Pigments tetrapyrrole

Interesting tautomeric possibilities exist in the xanthobilirubic acid series (cf. reference 57) which can be illustrated by the equilibrium 62 63, More complex examples of the same type are found among the linear tetrapyrrole pigments— the bilenes, bilidienes, and bili-trienes—and have been discussed by Stevens. Relatively little evidence is available concerning the fine structure of these compounds, although the formation of complexes has been advanced as evidence for the 0X0 structure in some cases. ... 

Systematic research of two-quantum excitation of tetrapyrrolic pigments (2,3) has made it evident that cooperative triplet-triplet excitation of metalloporphyrin upper electronic states successfully competes with the processes of stepwise two-quantum excitation. This made us pay greater attention to porphyrin TTA since high quantum yields of intersystem crossing to triplet Tj states is characteristic of these types of compounds. (It should be mentioned that interest in photoprocesses in excited states of tetrapyrrolic pigments has been greatly increased by development of solar energy converters based on metalloporphyrins (4,5)). 

As is known, the triplet and singlet tetrapyrrolic pigment states are caused by excitation of delocalized ir electrons of the heteroaromatic macrocycle (1 ). The most favourable structure of dimeric complexes for metal 1oporphyrin TTA seems to be a sandwich dimer as in the case when molecular orientation of plane to plane electronic shells reaches the maximum overlap. Quantum chemistry calculations of metalloporphyrin dimers indicates that for the dimeric emission process the 2 state is split into states of higher energy (SJ) and lower energy ( 2). In the case of a sandwich dimer of 04 symmetry, the following states are seen ... 

Equation (5) reflects a well-known experimental fact that TTA rate constant in liquid solutions is usually less than that of diffusion rate (for tetrapyrrolic pigments, for instance, k = (0.2 - 0.4) kd 119). 

Heme, an iron-containing tetrapyrrole pigment, is a component of 02-binding proteins (see p. 106) and a coenzyme of various oxi-doreductases (see p. 32). Around 85% of heme biosynthesis occurs in the bone marrow, and a much smaller percentage is formed in the liver. Both mitochondria and cytoplasm are involved in heme synthesis. 

Figure 24-24 The degradation of heme and the formation of open tetrapyrrole pigments.

Hemes (Fig. la) are a diverse group of tetrapyrrole pigments, being present as the prosthetic group of both the globins (hemoglobin and myoglobin Topic... 

The basic unit from which the tetrapyrrole pigments are derived is pyrrole. 

The chlorophylls are tetrapyrrole pigments in which the porphyrin ring is in the dihydro form and the central metal atom is magnesium. There are two chlorophylls, a and b, which occur together in a ratio of about 1 25. Chlorophyll b differs from chlorophyll a in that the methyl group on carbon 3 is replaced with an aldehyde group. The structural for-... 

The chemical nature of P-700 is difficult to establish. The absorption bleachings correspond approximately to the peaks of Chi a. which appears to be virtually the only tetrapyrrolic pigment in purified PS I particles. It has thus been assumed that P-700 is Chi a bound to a protein. A few recent results, however, may require this hypothesis to be refined. An examination of the spectroscopic and redox properties of P-700 led Wasielewski et al. [13] to propose that P-700 could be the enol form of Chi a where enolization was of the keto ester on ring V. This has not been confirmed by chemical extraction. Extraction experiments, however, have evidenced two other chlorophyll derivatives. A species named Chl-RC I has been isolated from PS I, at a nearly 1/1 molar ratio with P-700 and its structure shown to be a chlorinated derivative. It is not yet clear whether Chl-RC I is a native constituent of PS I or an extraction artefact. Chl-RC I has not been obtained in a recent chemical analysis by HPLC, which instead revealed two Chi a per P-700 [14]. 

We conclude that it is possible to determine the maturity of a sediment by examination of the properties of kerogen and tetrapyrrole pigments. In fact, a fairly detailed reconstruction of the chemical events surrounding a Miocene intrusion into a Cretaceous shale has been possible. 

Bugula dentata is a dark blue coloured bryozoan that is common in Japan. The colour has been ascribed to a tetrapyrrole pigment. 100, which also occurs in a bacterium and an ascidian (43). 

---