Forbes Matthew W, Bush Matthew F, Polfer Nick C, Oomens Jos, Dunbar Robert C, Williams Evan R, Jockusch Rebecca A
Department of Chemistry, University of Toronto, Toronto, Ontario M5S 3H6, Canada.
J Phys Chem A. 2007 Nov 22;111(46):11759-70. doi: 10.1021/jp074859f. Epub 2007 Nov 1.
The structures of cationized arginine complexes [Arg + M]+, (M = H, Li, Na, K, Rb, Cs, and Ag) and protonated arginine methyl ester [ArgOMe + H]+ have been investigated in the gas phase using calculations and infrared multiple-photon dissociation spectroscopy between 800 and 1900 cm-1 in a Fourier transform ion cyclotron resonance mass spectrometer. The structure of arginine in these complexes depends on the identity of the cation, adopting either a zwitterionic form (in salt-bridge complexes) or a non-zwitterionic form (in charge-solvated complexes). A diagnostic band above 1700 cm-1, assigned to the carbonyl stretch, is observed for [ArgOMe + H]+ and [Arg + M]+, (M = H, Li, and Ag), clearly indicating that Arg in these complexes is non-zwitterionic. In contrast, for the larger alkali-metal cations (K+, Rb+, and Cs+) the measured IR-action spectra indicate that arginine is a zwitterion in these complexes. The measured spectrum for [Arg + Na]+ indicates that it exists predominantly as a salt bridge with zwitterionic Arg; however, a small contribution from a second conformer (most likely a charge-solvated conformer) is also observed. While the silver cation lies between Li+ and Na+ in metal-ligand bond distance, it binds as strongly or even more strongly to oxygen-containing and nitrogen-containing ligands than the smaller Li+. The measured IR-action spectrum of [Arg + Ag]+ clearly indicates only the existence of non-zwitterionic Arg, demonstrating the importance of binding energy in conformational selection. The conformational landscapes of the Arg-cation species have been extensively investigated using a combination of conformational searching and electronic structure theory calculations [MP2/6-311++G(2d,2p)//B3LYP/6-31+G(d,p)]. Computed conformations indicate that Ag+ is di-coordinated to Arg, with the Ag+ chelated by both the N-terminal nitrogen and Neta of the side chain but lacks the strong M+-carbonyl oxygen interaction that is present in the tri-coordinate Li+ and Na+ charge-solvation complexes. Experiment and theory show good agreement; for each ion species investigated, the global-minimum conformer provides a very good match to the measured IR-action spectrum.
利用傅里叶变换离子回旋共振质谱仪，在800至1900厘米⁻¹之间通过计算和红外多光子解离光谱，对阳离子化精氨酸配合物[Arg + M]+（M = H、Li、Na、K、Rb、Cs和Ag）以及质子化精氨酸甲酯[ArgOMe + H]+的结构进行了气相研究。这些配合物中精氨酸的结构取决于阳离子的种类，采用两性离子形式（在盐桥配合物中）或非两性离子形式（在电荷溶剂化配合物中）。对于[ArgOMe + H]+和[Arg + M]+（M = H、Li和Ag），观察到一条高于1700厘米⁻¹的诊断带，归属于羰基伸缩振动，这清楚地表明这些配合物中的精氨酸是非两性离子的。相反，对于较大的碱金属阳离子（K⁺、Rb⁺和Cs⁺），测量的红外作用光谱表明精氨酸在这些配合物中是两性离子。[Arg + Na]+的测量光谱表明它主要以与两性离子精氨酸形成的盐桥形式存在；然而，也观察到了第二种构象（最可能是电荷溶剂化构象）的少量贡献。虽然银阳离子在金属 - 配体键距方面介于Li⁺和Na⁺之间，但它与含氧化合物和含氮配体的结合力与较小的Li⁺一样强甚至更强。[Arg + Ag]+的测量红外作用光谱清楚地表明只存在非两性离子精氨酸，证明了结合能在构象选择中的重要性。利用构象搜索和电子结构理论计算[MP2/6 - 311++G(2d,2p)//B3LYP/6 - 31+G(d,p)]相结合的方法，对精氨酸 - 阳离子物种的构象景观进行了广泛研究。计算得到的构象表明，Ag⁺与精氨酸形成双配位，Ag⁺被N端氮原子和侧链的Nε螯合，但缺乏三配位的Li⁺和Na⁺电荷溶剂化配合物中存在的强M⁺ - 羰基氧相互作用。实验和理论显示出良好的一致性；对于所研究的每种离子物种，全局最小构象与测量的红外作用光谱非常匹配。
