Oxidative chemistry of nickel porphyrins.

The oxidative chemistry of nickel(II) porphyrins is reviewed. Whether electron abstraction occurs from the metal to yield Ni(III) or from the porphyrin to yield Ni(II) pi cation radicals is discussed in terms of the relative energy levels of the metal and porphyrin orbitals. The effects of axial ligands in further modulating this ordering as well as the orbital occupancy of Ni(III) are also reviewed. Structural considerations, based on existing stereochemical data for Ni(I), high spin Ni(II) and related Ni(III) tetraaza complexes, are used to predict the metrics of Ni(III) porphyrins for which no structural data are available.

Ligated chlorophyll cation radicals: Their function in photosystem II of plant photosynthesis.

Magnesium tetraphenylchlorin, a synthetic model for chlorophyll, exhibits significant variations in the unpaired spin densities of its cation radicals with concomitant changes in oxidation potentials as a function of solvent and axial ligand. Similar effects are observed for chlorophyll (Chl) a and its cation radicals. Oxidation potentials for Chl --> Chl(+.) as high as +0.9 V (against a normal hydrogen electrode) are observed in nonaqueous solvents, with linewidths of the electron spin resonance signals of monomeric Chl(+.) ranging between 9.2 and 7.8 G in solution. These changes in electronic configuration and ease of oxidation are attributed to mixing of two nearly degenerate ground states of the radicals theoretically predicted by molecular orbital calculations. Comparison of the properties of chlorophyll in vitro with the optical, redox, and magnetic characteristics attributed to P-680, the primary donor of photosystem II which mediates oxygen evolution in plant photosynthesis, leads us to suggest that P-680 may be a ligated chlorophyll monomer whose function as a phototrap is determined by interactions with its (protein?) environment.

The cation radicals of free base and zinc bacteriochlorin, bacteriochlorophyll, and bacteriopheophytin.

One-electron oxidation of zinc tetraphenylbacteriochlorin and its metal-free base yielded stable cation radicals. Electron spin resonance hyperfine splittings were assigned by selective deuterations. These results indicate that the protons of the saturated rings of the bacteriochlorins carry large spin densities, in accord with molecular orbital calculations. Comparison in vitro of the optical spectra of bacteriochlorins and their cation radicals with those of bacteriochlorophyll show close correspondence and suggest that the electron spin resonance data from the former may also prove a guide to the biological molecule. The surprising similarity in properties between the radicals of free base and zinc bacteriochlorins is maintained in the chlorophylls: cation radicals of bacteriopheophytin and methyl pheophorbide (the free bases of bacteriochlorophyll and methyl chlorophyllide, respectively) exhibit electron spin resonance properties similar to those of their magnesium-containing derivatives. The possibility that metal-free chlorophylls participate in photosynthesis is discussed.

The pi-Cation Radical of Chlorophyll a.

Chlorophyll a undergoes reversible one-electron oxidation in dichloromethane and butyronitrile. Removal of the electron by controlled potential electrolysis or by stoichiometric charge transfer to a known cation radical yields a radical (epr line width = 9 gauss, g = 2.0025 +/- 0.0001) whose optical spectrum is bleached relative to that of chlorophyll. Upon electrophoresis this bleached species behaves as a cation. By comparison with the known properties of pi-cation radicals of porphyrins and chlorins, the chlorophyll radical is also identified as a pi-cation. Further correlation of optical and epr properties with published studies on photosynthesis leads to the conclusion that oxidized P700, the first photochemical product of photosystem I in green plants, contains a pi-cation radical of the chlorin component of chlorophyll a. This radical is the likely source of the rapidly-decaying, narrow epr signal of photosynthesis.

Design and synthesis of porphyrins bearing rigid hydrogen bonding motifs: highly versatile building blocks for self-assembly of polymers and discrete arrays.

Two aldehydes, 2,6-diacetamido-4-formylpyridine (7) and 1-butyl-6-formyluracil (11), are used to synthesize five pyridyl and four uracyl meso-subsituted porphyrins. With these complementary porphyrin building blocks, it is possible to build various types of multi-porphyrin supramolecules with different spatial relationships in predefined geometries. The formation and properties of self-complementary dimers and a closed tetrameric square are presented as a basis of comparison to the latter system in the solid state. An X-ray structure of 5,10-bis(4-tert-butylphenyl)-15,20-bis(3,5-diacetamido-4-pyridyl)porphyrin confirms its molecular structure and reveals a hydrogen-bonded supramolecular organization mediated by water molecules.

Compounds I of catalase and horse radish peroxidase: pi-cation radicals.

Two-electron oxidation of cobaltous octaethylporphyrin [Co(II)(Et)(8)P] yields a stable pi-cation radical [Co(III)(Et)(8)P](2+.), the optical spectrum of which exhibits spectral changes dependent upon the nature of the counterion. Comparison of these spectra with those of Compounds I of horseradish peroxidase and catalase leads us to propose that these Compounds I contain a pi-cation radical of the heme prosthetic group. This proposal explains the oxidation level, optical spectra, and stability of the primary compounds without recourse to properties such as stoichiometric mixtures of special porphyrins, stable Fe(V) porphyrins, or unique conformers of heme porphyrins. Explanations are advanced to account for the missing electron spin resonance signal of Compound I of horseradish peroxidase.

Primary charge separation in bacterial photosynthesis: oxidized chlorophylls and reduced pheophytin.

Bacteriopheophytin, the magnesium-free base of bacteriochlorophyll, undergoes reversible one-electron reduction in organic solvents to yield an anionic free radical with characteristic optical and electron spin resonance spectra. The reduction potential of bacteriopheophytin, E1/2 approximately --0.55 V against a normal hydrogen electrode, compared to E1/2 approximately --0.85 V for bacteriochlorophyll, renders it a likely electron acceptor in the primary charge separation of photosynthesis. Comparison of these data with picosecond optical changes recently observed upon pulsed laser excitation of bacterial reaction centers leads us to propose that bacteriopheophytin is indeed a transient electron acceptor and that the primary charge separation of bacterial photosynthesis occurs between the bacteriochlorophyll complex P870 and bacteriopheophytin to yield the radicals of the oxidized chlorophyll dimer cation and reduced pheophytin anion.

pi cation radicals of ferrous and free base isobacteriochlorins: Models for siroheme and sirohydrochlorin.

Theoretical and experimental optical, redox, and paramagnetic results are presented for models of siroheme, the iron isobacteriochlorin prosthetic group of nitrite and sulfite reductases, and of sirohydrochlorin, the metal-free siroheme that is an intermediate in the biosynthetic pathway to vitamin B(12). The facile oxidation of many isobacteriochlorins, which distinguishes them from porphyrins and chlorins, suggests that the siroheme macrocycle itself may undergo oxidation in the multi-electron enzymatic cycles that reduce nitrite to ammonia and sulfite to hydrogen sulfide. Extended Hückel MO calculations (i) help rationalize the redox properties of isobacteriochlorins compared with those of porphyrins and chlorins; (ii) indicate that Fe(II) pyridine carbonyl[(py) (CO)] complexes of isobacteriochlorins, unlike those of porphyrins and chlorins, should undergo oxidation from the macrocycle rather than the metal to yield pi cation radicals; (iii) suggest that, in hexacoordinated Fe(II) isobacteriochlorin complexes, the site of oxidation-i.e., the metal or the macrocycle-will depend on the ligand field induced by the axial ligands; and (iv) predict similar unpaired spin density profiles for metal-free and (py) (CO)Fe(II) isobacteriochlorin radicals. Experimental data for three isomeric free-base and (py) (CO)Fe(II) complexes of dimethyloctaethylisobacteriochlorins support the theoretical calculations and establish the existence of Fe(II) isobacteriochlorin pi cations in vitro.

Polytetrafluoroethylene microprosthesis in the arterial system of the rat.

The experimental behavior of a 1-mm internal diameter (i.d.) polytetrafluoroethylene (PTFE) microprosthesis, as a substitute for an abdominal aortic segment in the rat, was reviewed. Fifty Wistar rats were divided into four groups: Group I--12 rats with autotransplant of an abdominal aortic segment (AAS); Group II--12 rats with allotransplant of an AAS obtained from Long-Evans rats; Group III--12 rats with xenotransplant of an AAS taken from rabbit femoral arteries; and Group IV--14 rats with substitution of an AAS by a 1-mm i.d. PTFE microprosthesis. The rats were sacrificed at different time intervals ranging from five to 360 days, with previous aortography. In Group I, there was a 100 percent patency at a mean of 152.41 days; in Group II, a 91.6 percent patency at a mean of 100.08 days; in Group III, an 83.3 percent patency with a 75 percent aneurysmal dilation at a mean of 107.58 days; in Group IV, a 71.42 percent patency with two anastomotic aneurysms at a mean of 105 days (P less than 0.05, chi square) between Groups I and IV, autotransplant vs. PTFE). The 1-mm PTFE microprosthesis placed in the arterial system of the rat proved to be a reliable alternative for microvascular substitution.

Polytetrafluoroethylene microprosthesis in the venous system of the rat.

In this study, a 2 mm internal diameter (i.d.) polytetrafluoroethylene (PTFE) microprosthesis was used in the venous system of the rat, to determine whether or not it could serve as an acceptable microvenous substitute. Forty Long-Evans rats were divided into four groups: Group 1-10 rats with autotransplant of an inferior vena cava segment; Group 2-10 rats with a segment substitution of the inferior vena cava by a 2 mm i.d. PFTE microprosthesis; Group 3-10 rats with a laterolateral portacaval shunt; and Group 4-10 rats with a portacaval shunt and interposition of a 2 mm i.d. PTFE microprosthesis (new model). The rats were sacrificed at different time intervals up to 100 days, with cavography (femoral access) in Groups 1 and 2 and spleenoportography (direct puncture of the spleen) in Groups 3 and 4, before sacrifice. In Group 1, a 100 percent patency was observed at a mean of 49 days; in Group 2, a 70 percent patency with a 30 percent stenosis at a mean of 39.4 days (p less than 0.05); in Group 3, a 100 percent patency at a mean of 42.5 days; and in Group 4, a 30 percent patency at a mean of 38.4 days (p less than 0.01). Results showed that the 2 mm i.d. PTFE microprosthesis placed in the venous system of the rat is not an efficacious procedure, and that the search for better microvenous substitutes should focus on those of biologic origin.