Density functional theory investigation into the interaction of deep eutectic solvents with amino acids

Processing of amino acids (AAs) is a very cost-intensive process. Recently, Li et al. used supported liquid membranes (SLMs) based on deep eutectic solvents (DESs) for amino-acid extraction and found that [choline chloride][p-Toluenesulfonic acid] aChCl][p-TSA])) DES was efficiently separating Tryptophan (Trp (center dot)) amino acid. Inspired by the study, here we investigate the interaction of [ChCl] [p-TSA] DES with three forms of amino acids, including neutral(center dot), cationic(+), and anionic(-) forms using density functional theory (DFT) method at the M06-2X/6-311++G(d,p) level of theory. Our results indicate that the interaction of AAs(center dot/+/-) with [ChCl] [p-TSA] DES is governed by the hydrogen bonding interactions, which is consistent with the findings of H-1 NMR spectra by Li et al. Thermochemistry calculations indicate that the formation of [ChCl] [p-TSA]...AAs(center dot/+/-) complexes is an exothermic and favorable reaction and proceeds spontaneously. Among the three forms of AAs(center dot/+/-), cationic forms (AAs(+)) exhibited the most tendency to interact with the [ChCl][p-TSA] DES. The binding energy (Delta E-b) calculations show that Trp(center dot) has the strongest interaction with [ChCl][p-TSA] DES among the AAs(center dot), in agreement with the experimental results reported in the literature. The Gly(+) and Val(-) also have the highest Delta E-b value with [ChCl][pTSA] DES. A comparison between the calculated free energies and the extraction efficiency (%) reported by Li et al. for extraction of AAs(center dot) by [ChCl] [p-TSA] DES indicates that the higher the free energy value, the higher is the extraction efficiency (%) of neutral AAs(center dot). Furthermore, the natural bond orbital (NBO) analysis, atoms in molecules (AIM) theory, and noncovalent interaction (NCI) plots were performed to determine the nature and strength of hydrogen bonding interactions between the [ChCl][p-TSA] DES and AAs(center dot/+/-). These analyses indicate that the O-H..D, O-H...[Cl] , and N-H...[Cl] interactions in the complexes are stronger than the N-H...O and C-H...O interactions. Energy decomposition analysis (EDA) method revealed that the charged AAs(+/-) have a greater tendency to interact with [ChCl][p-TSA] DES due to the presence of positive and negative charges on the AAs(+/-), which leads to increased strength in the electrostatic interactions between the AAs(+/-) and [ChCl][p-TSA] DES. (C) 2021 Elsevier B.V. All rights reserved.

Automated summary

The study on amino acid extraction using deep eutectic solvents revealed strong hydrogen bonding interactions, mainly influenced by the charges present. AAs with positive and negative charges showed a greater tendency to interact with the deep eutectic solvent, indicating increased strength in electrostatic interactions.