25th International Meeting on Organic Geochemistry IMOG 2011

P-388
ESR spectroscopic study of humic acid-like materials obtained
by the autoxidation of caffeic and chlorogenic acid
Ruņica Nikoliš 1 , Tatjana AnŤelkoviš 1 , Goran Nikoliš 2
1 University of Niš, Faculty of Sciences, Department of Chemistry, Niš, Serbia, 2 University of Niš, Faculty of
Medicine, Department of Chemistry, Niš, Serbia (corresponding author:ruzicanf@yahoo.com)
Electron spin resonance (ESR) spectroscopy is
widely used technique for the investigation of both
natural humic acids and humic acid-like materials
obtained from various phenolic compounds [1,2].
Beside the characterization of organic radicals it is
also possible by using ESR to study interactions of
such samples with paramagnetic metal ions some of
which (eg. Fe 3+ , Cu 2+ , Mn 2+ ) are ubiquitous in natural
samples [3].
As far as organic radicals in humic acids and humic
acid-like materials are concerned the researchers
mainly reported ESR spectra consisting of one broad
symmetrical line with no hyperfine structure for both
solid samples and samples in solutions. These
organic radicals thus were characterized by the
linewidth of ESR signal and its g-value, and it was
possible to determine the free radical concentration in
solid samples. Only in few cases ESR spectra with
some hyperfine structure were obtained [1], but the
reason for that might be that modulation amplitudes
used for recording most of the ESR spectra reported
in the literature were to high (usually values between
0.1 and 0.2 mT were reported).
In this work we recorded ESR spectra of humic
acid-like materials obtained by the prolonged
autoxidation of 20 mM solutions of caffeic or
chlorogenic acid at pH 8.4 (Tris buffer) in the
presence of an excess of Mg ions. ESR
measurements were carried out in a quartz flat cell
using an X-band Bruker ESP-300 spectrometer by
employing microwave power of 2.0 mW and
modulation amplitude of 0.01 mT. The spectra
obtained immediately after starting the autoxidation
reaction displayed well resolved hyperfine structure
which enabled characterization of initially formed spinstabilized
ortho semiquinone radicals. After prolonged
autoxidation solutions became turbid and
corresponding ESR spectra lost some, but not all, of
its hyperfine structure. The ESR spectrum of system
based on caffeic acid is shown in Fig. 1. and its
appearance is very similar to the ESR spectrum of the
unfractionated Tsubame humic acid in solution [1].
For the chlorogenic acid system under the same
conditions we obtained similar ESR spectrum, but
with much more noise and less resolved hyperfine
structure. In some earlier experiments after prolonged
autoxidation of rutin under the same conditions we
obtained ESR spectra consisting of one
unsymmetrical line but without hyperfine structure [4].
Fig. 1. ESR spectrum of humic acid-like material
obtained by the 7 day autooxidation of caffeic acid in
the presence of Mg ions at pH 8.4 in aqueous
solution.
Our results suggest that ESR spectra of humic
acids and humic acid-like materials should be
recorded with the use of small enough values of
modulation amplitude in order to see whether some
hyperfine structure is present. This may help better
characterization of such samples.
References
[1] Watanabe, A., McPhail, D.B., Maie, N.,
Kawasaki, S., Anderson, H.A., Cheshire, M.V.
(2005) Org. Geochem. 36, 981-990.
[2] Giannakopoulos, E., Drosos, M., Deligiannakis,
Y. (2009) J. Coll. Int. Sci. 336, 59-66.
[3] Barriquello, M.F., da Costa Saab, S., Filho,
N.C.,Martin-Neto, L. (2010) J. Braz. Chem. Soc.
21, 2302-2307.
[4] Nikoliš, G.M., Nikoliš, R.S., Miliš, B.Lj.,
Ţanadanoviš-Brunet, J.M. (2000) Proceedings of
the 5th <strong>International</strong> Conference on Fundamental
and Applied Aspects of Physical Chemistry,
Belgrade, Yugoslavia, 123-125.
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