Vapor assisted mobility shift measurements were made with atmospheric pressure drift-tube ion mobility-mass spectrometry (IM-MS) to determine the thermodynamic properties of weakly bound ion-molecule clusters formed from protonated phenylalanine and neutral vapor molecules with hydroxyl functional groups. Relative binding energies and gas-phase association energies of amino acid ions clustered with small organic molecules have been established previously using high-pressure mass spectrometry. However, the issue of volatility largely prohibits the use of high-pressure mass spectrometry for the determination of gas-phase associations of amino acid ions clustered with neutral vapor molecules in many instances. In contrast, ion mobility measurements can be made at atmospheric pressure with volatile vapor additives near and above their boiling points, providing access to clustering equilibria not possible using high-vacuum techniques. In this study, we report the gas-phase association energies, enthalpies, and entropies for a protonated phenylalanine ion clustered with three neutral vapor molecules: 2-propanol, 1-butanol, and 2-pentanol based upon measurements at temperatures ranging from 120 to 180 °C. The gas-phase enthalpy and entropy changes ranged between -4 to -7 kcal/mol and -3 to 6 cal/(mol K), respectively. We found enthalpically favored ion-neutral cluster reactions for phenylalanine with entropic barriers for the formation of phenylalanine-1-butanol and phenylalanine-2-pentanol cluster ions, while phenylalanine-2-propanol cluster ion formation is both enthalpically and (weakly) entropically favorable. Under the measurement conditions examined, phenylalanine-vapor modifier cluster ion formation is clearly observed via shifts in the drift time for the three test vapor molecules. In comparison, negligible shifts in mobility are observed for protonated arginine exposed to the same vapor modifiers.

中文翻译：

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用离子迁移质谱法研究气相簇热力学：苯丙氨酸离子与气相醇的缔合能。

用大气压漂移管离子迁移质谱法（IM-MS）进行了蒸汽辅助迁移率迁移测量，以确定由质子化苯丙氨酸和具有羟基官能团的中性蒸气分子形成的弱结合离子分子簇的热力学性质。先前已经使用高压质谱法建立了与小有机分子簇集的氨基酸离子的相对结合能和气相缔合能。然而，在许多情况下，挥发性问题在很大程度上禁止使用高压质谱法测定与中性蒸气分子簇合的氨基酸离子的气相缔合。相反，可以在大气压下使用接近或高于其沸点的挥发性蒸汽添加剂进行离子迁移率测量，使用高真空技术无法实现聚类平衡。在这项研究中，我们基于在120至120℃的温度范围内的测量结果，报告了质子化苯丙氨酸离子与三个中性蒸气分子（2-丙醇，1-丁醇和2-戊醇）簇合的气相缔合能，焓和熵180℃。气相的焓和熵的变化分别在-4至-7 kcal / mol和-3至6 cal /（mol K）之间。我们发现苯丙氨酸的焓偏于离子中性簇反应具有形成苯丙氨酸-1-丁醇和苯丙氨酸-2-戊醇簇离子的熵屏障，而苯丙氨酸-2-丙醇簇离子的形成既对焓又对熵弱（弱） 。在检查的测量条件下，通过三个测试蒸气分子的漂移时间的变化，可以清楚地观察到苯丙氨酸蒸气改性剂簇离子的形成。相比之下，对于暴露于相同蒸气改性剂的质子化精氨酸来说，迁移率的变化可忽略不计。