A catechin analogue in which the geometry was constrained to be planar was synthesized. The planar catechin showed excellent radical-scavenging ability, comparable to that of quercetin, and efficient protection against DNA strand breakage induced by the Fenton reaction.

A kinetic study of a hydrogen-transfer reaction from (+)-catechin (1) to galvinoxyl radical (G•) has been performed using UV−vis spectroscopy in the presence of Mg(ClO4)2 in deaerated acetonitrile (MeCN). The rate constants of hydrogen transfer from 1 to G• determined from the decay of the absorbance at 428 nm due to G• increase significantly with an increase in the concentration of Mg2+. The kinetics of hydrogen transfer from 1 to cumylperoxyl radical has also been examined in propionitrile (EtCN) at low temperature with use of ESR. The decay rate of cumylperoxyl radical in the presence of 1 was also accelerated by the presence of scandium triflate [Sc(OTf)3 (OTf = OSO2CF3)]. These results indicate that the hydrogen-transfer reaction of (+)-catechin proceeds via electron transfer from 1 to oxyl radicals followed by proton transfer rather than via a one-step hydrogen atom transfer. The coordination of metal ions to the one-electron reduced anions may stabilize the product, resulting in the acceleration of electron transfer.

No abstract

Resveratrol (3,4′,5-trihydroxy-trans-stilbene) efficiently scavenges an oxygen radical via an electron transfer from resveratrol to the radical in deaerated acetonitrile, which is significantly accelerated by the presence of magnesium ion.

A planar catechin analogue (1H2), in which catechol and chroman moieties in (+)-catechin are constrained to be coplanar, is an efficient radical scavenger compared to the native catechin, and are nearly as effective as quercetin, a strong radical scavenger. The dianion (1(2-)) of 1H2 produced by the reaction of 1H2 with 2 equiv of tetramethylammonium methoxide reduced molecular oxygen (O2) to generate superoxide anion (O2*-). The resulting radical anion (1*-) from 1H2 underwent intramolecular proton transfer to give an o-semiquinone radical anion form of 1*-, which shows a characteristic ESR spectrum with g value of 2.0048. Although the same mechanism has also been shown for (+)-catechin, the rate constant of electron transfer (ket) from 1(2-) to O2 is about a half of that reported for (+)-catechin, indicating that the electron transfer from 1(2-) to O2 is slower than that from (+)-catechin dianion to O2. Together with efficient protection against DNA strand breakage induced by the Fenton reaction, the small ket value for 1H2 implies that, in physiologically relevant systems, there is less of a possibility of generating oxygen radicals responsible for prooxidant activity with 1H2 than that with (+)-catechin. The strong radical scavenging ability and less-efficient generation of O2*- suggest that the planar catechin analogue may be useful for the prevention and/or treatment of free-radical-associated diseases.

Electron-transfer reduction of molecular oxygen (O2) by the phenolate anion (1-) of a vitamin E model, 2,2,5,7,8-pentamethylchroman-6-ol (1H), occurred to produce superoxide anion, which could be directly detected by a low-temperature EPR measurement. The rate of electron transfer from 1- to O2 was relatively slow, since this process is energetically unfavourable. The one-electron oxidation potential of 1- determined by cyclic voltammetric measurements is sufficiently negative to reduce 2,2-bis(4-tert-octylphenyl)-1-picrylhydrazyl radical (DOPPH*) to the corresponding one-electron reduced anion, DOPPH-, suggesting that 1- can also act as an efficient radical scavenger.

Superoxide anion (O2•−) was generated via an electron-transfer oxidation of catechin dianion, which was produced in the reaction of catechin with two equivalents of methoxide anion, by molecular oxygen in acetonitrile. From the detailed spectroscopic and kinetic analyses was determined the rate constant for the formation of O2•− to be 5.8 × 10−2 mol−1 dm3 s−1.

No abstract