The role of ascorbate in antioxidant protection of biomembranes: Interaction with vitamin E and coenzyme Q

One of the vital roles of ascorbic acid (vitamin C) is to act as an antioxidant to protect cellular components from free radical damage. Ascorbic acid has been shown to scavenge free radicals directly in the aqueous phases of cells and the circulatory system. Ascorbic acid has also been proven to protect membrane and other hydrophobic compartments from such damage by regenerating the antioxidant form of vitamin E. In addition, reduced coenzyme Q, also a resident of hydrophobic compartments, interacts with vitamin E to regenerate its antioxidant form. The mechanism of vitamin C antioxidant function, the myriad of pathologies resulting from its clinical deficiency, and the many health benefits it provides, are reviewed.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/44796/1/10863_2004_Article_BF00762775.pd

Irreversibility analysis of cross flow heat exchangers

In this paper, a cross flow plate type heat exchanger, operating with unmixed fluids, was analyzed with balanced cross flow. For this aim, a cross flow plate type heat exchanger was developed and manufactured in the laboratory. The heat exchanger was tested with an applicable experimental set up, and temperatures, velocity of the air and the pressure losses occurring in the system were measured so that the effectiveness of the heat exchanger has been determined. The minimum entropy generation number has been taken into consideration with respect to the second law of thermodynamics for the heat exchanger in this analysis. The minimum entropy generation number depends on parameters such as optimum flow path length, dimensionless mass velocity, dimensionless heat transfer area and dimensionless heat transfer volume. The variations between the entropy generation number and these parameters were analyzed for the manufactured heat exchanger and introduced with their graphics

Second-law and experimental analysis of a cross-flow heat exchanger

This article presents a cross-flow plate-type heat exchanger that has been studied and manufactured in laboratory conditions because of its effective use in waste heat recovery systems. This new heat exchanger was tested with an applicable experimental setup, considering temperatures, velocity of the air, and the pressure losses occurring in the system. These variables were measured and the efficiency of the system was determined. The irreversibility of the heat exchanger was taken into consideration, while the design of the heat exchanger was such that the minimum entropy generation number was analyzed with respect to the second law of thermodynamics in the cross-flow heat exchanger. The minimum entropy generation number depends on the parameters called the optimum flow path length and dimensionless mass velocity. Variations of the entropy generation number with these parameters are analyzed. © 1999 Taylor & Francis Group, LLC

Antioxidant and co-antioxidant activity of vitamin C. The effect of vitamin C, either alone or in the presence of vitamin E or a water-soluble vitamin E analogue, upon the peroxidation of aqueous multilamellar phospholipid liposomes

Thermally labile azo-initiators, dissolved in either the aqueous or lipid phase, have been used to generate peroxyl radicals at a known, steady rate in an aqueous dispersion of dilinoleoylphosphatidylcholine multilamellar liposomes at 37\ub0 C in order to study the antioxidant behaviour of ascorbate itself and ascorbate in combination with either \u3b1-tocopherol or a water-soluble \u3b1-tocopherol analogue (TROLOX(-)). It is found that ascorbate is an effective inhibitor of peroxidations initiated in the aqueous phase, with each ascorbate terminating 0.6 radical chains (i.e., n = 0.6), but it is a very poor inhibitor of peroxidations initiated in the lipid phase. Peroxidations initiated in the lipid-phase in the presence of either \u3b1-tocopherol or TROLOX(-) indicate that ascorbate is an excellent synergist with both phenolic antioxidants (n = 0.4). In peroxidations initiated in the aqueous phase ascorbate acts as a co-antioxidant with TROLOX(-) (n = 0.7), but the interpretation of the approximately additive effect obtained in the presence of \u3b1-tocopherol is complicated by the fact that under the experimental conditions employed \u3b1-tocopherol alone does not give a distinct, measurable inhibition period. The latter problem is shown to be due to a non-uniform distribution of the water-soluble initiator within the liposome. Other examples of the complicating effects of non-uniform distributions of reactants in kinetic studies of the autoxidation of organic substrates dispersed in water are described. \ua9 1985.Peer reviewed: YesNRC publication: Ye

EPR spectra of some \u3b1-tocopherol model compounds. Polar and conformational effects and their relation to antioxidant activities

Peer reviewed: YesNRC publication: Ye

Hydrogen abstraction by methyl radicals in glasses

An experimental and theoretical study is reported of the abstraction of hydrogen atoms by methyl radicals from organic glasses, in particular methanol and several of its deuterated analogues. Rate constants are obtained for hydrogen and deuterium transfer as a function of temperature for a distribution of trapping sites of the radical in the glass. The site dependence is investigated by an analysis of the non-exponential decay of the methyl radical concentration based on the use of Laplace transforms. With the help of a recently developed theoretical model for hydrogen tunnelling, a relationship between tunnelling rate and tunnelling distance is established on the basis of these observations. The results yield a detailed picture of the structure of the trapping sites: roughly spherical cages in which the tumbling radical is surrounded on average by eleven rotating methyl groups, with the closest of which it reacts. The model yields a quantitative description of the rate of this reaction, expressed in terms of spectroscopic, thermodynamic and quantum-chemical input parameters.Peer reviewed: YesNRC publication: Ye