Infrared multiple photon dissociation spectroscopy was performed on protonated and cationized canavanine (Cav), a non-protein amino acid oxy-analog of arginine. Infrared spectra in the XH stretching region (3000 - 4000 cm-1) were obtained at the Centre Laser Infrarouge d'Orsay (CLIO) facility. Comparison of the experimental infrared spectra with scaled harmonic frequencies at the B3LYP/6-31+G(d,p) level of theory indicates that canavanine is in a canonical neutral form in CavH+, CavLi+, and CavNa+; therefore, these cations are charge-solvated structures. The infrared spectrum of CavK+ is consistent with a mixture of Cav in canonical and zwitterionic forms leading to both charge-solvated and salt-bridged cationic structures. The Cav moiety in CavCs+ is shown to be zwitterionic, forming a salt-bridged structure for the cation. Infrared spectra in the fingerprint region (1000 - 2000 cm-1) obtained at the FELIX Laboratory in Nijmegen, Netherlands support these assignments. These results show that that a single oxygen atom substitution in the side chain reduces the stability of the zwitterion compared to that of the protein amino acid arginine (Arg), which has been shown previously to adopt a zwitterionic structure in ArgNa+ and ArgK+. This difference can be explained in part due to the decreased basicity of Cav (PA = 1001 kJ/mol) as compared to arginine (PA = 1051 kJ/mol), but not entirely, as lysine, which has nearly the same proton affinity as Cav, (~993 kJ/mol) forms only canonical structures with Na+, K+, and Cs+. A major difference between the zwitterionic forms of ArgM+ and CavM+ is that the protonation site is on the side chain for Arg and on the N-terminus for Cav. This results in systematically weaker salt bridges in the Cav zwitterions. In addition, the presence of another hydrogen-bonding acceptor atom in the side chain contributes to the stability of the canonical structures for the smaller alkali cations.

中文翻译：

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阳离子化刀豆氨酸的红外多光子解离光谱：侧链取代影响气相两性离子形成

对质子化和阳离子化的刀豆氨酸 (Cav)（精氨酸的非蛋白质氨基酸氧类似物）进行了红外多光子解离光谱分析。XH 拉伸区域 (3000 - 4000 cm-1) 的红外光谱是在 Center Laser Infrarouge d'Orsay (CLIO) 设施中获得的。实验红外光谱与 B3LYP/6-31+G(d,p) 理论水平的缩放谐波频率的比较表明，刀豆氨酸在 CavH+、CavLi+ 和 CavNa+ 中处于规范的中性形式；因此，这些阳离子是电荷溶剂化结构。CavK+ 的红外光谱与经典和两性离子形式的 Cav 混合物一致，导致电荷溶剂化和盐桥接阳离子结构。CavCs+ 中的 Cav 部分显示为两性离子，形成阳离子的盐桥结构。在荷兰奈梅亨的 FELIX 实验室获得的指纹区域 (1000 - 2000 cm-1) 的红外光谱支持这些分配。这些结果表明，与蛋白质氨基酸精氨酸 (Arg) 相比，侧链中的单个氧原子取代降低了两性离子的稳定性，而蛋白质氨基酸精氨酸 (Arg) 先前已显示在 ArgNa+ 和 ArgK+ 中采用两性离子结构。这种差异可以部分解释为与精氨酸 (PA = 1051 kJ/mol) 相比 Cav (PA = 1001 kJ/mol) 的碱度降低，但不完全是因为赖氨酸，它具有几乎相同的质子亲和力Cav (~993 kJ/mol) 仅与 Na+、K+ 和 Cs+ 形成规范结构。ArgM+ 和 CavM+ 的两性离子形式之间的主要区别在于质子化位点在 Arg 的侧链上和 Cav 的 N 端。这导致 Cav 两性离子中系统性较弱的盐桥。此外，侧链中另一个氢键受体原子的存在有助于较小碱性阳离子的规范结构的稳定性。