Microscopic Picture of the Aqueous Solvation of Glutamic Acid

We present molecular dynamics simulations of glutamic acid and glutamate solvated in water, using both density functional theory (DFT) and the Gromos96 force field. We focus on the microscopic aspects of the solvation-particularly on the hydrogen bond structures and dynamics-and investigate the influence of the protonation state and of the simulation method. Radial distribution functions show that the hydrogen bonds are longer in the force field systems. We find that the partial charges of the solutes in the force field simulations are lower than the localized electron densities for the quantum simulations. This lower polarization decreases the hydrogen bond strength. Protonation of the carboxylate group renders glutamic acid a very strong and stable hydrogen bond donor. The donated hydrogen bond is shorter and lives longer than any of the other hydrogen bonds. The solute molecules simulated by the force field accept on average three more hydrogen bonds than their quantum counterparts. The life times of these bonds show the opposite result: the residence times are much longer (up to a factor 4) in the ab initio simulations.