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Alkaline-earth cations enhance ortho-quinone-catalyzed ascorbate oxidation
Antonio E Alegría1, Pedro Sanchez-Cruz, Lilyvet Rivas
1Department of Chemistry, University of Puerto Rico at Humacao, CUH Station, Humacao, PR 00791. ae_alegria@webmail.uprh.edu
Free Radical Biology & Medicine
|October 13, 2004
Summary
Ortho-quinones, unlike para-quinones, enhance ascorbate oxidation catalyzed by magnesium and calcium salts. This occurs because ortho-semiquinones form chelates with these cations, influencing redox cycling efficiency.
Area of Science:
- Biochemistry
- Chemical Kinetics
- Coordination Chemistry
Background:
- Quinones are versatile redox-active compounds with diverse biological roles.
- Ascorbate (vitamin C) is a crucial biological antioxidant.
- Metal cations can influence redox reactions, but their interaction with quinones and ascorbate requires further elucidation.
Purpose of the Study:
- To investigate the effect of different quinone types (ortho- vs. para-) on ascorbate oxidation in the presence of alkaline-earth metal salts.
- To determine the role of metal chelation by quinone derivatives in catalyzing ascorbate oxidation.
Main Methods:
- Spectrophotometric monitoring of ascorbate oxidation kinetics.
- Varying quinone structures (1,10-phenanthroquinone, beta-lapachone, naphthazarin, 1,4-naphthoquinone).
- Addition of magnesium chloride (MgCl2) and calcium chloride (CaCl2) at constant ionic strength.
Main Results:
- Ortho-quinones (1,10-phenanthroquinone, beta-lapachone) significantly enhanced ascorbate oxidation with MgCl2 and CaCl2.
- Para-quinones (naphthazarin, 1,4-naphthoquinone) did not show this enhancing effect.
- Chelation of alkaline-earth cations was observed for ortho-semiquinones, but not para-semiquinones, correlating with catalytic activity.
Conclusions:
- The catalytic enhancement of ascorbate oxidation by ortho-quinones in the presence of MgCl2 and CaCl2 is attributed to the formation of semiquinone-metal complexes.
- Redox cycling efficiency of quinones with ascorbate is dependent on both the quinone's redox potential and its ability to chelate alkaline-earth cations.