Interactions between plant cell wall materials and natural antioxidants
Abstract (Summary)Restricted Item. Print thesis available in the University of Auckland Library or available through Inter-Library Loan. The interactions between two dietary antioxidants (ascorbic acid and quercetin) and cell walls isolated from Pacific Rose apples (Malus domestica Borkh. cv. ‘Pacific Rose’) and onions (Allium cepa L. red skinned variety) were examined in detail. Do plant cell walls materials influence the activity of the antioxidants? Parenchyma cell walls isolated from apples were used either fresh or after freeze-drying. A portion of the fresh apple cell walls was then extracted sequentially with CDTA, sodium carbonate, 1 M KOH and 4 M KOH, to give polysaccharides such as pectic polysaccharides, xyloglucans and cellulose. Parenchyma cell walls were isolated from onions before and after steam or microwave cooking, and used either fresh or after freeze-drying. The chemical analysis of the isolated cell walls showed that although apple and onion cell walls both contained the same seven neutral monosaccharides (Ara, Gal, Glc, Fuc, Man, Rha and Xyl) and uronic acid, the relative proportions of these monosaccharides differed between the two species. The results of brightfield microscopy and cryo-SEM revealed that apple or onion cells were completely broken during the cell wall isolation. There are differences in the morphology of the cells between the two species, with onion cells being larger than the apple cells. Cells in the onion tissues were more densely packed with few intercellular spaces. In contrast, the apple tissue contained more loosely packed cells allowing greater observation of the external surfaces of the cells. The interactions of cell wall materials and dietary antioxidants (L-ascorbic acid or quercetin) were investigated. Apple cell walls and their cell wall fractions, cooked or raw onion cell walls, commercial pectin, polygalacturonic acid and sugars were incubated with L-ascorbic acid or quercetin for various times (2-24 h) at 37°C under a range of pH values (5.5-7.5). The supernatants of the incubation mixtures were analysed by ferric reducing antioxidant power (FRAP) assay, HPLC/LC-MS, cyclic voltammetry (CV) and solution 13C NMR spectroscopy, whereas, the solid pellets were investigated by solid-state CP/MAS 13C NMR spectroscopy. Results of FRAP assays showed that the antioxidant activity of ascorbic acid increased significantly, whereas that of quercetin decreased slightly, after the incubation with apple or onion cell walls. Reasons for these changes in activity were established by HPLC/LC-MS and cyclic voltammetry techniques. Onion cell walls stabilised the ascorbic acid content very effectively and apple cell walls to a lesser extent, but neither had protective effect on quercetin degradation. The results from NMR spectroscopy suggested that both the 2 h-incubation (pH 6.5, 37°C) and the presence of antioxidants could affect the extractability and mobility of polysaccharides (e.g. pectic polysaccharides). These effects might be due to direct interactions between antioxidants and cell wall monosaccharide residues on polysaccharides, or other indirect effects (e.g. cell wall materials exerted their effects on antioxidants via interactions with the inhibitor or accelerator of antioxidant activity). There was no detectable evidence to show that AA, quercetin or their degradation products bound to the cell walls. The interactions between cell walls and antioxidants probably occurred in a non-binding and/or in a liquid state. It was concluded that isolated apple and onion cell walls interact with antioxidants. The effect of this interaction may be beneficial or detrimental, depending on the antioxidants and the composition of cell wall materials. Results obtained bring a new stage of understanding to the interactions between plant cell wall materials and natural antioxidants at a molecular level, and indicate that a combination of ascorbic acid and plant cell wall materials helps to retain the antioxidant activity of ascorbic acid. Furthermore, the ability of CV and solid-state 13C NMR to provide information about the interactions between cell wall materials and dietary antioxidants is demonstrated for the first time. These interactions were reflected, not only in the changes of antioxidant activity (FRAP assay and CV), but also in the changes of the mobility of polysaccharides by antioxidants (solid-state 13C NMR).
School Location:New Zealand
Source Type:Master's Thesis
Date of Publication:01/01/2004