Raman and the antioxidants
The first report of Raman spectroscopy being used to look at chemical structures in olive oil has been published. The study establishes Raman as a rapid, non-destructive and reliable analytical technique for identifying bioactive components, such as biophenols in dietary extracts and surpasses other analytical methods.
Fátima Paiva-Martins and Vera Rodrigues of the Department of Chemistry and Biochemistry at the University of Porto and Department of Pharmacy, College of Health Technology, Coimbra, together with Rita Calheiros and Maria Marques of the University of Coimbra, Portugal, explain how olive oil contains numerous phenolic components, which are thought to endow it with many of the health benefits associated with a so-called Mediterranean diet. Indeed, there is some evidence that a diet containing olive oil might reduce the risk of cardiovascular distress and cancer because of its high antioxidant content.
Phenolic moieties
The major phenolic components of olives and virgin olive oil are hydroxytyrosol, oleuropein and the oleuropein aglycones 3,4-DHPEA-EA (3,4-dihydroxyphenylethanol-elenolic acid) and 3,4-DHPEA-EDA (the dialdehyde of elenolic acid linked with 3,4-dihydroxyphenylethanol the major derivative of oleuropein). There are numerous known metabolites of varying degrees of activity. The team has now used vibrational spectroscopy (Raman) in conjunction with theoretical methods to help them fully characterize these systems of compounds in order to quickly and reliably identify food samples.
"The discovery of antioxidant activities (in vitro, ex vivo and in animal models) for the most important olive oil phenolic derivatives has stimulated intensive research on their presence in olive oils and on their bioavailability, metabolism and disposition in humans. The basis for these studies is that the debate on the potential benefits of olive oil phenolic compounds for human health require the support of clinical studies on their bioavailability," the team explains. They add that a complex metabolism of the main antioxidant phenolics would suggest that, "the potential health benefits of olive oil phenolic derivatives may also be attributed to their conjugated metabolites or to the products of their enzymatic hydrolysis at the cellular level, which display a free phenolic moiety."
Olive oil sampled
However, very few studies have studied individual components of olive or oil or their metabolites. There are, in particular, few reports on structure-activity relationships. Nevertheless understanding the health effects of olive oil will depend critically on just such understanding and specifically knowing the profile of a given dietary extract or sample being studied "clinically".
Raman spectroscopy has now proven itself especially useful in the team's structural analysis of the main olive oil polyphenolic constituents hydroxytyrosol (2-(3,4-dihydroxyphenyl)ethanol, HT) and its metabolites homovanillyl alcohol (2-(4-hydroxy-3-methoxiphenyl)ethanol, MOPET), homovanillic acid (4-hydroxy-3-methoxiphenylethanoic acid, HVA) and 3,4-dihydroxyphenylatanoic acid (DOPAC), as well as of the secoiridoid derivatives oleuropein (OP) and the oleuropein aglycones 3,4-DHPEA-EA and 3,4-DHPEA-EDA. "The main conformational preferences of these systems were obtained by coupling Raman spectroscopic data to the results yielded by density functional theoretical (DFT) methods (at the quantum mechanical level)," the team adds. The theoretical studies offered a reliable structural model that was previously lacking.
"The information yielded by this type of study will help to understand the mechanisms through which phenols act in living systems, and hopefully establish structure-activity relationships governing this cytoprotective role," the team says. These various phenolic derivatives have been linked in vitro and in vivo to a chemopreventive effect that protects against oxidative-induced processes and so may have benefits on the cardiovascular system and in reducing the risk of neurodegenerative disorders and cancer.
This first reported study using Raman spectroscopy to investigate olive oil biophenols demonstrates its potential, its speed, and the non-destructive nature of the technique. It can distinguish between chemically similar phenolic molecules and so is extensible into the metabolic domain. "It can also surpass other analytical methods currently used, once it allows the concomitant identification of several olive oil components in a particular olive oil sample," the team concludes.
The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.
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