Department of Chemistry and Biochemistry, Ohio University, 391 Clippinger Laboratories, Athens, Ohio 45701, United States.
Department of Chemistry and Biochemistry, University of Missouri-St. Louis, 1 University Boulevard, St. Louis, Missouri 63121, United States.
J Am Soc Mass Spectrom. 2021 Jan 6;32(1):55-63. doi: 10.1021/jasms.0c00074. Epub 2020 Apr 20.
We investigate the structure and dissociation pathways of the deprotonated amphoteric peptide arginylglycylasparic acid, [RGD-H]. We model the pertinent gas-phase structures and fragmentation chemistry of the precursor anions and predominant sequence-informative bond cleavages (, , and peaks) and compare these predictions to our tandem mass spectra and infrared spectroscopy experiments. Formation of the anions requires rate-limiting intramolecular back biting to cleave the second amide bond and generate an anhydride structure. Facile cleavage of the newly formed ester bond with concerted expulsion of a cyclic anhydride neutral generates the product structure. IR spectroscopy supports this anion having structures that are essentially identical to C-terminally deprotonated arginylglycine, [RG-H]. Formation of the anion is predicted to require concerted expulsion of CO from the aspartyl side chain carboxylate and cleavage of the N-C bond to produce a proton-bound dimer of arginylglycinamide and acrylate. Proton transfers within the dimer then enable predominant detection of a anion with the negative charge nominally on the central, glycine nitrogen (amidate structure) as the proton affinity of this structure is predicted to be lower than acrylate by ∼27 kJ mol. Alternate means of cleaving the same N-C bond produce deprotonated -1,4-dibut-2-enoic acid anion structures. These lowest energy processes involve C-H proton mobilization from the aspartyl side chain prior to N-C bond cleavage consistent with proposals from the literature.
我们研究了两性肽精氨酸-甘氨酰-天冬氨酸去质子化后的结构和离解途径[RGD-H]。我们对前体阴离子的相关气相结构和碎片化化学进行建模，并对主要序列信息键断裂（[RGD-H]-，[RGD-H]-，[RGD-H]-和[RGD-H]-峰）进行预测，将这些预测与我们的串联质谱和红外光谱实验进行比较。形成[RGD-H]-阴离子需要限速的分子内回咬作用来切断第二个酰胺键并生成酐结构。新形成的酯键的易断裂与协同排出环状酐中性物生成产物结构。IR 光谱支持该[RGD-H]-阴离子具有与末端去质子化精氨酸-甘氨酸[RG-H]基本相同的结构。形成[RGD-H]-阴离子预计需要协同从天冬氨酸侧链羧酸盐中排出 CO 并切断 N-C 键，以产生精氨酰甘氨酰胺和丙烯酸盐的质子化二聚体。然后，二聚体内的质子转移使主要检测到带负电荷的[RGD-H]-阴离子，其负电荷名义上位于中心甘氨酸氮上（酰胺结构），因为该结构的质子亲和力预计比丙烯酸盐低约 27 kJ mol。切断相同 N-C 键的其他方法会产生去质子化的-1,4-二丁-2-烯酸[RGD-H]-阴离子结构。这些最低能量过程涉及在 N-C 键断裂之前，天冬氨酸侧链的 C-H 质子迁移，这与文献中的建议一致。
