Boutin Michel, Bich Claudia, Afonso Carlos, Fournier Françoise, Tabet Jean-Claude
Université Pierre et Marie Curie-Paris 6, UMR7613, Synthèse, Structure et Fonction de Molécules Bioactives, Paris, F-75005 France.
J Mass Spectrom. 2007 Jan;42(1):25-35. doi: 10.1002/jms.1127.
In aqueous solution, amino acids (AA) and peptides are known to exist as zwitterions over a large pH range. However, in the gas phase, i.e. in electrospray (ESI), the zwitterionic form becomes unfavorable owing to the absence of stabilizing effects from intermolecular solvation. Nevertheless, during mass spectrometry experiments, the presence of a metallic cation can reinforce the zwitterionic character of the molecule and thus influence its fragmentation under low energy collision-induced dissociation (CID) conditions. The M + Cu(II) complexes of six pentapeptides (YGGFL, YGGFL(NH(2)), YGGFK, YGGFQ, KYGGF and QYGGF) were analyzed by collision to highlight the presence of zwitterions. The experiments were performed on a 3D-ion trap equipped with an orthogonal ESI source. For each peptides studied, negative-charge driven fragmentations on globally positively charged ions were observed. These fragmentation mechanisms, generally observed in the negative mode, suggest the competitive deprotonation of the C-terminal carboxylic acid or of the tyrosine side-chain residue for each peptide studied and thus a zwitterionic form to preserve the charge balance. Moreover, the specific loss of (CH(3)--C(6)H(4)--O)() characterizes YGGFK compared to YGGFQ and the specific loss of styrene characterizes KYGGF compared to QYGGF. These results allow the differentiation of the two couples of isobaric pentapeptides. An unusual loss of NH(4) (+), which occurred from the N-terminus, was also observed for YGGFL, YGGFL(NH(2)), YGGFK and YGGFQ. Finally, the reduction of Cu(II) to Cu(I), concomitant with the (CH(3)--C(6)H(4)--O)() release, was pointed out for YGGFK.
在水溶液中，氨基酸（AA）和肽在较大的pH范围内以两性离子形式存在。然而，在气相中，即在电喷雾（ESI）中，由于缺乏分子间溶剂化的稳定作用，两性离子形式变得不利。尽管如此，在质谱实验中，金属阳离子的存在可以增强分子的两性离子特性，从而影响其在低能量碰撞诱导解离（CID）条件下的碎片化。通过碰撞分析了六种五肽（YGGFL、YGGFL(NH₂)、YGGFK、YGGFQ、KYGGF和QYGGF）的M + Cu(II)配合物，以突出两性离子的存在。实验在配备正交ESI源的三维离子阱上进行。对于所研究的每种肽，观察到全局带正电离子上的负电荷驱动碎片化。这些通常在负模式下观察到的碎片化机制表明，对于所研究的每种肽，C端羧酸或酪氨酸侧链残基存在竞争性去质子化，因此存在两性离子形式以保持电荷平衡。此外，与YGGFQ相比，(CH₃--C₆H₄--O)()的特定损失表征了YGGFK，与QYGGF相比，苯乙烯的特定损失表征了KYGGF。这些结果允许区分两对同量异位五肽。对于YGGFL、YGGFL(NH₂)、YGGFK和YGGFQ，还观察到从N端发生的不寻常的NH₄(+)损失。最后，指出了YGGFK中Cu(II)还原为Cu(I)并伴随(CH₃--C₆H₄--O)()释放的情况。
