FOM Institute for Plasma Physics "Rijnhuizen", Edisonbaan, MN, Nieuwegein, The Netherlands.
Anal Bioanal Chem. 2011 Mar;399(7):2463-73. doi: 10.1007/s00216-010-4582-y. Epub 2011 Jan 27.
Structural characterization of protonated gas-phase ions of cysteine and dopamine by infrared multiple photon dissociation (IRMPD) spectroscopy using a free electron laser in combination with theory based on DFT calculations reveals the presence of two types of protonated dimer ions in the electrospray mass spectra of the metabolites. In addition to the proton-bound dimer of each species, the covalently bound dimer of cysteine (bound by a disulfide linkage) has been identified. The dimer ion of m/z 241 observed in the electrospray mass spectra of cysteine has been identified as protonated cystine by comparison of the experimental IRMPD spectrum to the IR absorption spectra predicted by theory and the IRMPD spectrum of a standard. Formation of the protonated covalently bound disulfide-linked dimer ions (i.e. protonated cystine) from electrospray of cysteine solution is consistent with the redox properties of cysteine. Both the IRMPD spectra and theory indicate that in protonated cystine the covalent disulfide bond is retained and the proton is involved in intramolecular hydrogen bonding between the amine groups of the two cysteine amino acid units. For cysteine, the protonated covalently bound dimer (m/z 241) dominated the mass spectrum relative to the proton-bound dimer (m/z 243), but this was not the case for dopamine, where the protonated monomer and the proton-bound dimer were both observed as major ions. An extended conformation of the ethylammonium side chain of gas-phase protonated dopamine monomer was verified from the correlation between the predicted IR absorption spectra and the experimental IRMPD spectrum. Dopamine has the same extended ethylamine side chain conformation in the proton-bound dopamine dimer identified in the mass spectra of electrosprayed dopamine. The structure of the proton-bound dimer of dopamine is confirmed by calculations and the presence of an IR band due to the shared proton. The presence of the shared proton in the protonated cystine ion can be inferred from the IRMPD spectrum.
采用自由电子激光结合基于 DFT 计算的理论，通过红外多光子解离（IRMPD）光谱对半胱氨酸和多巴胺的气相质子化离子进行结构表征，在代谢物的电喷雾质谱中发现了两种类型的质子化二聚体离子。除了每种物质的质子结合二聚体外，还鉴定出了半胱氨酸的共价键合二聚体（通过二硫键键合）。通过将实验的 IRMPD 光谱与理论预测的红外吸收光谱和标准物质的 IRMPD 光谱进行比较，鉴定出在半胱氨酸的电喷雾质谱中观察到的 m/z 241 二聚体离子为质子化胱氨酸。半胱氨酸溶液的电喷雾形成质子化共价键合二硫键连接的二聚体离子（即质子化胱氨酸）与半胱氨酸的氧化还原性质一致。IRMPD 光谱和理论都表明，在质子化胱氨酸中，共价二硫键得以保留，质子参与两个半胱氨酸氨基酸单元的氨基之间的分子内氢键。对于半胱氨酸，质子化共价键合二聚体（m/z 241）相对于质子结合二聚体（m/z 243）在质谱中占主导地位，但多巴胺则不然，其中观察到质子化单体和质子结合二聚体都是主要离子。通过预测的红外吸收光谱与实验的 IRMPD 光谱之间的相关性，证实了气相质子化多巴胺单体中乙基铵侧链的扩展构象。在电喷雾多巴胺的质谱中鉴定出的质子结合多巴胺二聚体中存在相同的扩展乙基胺侧链构象。通过计算和共享质子的存在，确认了多巴胺的质子结合二聚体的结构。质子化胱氨酸离子中共享质子的存在可以从 IRMPD 光谱中推断出来。
