The paper by Canelas and da Costa (1) introduces students to the antioxidant rosmarinic acid, and its interaction with the free radical DPPH. Those two molecules are the featured species this month. The original paper shows the 2-dimensional structure of the cis isomer of rosmarinic acid, although the trans isomer exhibits very similar antioxidant properties. Calculations at the DFT/B3LYP 631-G(d) level show that the trans isomer is more stable than the cis isomer in the gas phase, a situation that is expected to carry over into solution. Many antioxidants are phenols, and rosmarinic acid has four such groups available for radical formation. A DFT study by Cao et al. (2) examines the relative stabilities of the radicals formed from loss of each of the phenolic hydrogens. That paper focuses on the trans isomer, and a useful student project would be to repeat the calculations with the cis isomer. An HPLC separation of the isomers of rosmarinic acid has been published (3), and might well lead to an extension of the experiment described in ref 1 in which relative antioxidant efficiencies of the two isomers could be evaluated.
cis-rosmarinic acid
trans-rosmarinic acid
DPPH has been used extensively as a standard for determining antioxidant activity. An examination of the molecular orbital occupied by the lone electron shows significant delocalization, providing a partial explanation for the stability of the neutral radical. Our gas phase structure for DPPH, also at the DFT/B3LYP 631-G(d) level, is quite consistent with several crystal structures on DPPH and DPPH in the presence of another species (4).
Caniova, A.; Brandstetrova, E. J. Liquid Chrom. and Related Tech. 2001, 24, 2647–2659.
See for example Williams, D. E. J. Am. Chem. Soc. 1967, 89, 4280 and Kiers, C. T.; de Boer, J. L.; Olthof, R.; Spek, A. L. Acta Crystallogr., Sect. B: Struct. Crystallogr. Cryst. Chem. 1976, 32, 2297.
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