Diffusion and structural behaviour of the dl-2-aminobutyric acid

Abstract

dl-2-Aminobutyric acid (α-aminobutyric acid or AABA) is a non-protein amino acid with important physiological properties, and with considerable relevance in different areas, such as fundamental research and pharmaceutical industry. In this paper, the binary mutual diffusion coefficients of AABA in non-buffered aqueous solutions (0.001–0.100) mol·dm−3 at 298.15 K were measured. From these diffusion data, limiting values at infinitesimal concentration, D0, as well as those of the thermodynamic factors, FT, and activity coefficients, γ, were estimated. These data for AABA were compared with the data previously obtained for dl-4-aminobutyric acid (GABA). The difference of the behaviour of these transport properties for both amino-acids was explained based on the structural differences for the monomeric zwitterionic species of AABA and GABA. Such structural differences are supported by both 1H and 13C NMR spectroscopy and theoretical calculations, which indicate that the predominant species of AABA in these solutions is the extended zwitterionic form in contrast with the curled one of GABA.