Vibrational spectroscopy of the cryogenically cooled O- and N-protomers of 4-Aminobenzoic acid: Tag effects, isotopic labels, and identification of the E,Z isomer of the O-protomer,International Journal of Mass Spectrometry

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Vibrational spectroscopy of the cryogenically cooled O- and N-protomers of 4-Aminobenzoic acid: Tag effects, isotopic labels, and identification of the E,Z isomer of the O-protomer
International Journal of Mass Spectrometry ( IF 1.6 ) Pub Date : 2020-11-01 , DOI: 10.1016/j.ijms.2020.116427
Thien Khuu ₁ , Nan Yang ₁ , Mark A Johnson ₁

Affiliation

4-Aminobenzoic acid (4ABA) is a biologically relevant, small organic molecule with two protonation sites: the amino group (N-protomer) and the carboxyl group (O-protomer). The O-protomer is energetically preferred in the gas-phase, while the higher energy N-protomer can be trapped using aprotic solvents such as acetonitrile during electrospray ionization. Here, we focus on the structure of the O-protomer, which can occur in three low-lying isomeric forms that result from different orientations of the OH groups relative to the benzene ring. We report the vibrational spectra of both N- and O-protomers of the cryogenically cooled ions in the gas phase over the spectral range 800-4000 cm-1. The bands arising from the OH stretches are isolated from the nearby NH stretching fundamentals using isotopic labeling as well as by analysis of the shifts in these fundamentals upon attachment of D2 and N2 molecules to the OH groups of the O-protomer. The spectra of isomers derived from the different locations of the adducts were isolated using two-color, IR-IR photofragmentation spectroscopy. The docking motifs by which the O-protomer binds to another 4ABA molecule is also explored and found to feature a bifurcated arrangement involving attachment of both OH groups of the protonated head group to the carbonyl group of the neutral partner.

中文翻译：

4-氨基苯甲酸低温冷却的 O-和 N-原体的振动光谱：标签效应、同位素标记以及 O-原体 E、Z 异构体的鉴定

4-氨基苯甲酸 (4ABA) 是一种生物学相关的有机小分子，具有两个质子化位点：氨基（N-原体）和羧基（O-原体）。O-原体在气相中在能量上是优选的，而较高能量的N-原体可以在电喷雾电离过程中使用非质子溶剂（例如乙腈）捕获。在这里，我们重点关注 O-原体的结构，它可以以三种低位异构体形式出现，这是由于 OH 基团相对于苯环的不同方向而产生的。我们报告了气相中低温冷却离子的 N 和 O 原体在 800-4000 cm-1 光谱范围内的振动光谱。使用同位素标记以及分析 D2 和 N2 分子附着到 O-原体的 OH 基团后这些基本原理的变化，将由 OH 拉伸产生的带与附近的 NH 拉伸基本原理分离。使用双色红外-红外光裂解光谱分离来自加合物不同位置的异构体的光谱。还探索了 O-原体与另一个 4ABA 分子结合的对接基序，发现其特征是分叉排列，涉及质子化头基的两个 OH 基团与中性伴侣的羰基的连接。

更新日期：2020-11-01

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