Asymmetric photosynthesis

An asymmetric photosynthesis may be performed inside a crystal of -cinnamide grown in the presence of E-cinnamic acid and considered in terms of the analysis presented before on the reduction of crystal symmetry (Section IV-J). We envisage the reaction as follows The amide molecules are interlinked by NH O hydrogen bonds along the b axis to form a ribbon motif. Ribbons that are related to one another across a center of inversion are enantiomeric and are labeled / and d (or / and d ) (Figure 39). Molecules of -cinnamic acid will be occluded into the d ribbon preferentially from the +b side of the crystal and into the / ribbon from the — b side. It is well documented that E-cinnamide photodimerizes in the solid state to yield the centrosymmetric dimer tnixillamide. Such a reaction takes place between close-packed amide molecules of two enantiomeric ribbons, d and lord and / (95). It has also been established that solid solutions yield the mixed dimers (Ila) and (lib) (Figure 39) (96). Therefore, we expect preferential formation of the chiral dimer 11a at the + b end of the crystal and of the enantiomeric dimer lib at the —b end of the crystal. Preliminary experimental results are in accordance with this model (97). 

Several studies have been undertaken to get optically active helicenes by means of an asymmetric photocyclization. Four types of asymmetric photosynthesis can be distinguished for helicenes ... 

This asymmetric photosynthesis might be explained by three possible mechanisms ... 

Enantioselective photoreactions in the solid state have many advantages as described in Section I. As reviewed in this chapter, highly enantioselective photoreactions can be accomplished in the solid state without using any chiral source. There are hopeful possibilities to find many other examples of similar absolute asymmetric photosynthesis in the solid state. Enantioselective photosynthesis in inclusion complex crystal with a chiral host compound will also be a new general method of asymmetric synthesis in future. By successful designing of many new chiral host compounds, the research field would also be developed widely. 

An interesting photocatalytic system was reported by Hamada and collaborators [36]. They applied the helical chiral ruthenium(II) complex, [Ru(( — )-men-bpy)3]2 +, to the asymmetric photosynthesis of bi-2-naphthol from 3-substituted-2-naphthol. 

When the ruthenium(II) complex is irradiated under an oxygen atmosphere, the oxidative quenching of the photoexcited ruthenium(II) complex by an oxygen molecule rapidly occurs, to afford the ruthenium(III) complex. Because the ruth-enium(III) complex is a powerful oxidizing reagent, it can oxidize some substrate. Such a catalytic cycle was applied to the asymmetric photosynthesis of Co(acac)3 from Co(acac)2 + acac . A- and A-[Ru(( — )-menbpy)3]2+ and A- and A-[Ru(R(or S)-PhEtbpy)3]2+ were used as photosensitizers for such asymmetric photosynthesis [38,39]. 

An important aspect of the function of photosynthetic complexes is their asymmetric arrangement in respect to the membrane and to the external and internal phases of the cellular compartments. This arrangement allows the catalysis of vectorial electron transfer and the performance of electrical work by promoting charge separation across the membrane dielectric barrier. It allows also in some cases the net translocation of protons across the membrane. These two processes are at the basis of the mechanism of energy conservation in photosynthesis coupled to the formation of ATP, which is added, in oxygenic photosynthesis, to the conservation of redox energy in the form of reduced pyridine nucleotide coenzymes. 

And the last remark, which is of principal character in my opinion. In terms of the proposed photoelectrochemical hypothesis of photosynthesis decomposition of v/ater is considered as a process taking place in the molecular electrolyser. Its anode and cathode areas are on the same surface of the membreine and, under these conditions, electrolysis has "asymmetric" character. 

The isotope data of Bassham el al. (36) strongly support the in vivo operation of the ribulose diphosphate cycle. In short-term photosynthesis PGA labeled in carbon atoms 1 would give rise to 3,4-labeled F-6-P. This would yield S-7-P and pentose phosphate labeled as is shown in Scheme 2. S-7-P would be expected to be labeled equally in carbon atoms 3, 4, and 5 and this is the result obtained by Bassham et al. (36) 5 seconds after the introduction of C Oj. The low activity of C-4, relative to C-3 and C-5, observed by these workers after 0.4 second in C K)2, is not explained by the known reactions, since asymmetric labeling in hexose (C-4>C-3) has not been observed by them. Furthermore, both C-3 and C-4 of S-7-P arise from C-3 of hexose (represented by the symbol +) and should always be equally labeled. ... 

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