Pheophytin degradation

Not-senescent and fresh-cut plants are almost devoid of degradation products like pheophytins and pheophorbides because chlorophylls associated with caro-... 

Studies of chlorophyll degradation in heated broccoli juices over the 80 to 120°C range revealed that chlorophylls degrade first to their respective pheophytins and then to other degradation products in what can therefore be described as a two-step process. Both chlorophyll and pheophytin conversions followed a first-order kinetics, but chlorophyll a was more heat sensitive and degraded at a rate approximately twice that of chlorophyll This feature had been observed by other authors. Temperature dependence of the degradation rate could adequately be described by the Arrhenius equation. ... 

Traditionally, dried or powdered plant material is used and extracts can be obtained by mixing the material with food-grade solvents like dichloromethane or acetone followed by washing, concentration, and solvent removal. The result is an oily product that may contain variable amounts of pheophytins and other chlorophyll degradation compounds usually accompanied by lipid-soluble substances like carotenoids (mainly lutein), carotenes, fats, waxes, and phospholipids, depending on the raw material and extraction techniques employed. This product is usually marketed as pheophytin after standardization with vegetable oils. 

Despite being a fast and relatively low-cost method, thin layer chromatography shows low resolution as it involves a large surface in contact with air, promoting photoxidation. Acidity of silica gels should be neutralized because it may trigger chlorophyll degradation by pheophytinization. ... 

As pheophytin a and pheophytin b are the major degradation derivatives formed during extraction, food processing, and storage, some authors reconunend converting chlorophylls into the more stable pheophytins by treatment with HCl, ion exchange resin, or oxalic acid to estimate the chlorophyll contents. ... 

Usually, HPLC analysis resolves four peaks identified by co-chromatography with authentic standards as copper pheophorbide a, Cn(II) chlorin e6, Cn(II) chlorin e4, Cu rhodin g7, and their degradation products, but a sum of other colored components can also be found, for example, native chlorophylls, pheophytins, pheophor-bides, and rodochlorins (free carboxyl forms of pheophorbides) besides epimers, allomers, and degradation products that have been only tentatively identified. 

Fig. 2.132. Chromatogram of spinach, stored frozen until analysis by HPLC (A) and after acidifying the same pigment extract with 0.2ml M HC1 per 10 ml extract and exposure to air and light for 15 h at 20°C (B). Zinc-phtalocyanine was used as an internal standard (IS). Peak identification 1 = chlorophyll-b 2 = chlorophyll-a x = unknown degradation product 3 = IS 4 = pheophytin-b 5 = pheophytin-a 6 = chlorophyll-b 7 = chlorophyll-a 8 = pheophytin-b 9 = pheophytin-a. Reprinted with permission from T. Bohn et al. [303].

Resuspension of bottom sediments presents a potential problem for flux estimates. However, our results suggest minimal resuspension during stratification. As a part of a separate study, Hurley (unpublished data) measured pigment fluxes to the sediment surface. Sediment trap material was dominated by chlorophyll a and pheophorbide a (a grazing indicator). Surface sediments, however, were dominated by pheophytin a, a relatively stable chlorophyll degradation product. The lack of any substantial amounts of pheophytin in trap material suggested that if resuspension of particulates from the surface sediment was important, it was probably minimal. 

The stability of olive oil compared with other vegetable oils is attributed to the high-to-low ratio of oleic to linoleic acid, and to the degradation of the chlorophylls to pheophytins (60). In addition, olive oil is also rich in antioxidative phenolic compounds such as hydroxytyrosol (66). 

Scheme 3. Chlorophyll a (R = CH3, la) or chlorophyll b (R = CH=0, lb) are degraded via chlorophyllide a (3a) to pheophorbide a (5a) and phytol (recovered as phytyl-acetate) alternatively, pheophytin a (4) is also hydrolyzed to 5a and...

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