Hydration of Gaseous m-Aminobenzoic Acid: Ionic vs Neutral Hydrogen Bonding and Water Bridges

Abstract

Hydration of a protonated amine and a neutral carboxylic acid were investigated for protonated m-aminobenzoic acid (MABAH(+)) with up to 15 water molecules attached using infrared photodissociation spectroscopy, laser-induced dissociation kinetics, and computational chemistry. A free COO-H stretch in the spectra of MABAH(+)·(H2O)1-5 indicates that water does not bind to the carboxylic acid H atom. This band is absent in the spectrum of MABAH(+) with six or more water molecules attached, and there is a hydrogen-bonded (HB) COO-H stretch indicating that water hydrogen bonds to the carboxylic acid H atom for these larger clusters. Photodissociation kinetic data for MABAH(+)·(H2O)6 indicate that greater than 74 ± 13% of the ion population consists of the HB COO-H isomer, consistent with this isomer being ≥0.5 kJ mol(-1) lower in energy than isomers where the carboxylic acid H atom does not donate a hydrogen bond. Calculations at the B3LYP/6-31+G** and MP2/6-31+G**//B3LYP/6-31+G** levels of theory indicate that this energy difference is 3-5 kJ mol(-1), in agreement with the experimental results. Lower effective ion heating rates, either by attenuation of the laser power or irradiation of the ions at a lower frequency, result in more time for interconversion between the free and HB COO-H isomers. These data suggest that the barrier to dissociation for the free COO-H isomer is less than that for the HB COO-H isomer but greater than the barrier for interconversion between the two isomers. These results show the competition between hydration of a primary protonated amine vs that of a neutral carboxylic acid and the effect of water bridging between the two functional groups, which provide valuable insight into the hydration of protonated amino acids and establish rigorous benchmarks for theoretical modeling of water-biomolecule interactions.

A computational study of the chlorination and hydroxylation of amines by hypochlorous acid

“…It is known that the inclusion of explicit solvent molecules is essential for stabilization of the zwitterionic form of urea, 21 amino, 22 sulfamic, 23 or m-aminobenzoic acids. 24 In our case, the stepwise hydration of structure 3 has resulted in the appearance of a zwitterionic structure as a genuine minimum on the PES. With two water molecules the zwitterionic amine oxide becomes competitive in energy to hydroxylamine, which is in line with the experimental findings.…”

Amine vs. carboxylic acid protonation in ortho-, meta-, and para-aminobenzoic acid: An IRMPD spectroscopy study

“…12 While the exact mechanism for this isomerization process has not been proven, it has been hypothesized that a protic bridge mechanism could facilitate the proton transfer from the amine to the carboxylic acid. 11,12 Infrared photodissociation spectroscopy and computational studies on partially hydrated protonated PABA 16 and MABA 17 show that in the case of PABA the addition of several water molecules can affect the energetics of proton attachment.…”

Competing Insertion and External Binding Motifs in Hydrated Neurotransmitters: Infrared Spectra of Protonated Phenylethylamine Monohydrate

“…[33] Hydration can also influence the protonation site [34][35][36] and, similar to the neutrals pecies, also the conformation of protonated biomolecules. [37][38][39][40] In contrast to amino acids, no spectrali nformation is availablef or H + PEA-(H 2 O) n clusters and related microhydrated protonated aromatic ethylamino neurotransmitters. Thus, the currentw ork on monohydratedH + PEA gives insight into the first step of the formation of its solvation shell, especially regarding the competition betweenintra-and intermolecular interactions.…”