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

Abstract

The complex dielec. permittivity spectra, .vepsiln.*(ν), of aq. glycine solns. at 25 °C were detd. in the frequency range of 0.1 ≤ ν/GHz≤89 using TDR and wave-guide interferometry. The concn. range covered amino acid mole fractions of 0 ≤ X ≤ 0.05, corresponding to molar concns. of 0 ≤ c/M ≤ 2.6. For all samples, a superposition of three Debye relaxation processes was required for a consistent description of .vepsiln.*(ν). The low-frequency dispersion (j = 1) of relaxation time τ1∼40 ps, assigned to the rotational diffusion of the zwitterionic amino acid mol., exhibits a linearly increasing amplitude, Δ.vepsiln.1(c) from which an effective dipole moment of μeff = 11.9 D was deduced for the glycine zwitterion. This value agrees well with the result (11.4 D) of MOPAC calcns. for the isolated mol. using the semiempirical AM1 force field, indicating that dipole-dipole correlations among glycine mols. are negligible even in concd. solns. where direct contacts of the hydrated solute mols. are likely. This is corroborated by the anal. of the glycine relaxation times. The intermediate (j = 2; τ2 ∼8-9 ps) and the fast (j = 3; τ ∼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" mols., resp. From the solvent amplitude an effective hydration no. of Zib(0) = 4.2 was detd. for glycine at infinite diln.