Naturally Occurring Bacteriochlorins

Another transformation observed with a naturally occurring bacteriochlorin is the tautomer-ization of the exocyclic C — C double bond in bacleriochlorophyll b (3) in acetone which gives 7,8-dehydrobacteriochlorophyll a (4), a chlorin-type product (configurations at C7 and of the 8-ethylidene group have not been established).112,12... 

The only representative of this class of compounds that is not involved in bacterial photosynthesis is the recently discovered tolyporphin 20. The unusual tolyporphin occurs in the Pacific cyanophyte tolypothrix nodosa. The substitution pattern, especially the tetrahydropyran moieties of the bacteriochlorin, is unique among naturally occurring... 

Hydroporphyrins. H. are derived from completely unsaturated porphyrins by reduction of the double bonds in the porphine skeleton. The spectrum of naturally occurring H. ranges from the dihydro derivatives chlorin and phlorin(s) through the tetrahydro derivatives bacteriochlorin and isobacteriochlorins, the hexahydro derivatives porphyrinogens and precor-rins to the more highly reduced corphins and factor F430. 

Because of its long-wavelength absorption close to 800 nm, naturally occurring bacteriochlorophylls are ideal candidates for PDT. However, due to their unstable nature, synthesis of stable bacteriochlorins has been a challenge for porphyrin chemists worldwide. 

The only naturally occurring bacteriochlorin that is not involved in photosynthesis is the tolyporphin 131 isolated by Prinsep et from the blue-green alga Tolypothrix nodosa. This compound, which enhances the cytotoxicity of adriamycin or vinblastine in SK-VLB cells at doses as low as 1 pg/ml, is characterized as a multidrug resistance-... 

Most of the naturally occurring bacteriochlorins (e.g., 132) have absorptions between 760 and 780 nm and are extremely sensitive to oxidation, resulting in a rapid transformation into the chlorin state 133 which generally has an absorption maximum at or below 660 nm (Scheme 37). Furthermore, if a laser is used to excite the bacteriochlorin in vivo, oxidation may result in the formation of a new chromophore absorbing outside the laser window, reducing its photodynamic efficacy. Due to the desirable photophysical properties of bacteriochlorins, there has been increasing interest in the synthesis of stable bacteriochlorins from bacteriochlorophyll a or other similar tetrapyrrolic systems. 

Isobacteriochlorins (8) with two adjacent saturated pyrrole rings are the constitutional isomers of bacteriochlorins, in which two opposite rings are partially reduced. In 1973 the first naturally occurring isobacteriochlorin, iron-containing siroheme (110), was isolated by Siegel et al (66) from a sulfite reductase of Escherichia coli. Later it was also discovered in sulfite and nitrite reductases of numerous bacteria and plants (67). 

More recently, syntheses of anionic boron hydride derivatives of naturally occurring porphyrin systems pyropheophorbide a [123], chlorine [22b,22c,124,125], and bacteriochlorin p [126] have been reported (Figures 9.13 through 9.15). 

Chls and all tetrapyrroles are heteroaromatic compounds and the aromatic character of the underlying tetrapyrrole moiety and the reactivity of the functional groups in the side chains govern their chemistry. Three different classes of tetrapyrroles, differentiated by their oxidation level, occur in nature porphyrins (11, e.g. hemes), chlorins (12, e.g. chls) and bacteriochlorins (13, e.g. bchls). As a cyclic tetrapyrrole with a fused five-membered ring, the overall reactivity of chi is that of a standard phytochlorin 7. Such compounds are capable of coordinating almost any known metal with the core nitrogen atoms. Together with the conformational flexibility of the macrocycle and the variability of its side chains, this accounts for their unique role in photosynthesis and applications ... 

---