Room-temperature aqueous solutions of the neurotransmitter -aminobutyric acid (GABA) and its isomer Y-aminobutyric acid (AABA) were studied with broad-band dielectric relaxation spectroscopy (DRS) and statistical mechanics (1D-RISM and 3D-RISM) calculations. Comparison of the first-shell coordination numbers from 3D-RISM with the total effective hydration numbers from DRS revealed that for both solutes only about half of the H2O molecules in direct contact with the solute are affected in their dynamics. Whereas for GABA 10 of them are moderately retarded by a factor of 2–3 compared to bulk water and only 1–2 effectively frozen at vanishing solute concentration, AABA appears to be more strongly hydrated with only 6 moderately retarded but 6 H2O frozen by the solute molecule. This is reflected in the effective solute dipole moments.
