Pheophytinization, porphyrins

The results of ref. 67 show that it is indeed possible to use solutions of porphyrin-quinone compounds in electron donor solvents for modelling the stage of electron transfer from pheophytin to quinone during photosynthesis (cf. Chap. 8, Sect. 1.2). Further research on these relatively simple model systems may provide still deeper insight into the mechanisms of this stage of photosynthesis. 

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,... 

Chlorophyll-Based Triads. The cyclic tetrapyrroles in natural photosynthetic reaction centers are chlorophylls and pheophytins, and therefore chlorins or other partially reduced porphyrin species. The artificial photosynthetic systems discussed so far are based on porphyrins. It was thus of interest to prepare a chlorophyll-based triad species and to investigate its properties. 

Chlorophylls a and b are found in almost all photosynthesizing organisms. They possess a complex cyclic structure (called a porphyrin) with a magnesium atom at its center. Chlorophyll a possesses a methyl group attached to ring II of the porphyrin, whereas chlorophyll b has an aldehyde group attached to the same site. Pheophytin a is similar in its structure to chlorophyll a. The magnesium atom is replaced by 2 protons. Chlorophylls a and b and pheophytin a all possess a phytol chain esterified to the porphyrin. The phytol chain extends into and anchors the molecule to the membrane. Lutein and /5-carotene are the most abundant carotenoids in thylakoid membranes. 

In the photosynthesis of green plants, photosystems I and II (PS I, PS II) contain chlorophyll a, a Mg(II)-porphyrin, as an antenna system for light absorption and energy transfer to the reaction centers of PS I and PS II. PS II consists of a dimeric chlorophyll a as reaction center, pheophytin a, a metal-free chlorophyll a as electron transfer system to PS I and - on the other side - a water-oxidizing Mn cluster. The electron connection between PS II and PS I is carried out by a cyth/f complex (heme complexes and an FeS protein). The reaction center of PS I is also a dimeric chlorophyll (perhaps together with other chlorophylls), and chlorophyll and several FeS proteins for electron transfer. 

Designate the molecules of surfactant in the bulk as A, and in the monolayer as B. At the interface, aggregation of surfactant molecules can take place, B rA, such as dimerization of porphyrin or pheophytin molecules at the octane-water interface. Let the surfactant B replace p quasimolecules at the interface. Therefore, one can write... 

In photosynthesis, the chlorophylls and pheophytins (close cousins of metalloporphyrins and porphyrins, respectively) play key roles in adsorbing light energy over a wide spectral range and converting it into the highly directional transfer of electrons. It is a marvelous but highly complex process... 

Pheophytins and pheophorbides isomerize readily to porphyrins, the vinyl group accepting the H-atoms of the dihydropyrrole ring. The synthesis of chlorophyll a was carried out by Woodward in 1960 [27]. 

Fig. 3 Absorption spectra of some of the chromophores from Figs. 1 and 2. Upper part some natural chromophores. Pheo a stands for pheophytin a, the free base of Chi a after replacement of the magnesium ion by two protons Lower part some synthetic chromophores dark green trace - nickel tetrasulfonated phthalocyanine (PcS4) dissolved in a water DMSO mixture (note the shoulders at 640 and 600 nm due to dimers and H-aggregates, respectively) magenta trace - meso-tetratolyl-porphyrin cyan trace - zinc tetratoljd-porphyrin. Note the sharp 420 nm (Soret) bands of the porphyrins and their very low visible absorptions (the Q bands) in comparison to Chls and phthalocyanines.

From these fractions, chlorophylls, pheophytins, sterols and carotenoids were identified (data not shown), and the suppressive effects of identified conq>ounds on transformation of AhR by TCDD were examined by EMSA as shown in Table. Chlorophylls showed a strong suppressive effect, though pheophytins showed weak effects. Since both chlorophylls and pheophytins have porphyrin rings, the structure of the rings may not be associated with their suppressive effects. Regarding carotenoids contained in fraction-7, -8 and -9, lutein suppressed effectively, while sterols and carotenes did not affect. 

Chlorophylls and pheophytins. Chlorophyll pigments are widely spread in the natural environment, chlorophylls a and b being the most abundant. Chlorophylls are included in the porphyrin group. They have a basic common stmcture, the porphin, with four units of pyrrole linked at the alpha positions by methylene bridges in a planar aromatic system, which is highly stable and amenable to the formation of chelates with metal ions. For the chlorophylls, the metal ion that forms the complex is Mg (Shahidi, 1997). 

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