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PubMed2005

Light-assisted synthesis of Pt-Zn porphyrin nanocomposites and their use for electrochemical detection of organohalides.

Pt-Zn porphyrin nanocomposites have been synthesized using zinc porphyrin and dihydrogen hexachloroplatinate in the presence of light and ascorbic acid. TEM and AFM imaging revealed that Pt nanoparticles with an average diameter of approximately 3.5 nm were embedded within the Zn porphyrin matrix. The glassy carbon electrode was modified with Nafion-stabilized Pt-Zn porphyrin nanocomposites and used for dehalogenation of carbon tetrachloride, chloroform, pentachlorophenol, chlorobenzene, and hexachlorobenzene as five test models. The Pt-Zn porphyrin nanocomposite-modified electrode exhibited catalytic activity for the reduction of organohalides at -1.0 V versus Ag/AgCl. Raman signatures confirmed the dehalogenation of chlorobenzene by the nanocomposite-modified electrode. The above two aliphatic and three aromatic organohalides had detection limits of 0.5 microM with linearity up to 8 microM. The modified electrode was good for at least 80 repeated measurements of 4 microM chlorobenzene with a storage stability of 1 month at room temperature. The deactivation of the electrode activity was associated with the loss of platinum nanoparticles from the nanocomposite structure.

Electrochemistry↗
PubMedDate not supplied

Porphyrin, chlorin, and bacteriochlorin isothiocyanates: useful reagents for the synthesis of photoactive bioconjugates.

A novel method for conjugating porphyrins and related molecules to proteins has been developed. The method, which involves synthesizing porphyrins, chlorins, and bacteriochlorins bearing a single amine-reactive isothiocyanate group represents a facile system for protein labeling with these photoactive species. Problems associated with the noncovalent binding of porphyrins to proteins are highlighted, and a method for purifying conjugates to yield exclusively covalently bound porphyrin protein species is demonstrated. Biological activity of porphyrin-bovine serum albumin conjugates formed and purified by these methods is demonstrated using laser scanning confocal microscopy.

Animals↗
PubMed1987

Influence of heme-surrounding amino acid residues on the manganese (V)-nitrido bond in manganese-substituted hemoproteins: resonance Raman evidence for porphyrin core expansion and reduction of the manganese(V)-nitrido stretching force constant.

Nitridomanganese(V) protoporphyrin IX was prepared by hypochlorite oxidation of the corresponding manganese(III) protoporphyrin IX derivative in the presence of ammonium ion and by photolysis of the corresponding azidomanganese(III) complex. Myoglobin and horseradish peroxidase containing this novel protoporphyrin derivative were prepared for the first time. These remarkably stable species were examined by electronic absorption, electron paramagnetic resonance, and resonance Raman spectroscopies. The MnV-N stretching modes of the nitridomanganese(V)-substituted myoglobin and horseradish peroxidase were observed at 1010 and 1003 cm-1, respectively, by resonance Raman spectroscopy, while the MnV-N stretching frequency for nitridomanganese(V) protoporphyrin IX in 0.1 N aqueous NaOH was found at 1046 cm-1. The equilibrium dissociation energies of MnV-N bonds in these complexes were estimated from vibrational overtone spacings by introducing the Morse potential energy function, were found to be around 4.5 eV, and seemed independent of the surroundings of the manganese porphyrin, although its force constant decreased from 7.3 to 6.7 mdyn/A upon incorporation into apoprotein. The porphyrin ring modes of these nitridomanganese(V) derivatives were influenced greatly upon incorporation into apoproteins, suggestive of the occurrence of porphyrin core expansion. Upon this core expansion the MnV center moves into the mean plane of porphyrin plane, but the access of nitrido (N) toward MnV is restricted due to a steric hindrance from porphyrin pyrrole nitrogens. The resulting stretched MnV-N bond might cause lowering of the MnV-N stretching frequency upon incorporation into apoprotein.

Binding Sites↗
PubMed1990

Drug binding by branched DNA: selective interaction of tetrapyridyl porphyrins with an immobile junction.

The differential binding of a number of water-soluble cationic porphyrins to a branched DNA molecule is reported. Tetrakis(4-N-methylpyridiniumyl)porphine (H2TMpyP-4) interacts near the branch point with an immobile DNA junction formed from four 16-mer strands. Its Cu(II) and Ni(II) derivatives show stronger preferential binding in the neighborhood of the branch point. Axially liganded derivatives, Zn, Co, and Mn, also interact near this branch point, but in a different way. We use the reagents methidiumpropyl-EDTA.Fe(II) [MPE.Fe(II)] and bis(o-phenanthroline)copper(I) [(OP)2Cu(I)] to cleave complexes of DNA duplex controls and the junction with these porphyrins. The resulting cleavage patterns are consistent with previous evidence that the branch point provides a strong site for intercalative binding agents, which is not available in unbranched duplexes of identical sequence. The preferential scission by (OP)2Cu(I) in the presence of Ni and Cu porphyrins near the branch point exceeds that seen for any agents we have studied. This hyperreactivity is not seen in the case of porphyrins with axial ligands, ZnTMpyP-4, CoTMpyP-4, and MnTMpyP-4, although these also interact near the branch point. The Zn derivative tends to protect sites close to the branch point from cutting, while the Co and Mn porphyrins moderately enhance cleavage of sites in this region.

Base Sequence↗
PubMed1981

Protein influences on porphyrin structure in cytochrome c: evidence from Raman difference spectroscopy.

To probe the details of protein heme interactions, we have developed a Raman difference spectroscopic technique, which allows reliable detection of very small, approximately equal to 0.01 cm-1, frequency differences. When this technique is applied to heme proteins, structural differences in the protein which perturb the porphyrin macrocycle may be examined by obtaining Raman difference data on the porphyrin vibrational modes which are strongly enhanced in the Raman spectrum produced with visible laser excitation. We report here Raman difference spectroscopic data on cytochromes c from 24 species. The differences in the Raman spectrum of the porphyrin between the cytochromes c of any two species are small, confirming that all of the cytochromes we have examined have the same "cytochrome fold". However, many small (0.02-2 cm-1) but systematic differences were detected which indicate structural differences among these proteins. These differences could be classified into three different groups and interpreted in terms of different types of structural variations resulting from specific differences in the amino acid sequences. First, direct interactions between near-heme residues and the porphyrin influence the electron density in the pi orbitals of the porphyrin macrocycle. Second, variation in the residue at position 92, far removed from the heme, affects the frequency of the core-size marker line at 1584 cm-1. Third, the conformation near cysteine 14 affects the shape of the Raman mode which is sensitive to the pyrrole ring substituents (approximately 1313 cm-1). From these data we conclude that there are several ways in which the protein amino acid sequence may regulate the oxidation-reduction potential and several ways in which the sequence can modify the binding site between cytochrome c and its redox partners.

Animals↗
PubMed2002

Binding of protoporphyrin IX and metal derivatives to the active site of wild-type mouse ferrochelatase at low porphyrin-to-protein ratios.

Resonance Raman (RR) spectroscopy is used to examine porphyrin substrate, product, and inhibitor interactions with the active site of murine ferrochelatase (EC 4.99.1.1), the terminal enzyme in the biosynthesis of heme. The enzyme catalyzes in vivo Fe(2+) chelation into protoporphyrin IX to give heme. The RR spectra of native ferrochelatase show that the protein, as isolated, contains varying amounts of endogenously bound high- or low-spin ferric heme, always at much less than 1 equiv. RR data on the binding of free-base protoporphyrin IX and its metalated complexes (Fe(III), Fe(II), and Ni(II)) to active wild-type protein were obtained at varying ratios of porphyrin to protein. The binding of ferric heme, a known inhibitor of the enzyme, leads to the formation of a low-spin six-coordinate adduct. Ferrous heme, the enzyme's natural product, binds in the ferrous high-spin five-coordinate state. Ni(II) protoporphyrin, a metalloporphyrin that has a low tendency toward axial ligation, becomes distorted when bound to ferrochelatase. Similarly for free-base protoporphyrin, the natural substrate of ferrochelatase, the RR spectra of porphyrin-protein complexes reveal a saddling distortion of the porphyrin. These results corroborate and extend our previous findings that porphyrin distortion, a crucial step of the catalytic mechanism, occurs even in the absence of bound metal substrate. Moreover, RR data reveal the presence of an amino acid residue in the active site of ferrochelatase which is capable of specific axial ligation to metals.

Animals↗
PubMed2004

Efficient synthesis and photodynamic activity of porphyrin-saccharide conjugates: targeting and incapacitating cancer cells.

Since the role of saccharides in cell recognition, metabolism, and cell labeling is well-established, the conjugation of saccharides to drugs is an active area of research. Thus, one goal in the use of saccharide-drug conjugates is to impart a greater specificity toward a given cell type or other targets. Although widely used to treat some cancers and age related macular degeneration, the drugs used in photodynamic therapy (PDT) display poor chemical selectivity toward the intended targets, and uptake by cells most likely arises from passive, diffusional processes. Instead, the specific irradiation of the target tissues, and the formation of the toxic species in situ, are the primary factors that modulate the selectivity in the present mode of PDT. We report herein a two-step method to make nonhydrolyzable saccharide-porphyrin conjugates in high yields using a tetra(pentafluorophenyl)porphyrin and the thio derivative of the sugar. As a demonstration of their properties, the selective uptake (and/or binding) of these compounds to several cancer cell types was examined, followed by an investigation of their photodynamic properties. As expected, different malignant cell types take up one type of saccharide-porphyrin conjugate preferentially over others; for example, human breast cancer cells (MDA-MB-231) absorb a tetraglucose-porphyrin conjugate over the corresponding galactose derivative. Doseametric studies reveal that these saccharide-porphyrin conjugates exhibit varying PDT responses depending on drug concentration and irradiation energy. (1) Using 20 microM conjugate and greater irradiation energy induces cell death by necrosis. (2) When 10-20 microM conjugate and less irradiation energy are used, both necrosis and apoptosis are observed. (3) Using 10 microM and the least irradiation energy, a significant reduction in cell migration is observed, which indicates a reduction in aggressiveness of the cancer cells.

Animals↗
PubMed2001

Structural and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin.

A quantitative study of the structure and electronic properties of Co-corrole, Co-corrin, and Co-porphyrin, using density functional theory, is reported. The structure of each macrocycle is optimized, with no symmetry constraints, by considering different spin states. The ground-state structures and spin states (S = 1 for Co-corrole, S = 0 for Co-corrin and S = 1/2 for Co-porphyrin) are in good agreement with the experimental data available. The trends in the sizes of the coordination cavities upon varying the inner metal atom and/or the macrocycle are analyzed and compared with those for the Fe-porphyrin we studied previously. Our results reveal that most of the distortion of the Co-corrin core in the B12 coenzyme is due to the inherent properties of Co-corrin. Quite different behaviors are found between corrinoids and porphyrins upon varying the spin state. While an increase in the metal-nitrogen (M-N) distance with spin state occurs in the porphyrins, the corrinoids show little variation in the M-N distance and, in some cases, undergo small structural changes in the ring structure. These results aid in understanding the often discussed question of why nature has chosen corrin/porphyrin for carrying out the particular biological functions identified in the B12 coenzyme.

Cobalt↗
PubMed2001

Synthesis and DNA binding of novel water-soluble cationic methylcobalt porphyrins.

Organocobalt derivatives of tetracationic water-soluble porphyrins are difficult to prepare via the typical reductive alkylation of the Co(II)(por) (porH(2) = porphyrin ligand). None have been reported. The problem may arise because the porphyrin core is made relatively electron poor by the positively charged peripheral groups. We have circumvented this problem by using the [Co(III)(NH(3))(5)CH(3)](2+) reagent, which inserts the Co(III)-CH(3) moiety directly into porH(2) in water under basic conditions. The method afforded two new [CH(3)Co(por)](4+) derivatives, [CH(3)CoTMpyP(4)](4+) and [CH(3)CoTMAP](4+), where [TMpyP(4)](4+) and [TMAP](4+) are the coordinated, NH-deprotonated forms of meso-tetrakis(N-methyl-4-pyridiniumyl)porphyrin and meso-tetrakis(N,N,N-trimethylaniliniumyl)porphyrin, respectively. The binding of the two new [CH(3)Co(por)](4+) cations to DNA and to the synthetic DNA polymers [poly(dA-dT)](2) and [poly(dG-dC)](2) was studied. Using published criteria by which changes in DNA viscosity and in the visible and CD spectra in the Soret region can be used to assess DNA binding, we conclude that both are outside binders. A large hypochromicity of the Soret bands of the [CH(3)Co(por)](4+) cations observed upon outside binding to DNA may indicate a high degree of self-stacking. The visible absorption and CD spectra of the [CH(3)Co(por)](4+) cations in the presence of 1:1 mixtures of [poly(dA-dT)](2) and [poly(dG-dC)](2) are nearly identical to those with [poly(dA-dT)](2) alone and are very different from those of [poly(dG-dC)](2) alone. Thus, both cations show a high preference for outside binding at AT-rich over GC-rich DNA sites. Upon binding of each of the [CH(3)Co(por)](4+) cations to all of the DNA polymers, the Soret bands exhibit blue shifts, whereas the Soret bands of the corresponding [(H(2)O)(2)Co(por)](5+) cations exhibit red shifts. The blue shifts strongly suggest that the [CH(3)Co(por)](4+) cations, particularly [CH(3)CoTMAP](4+), become five-coordinate forms to some extent on DNA binding; this result is the first good evidence for the presence at equilibrium of five-coordinate CH(3)Co(III)(N(4)) forms in water.

Alkylation↗
PubMed2005

Electronic structures of six-coordinate ferric porphyrin complexes with weak axial ligands: usefulness of 13C NMR chemical shifts.

1H NMR, (13)C NMR, and EPR spectra of six-coordinate ferric porphyrin complexes [Fe(Por)L2]ClO4 with different porphyrin structures are presented, where porphyrins (Por) are planar 5,10,15,20-tetraphenylporphyrin (TPP), ruffled 5,10,15,20-tetraisopropylporphyrin (TiPrP), and saddled 2,3,7,8,12,13,17,18-octaethyl-5,10,15,20-tetraphenylporphyrin (OETPP), and axial ligands (L) are weak oxygen ligands such as pyridine-N-oxide, substituted pyridine-N-oxide, DMSO, DMF, MeOH, THF, 2-MeTHF, and dioxane. These complexes exhibit the spin states ranging from an essentially pure high-spin (S = 5/2) to an essentially pure intermediate-spin (S = 3/2) state depending on the field strength of the axial ligands and the structure of the porphyrin rings. Reed and Guiset reported that the pyrrole-H chemical shift is a good probe to determine the spin state in the spin admixed S = 5/2,3/2 complexes (Reed, C. A.; Guiset, F. J. Am. Chem. Soc. 1996, 118, 3281-3282). In this paper, we report that the chemical shifts of the alpha- and beta-pyrrole carbons can also be good probes to determine the spin state because they have shown good correlation with those of the pyrrole-H or pyrrole-C(alpha). By putting the observed or assumed pyrrole-H or pyrrole-C(alpha) chemical shifts of the pure high-spin and pure intermediate-spin complexes into the correlation equations, we have estimated the carbon chemical shits of the corresponding complexes. The orbital interactions between iron(III) and porphyrin have been examined on the basis of these chemical shifts, from which we have found that both the d(xy)-a(2u) interaction in the ruffled Fe(T(i)PrP)L2+ and d(xy)-a(1u) interaction in the saddled Fe(OETPP)L2+ are quite weak in the high-spin and probably in the intermediate-spin complexes as well. Close inspection of the correlation lines has suggested that the electron configuration of an essentially pure intermediate-spin Fe(T(i)PrP)L2+ changes from (d(xy), d(yz))3(d(xy))1(d(z)2)1 to (d(xy))2(d(xz), d(yz))2(d(z)2)1 as the axial ligand (L) changes from DMF to MeOH, THF, 2-MeTHF, and then to dioxane. Although the DFT calculation has indicated that the highly saddled intermediate-spin Fe(OETPP)(THF)2+ should adopt (d(xy), d(yz))3(d(xy))1(d(z)2)1 rather than (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the strong d(xy)-a(1u) interaction (Cheng, R.-J.; Wang, Y.-K.; Chen, P.-Y.; Han, Y.-P.; Chang, C.-C. Chem. Commun. 2005, 1312-1314), our 13C NMR study again suggests that Fe(OETPP)(THF)2+ should be represented as (d(xy))2(d(xz), d(yz))2(d(z)2)1 because of the weak d(xy)-a(1u) interaction. The contribution of the S = 3/2 state in all types of the spin admixed S = 5/2,3/2 six-coordinate complexes has been determined on the basis of the (13)C NMR chemical shifts.

Carbon Isotopes↗
PubMed2006

Mixed substituted porphyrins: structural and electrochemical redox properties.

To examine the influence of mixed substituents on the structural, electrochemical redox behavior of porphyrins, two new classes of beta-pyrrole mixed substituted free-base tetraphenylporphyrins H2(TPP(Ph)4X4) (X = CH3, H, Br, Cl, CN) and H2(TPP(CH3)4X4) (X = H, Ph, Br, CN) and their metal (M = Ni(II), Cu(II), and Zn(II)) complexes have been synthesized effectively using the modified Suzuki cross-coupling reactions. Optical absorption spectra of these porphyrins showed significant red-shift with the variation of X in H2(TPPR4X4), and they induce a 20-30 nm shift in the B band and a 25-100 nm shift in the longest wavelength band [Q(x)(0,0)] relative to the corresponding H2TPPR4 (R = CH3, Ph) derivatives. Crystal structure of a highly sterically crowded Cu(TPP(Ph)4(CH3)4).2CHCl3 complex shows a combination of ruffling and saddling of the porphyrin core while the Zn(TPP(Ph)4Br4(CH3OH)).CH3OH structure exhibits predominantly saddling of the macrocycle. Further, the six-coordinated Ni(TPP(Ph)4(CN)4(Py)2).2(Py) structure shows nearly planar geometry of the porphyrin ring with the expansion of the core. Electrochemical redox behavior of the MTPPR4X4 compounds exhibit dramatic cathodic shift in first ring oxidation potentials (300-500 mV) while the reduction potentials are marginally cathodic in contrast to their corresponding MTPPX4 (X = Br, CN) derivatives. The redox potentials were analyzed using Hammett plots, and the highest occupied molecular orbital-lowest unoccupied molecular orbital (HOMO-LUMO) gap decreases with an increase in the Hammett parameter of the substituents. Electronic absorption spectral bands of H2TPPR4X4 are unique that their energy lies intermediate to their corresponding data for the H2(TPPX8) (X = CH3, Ph, Br, Cl) derivatives. The dramatic variation in redox potentials and large red-shift in the absorption bands in mixed substituted porphyrins have been explained on the basis of the nonplanarity of the macrocycle and substituent effects.

Crystallography, X-Ray↗
PubMed2006

Fast catalytic hydroxylation of hydrocarbons with ruthenium porphyrins.

Ruthenium porphyrin complexes such as carbonylruthenium(II) tetrakispentafluorophenylporphyrin [Ru(II)(TPFPP)(CO)] were found to be efficient catalysts for the hydroxylation of alkanes in the presence of 2,6-dichloropyridine N-oxide as the oxidant under mild, nonacidic conditions. Up to 14 800 turnovers (TO) and rates of 800 TO/min were obtained for the hydroxylation of adamantane. The hydroxylation of cis-decalin afforded cis-9-decalol and cis-decalin-9,10-diol, exclusively, thus, excluding a long-lived radicals mechanism. The kinetics of product evolution in a typical catalytic oxygenation showed an initial induction period followed by a fast, apparently zero-order phase with maximum rates and high efficiencies. Deuterium isotope effects (kH/kD) in the range of 4.2-6.4 were found for the hydroxylation of alkanes. A Hammett treatment of the data for the oxidation of para-substituted toluene derivatives showed a linear correlation with a highly negative rho+ value of -2.0. On the basis of kinetic and spectroscopic evidence, Ru(VI)(TPFPP)(O)2, Ru(II)(TPFPP)(CO), and Ru(IV)(TPFPP)Cl2 observed during catalysis were ruled out as candidates for the active catalyst responsible for the high efficiencies and turnover rates in the oxidation reactions. The fastest rates of adamantane hydroxylation with 2,6-dichloropyridine N-oxide were achieved by the reductive activation of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. This redox activation is consistent with the formation of an active Ru(III) intermediate in situ by a one-electron reduction of the Ru(IV) porphyrin. EPR spectra characteristic of Ru(III) have been observed upon the reduction of Ru(IV)(TPFPP)Cl2 with a zinc amalgam. In the adamantane oxidation mediated with Ru(III)(TPFPP)(OEt), it was found that, during this process, the Ru(III) porphyrin was gradually converted to a dioxoRu(VI) porphyrin. Concomitant with this conversion, the reaction rates decreased. Catalyst activation was also stimulated by autoxidation of the solvent CH2Cl2. On the basis of these data, a mechanism is proposed that incorporates a "fast" cycle involving metastable Ru(III) and oxoRu(V) intermediates and a "slow" oxidation cycle, mediated by oxoRu(IV) and trans-dioxoRu(VI) porphyrin complexes.

Adamantane↗
PubMed2006

Electron spin-spin exchange coupling mediated by the porphyrin pi system.

The syntheses and electron paramagnetic resonance (EPR) spectral characterizations of porphyrins (1-3) substituted with two radical groups bound to trans-meso positions are described. One of these compounds, 3, has been studied by variable-temperature magnetic susceptibility and has been structurally characterized. Biradical porphyrin 3 is monoclinic, space group P2(1)/n, with a = 12.239(2) A, b = 17.819(3) A, c = 34.445(7) A, alpha = 90 degrees , beta = 97.466(3) degrees , gamma = 90 degrees , and Z = 2. The bis(nitroxide) porphyrins 1 and 2 exhibit fluid solution EPR spectra consistent with |J| >> |a|. No evidence was observed for conformational modulation of J by rotation about single bonds as shown by the lack of change of the EPR spectra as a function of temperature. The bis(semiquinone) porphyrin 3 exhibits frozen-solution EPR spectra with zero-field splitting and a Deltam(s) = 2 transition characteristic of a triplet state. The intensity of the Deltam(s) = 2 transition of 3 was measured as a function of temperature, and the data fit according to a singlet-triplet model to yield J(3,solution) = -75 cm(-1) (H = - 2Jŝ1.ŝ2). Polycrystalline samples of porphryin 3 were examined by variable-temperature magnetometry. The paramagnetic susceptibility data were fit using a modified Bleaney-Bowers equation to give J(3,solid) = -29 cm(-1) (H = - 2Jŝ(1).ŝ(2)). The antiferromagnetic J values are consistent with the pi topology of the porphyrin ring.

Chemistry↗
PubMed2001

Photoswitched singlet energy transfer in a porphyrin-spiropyran dyad.

A photochromic nitrospiropyran moiety (Sp) has been covalently linked to a zinc (PZn) and to a free-base (P(H2)) porphyrin. In the resulting dyads (P(Zn)-Sp(c) and P(H2)-Sp(c)), the porphyrin first excited singlet states are unperturbed by the closed form of the attached spiropyran. Excitation of the spiropyran moiety of either dyad in the near-UV region results in ring opening to a merocyanine form (P-Sp(o)) that absorbs at 600 nm. The open form re-closes thermally in 2-methyltetrahydrofuran with a time constant of 20 s, or following irradiation into the 600 nm band. Excitation of the zinc porphyrin moiety in the merocyanine form of the dyad yields 1PZn-Sp(o). The lifetime of the zinc porphyrin excited state is reduced from its usual value of 1.8 ns to 130 ps by singlet-singlet energy transfer to the merocyanine moiety to give PZn-1Sp(o). The quantum yield of energy transfer is 0.93. Quenching is also observed in the free base dyad, where 1P(H2)-Sp(o) and P(H2)-1Sp(o) exchange singlet excitation energy. This photoswitchable quenching phenomenon provides light-activated control of the porphyrin excited states, and consequently control of any subsequent energy or electron-transfer processes that might be initiated by these excited states in more complex molecular photonic or optoelectronic devices.

Benzopyrans↗
PubMed2004

The dithianyl group as a synthon in porphyrin chemistry: condensation reactions and preparation of formylporphyrins under basic conditions.

Vilsmeier formylation is one of the most widely used substitution reactions for the functionalization of porphyrins. However, its utility is limited by the electrophilic/acidic reaction conditions, deactivation of the aromatic system and regiochemical problems, the requirement for metal complexes and necessity for subsequent demetalation under harsh conditions, and low functional group tolerance. To overcome these limitations, the dithianyl group has been utilized as a latent formyl synthon in porphyrin chemistry. 2-Formyl-1,3-dithiane can be used directly in pyrrole condensation reactions to regioselectively yield porphyrins with up to four dithianyl residues. Likewise, 5-dithianyldipyrromethane could be prepared quantitatively as a key building block for various porphyrin condensation reactions yielding the respective free base formylporphyrins after deprotection. Additionally, dithianyllithium can be used as a reagent for the direct aromatic substitution of metallo- and free base porphyrins under nucleophilic conditions.

Formates↗
PubMed2005

Dendrimers made of porphyrin cores and carbazole chromophores as peripheral units. Absorption spectra, luminescence properties, and oxidation behavior.

Luminescent and redox-active porphyrin-based dendrimers of first and second generation have been synthesized, and their absorption spectra, photophysical properties, and oxidation behavior have been investigated, together with those of the corresponding aldehyde carbazole precursors. All the dendrimers contain a porphyrin core and carbazole-based chromophores as branches. The structural formulas of the new species are represented in Figures 1 and 2, with the corresponding schematizations. The absorption spectra of the aldehyde carbazole precursors A1-A6 in dichloromethane exhibit intense transitions in the UV region, centered on the carbazole and benzaldehyde subunits. The lowest-energy absorption bands receive contribution from charge-transfer transitions. Compounds A1-A6 are luminescent at room temperature in fluid solution; such a luminescence is attributed to twisted intramolecular charge-transfer excited states. The luminescence at 77 K in a rigid matrix is blue-shifted with respect to room-temperature emission and is assigned to locally excited states. Absorption spectra of the porphyrin-cored dendrimers P1-P6 appear additive as they are constituted by visible bands due to porphyrin absorption and bands in the UV region due to transitions centered on the carbazole-based branches. Emission spectra of P1-P6 both at 77 K and at room temperature are typical of porphyrin species and independent of excitation wavelength, indicating that the light collected by the peripheral chromophores is quantitatively transferred to the core. All the compounds exhibit a rich oxidation behavior in 1,2-dichloroethane solution, with reversible processes centered on the different carbazole subunits. Interaction between the different carbazole centers depends on the size of the spacer interposed.

Carbazoles↗
PubMed2006

Thermodynamics of interactions of water-soluble porphyrins with RNA duplexes.

We characterized the interactions of meso-tetrakis(4N-(2-hydroxyethyl)pyridinium-4-yl) porphyrin (TEtOHPyP4), meso-tetrakis(4N-allylpyridinium-4-yl) porphyrin (TAlPyP4), and meso-tetrakis(4N-metallylpyridinium-4-yl) porphyrin (TMetAlPyP4) with the poly(rA)poly(rU) and poly(rI)poly(rC) RNA duplexes between 18 and 45 degrees C by employing circular dichroism, light absorption, and fluorescence intensity spectroscopic measurements. Our results suggest that TEtOHPyP4 and TAlPyP4 intercalate into the poly(rA)poly(rU) and poly(rI)poly(rC) host duplexes, while TMetAlPyP4 associates with these RNA duplexes by forming outside-bound, self-stacked aggregates. We used our temperature-dependent absorption titration data to determine the binding constants and stoichiometry for each porphyrin-RNA binding event studied in this work. From the temperature dependences of the binding constants, we calculated the binding free energies, DeltaG(b), enthalpies, DeltaH(b), and entropies, DeltaS(b). For each RNA duplex, the binding enthalpy, DeltaH(b), is the most favorable for TEtOHPyP4 (an intercalator) followed by TAlPyP4 (an intercalator) and TMetAlPyP4 (an outside binder). On the other hand, for each duplex, external self-stacking of TMetAlPyP4 produces the most favorable change in entropy, DeltaS(b), followed by the intercalators TAlPyP4 and TEtOHPyP4. Thus, our results suggest that the thermodynamic profile of porphyrin-RNA binding may correlate with the binding mode. This correlation reflects the differential nature of molecular forces that stabilize/destabilize the two modes of binding-intercalation versus external self-stacking along the host duplex.

Kinetics↗
PubMed2006

Calix[4]arene-linked bisporphyrin hosts for fullerenes: binding strength, solvation effects, and porphyrin-fullerene charge transfer bands.

A calix[4]arene scaffolding has been used to construct bisporphyrin ("jaws" porphyrin) hosts for supramolecular binding of fullerene guests. Fullerene affinities were optimized by varying the nature of the covalent linkage of the porphyrins to the calixarenes. Binding constants for C60 and C70 in toluene were explored as a function of substituents at the periphery of the porphyrin, and 3,5-di-tert-butylphenyl groups gave rise to the highest fullerene affinities (26,000 M(-1) for C60). The origin of this high fullerene affinity has been traced to differential solvation effects rather than to electronic effects. Studies of binding constants as a function of solvent (toluene < benzonitrile < dichloromethane < cyclohexane) correlate inversely with fullerene solubility, indicating that desolvation of the fullerene is a major factor determining the magnitude of binding constants. The energetics of fullerene binding have been determined in terms of DelatH and DeltaS and are consistent with an enthalpy-driven, solvation-dependent process. A direct relationship between supramolecular binding of a fullerene guest to a bisporphyrin host and the appearance of a broad NIR absorption band have been established. The energy of this band moves in a predictable manner as a function of the electronic structure of the porphyrin, thereby establishing its origin in porphyrin-to-fullerene charge transfer.

Algorithms↗