Allomerized chlorophylls

Allomerized chlorophylls are oxidized compounds at C-13, forming theC-13 OH-chlorophyU catabolites. Other common positions for modifications and/or oxidation have been found at C-3, C-7, and C-8. Pyroderivatives of chlorophylls and their degradation products, usually found in heated and processed vegetables, lack the carbomethoxy group (-COOCH3) at C-13 of ring E, which is replaced by hydrogen. 

Allomerized chlorophylls— These are prepared by dissolving 1-2 p,mol of chlorophyll a or in 5 mL of methanol, and keeping the solution at room temperature and in darkness with magnetic stirring for 3 days. The progress of the reaction is followed by HPLC [218]. 

The chlorophylls produced by bacteria, algae and plants are a natural source of chlorins. The isolation of chlorophylls from natural material is known to be difficult because of their extreme sensitivity to various reactions, such as enolization, epimerization, allomerization, de-methoxycarbonylation, solvolysis, demctalation, dephytylation, photooxidation, etc. Often the... 

Due to the high mass, low volatility, and thermal instability of chlorophylls and derivatives, molecular weight determination by electron impact (El) MS is not recommended. Desorption-ionization MS techniques such as chemical ionization, secondary ion MS, fast-atom bombardment (FAB), field, plasma- and matrix-assisted laser desorption have been very effective for molecular ion detection in the characterization of tetrapyrroles. These techniques do not require sample vaporization prior to ionization and they are effective tools for allomerization studies. 

Several productive degradative transformations of chlorophyll a and b depend upon the ready oxidation of the ring E /3-keto ester. The so-called phase test involves aerial oxidation of the enolate anion produced by treatment of the ring e keto ester with alkali. One molecule of oxygen is consumed in this allomerization reaction to produce a hydroperoxide (111) which can fragment to give (112) after acidification, the unstable chlorin (113) is obtained, and this can be transformed into purpurin-18 (114) by evaporation, or into purpurin-7 trimethyl ester (115) by esterification with diazomethane. 

Successful formation ofCu2+ or Zn2+ pheophytin would result in only one major chromatographic peak (>90% of total peak area). The presence of other peaks most likely results from incomplete complexation or allomerization (oxidation) of the chlorophyll molecules. In these instances further chromatographic purification (based on A Iternate Protocol) may be necessary in order to achieve the desired degree of analytical purity. 

The oxidative scission of the isocyclic ring on the nucleus of chlorophyll and its derivatives, via allomerization, consists of a series of well documented reactions (6-7) and leads to such products as chlorin (e.g. -ea.-eg.-PeT nd purpurin (e.g. 

HYNNINEN p H (1981), Mechanism of the allomerization of chlorophyll inhibition of the allomerization by carotenoid pigments , Z Naturforsch, 36b, 1010-1016. 

In methanol, allomerization of chlorophyll to the corresponding 10-hydroxy- and 10-methoxylactone compounds has been reported to occur instantaneously Schaber et al., 1984). This is the main reason why methanol should be avoided as the extraction agent. Lyo-philisation of filtered material results in pigment losses Riaux-Gobin et al., 1987). 

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