The gas-phase infrared photodissociation (IRPD) spectra of solvent-tagged small biomolecules are studied in a cryogenic ion trap at 17 K. In this study para-aminobenzoic acid (PABA) and tyramine molecules are noncovalently tagged with water or acetonitrile in the electrospray ionization (ESI) source. The complexes are then cooled in the cryogenic trap prior to spectroscopic measurements. These molecules provide two putative sites for solvent attachment: the protonated amine (NH₃⁺) and the OH groups. Comparisons of the experimental IR spectra to theoretical spectra obtained with density functional theory show that the NH₃⁺ site is mainly favored. Evidence for the formation of both NH₃-bound and OH-bound conformers is found only in tyramine, despite having similar solution- and gas-phase energetics to that of PABA. Since the structures cannot interconvert in the gas phase, this suggests an isomerization during the electrospray process.

中文翻译：

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使用低温红外光解离光谱技术研究电喷雾电离中形成的溶剂复杂离子的结构

在17 K的低温离子阱中研究了溶剂标记的小生物分子的气相红外光解离（IRPD）光谱。在这项研究中，对位氨基苯甲酸（PABA）和酪胺分子在电喷雾中与水或乙腈非共价标记。电离（ESI）源。然后，在进行光谱测量之前，将复合物在低温阱中冷却。这些分子为溶剂连接提供了两个假定的位置：质子化的胺（NH ₃ ⁺）和OH基团。实验红外光谱与密度泛函理论获得的理论光谱的比较表明，NH ₃ ⁺位点是最有利的。形成NH _3的证据尽管在溶液和气相中的能量与PABA相似，但仅在酪胺中发现了与OH结合的构象异构体。由于结构不能在气相中相互转化，因此表明在电喷雾过程中发生了异构化。