Hu Yongjun, Bernstein Elliot R
Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523-1872, USA.
J Chem Phys. 2008 Apr 28;128(16):164311. doi: 10.1063/1.2902980.
The aliphatic amino acids glycine, valine, leucine, and isoleucine are thermally placed into the gas phase and expanded into a vacuum system for access by time of flight mass spectroscopy and infrared (IR) spectroscopy in the energy range of 2500-4000 cm(-1) (CH, NH, OH, and stretching vibrations). The isolated neutral amino acids are ionized by a single photon of 10.5 eV energy (118 nm), which exceeds by less than 2 eV their reported ionization thresholds. As has been reported for many hydrogen bonded acid-base systems (e.g., water, ammonia, alcohol, acid clusters, and acid molecules), the amino acids undergo a structural rearrangement in the ion state (e.g., in simplest form, a proton transfer) that imparts sufficient excess vibrational energy to the ion to completely fragment it. No parent ions are observed. If the neutral ground state amino acids are exposed to IR radiation prior to ionization, an IR spectrum of the individual isomers for each amino acid can be determined by observation of the ion intensity of the different fragment mass channels. Both the IR spectrum and fragmentation patterns for individual isomers can be qualitatively identified and related to a particular isomer in each instance. Thus, each fragment ion detected presents an IR spectrum of its particular parent amino acid isomer. In some instances, the absorption of IR radiation by the neutral amino acid parent isomer increases a particular fragmentation mass channel intensity, while other fragmentation mass channel intensities decrease. This phenomenon can be rationalized by considering that with added energy in the molecule, the fragmentation channel populations can be modulated by the added vibrational energy in the rearranged ions. This observation also suggests that the IR absorption does not induce isomerization in the ground electronic state of these amino acids. These data are consistent with theoretical predictions for isolated amino acid secondary structures and can be related to previous IR spectra of amino acid conformers.
脂肪族氨基酸甘氨酸、缬氨酸、亮氨酸和异亮氨酸被热置于气相中，并扩展到真空系统，以便通过飞行时间质谱和红外（IR）光谱在2500 - 4000 cm⁻¹能量范围内（CH、NH、OH和伸缩振动）进行检测。分离出的中性氨基酸通过能量为10.5 eV（118 nm）的单个光子进行电离，该能量比其报道的电离阈值高出不到2 eV。正如许多氢键酸碱体系（如水、氨、醇、酸簇和酸分子）所报道的那样，氨基酸在离子状态下会发生结构重排（例如，以最简单的形式，质子转移），从而赋予离子足够的过剩振动能量使其完全碎片化。未观察到母离子。如果中性基态氨基酸在电离之前暴露于红外辐射下，通过观察不同碎片质量通道的离子强度，可以确定每种氨基酸各个异构体的红外光谱。各个异构体的红外光谱和碎片化模式在每种情况下都可以定性识别并与特定异构体相关联。因此，检测到的每个碎片离子都呈现出其特定母氨基酸异构体的红外光谱。在某些情况下，中性氨基酸母异构体对红外辐射的吸收会增加特定碎片化质量通道的强度，而其他碎片化质量通道的强度则会降低。通过考虑分子中添加能量后，重排离子中添加的振动能量可以调节碎片化通道的分布，这种现象可以得到合理的解释。该观察结果还表明，红外吸收不会在这些氨基酸的基态电子态中诱导异构化。这些数据与孤立氨基酸二级结构的理论预测一致，并且可以与氨基酸构象体的先前红外光谱相关联。
