Water-Soluble Porphyrins

P-substituted porphyrins in organic solvents or membranes is the shifted face-to-face dimer found in crystal structures (Fig. 6.2.15a). Here one usually finds that an electron-rich pyrrole ring of one porphyrin sits atop the electron-poor center of the other porphyrin. Water-soluble substituents, in particular the most common acetic and propionic acid side chains, lead to amphiphilic bilayers (Fig. 6.2.15c) (Fuhrhop, 1976 Hunter and Sanders, 1990). 

The desired porphyrin water-soluble 187a was obtained when the protecting groups of the carbohydrate moieties of 187b were removed by treatment with cation exchange resin in refluxing water saturated with chloroform for 5 h. 

Nitro ilkenes derived from galdctose or other carbohydrates are converted directly into pyrroles siibsdnited v/ith such carbohydrates at the fi-posidon. They are important precursors for water-soluble porphyrins fEq. 10.29. Such kmds of porphyrins are good candidates for photodynamic therapy of cancer and have been extensively snithed. 

Cobalt(II) complexes of three water-soluble porphyrins are catalysts for the controlled potential electrolytic reduction of H O to Hi in aqueous acid solution. The porphyrin complexes were either directly adsorbed on glassy carbon, or were deposited as films using a variety of methods. Reduction to [Co(Por) was followed by a nucleophilic reaction with water to give the hydride intermediate. Hydrogen production then occurs either by attack of H on Co(Por)H, or by a disproportionation reaction requiring two Co(Por)H units. Although the overall I easibility of this process was demonstrated, practical problems including the rate of electron transfer still need to be overcome. " " ... 

At another level, water-soluble polyphosphazenes are of interest as plasma extenders. In addition, specific polymers with pendent imidazolyl units have been studied as carrier macromolecules for heme and other iron porphyrins (structures and (44,45). (In structures M and the ellipse and Fe symbol represent heme, hemin, or a synthetic heme analog.)... 

Su YO, Kuwana T, Chen SM. 1990. Electrocatalysis of oxygen reduction by water-soluble iron porphyrins. Thermodynamic and kinetic advantage studies. J Electroanal Chem 288 177. 

FIG. 4 (a) Latimer diagram of the water soluble zinc tetra-Af-methyl-4-pyridium porphyrin (ZnTMPyP ). (Reprinted with permission from Ref. 47.) (b) Schematic representation of a photosynthetic process based on porphyrin sensitized water-organic interface. Dotted line corresponds to the back electron-transfer process. (Reprinted from Ref. 51 with permission from Elsevier Science.)... 

DCE interface in the presence of TPBCl [43,82]. The accumulation of products of the redox reactions were followed by spectrophotometry in situ, and quantitative relationships were obtained between the accumulation of products and the charge transfer across the interface. These results confirmed the higher stability of this anion in comparison to TPB . It was also reported that the redox potential of TPBCP is 0.51V more positive than (see Fig. 3). However, the redox stability of the chlorinated derivative of tetra-phenylborate is not sufficient in the presence of highly reactive species such as photoex-cited water-soluble porphyrins. Fermin et al. have shown that TPBCP can be oxidized by adsorbed zinc tetrakis-(carboxyphenyl)porphyrin at the water-DCE interface under illumination [50]. Under these conditions, the fully fluorinated derivative TPFB has proved to be extremely stable and consequently ideal for photoinduced ET studies [49,83]. Another anion which exhibits high redox stability is PFg- however, its solubility in the water phase restricts the positive end of the ideally polarizable window to < —0.2V [85]. 

Although the correlation between ket and the driving force determined by Eq. (14) has been confirmed by various experimental approaches, the effect of the Galvani potential difference remains to be fully understood. The elegant theoretical description by Schmickler seems to be in conflict with a great deal of experimental results. Even clearer evidence of the k t dependence on A 0 has been presented by Fermin et al. for photo-induced electron-transfer processes involving water-soluble porphyrins [50,83]. As discussed in the next section, the rationalization of the potential dependence of ket iti these systems is complicated by perturbations of the interfacial potential associated with the specific adsorption of the ionic dye. 

A rather important aspeet that should be eonsidered is that interfaeial quenching of dyes does not neeessarily imply an eleetron-transfer step. Indeed, many photoehemieal reactions involving anthracene oeeur via energy transfer rather than ET [128]. A way to discern between both kinds of meehanisms is via monitoring the accumulation of photoproducts at the interfaee. Eor instance, heterogeneous quenehing of water-soluble porphyrins by TCNQ at the water-toluene interfaee showed a elear accumulation of the radical TCNQ under illumination [129]. This system was also analyzed within the framework of the exeited-state diffusion model where time-resolved absorption of the porphyrin triplet state provided a quenehing rate eonstant of the order of 92M ems. ... 

More recently, a series of papers based on photocurrent responses involving water soluble porphyrin species has allowed to address the various aspects involved in the mechanism of Fig. 11 [50,73,83]. Photocurrent transients at the water-DCE interface in the presence of zinc tetrakis(carboxyphenyl) porphyrin (ZnTPPC" ) and Fc under monochromatic light are shown in Fig. 15 at various Galvani potential differences [50]. The... 

Polar substituents have been attached to the porphyrin core in order to achieve water-solubility. (685) has been synthesized by reaction of the Ni11 complex of tetrakis(pentafluorophenyl)-porphyrin with dimethylammonium hydrochloride in DMF, followed by methylation with methyl trifluoromethanesulfonate (triflate) in trimethyl phosphate.1778 The triflate and chloride salts are... 

In several cases, the porphyrins (chlorophyll, haem and F-430 but not vitamin B12) are not water-soluble and are often placed in membranes, which is not so easily managed with simple ions. 

A number of metal porphyrins have been examined as electrocatalysts for H20 reduction to H2. Cobalt complexes of water soluble masri-tetrakis(7V-methylpyridinium-4-yl)porphyrin chloride, meso-tetrakis(4-pyridyl)porphyrin, and mam-tetrakis(A,A,A-trimethylamlinium-4-yl)porphyrin chloride have been shown to catalyze H2 production via controlled potential electrolysis at relatively low overpotential (—0.95 V vs. SCE at Hg pool in 0.1 M in fluoroacetic acid), with nearly 100% current efficiency.12 Since the electrode kinetics appeared to be dominated by porphyrin adsorption at the electrode surface, H2-evolution catalysts have been examined at Co-porphyrin films on electrode surfaces.13,14 These catalytic systems appeared to be limited by slow electron transfer or poor stability.13 However, CoTPP incorporated into a Nafion membrane coated on a Pt electrode shows high activity for H2 production, and the catalysis takes place at the theoretical potential of H+/H2.14... 

Water soluble iron porphyrins [Fem(TPPS)(H20) ]3-330 and [Fem(TMPy)(H20)2]5+ 331 332 (TPPS = maso-tetrakis(/ -sulfonatophenyl)porphyrin, TMPyP = / /e.vo-tetrakis(7V-methyl-4-pyridi-nium)porphyrin331 or maso-tetrakis (A -methyl-2-pyridinium)porphyrin332 dications) act as effective electrocatalysts for the reduction of nitrite to ammonia in aqueous electrolytes (Equation (64) Ei/2= 0.103 V vs. SCE at pH 7), with NH2OH or N20 also appearing as products depending on the reaction conditions. Nitric oxide then ligates to the iron(III) porphyrin to form a nitrosyl complex [Fen(P)(NO+)] (P = porphyrin) as intermediate. 

For systemic administration, the photosensitizer usually has to be delivered into the bloodstream by intravenous injection. Since the photosensitizer is a solid, this means that a solution or a stable suspension has to be provided. Metal complexes of the basic porphyrin and phthalocyanine nuclei are insoluble in water, so that some effort has to be made to render the system water soluble, or at least amphiphilic, by placing various substituents (e.g., S03H, C02H, OH, NR3+, polyether, aminoacid, sugar) on the periphery of the molecule. The aromatic character of the ligand offers a suitable opportunity for such substitutions to be made. Examples will appear frequently in the following sections. 

Coordinated "mTc has featured in attempts to follow the distribution and tumor uptake of the water-soluble sensitizers PcS4147 and 5,10,15,20-tetrakis[3,4-bis(carboxymethyleneoxy)phenyl]-porphyrin 148 and of haematoporphyrin.29... 

BNCT sensitisers are designed with neutron capture in mind, but those that are porphyrin/ phthalocyanine-based may show PDT activity. An example is the disodium (or dipotassium) salt of the tetrakis-carborane ester (58) derived from 3,8-bis(l,2-dihydroxyethyl)deuteroporphyrin.298 This is water soluble and tumor selective, showing high tumor/normal brain ratios.299 Photophysical properties are similar to other water-soluble porphyrins in rat glioma cells, specific localization in mitochondria is observed.300... 

Although the majority of metal complexes developed for PDT have porphyrin-style ligands, there are exceptions (e.g., compounds (53) and (54) to (57)). The aluminum(III) complex of hypocrellin B (59) provides another example. Compound (59) is an oligomeric system where n is about 9 the material is water soluble, has Amax(DMSO) 614 nm, and generates both superoxide and singlet oxygen on irradiation ([Pg.987]

The two water-soluble porphyrins, 5,10,15,20-tetra(4-N-methyl-pyridiniumyl) porphyrin (H2TMPyP4+) and 5,10,15,20-tetra(4-sulfonato) porphyrin (H2TPPS4 ), were immobilized in the silica sol-gel matrix and served as the pH sensors as well65. The sol-gels doped with porphyrin were... 

Delmarre D., Meallet-Renault R., Bied-Charreton C., Pasternack R., Incorporation of water-soluble porphyrins in sol-gel matrices and application to pH sensing, Anal. Chim. Acta 1999 401 125-128. 

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