Dielectric relaxation spectroscopy of
aqueous amino acid solutions: dynamics and interactions in aqueous glycine
Takaaki Satoa, Richard Buchner b, Šarka
Fernandezb,Akio Chibac and Werner
Kunzb
J. Mol. Liq 117 (2005) 93-98.
a Division of Pure and Applied Physics, Graduate School of
Science and Engineering,
Waseda University, Okubo, Shinjuku-ku, Tokyo 169-8555,Japan.
b Institut für Physikalische und Theoretische Chemie,
Universität Regensburg, D-93040 Regensburg, Germany
c Department of Applied Physics, School of Science and
Engineering, Waseda University,
Universität Okubo, Shinjuku-ku, Tokyo 169-8555,
Japan
Abstract
The complex dielectric permittivity spectra, e*(n), of aqueous
glycine solutions at 25°C were determined in the frequency range of 0.1
<=n/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 e*(n). The
low-frequency dispersion (j=1) of relaxation time t1 ~40 ps, assigned to the rotational diffusion
of the zwitterionic amino acid molecule, exhibits a linearly increasing
amplitude, De1(c) from which an effective dipole moment of
meff 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 semi-empirical 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; t2~8-9ps) and the fast (j=3;
t3~1-2ps) 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 Zib (0)=4.2
was determined for glycine at infinite dilution. |
