Zinc bacteriochlorin

Figure 13.67. Zinc bacteriochlorin aggregates within a lipid bilayer are spectroscopically similar to bacteriochlorophyll aggregates.

Figure 13.67. As with most of the approaches in this section, the design of the assembly very closely resembles the features of LHi and LH2 natural systems. Aggregates of zinc bacteriochlorins are trapped in a lecithin or Triton-X bilayer to generate chromophore assemblies with spectroscopic behavior that resembles that of the bacteriochlorophyll c aggregates in the antennae systems The aim of the work is indeed to mimic the function of chlorosomes. Chlorosomes are aggregates of BChl c, d, and e that are embedded in lipid monolayers.

Chlorins, e.g. 14, form adducts with osmium(VIII) oxide, which can be hydrolyzed in aqueous sodium sulfide to bacteriochlorindiols, e g. 2, or isobacteriochlorindiols, e.g. 3. Thus, similar to diimide reductions of chlorins, metal-free tetraphenylchlorin 14 (M = 2H) is selectively oxidized to a corresponding bacteriochlorin 2 whereas the zinc chlorin gives an isobac-teriochlorin 3 on oxidation with osmium(VIII) oxide.40 With less symmetrical chlorins, very complex mixtures of constitutional isomers and stereoisomers are formed by /i-bishydroxyla-tion.17... 

In the case of the osmium(VIII) oxide oxidation of 5,10,15,20-tetraphenylchlorin,3b the metal-free macrocycle gives the bacteriochlorin 6, whereas the corresponding zinc derivative leads to an isobacteriochlorin. 

Isobacteriochlorins, since they are tetrahydroporphyrins, can be obtained by tetrahydrogena-tion of porphyrins and dihydrogenation of chlorins. However, alkali-metal reduction of porphyrins and metalloporphyrins always gives a mixture of chlorins, bacteriochlorins or isobacteriochlorins.14 The method of choice for the preparation of pure isobacteriochlorins, e.g. 2, is the diimide reduction of zinc(II) chlorins, e.g. l.15a,b... 

It is important to use a zinc(ll) chlorin because with the metal-free chlorin bacteriochlorins... 

Similiar problems of regioselectivity as in reduction reactions are encountered in oxidation reactions of porphyrins and chlorins. The oxidation of chlorins to isobacteriochlorins can be directed by insertion of zinc(II) or nickel(II) into the macrocycle. Again here, the metal-free chlorins give the bacteriochlorins whereas the metal chlorins, e.g. 1, give isobacteriochlorins, e.g. 3.15a,b I 7... 

A study of the hyperfine splitting of the e.s.r. spectrum of the cation radical of oxidized zinc(n) bacteriochlorin, together with selective deuteriation, has allowed assignment of the electron density to the atoms of the system.279 There is high spin density on the protons of the saturated rings of the bacteriochlorin with or without the zinc. A similar situation may pertain in metal-free bacteriochlorophyll. 

Whitlock et al. developed a new method to synthesize bacteriochlorins and isobacteriochlorins selectively it involved diimide reduction of porphyrins. 2 Di-imide reduction of metal-free tetraphenyl-chlorin H2(TPC) afforded tetraphenyl-bacteriochlorin H2(TPBC) 140 in 84% yield, while reduction of zinc tetraphenyl-chlorin Zn(Xrc) produced the Zn(II) complex of tetraphenyl-isobacteriochlorin 141 in 57% yield (Scheme 40). 

Miyatake, T., H. Tamiaki, A.R. Holtzwarth, and K. Schaffner (1999). Artificial lightharvesting antennae Singlet excitation energy transfer from zinc chlorin aggregate to bacteriochlorin in homogeneous hexane solution. Photochem. Photobiol. 69(4), 448 56. 

Recent reconstitution experiments open the possibility of selectively introducing other metallo bacteriochlorins (or chlorins) at the acceptor BChl or BPheo sites. Zinc substitutions are obvious choices since the optical and coordination properties of Zn complexes resemble those of Mg derivatives but the former are easier to reduce (and harder to oxidize) than the latter. Perhaps a less obvious candidate for metal replacement is nickel which offers several advantages Ni complexes are also easier to reduce and harder to oxidize than Mg compounds but, in addition, redox potentials of Ni porphyrins and chlorins are close to those of the metal-free derivatives the optical properties of Ni chlorins are similar to those of Mg ones they possess shortlived excited states as a result of which they have proved to be valuable for resonance Raman studies. A more practical advantage of Ni complexes is their enhanced stability towards demetallation, compared to Mg or Zn complexes, a property of merit if the chromophores are to be used in reconstitution reactions. (Ni protoporphyrins IX have been successfully exchanged for the histidine-bound hemes in hemoglobin.) ... 

Almost 30 years ago, Whitlock et al. ° developed an efficient diimide reduction method for the synthesis of bacteriochlorins and isobacteriochlorins from porphyrins. Diimide reduction of metal-free tetraphenyl chlorin afforded tetraphenyl bacteriochlorin, while reduction of the corresponding zinc analog produced... 

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