Abstract
Gallic acid (3,4,5-trihydroxybenzoic acid, GA) is one of the most abundant phenolic acids found in the plant kingdom. In this work, the electron paramagnetic resonance (EPR) spectrum of the gallate semiquinone radical tri-anion (GAS) derived from GA by air oxidation was measured and analyzed by advanced simulation procedures. The observed main spectrum was surrounded by five satellite spectra from which a thorough analysis led to determination of hyperfine splittings (HFS) from five chemically different 13C nuclei in natural abundance. The spectra were further characterized by detailed linewidth analyses. The assignment of the 13C HFS constants was supported by the results of theoretical calculations, using the classical, semi-empirical Karplus-Fraenkel approach, as well as quantum chemical procedures based on density functional theory (DFT), representing the influence of the solvent by polarizable continuum models (PCM). The combined results suggest a consistent assignment of positions and signs for all five 13C constants of GAS, providing a unique insight into the electron spin structure of this radical.
Highlights
Gallic acid (3,4,5-trihydroxybenzoic acid, GA) and many of its derivatives are widely present in numerous fruits and plants, where they represent a large family of secondary metabolites
We present the results of a detailed investigation of gallate semiquinone radical tri-anion (GAS) by electron paramagnetic resonance (EPR) spectroscopy and by theoretical calculations
The determination of the spin-density distribution in aromatic free radicals has in many studies been based on proton hyperfine splittings (HFS) constants determined by EPR
Summary
Gallic acid (3,4,5-trihydroxybenzoic acid, GA) and many of its derivatives are widely present in numerous fruits and plants, where they represent a large family of secondary metabolites. At an elevated pH, GA is oxidized by air to yield the unusually stable gallate semiquinone radical tri-anion [1,2] (GAS, Scheme 1). The spectrum was stored digitally and “best fit” parameters were obtained by an iterative optimization procedure as described earlier [12] By this procedure, the splitting constants, relative intensities, as well as different peak-to-peak linewidths for high and low field triplets for each individual spectrum were obtained (Table 1)
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