Michael Antolovich

Phenolic content and antioxidant activity of olive extracts

The phenolic component of freeze-dried olive fruit was fractionated by high-performance liquid chromatography using ultraviolet, atmospheric pressure chemical ionisation (APCI) and electrospray ionisation (ESI) detection. The fractions together with several standards were tested for antioxidant activity in an aqueous and a lipid system. The negative ion mode of APCI and ESI showed less fragmentation than positive ion mode. The latter was generally more useful in obtaining fragmentation data and hence structural information. Some olive phenolics notably tyrosol exhibited a low ionisation efficiency in both APCI and ESI. There was no simple relationship between antioxidant activity and chemical structure. The ranking of antioxidant activity was strongly dependent on both the test system and on the substrate demonstrating the need to examine activity in both aqueous and lipid systems. Significant antioxidant activity was seen in most olive fractions and this was related to phenolic content. The kinetics of the oxidation process are complex and suggest that multiple pathways may be involved at different antioxidant concentrations.

Methods for testing antioxidant activity

Antioxidant activity has been assessed in many ways. The limitation of many newer methods is the frequent lack of an actual substrate in the procedure. The combination of all approaches with the many test methods available explains the large variety of ways in which results of antioxidant testing are reported. The measurement of antioxidant activities, especially of antioxidants that are mixtures, multifunctional or are acting in complex multiphase systems, cannot be evaluated satisfactorily by a simple antioxidant test without due regard to the many variables influencing the results. Several test procedures may be required to evaluate such antioxidant activities. A general method of reporting antioxidant activity independent of the test procedure is proposed.

Sample preparation in the determination of phenolic compounds in fruits

Phenolic compounds occur as secondary metabolites in all plants.1 They embrace a considerable range of substances possessing an aromatic ring bearing one or more hydroxy substituents, although a more precise definition is based on metabolic origin as those substances derived from the shikimate pathway and phenylpropanoid metabolism.2 A convenient classification of the plant phenols distinguishes the number of constitutive carbon atoms in conjunction with the structure of the basic phenolic skeleton (Table 1). The range of known phenolics is thus vast and also includes polymeric lignins and condensed tannins. Some plant phenols may be involved in primary metabolism whereas others have an effect on plant growth or protect the more vulnerable cell constituents against photooxidation by ultraviolet light by virtue of their strong UV absorption.3 Plant phenols also play an important role in disease resistance in the plant. Intense interest in fruit phenolics is also related to their physiological activity which depends on their antioxidant activity, the ability to scavenge both active oxygen species and electrophiles, the ability to inhibit nitrosation and to chelate metal ions, the potential for autooxidation and the capability to modulate certain cellular enzyme activities.4–7 Thus, knowledge of the levels of these compounds in plants is of considerable interest but is limited by problems of analysis. The structural diversity of the phenolics and its effect on physicochemical behaviour such as solubility and analyte recovery presents a challenging analytical problem. Moreover, a number of phenolic compounds are easily hydrolysed and many are relatively easily oxidized, which further complicates sample handling.8,9 This review emphasises the importance of sample preparation in the determination of phenolic compounds in plant materials particularly fruits. Fruits are an important dietary source of phenolic substances although interest is also shifting to other parts of the plant as potential commercial sources of phenols. Sample preparation is a critical step in analysis and this is even more significant with real samples where the matrix components are biologically active and the analytes represent a diverse spectrum of numerous compounds, many having an unknown identity. Thus, methods of extraction of phenols from fruits are generally dependent on several factors while the usual quantification procedures involve the separation sciences and are universally applicable. Soleas et al.10 illustrated this point. They developed a derivatization procedure for determination of 15 phenolic constituents in solid vitaceous plant materials and concluded that the method ‘should be suitable to measure polyphenols in fruit, vegetables, and other foods provided that efficient extraction techniques are employed’. Such statements are seen frequently in the analytical literature but they tend to belittle the importance of this step (or perhaps they serve to underline its critical importance). Rhodes and Price11 observed that the determination of phenolic species in foods is an important outstanding problem and reviewed methods for the extraction and purification of phenolic antioxidants as the conjugated forms that exist in plant foods. Knowledge of the extraction of phenolics is also desirable outside the analytical context for it has important practical applications in the food industry. For instance, the mechanism and kinetics of phenolic extraction from wood to wine during ageing in barrels12 has significant consequences for the production of quality wines.

Biotransformations of phenolic compounds in Olea europaea L.

Phenolic compounds are a diverse range of secondary metabolites derived from the shikimate pathway and phenylpropanoid metabolism. Olea europaea L. contains a number of unusual phenolics including various oleosides. The amounts and types of phenolics vary markedly between leaf, fruit, stone, and seed. The metabolic relationships between the various parts and phenolic content are poorly understood. Interest in this area is related to the importance of the phenolic profile to the aesthetics and quality of olive products, and to the use of olive leaves in phytomedicines.

Liquid chromatography with electrospray ionisation mass spectrometric detection of phenolic compounds from Olea europaea

The results demonstrate the potential of electrospray ionisation mass spectrometry for the specific detection of phenolic species in olives. Phenolic compounds were detected with greater sensitivity in the negative ion mode, but results from positive and negative ion modes were complementary with the positive ion mode showing structurally significant fragments. This is demonstrated by the identification of oleuropein and isomers of verbascoside. The structure of the latter were confirmed by retention, mass spectral and nuclear magnetic resonance data. These isomers have not previously been reported in olive.

Characterisation of citrus by chromatographic analysis of flavonoids

Flavonoid glycosides in citrus were characterised by high-performance liquid chromatography using both ultraviolet and fluorescence detection. The effects of sample preparation on the chromatographic profiles are reported. Key variables in the profiles useful as chemotaxonomic markers were identified with the aid of pattern recognition, which was also used to create sample categories. LC–MS data are presented and the advantages of mass spectrometric detection are demonstrated.

Varietal and processing effects on the volatile profile of Australian olive oils

The volatile profile of virgin olive oils was established using SPME and gas chromatography(-mass spectrometry). The major volatile in approximately 50% of the oils was E-hex-2-enal in contrast with European oils. The minor contribution of C5 compounds to the volatile profiles also contrasted with data on European oils. Hierarchical Cluster Analysis (HCA) implicates variety as the single-most important factor in determining volatile profile whilst malaxation time and temperature exerted a minor secondary effect on the volatile profile.

Comparative molecular mechanics study of the low-spin nickel(II) complexes of an extended series of tetraaza macrocycles

Molecular mechanics calculations of metal complex structures are often not straight forward because of uncertainties concerning the appropriate force-field parameters for structural elements involving the metal. A systematic study aimed at extending the Allinger MM2 force field for use with N 4 -macrocyclic complexes of low-spin Ni(II) is reported. The application of the extended force field to the re-examination of the configurational isomers of [Ni(cyclam)] 2+ and their N,N,N,N-tetramethylated derivatives is reported

Applications of mass spectrometry to plant phenols

Abstract Plant phenols embrace a considerable range of compounds and are defined as those substances derived from the shikimate acid pathway and phenylpropanoid metabolism. The present article examines the application of mass spectrometry to the analysis of these compounds and traces the chronological development of analyte ionisation methods.

The Role of the Peripheral Benzodiazepine Receptor in the Apoptotic Response to Photodynamic Therapy

Abstract Several previous studies have suggested that the peripheral benzodiazepine receptor (PBR) on the mitochondrial surface was an important target for photodynamic therapy (PDT). In this study we compared PBR affinity vs photodynamic efficacy of protoporphyrin-IX (PP-IX) and two structural analogs, PP-III and PP-XIII, using murine leukemia L1210 cells in culture. The results indicate that the three agents have approximately equal hydrophobicity, affinity for L1210 cells and ability to initiate photodamage leading to an apoptotic response. But only PP-IX had significant affinity for the PBR. These data indicate that the relationship between PDT efficacy and PBR affinity may hold only for sensitizers with the PP-IX configuration.