Porphyrin vesicle

The sterols that were chosen as substrates contained two double bonds, one at various positions in the side chain and A5 in the steroid nucleus. Whereas the latter double bond was never touched in reactions with the Fe(III) porphyrin vesicle system 183 in the presence of PhIO, the side chain double bonds of desmosterol 186 and fucosterol 187 were epoxidized to 188 and 189 in 32% and 22% yield, respectively (Fig. 31). In contrast, stigmasterol 190 was not reactive, since the double bond cannot approach the reactive iron-oxo intermediate. 

Shape selective reactions are typically carried out over zeolites, molecular sieves and other porous materials. There are three major classifications of shape selectivity including (1) reactant shape selectivity where reactants of sizes less than the pore size of the support are allowed to enter the pores to react over active sites, (2) product shape selectivity where products of sizes smaller than the pore dimensions can leave the catalyst and (3) transition state shape selectivity where sizes of pores can influence the types of transition states that may form. Other materials like porphyrins, vesicles, micelles, cryptands and cage complexes have been shown to control product selectivities by shape selective processes. 

Tsuchida has developed a very elegant approach based on the use of amphiphilic porphyrins in which the lipophilic moiety of the amphiphile is the porphyrin and the polar head group is a classical phospholipid moiety The compounds 125(M) represented in Figure 13.66 are able to form vesicles containing up to 23,000 porphyrins (vesicle diameter 100-150 nm). This is without a doubt the largest structurally characterized multiporphyrinic assembly characterized to date. 

As mentioned earlier, a great deal of literature has dealt with the properties of heterogeneous liquid systems such as microemulsions, micelles, vesicles, and lipid bilayers in photosynthetic processes [114,115,119]. At externally polarizable ITIES, the control on the Galvani potential difference offers an extra variable, which allows tuning reaction paths and rates. For instance, the rather high interfacial reactivity of photoexcited porphyrin species has proved to be able to promote processes such as the one shown in Fig. 3(b). The inhibition of back ET upon addition of hexacyanoferrate in the photoreaction of Fig. 17 is an example of a photosynthetic reaction at polarizable ITIES [87,166]. At Galvani potential differences close to 0 V, a direct redox reaction involving an equimolar ratio of the hexacyanoferrate couple and TCNQ features an uphill ET of approximately 0.10 eV (see Fig. 4). However, the excited state of the porphyrin heterodimer can readily inject an electron into TCNQ and subsequently receive an electron from ferrocyanide. For illumination at 543 nm (2.3 eV), the overall photoprocess corresponds to a 4% conversion efficiency. 

A trianionic zinc porphyrin anchored to a membrane by an imidazole link has been used to bind cytochrome c at the membrane surface. UV spectra confirmed the insertion of the zinc porphyrin into the phospholipid vesicle and was used to study surface association of cytochrome c. 

In such vesicle systems, the electrons are transported through the membrane. Electron carriers such as quinones or alloxazines in the vesicle wall enhance remarkably the rate of photoinduced charge separation. The vesicle system shown in Fig. 6 contains the surfactant Zn-porphyrine complex (ZnC12TPyP) in the wall 23). 

Vesicles are known to exhibit catalytic activity. An impressive example of this kind reported by Grooves and Neumann is presented in Section 6.4.2 [33]. The catalytic oxidation of the inactive carbon atom mimicking the action of cytochrome P-450 enzyme by a porphyrin derivative in the presence ofvesicles... 

Figure 6.8 Schematic representation of a cytochrome P450 mimic in which catalytic manganese porphyrins are captured in the bilayer of polymerized vesicles. Colloidal platinum encapsulated in the vesicles in combination with molecular hydrogen serves as a reductant.

Several additional studies were carried out to obtain information about the precise behavior of the various components in the model system. The interplay between the manganese porphyrin and the rhodium cofactor was found to be crucial for an efficient catalytic performance of the whole assembly and, hence, their properties were studied in detail at different pH values in vesicle bilayers composed of various types of amphiphiles, viz. cationic (DODAC), anionic (DHP), and zwitterionic (DPPC) [30]. At pH values where the reduced rhodium species is expected to be present as Rh only, the rate of the reduction of 13 by formate increased in the series DPPC < DHP < DODAC, which is in line with an expected higher concentration of formate ions at the surface of the cationic vesicles. The reduction rates of 12 incorporated in the vesicle bilayers catalyzed by 13-formate increased in the same order, because formation of the Rh-formate complex is the rate-determining step in this reduction. When the rates of epoxidation of styrene were studied at pH 7, however, the relative rates were found to be reversed DODAC DPPC < DHP. Apparently, for epoxidation to occur, an efficient supply of protons to the vesicle surface is essential, probably for the step in which the Mn -02 complex breaks down into the active epoxidizing Mn =0 species and water. Using a-pinene as the substrate in the DHP-based system, a turnover number of 360 was observed, which is comparable to the turnover numbers observed for cytochrome P450 itself. 

Using vesicles where the amphiphilic chromophore is embedded only in the outer wall, electron transfer across the bilayer is not observed in the absence of mediators whether the EDTA is in the inner pools with MV2+ in the bulk phase or vice versa.338 339 With EDTA in the inner pools, electron transfer can, however, be effected on addition of, for example, ubiquinone, ZnTPP or H2TPP to the bilayer. In the last two cases, electron transfer is much more efficient than when the amphiphilic zinc porphyrin is omitted.339... 

Another mechanism of the transmembrane electron transfer in the system under discussion has been proposed by Khairutdinov and co-workers [60]. This mechanism assumes the two-quantum ionization of porphyrin dimer located in the inner membrane monolayer with the capture of the electron by MV2+ dication to be the primary redox photochemical process. However, this interpretation is based on the data obtained when studying vitreous suspensions of vesicles at 77 K, and further experiments seem to be needed to justify their applicability to liquid suspensions at ambient temperatures. 

Ru(bpy)3+ complex placed into the inner cavity of the vesicle was used as such antenna . The lifetime of the triplet-excited state of this complex ( 0.6 ps) is sufficiently long, so that before its deactivation it can experience numerous collisions with the inner surface of the vesicle membrane and thus with the porphyrin molecules embedded into the membrane. Indeed, it was found that the introduction of Ru(bpy)2 + into the inner volume of the vesicle leads to the sixfold increase of the rate of the transmembrane PET [58, 61]. This effect results, first, from the spectral sensitization due to the light absorption by the ruthenium complex in the spectral region where porphyrin does not absorb, and, second, from the two-three fold increase of transfer from 3Ru(bpy)i+ to ZnTPPin. 

Synthetic or natural porphyrins are most widely used as photosensitizers of PET across the membranes. It is well known that in homogeneous solutions the electron excited states of porphyrins are efficiently quenched upon the increase of the concentration of the porphyrins [129]. If porphyrins are located in the membranes of the vesicles, these processes are expected to manifest themselves especially strongly due to rather high local concentration of the porphyrins. Note that this concentration may be high enough for the quenching even when only a few molecules of a porphyrin are located in the membrane. 

The non-exponential decay of the triplet excited states of the photosensitizers is observed for Chi-, Phe- and ZnTPP-containing vesicles [132-135], The reason for the non-exponentiality may be, first, a statistical distribution of the concentrations of porphyrin molecules in the membrane, and, second, a simultaneous decay of the triplet excited states via several parallel channels such as spontaneous deactivation, concentration quenching and triplet-triplet annihilation which are known to be characteristic of porphyrins in organic solvents [129]. For ZnTPP and Phe in vesicles, the process of triplet-triplet annihilation is indeed observed [56, 134], while according to [132] this process is surprisingly absent for Chi. 

Cationic vesicles have been used to accomplish charge separation (Mon-serrat and Gratzel, 1981). The photosensitiser was a water-soluble porphyrin and electron acceptor was a modified, water-soluble viologen. The porphyrin photo-reduced the viologen which in its reduced form is lipid soluble but water insoluble. Consequently, the reduced species enters the vesicle. So effective is the charge separation that multimer formation of the reduced species in the vesicle can be observed. Another method which has been employed is to immobilise donors and acceptors on the surface of latex particles (Frank et al., 1979). 

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