Department of Chemistry, Wayne State University, Detroit, MI, 48202, USA.
J Am Soc Mass Spectrom. 2013 Oct;24(10):1523-33. doi: 10.1007/s13361-013-0689-7. Epub 2013 Jul 27.
The gas-phase structures of alkali metal cation-cytosine complexes generated by electrospray ionization are probed via infrared multiple photon dissociation (IRMPD) action spectroscopy and theoretical calculations. IRMPD action spectra of five alkali metal cation-cytosine complexes exhibit both similar and distinctive spectral features over the range of ~1000-1900 cm(-1). The IRMPD spectra of the Li(+)(cytosine), Na(+)(cytosine), and K(+)(cytosine) complexes are relatively simple but exhibit changes in the shape and shifts in the positions of several bands that correlate with the size of the alkali metal cation. The IRMPD spectra of the Rb(+)(cytosine) and Cs(+)(cytosine) complexes are much richer as distinctive new IR bands are observed, and the positions of several bands continue to shift in relation to the size of the metal cation. The measured IRMPD spectra are compared to linear IR spectra of stable low-energy tautomeric conformations calculated at the B3LYP/def2-TZVPPD level of theory to identify the conformations accessed in the experiments. These comparisons suggest that the evolution in the features in the IRMPD action spectra with the size of the metal cation, and the appearance of new bands for the larger metal cations, are the result of the variations in the intensities at which these complexes can be generated and the strength of the alkali metal cation-cytosine binding interaction, not the presence of multiple tautomeric conformations. Only a single tautomeric conformation is accessed for all five alkali metal cation-cytosine complexes, where the alkali metal cation binds to the O2 and N3 atoms of the canonical amino-oxo tautomer of cytosine, M(+)(C1).
通过红外多光子解离（IRMPD）作用光谱和理论计算研究了电喷雾电离生成的碱金属阳离子-胞嘧啶配合物的气相结构。五种碱金属阳离子-胞嘧啶配合物的 IRMPD 作用光谱在~1000-1900cm(-1)范围内表现出相似和独特的光谱特征。Li(+)(胞嘧啶)、Na(+)(胞嘧啶)和 K(+)(胞嘧啶)配合物的 IRMPD 光谱相对简单，但形状发生变化，几个带的位置发生位移，与碱金属阳离子的大小有关。Rb(+)(胞嘧啶)和 Cs(+)(胞嘧啶)配合物的 IRMPD 光谱要丰富得多，观察到了独特的新 IR 带，几个带的位置继续随金属阳离子的大小而移动。将测量的 IRMPD 光谱与在 B3LYP/def2-TZVPPD 理论水平上计算的稳定低能互变异构构象的线性 IR 光谱进行比较，以确定实验中访问的构象。这些比较表明，IRMPD 作用光谱中随金属阳离子大小变化的特征的演变，以及较大金属阳离子的新带的出现，是由于这些配合物可以被生成的强度以及碱金属阳离子-胞嘧啶结合相互作用的强度的变化，而不是由于多种互变异构构象的存在。对于所有五个碱金属阳离子-胞嘧啶配合物，只有一种互变异构构象被访问，其中碱金属阳离子与胞嘧啶的典型氨基-氧互变异构体的 O2 和 N3 原子结合，M(+)(C1)。
