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

The complex dielectric permittivity spectra, epsilon*(nu), of aqueous glycine solutions at 25 degreesC were determined in the frequency range of 0.1less than or equal tonu/GHzless than or equal to89 using TDR and wave-guide interferometry. The concentration range covered amino acid mole fractions of 0less than or equal toXless than or equal to0.05, corresponding to molar concentrations of 0less than or equal toc/Mless than or equal to2.6. For all samples, a superposition of three Debye relaxation processes was required for a consistent description of epsilon*(nu). The low-frequency dispersion (j=1) of relaxation time tau(1)similar to40 ps, assigned to the rotational diffusion of the zwitterionic amino acid molecule, exhibits a linearly increasing amplitude, Deltaepsilon(1)(c) from which an effective dipole moment of mu(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 AMI 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; tau(2)similar to8-9 ps) and the fast (j=3; tau(3)similar to1-2 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. (C) 2004 Published by Elsevier B.V.