The structure and diffusion behaviour of the neurotransmitter γ-aminobutyric acid (GABA) in neutral aqueous solutions

GABA (γ-aminobutyric acid) is a non-protein amino acid with important physiological properties, and with considerable relevance to the food and pharmaceutical industries. Particular interest has focused on its role as an inhibitory neurotransmitter in the mammalian cerebral cortex. In this paper, we report density and mutual diffusion coefficients of GABA in non-buffered aqueous solutions (0.001-0.100) mol·dm−3 at 298.15 K. Under these conditions, 1H and 13C NMR spectroscopy and pH measurements show that it is present predominantly as a monomeric zwitterionic species. Diffusion coefficients have been computed assuming that this behaves as the binary system GABA/water. From density and intermolecular diffusion coefficients measurements, the molar volume, hydrodynamic radii, Rh, diffusion coefficients at infinitesimal concentration, D0, activity coefficients and the thermodynamic factors, FT, have been estimated. Within experimental error, the hydrodynamic volume calculated from this is identical to the molar volume obtained from density measurements. From the NMR spectra and literature data, it is suggested that this amino acid diffuses in aqueous solution as a curved, coil-like hydrated zwitterionic entity.