Porphyrin bacteriochlorophyll

Bacteriochlorophyll e 456, 646" Green sulfur bacteria Green -C2CH3-0H Porphyrins -CHO -CH2CH3 -CH CHjCOO-farnesyl Single Single... 

Finally, we should note that a particularly important area of application where density functional techniques, in spite of the deficiencies noted above, are virtually without competition is provided by biochemically relevant molecules, such as enzymes or nucleic acids. The techniques discussed in this section are virtually the only quantum chemical methods which can be applied in this context due to their outstanding price/performance ratio. For example, the 13C and 15N chemical shifts in bacteriochlorophyll A have been studied by Facelli, 1998, and in another investigation the 57Fe, 13C and 170 shifts in iron porphyrin derivatives gave important clues as to the structural details of these species, as shown by McMahon et al 1998. 

Analogies of Rare Earth Porphyrin Doubledeckers with the Special Pair of Bacteriochlorophylls in Bacterial Photosynthesis... 

Metal ion chelates of various porphyrins, differing in their substituents at positions 1-8, are intimately involved in a great number of life processes. Iron protoporphyrin (13) is the most common form and serves as the cofactor of a large number of enzymes. Usually (13) is non-covalently bound to its conjugate apoenzymes. Examples of covalently attached (13) are provided by c-type cytochromes, the attachment being between two vinyl side chains of (13) and two cysteine residues of the protein. Other biologically important derivatives of porphyrin include chlorophyll a (14), bacteriochlorophyll a and heme a (B-79MI11002). 

Though most non-experts are familiar only with the major chlorophylls of the plant world, chlorophyll a (25) and chlorophyll b (26) (which normally co-exist in approximately a 3 to 1 ratio), there exists a tremendous variety of other, less abundant, chlorophylls and bacteriochlorophylls. Chlorophyll c is found in certain marine algae, brown seaweeds and fresh water diatoms, but never in terrestrial life forms, and consists of a mixture of the two porphyrin acrylates (27) and (28). Certain species of Rhodophyceae possess chlorophyll d (29) and this pigment resembles chlorophyll a (25), except that the 3-vinyl in the latter is a formyl group in (29). 

The facility of metal complex formation is underscored by the fact that most porphyrin systems with any type of physiological function occur as metal complexes (e.g. Fe in hemoglobins, myoglobins, cytochromes, catalases and peroxidases Mg in chlorophylls and bacteriochlorophylls Co in vitamin B12). 

Chlorins (2) are undoubtedly the most important dihydroporphyrins, since the chlorin chromophore is found in chlorophylls and some bacteriochlorophylls and, as the magnesium complex, is the catalyst in photosynthesis. The method of choice for formation of trans-chlorins involves reduction of iron porphyrins with sodium in boiling isopentyl alcohol (57JCS3461), but methods involving photochemical reduction of tin(IV) porphyrins, isomerization of phlorins, reduction of metalloporphyrins with sodium anthracenide followed by protonolysis, heating with sodium ethoxide, and photoreductions of zinc(II) porphyrins in the presence of ascorbic acid have also been employed. The best method for formation of c/s-chlorins (note that all natural chlorophylls possess the trans arrangement) appears to... 

The chemistry of porphyrins is well documented (75MI1) (78MI1) and chlorophylls (66MI1) as well as bacteriochlorophylls (78MI10) have been reviewed extensively. A review on hydroporphyrins (78MI3 78MI4) comprises publications up to 1975. Since the field has expanded enormously since then, a new review seems appropriate. Subjects that were dealt with previously will be treated only briefly here unless there have been important new developments. 

Porphyrins and their derivatives play critical roles in many biological functions. Some of the most remarkable examples are protoporphyrin IX and its iron complex that constitutes the heme prosthetic group, and the magnesium complexes of pheophytin a and bacteriopheophytin a that are known as chlorophyll a and bacteriochlorophyll a, respectively. These natural compounds are illustrated in Fig. 1 together with the structure of porphin,... 

The studies described in this section were started shortly after the X-ray crystal structure of the RC of Rh. viridis was disclosed [73]. During these years, the role of the so-called accessory bacteriochlorophyll BCh was under debate [79]. In particular, the possibility was considered that it could play the role of a superexchange relay between SP and BPh (see Figure 22). In this respect, the copper(I)-complexed [2]rotaxane Cu.20+ represented a functional artificial model of the SP/BPh/BCh triad, the central Cu complex fragment between the Zn porphyrin donor and the Au porphyrin acceptor mimicking the function of BCh between SP and BCh. However, the kinetic scheme shown in Figure 22a has been revised, being now quite firmly established that (at least at room temperature) BCh is directly involved in the electron transfer reaction the transfer from the electronically excited special pair SP to BCh takes about 3 ps, and the next transfer step to the BPh, 0.65 ps [80]. In the earlier experiments, detection of the intermediate state SP+BCh was prevented by its relatively slow population and fast decay. 

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