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 degrees 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 greater than or similar to 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.