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For these experiments, EPR spectra were obtained at room temperature.
Bruker epr shifting center field free#
In the experiments to determine the second p K a of the gallate free radical 5.0 mL of 10 mM gallic acid (final concentration) was spiked with various amounts of 1 M NaOH to obtain EPR spectra of the gallate free radical at different pH-values. Spectral simulations of EPR spectra were performed using the WinSim program developed at the NIEHS by Duling. Typical EPR parameters were as follows: 3510 G center field 10 G sweep width 9.852 GHz microwave frequency 20 mW power receiver gain varied as needed modulation frequency of 100 kHz modulation amplitude of 0.10 G conversion time of 40.96 ms and time constant of 20.48 ms with 20 X-scans for each 1024 point spectrum. Spectra were obtained at room temperature (24-27☌). All solutions above neutral pH (alkaline solutions) were prepared by adjusting with 1 M sodium hydroxide solution.ĮPR spectroscopy was done using a Bruker EMX spectrometer equipped with a high-sensitivity cavity and an Aqua-X sample holder. The final concentration of gallic acid was 1 mM, unless noted otherwise. All experiments were carried out in 100 mM potassium phosphate. Gallic acid (3,4,5-trihydroxybenzoic acid, CAS No.: 149-91-7) was obtained from Sigma, USA sodium hydroxide was from Fisher. This simple and novel approach permitted the understanding of the prototropic equilibrium of the semiquinone radicals generated by gallic acid, a ubiquitous compound, allowing new insights into its oxidation and subsequent reactions. The p K a of this proton has been determined to be 10. a H = 0.28 G) at lower pH is a slowly exchanging hydron, participating in hydrogen bonding with two oxygens in ortho position on the gallate ring. Using D 2O as a solvent, we demonstrate that the third hydrogen ( i.e.
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However, in a more alkaline environment, pH >10, the hyperfine splitting pattern transforms into a 1:2:1 pattern (a H (2) = 1.07 G). This is consistent with three hydrogens providing hyperfine splitting. In the pH range between 7-10, the spectrum of the gallate free radical is a doublet of triplets (a H = 1.00 G, a H = 0.23 G, a H = 0.28 G). We found that gallic acid produces two different radicals as a function of pH. In this work electron paramagnetic spectroscopy (EPR) was used to detect the free radicals generated by the air-oxidation of gallic acid. The gallate moiety is a key component of many functional phytochemicals. Gallic acid (3,4,5-trihydroxybenzoic acid) is found in a wide variety of plants it is extensively used in tanning, ink dyes, as well as in the manufacturing of paper.