Glycosylation of resveratrol protects it from enzymic oxidation.

Plant polyphenols, including dietary polyphenols such as resveratrol, are important components in the plant antioxidant and defence systems. They are also known to exert beneficial effects on human health through diet. As they are produced, these polyphenols may be subjected to deleterious enzymic oxidation by the plant polyphenol oxidases. They are generally synthesized as glycosides like 5,4'-dihydroxystilbene-3-O-beta-D-glucopyranoside, the 3-glucoside of resveratrol. The effects of the glycosylation and methylation of the parent resveratrol on its enzymic oxidation were studied. Methyl and glucosyl derivatives were synthesized using simple one-step methodologies. The kinetics of their enzymic oxidation by tyrosinases were defined. Substitution at the p-hydroxy group, by either glucose or methyl, abolished enzymic oxidation by both mushroom and grape tyrosinases. Substitution at the m-hydroxy group with methyl had a small effect, but substitution with glucose resulted in a much lower affinity of the enzymes for the glycoside. We suggest that glycosylation of polyphenols in nature helps to protect these vital molecules from enzymic oxidation, extending their half-life in the cell and maintaining their beneficial antioxidant capacity and biological properties

Water oxidation at low potential exploiting a nitroxide/oxoammonium ion redox couple as mediator

The oxoammonium cation (OAC) of 3-carboxy proxyl, a nitroxide radical (NitR), could be produced either by chemical or by electrochemical oxidation (0.8–1.0 V vs. NHE) of the radical. We have determined that in dilute aqueous basic solutions (pH Z 9), OAC is reduced quantitatively to the original radical with con- comitant formation of molecular oxygen in a ratio ca. 4 : 1 (4 moles of OAC reduced per 1 mole of O_2), and the redox cycle can be repeated. The low electrolysis potential (0.8 V) contrasts with the high redox potential of the bare OH anion (2–2.6 V vs. NHE for the first outer-sphere electron transfer). This appar- ent thermodynamic paradox was solved by a careful study of its possible mechanism. In our opinion, OAC/NitR’s may represent a new class of redox mediators for a novel approach to water oxidation (and generation of molecular oxygen) at a low potential

Nitroxyl Radicals for Studying Electron Transfer

Electron-transfer (ET) rates are measured by use of time-resolved EPR spectroscopy, involving photooxidation of nitroxyl radicals by a ruthenium bipyridyl complex. This permits acquisition of the fundamental characteristics of ET in solution. The method was used on two spin-labeled derivatives of bacteriorhodopsin, and is applicable to proteins, nucleic acids, and biological membranes

Systematic Quantification of Electron Transfer in a Bare Phospholipid Membrane using Nitroxide-Labelled Stearic Acids: Distance Dependence, Kinetics, and Activation Parameters

In this report, we present a method to characterize the kinetics of electron transfer across the bilayer of a unilamellar liposome composed of 1,2-dimyristoyl-sn-glycero-3-phosphocholine. The method utilizes synthetic phospholipids containing non-invasive nitroxide spin labels having the >N-O\u2022 moiety at well-defined distances from the outer surface of the liposome to serve as reporters for their local environment and, at the same time, permit measurement of the kinetics of electron transfer. We used 5-doxyl and 16-doxyl stearic acids. The paramagnetic >N-O\u2022 moiety is photo-oxidized to the corresponding diamagnetic oxoammonium cation (OAC) by a ruthenium electron acceptor formed in the solution. Electron transfer is monitored by three independent spectroscopic methods: by both steady-state and time-resolved electron paramagnetic resonance, and by optical spectroscopy. These techniques allowed us to differentiate between the electron transfer rates of nitroxides located in the outer leaflet of the phospholipid bilayer and of those located in the inner leaflet. Measurement of electron transfer rates as a function of temperature revealed a low-activation barrier ( 06G\u2021 ~ 40 kJ/mol) that supports a tunneling mechanism