Porphyrin micelles

Utilizing FT-EPR teclmiques, van Willigen and co-workers have studied the photoinduced electron transfer from zinc tetrakis(4-sulfonatophenyl)porphyrin (ZnTPPS) to duroquinone (DQ) to fonn ZnTPPS and DQ in different micellar solutions [34, 63]. Spin-correlated radical pairs [ZnTPPS. . . DQ ] are fomied initially, and the SCRP lifetime depends upon the solution enviromnent. The ZnTPPS is not observed due to its short T2 relaxation time, but the spectra of DQ allow for the detemiination of the location and stability of reactant and product species in the various micellar solutions. While DQ is always located within the micelle, tire... 

Figrue BE 16.20 shows spectra of DQ m a solution of TXlOO, a neutral surfactant, as a function of delay time. The spectra are qualitatively similar to those obtained in ethanol solution. At early delay times, the polarization is largely TM while RPM increases at later delay times. The early TM indicates that the reaction involves ZnTPPS triplets while the A/E RPM at later delay times is produced by triplet excited-state electron transfer. Calculation of relaxation times from spectral data indicates that in this case the ZnTPPS porphyrin molecules are in the micelle, although some may also be in the hydrophobic mantle of the micelle. Furtlier,... 

Mechanisms of micellar reactions have been studied by a kinetic study of the state of the proton at the surface of dodecyl sulfate micelles [191]. Surface diffusion constants of Ni(II) on a sodium dodecyl sulfate micelle were studied by electron spin resonance (ESR). The lateral diffusion constant of Ni(II) was found to be three orders of magnitude less than that in ordinary aqueous solutions [192]. Migration and self-diffusion coefficients of divalent counterions in micellar solutions containing monovalent counterions were studied for solutions of Be2+ in lithium dodecyl sulfate and for solutions of Ca2+ in sodium dodecyl sulfate [193]. The structural disposition of the porphyrin complex and the conformation of the surfactant molecules inside the micellar cavity was studied by NMR on aqueous sodium dodecyl sulfate micelles [194]. 

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. 

IrCl which occurs readily in homogeneous solution. The reaction is strongly inhibited in NaLS or CTAB micelles and is explained as being due to a shielding of the edge of the porphyrin ring by the micelle, such that e transfer is retarded (12). 

Porphyrins are also able to directly interact with the nanotube sidewalls. For example, tetraphenyl porphyrin (H2-TPP) has been reported to interact with nanotubes to form TPPs/CNTs compounds that are stable for days. Stability has been enhanced by using a micelle-assisted approach, leading to stable structures with potential applications in light harvesting devices [76]. 

It is essential to characterize the reactant species in solution. One of the problems, for example, in interpreting the rate law for oxidation by Ce(IV) or Co(III) arises from the difficulties in characterizing these species in aqueous solution, particularly the extent of formation of hydroxy or polymeric species. We used the catalyzed decomposition of HjOj by an Fe(III) macrocycle as an example of the initial rate approach (Sec. 1.2.1). With certain conditions, the iron complex dimerizes and this would have to be allowed for, since it transpires that the dimer is catalytically inactive. In a different approach, the problems of limited solubility, dimerization and aging of iron(III) and (Il)-hemin in aqueous solution can be avoided by intercalating the porphyrin in a micelle. Kinetic study is then eased. 

Fig. 8. Structure of dendritic micelle with a porphyrin core, 33...

In contrast to the above results, all three "picket fence" porphyrins are solubilized in an oil-in-water microemulsion to yield a clear solution having a Soret band at 419-421 nm resembling that of H2PF,TPro solubilized in micelles. In this case the microemulsion (composed of SDS, n-pentanol and dodecane) consists of oil "droplets" dissolved in bulk water the radius of the droplet has been estimated to be 37 A ( ), well over twice that estimated for an SDS micelle (16 A). Since the droplet in the microemulsion contains a much larger "interior", it is reasonable that it may be a better medium for solubilizing the porphyrin. 

The most elegant example which demonstrates this aspect of proton transfer equilibrium of heme proteins is the study reported on six coordinated aqua (pyridine) iron(III) porphyrin complexes encapsulated in aqueous micelles [27]. The ferric ion in these complexes is axially co-ordinated to a water and a pyridine molecule, thus having a coordination geometry similar to that of the heme in metmyoglobin. The pH dependence of the absorption spectra of the... 

The pKa for the substituted hemins in aqueous pyridine solutions is 10.5 [18] while in the micelles it ranges from 7.1 to 10.3 depending on the micelle and the porphyrin substitution (Table 1). The changes in the pKa between the aqueous pyridine and micellar solutions have been shown not to be associated with any dimer-monomer equilibrium of the heme complexes, since it has been shown that in the concentration range of the reported study the aqua (pyridinato) complexes exit as monomers even in the absence of micelles in the pH range 6.5-12.5. The changes in the pKa have also been shown to be... 

The range of pKa for the acid base equilibrium of aqua (pyridinato) iron(III) porphyrins incorporated in the micelles encompasses the values reported for various ferrihemoglobins and ferrimyoglobins (Table 1). This appears to suggest that the sensitivity of pKa to the structure of the hemoproteins may be simulated by changing the nature of the hydrophobic interactions in the micelles and the substitutions on the porphyrin ring. 

The diaqua and aqua (hydroxo) hemin complexes encapsulated in the micelles [20] are found to be high-spin (peff = 5.7 — S.Sps). Their high-spin nature is further confirmed from the ESR spectra at 4.2 K (Fig. 4). The spectra are characteristic of high-spin ferric porphyrins with a large zero-field-split Ai ground state with Mg = 1/2 lying lowest. The spectra are axially symmetric (gf = 2.05, = 6.0) for the diaqua complex, while for the aqua (hydroxo)... 

The HNMR spectra of the diaqua and aqua (hydroxo) hemin complexes encapsulated in micelles have been reported [20] (Fig. 5). The heme methyl resonances in the diaqua species lie in the same region as those of the high-spin bis(dimethyl sulphoxide) iron (III) porphyrin complex [37-39], while those of the aqua (hydroxo) complex appear in a more upheld region. The positions and linewidths of the heme methyl resonances in these complexes are similar to those observed in the aqua and hydroxo hemoproteins [19,40]. The broadness of the ring methyl resonances of both the diaqua and aqua (hydroxo) species in micelles has been ascribed to arise from the hindered rotational tumbling motion of the heme inside the micelles. The spread and linewidth of these resonances are much larger than those of similar high-spin model heme complexes in simple solution [3]. 

The dithionite reduction of the micelle encapsulated aqua (hydroxo) ferric hemes at pH 10 (in inert atmosphere) gives an iron (II) porphyrin complex whose optical spectrum [21] shows two well-defined visible bands at 524 and 567 nm and a Soret band split into four bands (Fig. 10). Such spectral features are typical of four-coordinate iron (II) porphyrins. The magnetic moment (p = 3.8 + 0.2 Pb) of this sample in the micellar solution is also typical of intermediate spin iron(II) system and is similar to that reported for four-coordinate S = 1 iron(II) porphyrins and phthalocyanine [54-56]. The large orbital-contribution (ps.o. = 2.83 p for S = 1) observed in this iron(II) porphyrin... 

The HNMR spectra of the Fe(II) porphyrins in aqueous micelles are however characterized by two important differences from those in benzene [12,13]. The porphyrin proton resonances are much broader in aqueous micelles than in the benzene solution, and the spread of the heme methyl resonances is also larger (see Table 2). These will be discussed in detail in the next sections. 

The ferrous complex of octaethyl porphyrin in SDS micelles has been characterized as four coordinated (S = 1) ferrous heme species and is similar to that observed for the ferrous protoheme complex in CTAB. It is noted that ferrous complexes of natural porphyrins cannot be stabilized in aqueous SDS micelles, and much larger aqueous micelles like CTAB were needed to stabilize various ferrous protohemes. This indicates that the environment around the octaethyl porphyrin complex in aqueous SDS is more hydrophobic than that of the analogous natural heme species, suggesting that the OEP moiety is embedded much deeper inside the micellar hydrophobic cavity than the protoporphyrin analogue. 

Although there is a similarity in the pattern of the isotropic shifts of the high-spin iron(II) porphyrins in the aqueous micellar and benzene solutions, some differences are also noticeable. First, the heme proton resonances in the micelle are much broader than in benzene, and resemble those reported for deoxymyoglobin [62]. Second, the downfield shift of the methyl resonances in... 

The micelle-encapsulated six coordinated bis(pyridinato) iron(II) complexes of protoporphyrin and OEP have been reported by addition of pyridine to the four coordinate ferrous complex in aqueous micellar solution. The optical spectrum of [Fe(II)(PP)(Py)2] in micelle (Fig. 10) is identical to S = 0 six-coordinate bis(pyridinato) iron(II) porphyrin complex [3]. The magnetic moment measurements in solution confirm their diamagnetic nature. The HNMR spectra are also characteristic low-spin iron(II) resonances (S = 0) with shifts lying in the diamagnetic region (Table 2). 

Assuming a smooth spherical structure of the micellar surface [73] with a uniform density distribution, the average distances of the micellar carbon and the ironcenter of the porphyrin complex were evaluated [67] and the disposition of porphyrin molecule in the micelle was determined (Fig. 15). The propionic acid side chains in the heme molecule have been proposed [67] to be directed... 

Fig. 15. Schematic diagram of the disposition of ferric porphyrin complex inside micelle. (Taken from Ref. 67)...

Attempts to support colloidal platinum particles with surfactant zinc porphyrin micelles allow hydrogen production at lower platinum concentrations than when the porphyrin and the particle are separate but an irreversible electron donor is still necessary for hydrogen production.116... 

The simplest use of micelles is in the solubilization of hydrophobic species and early experiments showed that [ZnTPP] in TritonX-100 micelles could act as a cliromophore in the photochemical reduction of MV2+ by ethanethiol or TEOA. Hydrogen was produced if hydrogenase or colloidal Pt were added.331 Zinc porphyrins in neutral micelles (TritonX-100) produce332 hydrogen on photolysis in the presence of TEOA, bipy and K2[PtCl6J. 

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