On the issue of closed versus open forms of gamma-aminobutyric acid (GABA) in water: Ab initio molecular dynamics and metadynamics studies

DFT investigation on the structural and vibrational behaviours of the non-protein amino acids in hybrid explicit/continuum solvent: a case of the zwitterions γ-aminobutyric and α - aminoisobutyric acids

BACKGROUND

The influence of hybrid solvation models on the molecular structures and vibrational characteristics of g-aminobutyric acid (GABA) and a-aminoisobutyric acid (AIB) zwitterions was assessed by employing a variety of Density Functional Theory (DFT). The quantum chemical methods included the B3LYP and B3PW91 hybrid functionals and the 6‑311++G(d,p) basis set.

METHODS

The most stable conformation derived from the potential energy surface (PES) scans using the B3LYP/6-311++G(d,p) model chemistry for each studied molecule was predicted within a continuum environment represented by the COSMO and SMD solvation models. The stable structures were subsequently immersed in explicit/COSMO and explicit/SMD hybrid solvation models, where 10 and 8 water molecules were explicitly positioned around the functional groups of the GABA and AIB zwitterions, respectively. The number of water molecules chosen was sufficient to prevent proton transfer among the carboxylate group (COO-) and the ammonium group (NH3+) within each molecule under investigation. After optimizing the geometry of each hydrated complex, the normal vibrational modes were determined. The scaled theoretical frequencies obtained from the various model chemistries were then compared to available experimental data from infrared (IR) and Raman spectroscopy.

RESULTS

In the case of GABA and AIB molecules, the comparisons revealed that the B3LYP/6-311++G(d,p) model chemistry yielded wavenumber values that closely matched the experimental IR and Raman data, particularly when the explicit/SMD solvent was employed. The computed results indicate deviations of less than 4% when compared to the experimental data for the two molecules under investigation.

Temperature dependence of the microwave dielectric properties of [Formula: see text]-aminobutyric acid

The GABA molecule is the major inhibitory neurotransmitter in the mammalian central nervous system. Through binding to post-synaptic neurons, GABA reduces the neuronal excitability by hyperpolarization. Correct binding between the GABA molecules and its receptors relies on molecular recognition. Earlier studies suggest that recognition is determined by the geometries of the molecule and its receptor. We employed dielectric relaxation spectroscopy (DRS) to study the conformation and dielectric properties of the GABA molecule under physiologically relevant laboratory conditions. The dielectric properties of GABA investigated have given us new insights about the GABA molecule, such as how they interact with each other and with water molecules at different temperatures (22°C and 37.5°C). Higher temperature leads to lower viscosity, thus lower relaxation time. The change in the GABA relaxation time due to concentration change is more associated with the solution viscosity than with the GABA dipole moment. A resonance behavior was observed with high GABA concentrations at physiological temperature, where there might be a phase transition at a certain temperature for a given GABA concentration that leads to a sudden change of the dielectric properties.