Ramesh V, Srinivas R, Sharma G V M, Jayaprakash P, Kunwar A C
National Centre for Mass Spectrometry, Indian Institute of Chemical Technology, Hyderabad 500 007, India.
J Mass Spectrom. 2008 Sep;43(9):1201-14. doi: 10.1002/jms.1393.
A new series of Boc-N-beta(3), gamma(4)-/gamma(4), beta(3)-isomeric hybrid peptides (containing repeats of beta(3)-Caa and gamma(4)-Caa's, Caa = C-linked carbo beta(3)-/gamma(4)-amino acids derived from D-xylose) have been differentiated by both positive and negative ion electrospray ionization (ESI) ion-trap and high resolution quadrupole time-of-flight/tandem mass spectrometry (Q-TOF MS/MS). MS(n) of protonated isomeric peptides and M+H-Boc+H produce characteristic fragmentation involving the peptide backbone, the Boc-group, and the side chain. The positional isomers are differentiated from one another by the presence of y(n)(+), b(n)(+), and other fragment ions of different m/z values. It is observed that the peptides with beta-Caa at the N-terminus produce extensive fragmentation, whereas gamma-Caa gave rise to much less fragmentation. Peptides with gamma-Caa at the N-terminus lose NH(3), whereas this process is absent for the carbopeptides with beta-Caa at the N-terminus. Two pairs of dipeptide diastereomers are clearly differentiated by the collision-induced dissociation (CID) of their protonated molecules. The loss of 2-methylprop-1-ene is more pronounced for Boc-NH-(R)-beta-Caa-(R)-gamma-Caa-OCH(3) (6) and Boc-NH-(R)-gamma-Caa-(R)-beta-Caa-OCH(3) (12), whereas it is insignificant or totally absent for its protonated diastereomeric pair Boc-NH-(S)-beta-Caa-(S)-gamma-Caa-OCH(3) (1) and Boc-NH-(S)-gamma-Caa-(S)-beta-Caa-OCH(3) (7). Further, ESI negative ion tandem mass spectrometry has also been found to be useful for differentiating these isomeric peptide acids.
通过正离子和负离子电喷雾电离（ESI）离子阱以及高分辨率四极杆飞行时间串联质谱（Q-TOF MS/MS），已鉴别出一系列新的Boc-N-β(3),γ(4)-/γ(4),β(3)-异构杂合肽（包含β(3)-Caa和γ(4)-Caa的重复序列，Caa = 源自D-木糖的C-连接碳β(3)-/γ(4)-氨基酸）。质子化异构肽和M+H-Boc+H的MS(n)产生涉及肽主链、Boc基团和侧链的特征性碎片化。位置异构体通过不同m/z值的y(n)(+)、b(n)(+)和其他碎片离子的存在而相互区分。观察到N端带有β-Caa的肽产生广泛的碎片化，而γ-Caa导致的碎片化要少得多。N端带有γ-Caa的肽会失去NH(3)，而N端带有β-Caa的碳肽则不存在此过程。通过其质子化分子的碰撞诱导解离（CID），两对二肽非对映异构体得到了清晰的区分。对于Boc-NH-(R)-β-Caa-(R)-γ-Caa-OCH(3)（6）和Boc-NH-(R)-γ-Caa-(R)-β-Caa-OCH(3)（12），2-甲基丙烯的损失更为明显，而其质子化非对映异构体对Boc-NH-(S)-β-Caa-(S)-γ-Caa-OCH(3)（1）和Boc-NH-(S)-γ-Caa-(S)-β-Caa-OCH(3)（7）的这种损失不明显或完全不存在。此外，ESI负离子串联质谱也被发现可用于区分这些异构肽酸。
