Natural pigments porphyrins

Porphyrins and chlorophylls are the most widespread natural pigments. They are associated with the energy-converting processes of respiration and photosynthesis in living organisms, and the synthesis of specific porphyrin derivatives is often motivated by the desire to perform similar processes in the test tube. The structurally and biosynthetically related corrins (e.g. vitamin B,j) catalyze alkylations and rearrangements of carbon skeletons via organocobalt intermediates. The biosyntheses of these chromophores are also of topical interest. 

Stable dispersion of water-insoluble and/or hydrophobic natural pigment such as carotenoid, curcumin, porphyrin pigment, or vegetable carbon black in form of bodies of average size of 10 ram Addition of 0.5 ppm P-carotene to yogurt containing 200 ppm riboflavin color did not change after 40 days at 6°C compared with control (decoloration at 1 day)... 

Various natural pigment classes, such as llavonoids, an-thocyanins, carotenoids, chlorophylls and chlorophyll derivatives, porphyrins, quinones, anthraqinones, be-talains, and so forth are abundant in many famihes of the vegetable and animal kingdoms. As consumers generally dislike the color of synthetic dyes, the concentration and composition of pigments in foods and food products exert a considerable impact on the consumer acceptance and, consequently, on the commercial value of the products. It has been proven many times that one of the main properties employed for the commercial evaluation of the quahty of products is their color that is, an adequate color is an important requirement of marketabihty. 

Identification and Assignment of Structure The identity of a synthetic product is often established by comparison of its absorption curve with that of the natural product or another standard sample. The absorption curves of synthetic and natural vitamin A2 are shown in Figure 7.1. The identification pf coloured substances is easier since colour can almost always be correlated with the presence of certain types of chromo-phoric groups. Thus, visible absorption spectra are employed for the detection and identification of the natural pigments (e.g. carotenoids, anthocyanins md porphyrins), dyes and other colouring matters. Hie absorption spectra of these colouring principles will be discussed separately in Chapter 12. 

There is an important class of natural pigments which contains the porphyrin ring system (e.g. chlorophyll and haemin). The porphyrin ring, IV, contains four pyrrole nuclei connected by four C—H bridges and is found to... 

Porphyrins - Natural pigments containing a fundamental skeleton of four pyrrole nuclei united through the a-positions by four methine groups to form a macrocyclic structure (porphyrin is designated porphine in Chemical Abstracts indexes). [5]... 

In our earlier article we reviewed the main methods then available for the synthesis of naturally occurring porphyrins and bile pigments, and the structurally and biosynthetically related vitamin (cobalamin). The review essentially covered material published up to the middle of 1971. In the present article we describe new developments during the subsequent nine years. In order to preserve continuity with the earlier article and for ease of cross-referencing we have used a similar format and order of topics. 

As indicated in our earlier article, the direct polymerization of monopyrroles to porphyrins is of little synthetic value as far as Ae unsymmetrical natural pigments are concerned. However, this might provide a feasible approach at l( ast to the copro- and uroporphyrins if sufficiently sophisticated methods could be developed for their chromatographic separation on a preparative basis. This has already been achieved analytically by high performance liquid chromatography (hplc). ... 

Porphyrins. Collective name derived from Greek porphyra=purple, purple snail, for the widely distributed purple (red) natural pigments formally prepared from the parent skeleton of porphine by substitution of the macrocylic tetrapyrrole framework with methyl, vinyl, acetic acid, propanoic acid units, or other substituents. 

Some natural pigments, including chlorophylls, porphyrins. riboflavin, bilirubin, carotene, NADH, and FAD. have strong fluorecence in the UV and visible ranges. They were exploited for studies of enzyme active sites (NADH, FAD), enzyme kinetics, photosynthesis, and electron-transport processes (chlorophylls, porphyrins). 

Figure 2 Structures of some naturally occurring porphyrin pigments. (a) Heme b, (b) chlorophyll a, and (c) the macrocyclic core of vitamin B12.

Kirchner (1978) has discussed the utility of thin-layer chromatography for the following natural pigments chlorophylls, carotenoids, xanthophylls, flavonoids, anthocyanins, porphyrins, and bile. From the standpoint of practical TLC, the most important of these pigments are the chlorophylls, carotenoids, xanthophylls, and anthocyanins, and it is these pigments that are considered further in this chapter. For practical TLC of the flavonoids, consult Harbome (1984, 1992). For information on the TLC of porphyrins, see Doss (1972), Dolphin (1983), and Jacob (1992). Jain (1996) has provided useful information on the examination of porphyrins (in studies on clinical porphyrias) by TLC in clinical chemistry. 

Isaksen (1991) provided an extensive review on the TLC of natural pigments. He stated that TLC is the most common method used to analyze natural pigments. Pigments considered by Isaksen include the flavonoids, anthocyanins, carotenoids, chlorophylls, chlorophyll derivatives, and porphyrins. Optimal chromatographic systems (i.e., sorbents and mobile phases for each pigment group) are considered in detail in the review. The Isaksen (1991) review was revised and updated by Andersen and Francis (1996). 

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