Cation-Binding of Glutamate in Aqueous Solution

Interactions of the cations Li⁺, Na⁺, Mg²⁺, and Ca²⁺ with L-glutamate (Glu^-) in aqueous solution were studied at room temperature with dielectric relaxation spectroscopy in the gigahertz region. Spectra of ∼0.4 M NaGlu with added LiCl, NaCl, MgCl₂, or CaCl₂ (c(MCl) ≤ 1.5 M) were evaluated and experiments supplemented by density functional theory and 3D reference interaction site model (3D-RISM) calculations. In addition to the modes found for aqueous NaGlu, namely, the reorientation of free Glu^- ions (peaking at ∼1.6 GHz), of moderately retarded H₂O molecules hydrating the carboxylate moieties of Glu^- (∼8.4 GHz), of the cooperative resettling of the H-bond network of bulk water (∼20 GHz), and its preceding fast H-bond flip (∼400 GHz), an additional low-frequency relaxation at ∼0.4 GHz was detected upon the addition of the four salts. In the case of NaGlu + MgCl₂(aq) and NaGlu + CaCl₂(aq), this mode could be unequivocally assigned to an ion pair formed by the cation and the side-chain carboxylate moiety of Glu^-. For NaGlu + LiCl(aq), either this species or a backbone-[Li⁺-H₂O-Cl^--Glu^-] triple ion is formed. Binding constants increase in the order Li⁺2+2+. For NaGlu + NaCl(aq), an assignment of the ∼0.4 GHz mode to ion pairs or triples was not plausible. Accordingly, its origin remains speculative here.