Hydration and dynamics of L-glutamate ion in aqueous solution

Aqueous solutions of sodium l-glutamate (NaGlu) in the concentration range 0 < c/M ≤ 1.90 at 25 °C were investigated by dielectric relaxation spectroscopy (DRS) and statistical mechanics (1D-RISM and 3D-RISM calculations) to study the hydration and dynamics of the l-glutamate (Glu-) anion. Although at c → 0 water molecules beyond the first hydration shell are dynamically affected, Glu- hydration is rather fragile and for c ⪆ 0.3 M apparently restricted to H2O molecules hydrogen bonding to the carboxylate groups. These hydrating dipoles are roughly parallel to the anion moment, leading to a significantly enhanced effective dipole moment of Glu-. However, l-glutamate dynamics is determined by the rotational diffusion of individual anions under hydrodynamic slip boundary conditions. Thus, the lifetime of the hydrate complexes, as well as of possibly formed [Na+Glu-]0 ionpairs and l-glutamate aggregates, cannot exceed the characteristic timescale for Glu- rotation.