Dielectric relaxation spectroscopy of aqueous amino acid solutions: dynamics and interactions in aqueous glycine

Abstract

The complex dielectric permittivity spectra, ɛ*( ν), of aqueous glycine solutions at 25 °C were determined in the frequency range of 0.1≤ ν/GHz≤89 using TDR and wave-guide interferometry. The concentration range covered amino acid mole fractions of 0≤ X≤0.05, corresponding to molar concentrations of 0≤ c/M≤2.6. For all samples, a superposition of three Debye relaxation processes was required for a consistent description of ɛ*( ν). The low-frequency dispersion ( j=1) of relaxation time τ 1∼40 ps, assigned to the rotational diffusion of the zwitterionic amino acid molecule, exhibits a linearly increasing amplitude, Δ ɛ 1( c) from which an effective dipole moment of μ eff=11.9 D was deduced for the glycine zwitterion. This value agrees well with the result (11.4 D) of MOPAC calculations for the isolated molecule using the semiempirical AM1 force field, indicating that dipole–dipole correlations among glycine molecules are negligible even in concentrated solutions where direct contacts of the hydrated solute molecules are likely. This is corroborated by the analysis of the glycine relaxation times. The intermediate ( j=2; τ 2∼8–9 ps) and the fast ( j=3; τ 3∼1–2 ps) dispersions originate from the contribution of water, assigned to the cooperative dynamics of bulk water and a fast localized motion of “free water” molecules, respectively. From the solvent amplitude an effective hydration number of Z ib(0)=4.2 was determined for glycine at infinite dilution.