.1016/j.aca.2008.03.028. Epub 2008 Mar 19.
In a previous report [J.W. de Beukelaar, J.W. Gratama, P.A. Sillevis Smitt, G.M. Verjans, J. Kraan, Th.M. Luider, P.C. Burgers, Rapid Commun. Mass Spectrom. 21 (2007) 1282] on the quality assessment of synthetic peptides used in protein-spanning peptide pools by matrix-assisted laser desorption/ionization Fourier transform mass spectrometry (MALDI-FTMS) we noted that certain peptides showed remarkably intense signals for their calcium-containing analogues. Here we report on a detailed mass spectrometric study of the unimolecular chemistry of these calcium-containing peptides. By integration of the experimental findings with computational results derived from DFT and the CBS-QB3 model chemistry, we have traced the processes induced by Ca(2+) attachment in the peptide ions. Key to our analysis is the observation that all of the studied calcium-bound peptides containing a threonine or serine residue show prominent losses of CH(3)CHO (from threonine) and/or CH(2)O (from serine) in both the positive and the negative ion mode. In the first step, Ca(2+) attaches itself to a negatively charged in-chain carboxylate group. Next, electrophilic attack of the calcium ion on the CH(R)OH group of threonine (RCH(3)) or serine (RH) releases the hydroxyl proton which can then move to a suitable acceptor site, viz. a peptide bond. This leads to the formation of a very stable ionic bidentate structure. Upon collisional activation (MS/MS), this bidentate opens up leading to the loss of the exposed acetaldehyde or formaldehyde molecule, to yield another bidentate structure. MS/MS spectra of selected peptides interacting with other metal ions have also been investigated and it is found that only divalent ions follow the Ca(2+)-induced transformations.
在之前一篇关于通过基质辅助激光解吸/电离傅里叶变换质谱法（MALDI-FTMS）对用于跨蛋白质肽库的合成肽进行质量评估的报告[J.W. 德·贝克拉尔、J.W. 格拉塔马、P.A. 西勒维斯·斯米特、G.M. 韦尔扬斯、J. 克兰、Th.M. 吕德、P.C. 伯格斯，《快速通信：质谱分析》21 (2007) 1282]中，我们注意到某些肽的含钙类似物显示出异常强烈的信号。在此，我们报告对这些含钙肽单分子化学的详细质谱研究。通过将实验结果与源自DFT和CBS-QB3模型化学的计算结果相结合，我们追踪了肽离子中Ca(2+)附着所引发的过程。我们分析的关键在于观察到，所有研究的含苏氨酸或丝氨酸残基的钙结合肽在正离子和负离子模式下均显示出CH(3)CHO（来自苏氨酸）和/或CH(2)O（来自丝氨酸）的显著损失。第一步，Ca(2+)自身附着于带负电荷的链内羧酸盐基团。接下来，钙离子对苏氨酸（RCH(3)）或丝氨酸（RH）的CH(R)OH基团进行亲电攻击，释放出羟基质子，然后该质子可移动至合适的受体位点，即肽键。这导致形成非常稳定的离子双齿结构。经碰撞活化（MS/MS）后，该双齿结构打开，导致暴露的乙醛或甲醛分子损失，从而产生另一种双齿结构。还研究了选定肽与其他金属离子相互作用的MS/MS谱，发现只有二价离子遵循Ca(2+)诱导的转化过程。
