Department of Experimental Physics, Faculty of Mathematics, Physics and Informatics, Comenius University, Mlynská dolina, 84248 Bratislava, Slovakia.
J Chem Phys. 2012 Sep 14;137(10):105101. doi: 10.1063/1.4749244.
We present experimental and theoretical study of electron ionization and dissociative ionization to the gas phase amino acids valine, leucine, and isoleucine. A crossed electron/molecular beams technique equipped with quadrupole mass analyzer has been applied to measure mass spectra and ion efficiency curves for formation of particular ions. From experimental data the ionization energies of the molecules and the appearance energies of the fragment ions were determined. Ab initio calculations (Density Functional Theory and G3MP2 methods) were performed in order to calculate the fragmentation paths and interpret the experimental data. The experimental ionization energies of parent molecules P 8.91 ± 0.05, 8.85 ± 0.05, and 8.79 ± 0.05 eV and G3MP2 ionization energies (adiabatic) of 8.89, 8.88, and 8.81 eV were determined for valine, leucine, and isoleucine, respectively, as well as the experimental and theoretical threshold energies for dissociative ionization channels. The comparison of experimental data with calculations resulted in identification of the ions as well as the neutral fragments formed in the dissociative reactions. Around 15 mass/charge ratio fragments were identified from the mass spectra by comparison of experimental appearance energies with calculated reaction enthalpies for particular dissociative reactions.
我们对气相氨基酸缬氨酸、亮氨酸和异亮氨酸进行了电子电离和离解电离的实验和理论研究。采用交叉电子/分子束技术和四极杆质量分析器，测量了形成特定离子的质谱和离子效率曲线。从实验数据中确定了分子的电离能和碎片离子的出现能。为了计算碎片路径并解释实验数据，进行了从头算（密度泛函理论和 G3MP2 方法）计算。实验确定了母体分子P的电离能为 8.91 ± 0.05、8.85 ± 0.05 和 8.79 ± 0.05 eV，以及缬氨酸、亮氨酸和异亮氨酸的 G3MP2 电离能（绝热）分别为 8.89、8.88 和 8.81 eV，以及离解电离通道的实验和理论阈值能。将实验数据与计算结果进行比较，确定了在离解反应中形成的离子和中性碎片。通过将实验出现能与特定离解反应的计算反应焓进行比较，从质谱中鉴定出约 15 个质量/电荷比碎片。
