Allomerism

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... 

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. 

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. 

ALLOMERISM. A property of substances that differ in chemical composition but have the same crystalline form. 

The word polyallomer is derived from the Greek words alios, meros. and poly. Alios means oilier and denotes a differentiation from the normal. The word meros means parts and the prefix poly is added to show that these materials are polymeric. Since allomerism is defined as a constancy of crystalline form with a variation in chemical composition, the polyallomers are examples of allomerism in polymer chemistry. 

Figure F4.4.3 HPLC chromatogram of Zn2+ pheophytins separated using the Alternate Protocol. Peak identifications A, allomerized Zn2+ pheophytin b B, Zn2+ pheophytin b C, allomerized Zn2+ pheophytin a D, Zn2+ pheophytin a. Reproduced from Schwartz (1984) with permission from Marcel Dekker, Inc.

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. 

During very early diagenesis and, to some extent, in the water column, 2 key competing or fating reactions appear to direct subsequent alteration, i.e. allomerization versus loss of the 10-carbomethoxy moiety. 

In the presence of oxygen, allomerization leads to the oxidative scission of the isocyclic ring. Products found in oxidative environments include the purpurin and chlorin acids (Figure 11). Tracing the down-hole fate of these compounds has proved futile, to date. That is, in cases were oxidation dominates, such as in sediments low in metabolizable organics (i.e. Philippine Sea. 

Block copolymers and polyallomers require separate treatment in the field of analytical applications of IR spectroscopy to C2-C3 copolymers. Polyallomers are copolymers synthesized from two monomers but exhibiting a degree of crystallinity normally associated only with homopolymers (32) indeed, allomerism denotes constancy of crystalline form with variation in chemical composition. Though crystalline, polyallomers have properties that differ not only from crystalline homopolymers, but also from blends of homopolymers containing the same proportions of the two monomers. The analytical problems are thus somewhat different from those concerning the elastomeric C2-C3 copolymers and a number of methods have been used to determine their composition. 

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

The isolation of chlorin Pe and mesochlorin Pe and possibly the purpurin 18 intermediates suggests that allomerization is an important process for chlorin formation by oxidative ring opening (Figure 1.7). It is also possible that petroporphyrins of the etio and phyllo type can be derived from these chlorins. 

In 1974, Porra and Grimme (8) extracted Chls aerobically from "recalcitrant" Chlorella fusca cells, with 2.1M pyridine in 0.35M NaOH. The Chls were oxidized to and measured as their allomerization products, Mg-cyclic hydroxylactones a and b (Fig. 2 V). Recently, a... 

Allomerism n. A similarity of crystalline form with a difference in chemical composition. 

Until recent years, bile salts were considered to be exclusively 5 8 steroids. The first recognition of the occurrence of 5a or alio bile salts came from the studies of Anderson and Haslewood (165) who synthetized allocholic acid and identified it as a previously unidentified naturally occurring acid from fish bile. The earliest isolation of allocholic acid appears to have been that by Ohta from the bile of the Gigi fish (166). Since that time allocholic acid has been isolated from the bile of a large number of fishes, birds, and, to a smaller extent, mammals (4). It has also been detected in human feces (121). Allocholic acid may be prepared from cholic acid by allomerization of the 6,7-ketol in hot alkali (165) or by treatment of the 3-keto acid with Raney nickel in boiling isopropyl benzene (167). 

Goto (88) obtained an allo-acid designated as 3a,7 -dihydroxy-5a-cholanoic acid in a manner somewhat analogous to the procedure of Anderson and Haslewood for the preparation of allocholic acid. Methyl 3 -cathyl-7,12-diketo-6a-bromo-5/3-cholanoate was allomerized with alkali to provide three ketolic acids. One of these designated as 3a,7/3-dihydroxy-6,12-diketo-5a-cholanoic acid, m.p. 235 °C, was reduced by the Wolff-Kishner procedure to provide equal quantities of allocholanic acid and alloursodeoxycholic acid, m.p. 223-224°C. 

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