Activation energies for oxidation of porphyrin monolayers anchored to Au(111)

The activation energy for the oxidation of porphyrin monolayers anchored to gold surfaces is determined via measurement of the temperature dependence of the electron-transfer rates. The activation energy (1) increases with increasing surface concentration of the porphyrin and (2) is significantly lower (8.1-17 versus 37-49 kJ mol(-1)) when smaller, more mobile counterions (Cl(-) versus PF(6)(-)) are used as the supporting electrolyte. Regardless, the lower activation energies do not result in radically different electron-transfer rates for the different types of counterions owing to compensating entropic effects.

Light-harvesting antenna system from the phototrophic bacterium Roseiflexus castenholzii

Photosynthetic organisms have evolved diverse light-harvesting complexes to harness light of various qualities and intensities. Photosynthetic bacteria can have (bacterio)chlorophyll Q(y) antenna absorption bands ranging from approximately 650 to approximately 1100 nm. This broad range of wavelengths has allowed many organisms to thrive in unique light environments. Roseiflexus castenholzii is a niche-adapted, filamentous anoxygenic phototroph (FAP) that lacks chlorosomes, the dominant antenna found in most green bacteria, and here we describe the purification of a full complement of photosynthetic complexes: the light-harvesting (LH) antenna, reaction center (RC), and core complex (RC-LH). By high-performance liquid chromatography separation of bacteriochlorophyll and bacteriopheophytin pigments extracted from the core complex and the RC, the number of subunits that comprise the antenna was determined to be 15 +/- 1. Resonance Raman spectroscopy of the carbonyl stretching region displayed modes indicating that 3C-acetyl groups of BChl a are all involved in molecular interactions probably similar to those found in LH1 complexes from purple photosynthetic bacteria. Finally, two-dimensional projections of negatively stained core complexes and the LH antenna revealed a closed, slightly elliptical LH ring with an average diameter of 130 +/- 10 A surrounding a single RC that lacks an H-subunit but is associated with a tetraheme c-type cytochrome.

Probing the rate of hole transfer in oxidized synthetic chlorin dyads via site-specific (13)C-labeling

Understanding electronic communication among interacting constituents of multicomponent molecular architectures is important for rational design in diverse fields including artificial photosynthesis and molecular electronics. One strategy for examining ground-state hole/electron transfer in an oxidized tetrapyrrolic array relies on analysis of the hyperfine interactions observed in the EPR spectrum of the pi-cation radical. This strategy has been previously employed to probe the hole/electron-transfer process in oxidized multiporphyrin arrays of normal isotopic composition, wherein (1)H and (14)N serve as the hyperfine "clocks", and in arrays containing site-specific (13)C-labels, which serve as additional hyperfine clocks. Herein, the hyperfine-clock strategy is applied to dyads of dihydroporphyrins (chlorins). Chlorins are more closely related structurally to chlorophylls than are porphyrins. A de novo synthetic strategy has been employed to introduce a (13)C label at the 19-position of the chlorin macrocycle, which is a site of large electron/hole density and is accessible synthetically beginning with (13)C-nitromethane. The resulting singly (13)C-labeled chlorin was coupled with an unlabeled chlorin to give a dyad wherein a diphenylethyne linker spans the 10-positions of the two zinc chlorins. EPR studies of the monocations of both the natural abundance and (13)C-labeled zinc chlorin dyads and benchmark zinc chlorin monomers reveal that the time scale for hole/electron transfer is in the 4-7 ns range, which is 5-10-fold longer than that in analogous porphyrin arrays. The slower hole/electron transfer rate observed for the chlorin versus porphyrin dyads is attributed to the fact that the HOMO is a(1u)-like for the chlorins versus a(2u)-like for the porphyrins; the a(1u)-like orbital exhibits little (or no) electron/hole density at the site of linker attachment whereas the a(2u)-like orbital exhibits significant electron/hole density at this site. Collectively, the studies of the chlorin and porphyrin dyads provide insights into the structural features that influence the hole/electron-transfer process.

In vitro photodynamic therapy and quantitative structure-activity relationship studies with stable synthetic near-infrared-absorbing bacteriochlorin photosensitizers

Photodynamic therapy (PDT) is a rapidly developing approach to treating cancer that combines harmless visible and near-infrared light with a nontoxic photoactivatable dye, which upon encounter with molecular oxygen generates the reactive oxygen species that are toxic to cancer cells. Bacteriochlorins are tetrapyrrole compounds with two reduced pyrrole rings in the macrocycle. These molecules are characterized by strong absorption features from 700 to >800 nm, which enable deep penetration into tissue. This report describes testing of 12 new stable synthetic bacteriochlorins for PDT activity. The 12 compounds possess a variety of peripheral substituents and are very potent in killing cancer cells in vitro after illumination. Quantitative structure-activity relationships were derived, and subcellular localization was determined. The most active compounds have both low dark toxicity and high phototoxicity. This combination together with near-infrared absorption gives these bacteriochlorins great potential as photosensitizers for treatment of cancer.

Stable synthetic bacteriochlorins overcome the resistance of melanoma to photodynamic therapy

Cutaneous malignant melanoma remains a therapeutic challenge, and patients with advanced disease have limited survival. Photodynamic therapy (PDT) has been successfully used to treat many malignancies, and it may show promise as an antimelanoma modality. However, high melanin levels in melanomas can adversely affect PDT effectiveness. Herein the extent of melanin contribution to melanoma resistance to PDT was investigated in a set of melanoma cell lines that markedly differ in the levels of pigmentation; 3 new bacteriochlorins successfully overcame the resistance. Cell killing studies determined that bacteriochlorins are superior at (LD(50) approximately 0.1 microM) when compared with controls such as the FDA-approved Photofrin (LD(50) approximately 10 microM) and clinically tested LuTex (LD(50) approximately 1 microM). The melanin content affects PDT effectiveness, but the degree of reduction is significantly lower for bacteriochlorins than for Photofrin. Microscopy reveals that the least effective bacteriochlorin localizes predominantly in lysosomes, while the most effective one preferentially accumulates in mitochondria. Interestingly all bacteriochlorins accumulate in melanosomes, and subsequent illumination leads to melanosomal damage shown by electron microscopy. Fluorescent probes show that the most effective bacteriochlorin produces significantly higher levels of hydroxyl radicals, and this is consistent with the redox properties suggested by molecular-orbital calculations. The best in vitro performing bacteriochlorin was tested in vivo in a mouse melanoma model using spectrally resolved fluorescence imaging and provided significant survival advantage with 20% of cures (P<0.01).

Imidazole metalloporphyrins as photosensitizers for photodynamic therapy: role of molecular charge, central metal and hydroxyl radical production

The in vitro photodynamic therapy activity of four imidazole-substituted metalloporphyrins has been studied using human (HeLa) and mouse (CT26) cancer cell lines: an anionic Zn porphyrin and a homologous series of three cationic Zn, Pd or InCl porphyrins. A dramatic difference in phototoxicity was found: Pd cationic>InCl cationic>Zn cationic>Zn anionic. HeLa cells were more susceptible than CT26 cells. Induction of apoptosis was demonstrated using a fluorescent caspase assay. The anionic Zn porphyrin localized in lysosomes while the cationic Zn porphyrin localized in lysosomes and mitochondria, as assessed by fluorescence microscopy. Studies using fluorescent probes suggested that the cationic Pd porphyrin produced more hydroxyl radicals as the reactive oxygen species. Thus, the cationic Pd porphyrin has high potential as a photosensitizer and gives insights into characteristics for improved molecular designs.

Energy- and hole-transfer dynamics in oxidized porphyrin dyads

The mechanisms and dynamics of quenching of a photoexcited free base porphyrin (Fb*) covalently linked to a nearby oxidized zinc porphyrin (Zn(+)) have been investigated in a set of five dyads using time-resolved absorption spectroscopy. The dyads include porphyrins joined at the meso-positions by a diphenylethyne linker or a diarylethyne linker with 2,6-dimethyl substitution on either one or both of the aryl rings. Another dyad is linked at the beta-pyrrole positions of the porphyrins via a diphenylethyne linker. The type of linker and attachment site modulate the interporphyrin through-bond electronic coupling via steric hindrance (porphyrin-linker orbital overlap) and attachment motif (porphyrin electron density at the connection site). For each ZnFb dyad, the zinc porphyrin is selectively electrochemically oxidized (to produce Zn(+)Fb), the free base porphyrin is selectively excited with a 130 fs flash (to produce Zn(+)Fb*), and the subsequent dynamics monitored. The Zn(+)Fb* excited state has a lifetime of approximately 3 to approximately 30 ps (depending on the linker steric hindrance and attachment site) and decays by parallel excited-state energy- and hole-transfer pathways. The relative yields of the two channels depend on a number of factors including the linker-mediated through-bond electronic coupling and a modest (< or =20%) Forster through-space contribution for the energy-transfer route. One product of Zn(+)Fb* decay is the metastable ground-state ZnFb(+), which decays to the Zn(+)Fb preflash state by ground-state hole transfer with a linker-dependent rate constant of (20 ps)(-1) to (150 ps)(-1). Collectively, these results provide a detailed understanding of the mechanism and dynamics of quenching of excited porphyrins by nearby oxidized sites, as well as the dynamics of ground-state hole transfer between nonequivalent porphyrins (Zn and Fb). The findings also lay the foundation for the study of ground-state hole transfer between identical porphyrins (e.g., Zn/Zn, Fb/Fb) in larger multiporphyrin arrays wherein a hole is selectively placed via electrochemical oxidation.

Chlorin-bacteriochlorin energy-transfer dyads as prototypes for near-infrared molecular imaging probes: controlling charge-transfer and fluorescence properties in polar media

The photophysical properties of two energy-transfer dyads that are potential candidates for near-infrared (NIR) imaging probes are investigated as a function of solvent polarity. The dyads (FbC-FbB and ZnC-FbB) contain either a free base (Fb) or zinc (Zn) chlorin (C) as the energy donor and a free base bacteriochlorin (B) as the energy acceptor. The dyads were studied in toluene, chlorobenzene, 1,2-dichlorobenzene, acetone, acetonitrile and dimethylsulfoxide (DMSO). In both dyads, energy transfer from the chlorin to bacteriochlorin occurs with a rate constant of approximately (5-10 ps)(-1) and a yield of >99% in nonpolar and polar media. In toluene, the fluorescence yields (Phif=0.19) and singlet excited-state lifetimes (tau approximately 5.5 ns) are comparable to those of the benchmark bacteriochlorin. The fluorescence yield and excited-state lifetime decrease as the solvent polarity increases, with quenching by intramolecular electron (or hole) transfer being greater for FbC-FbB than for ZnC-FbB in a given solvent. For example, the Phif and tau values for FbC-FbB in acetone are 0.055 and 1.5 ns and in DMSO are 0.019 and 0.28 ns, whereas those for ZnC-FbB in acetone are 0.12 and 4.5 ns and in DMSO are 0.072 and 2.4 ns. The difference in fluorescence properties of the two dyads in a given polar solvent is due to the relative energies of the lowest energy charge-transfer states, as assessed by ground-state redox potentials and supported by molecular-orbital energies derived from density functional theory calculations. Controlling the extent of excited-state quenching in polar media will allow the favorable photophysical properties of the chlorin-bacteriochlorin dyads to be exploited in vivo. These properties include very large Stokes shifts (85 nm for FbC-FbB, 110 nm for ZnC-FbB) between the red-region absorption of the chlorin and the NIR fluorescence of the bacteriochlorin (lambdaf=760 nm), long bacteriochlorin excited-state lifetime (approximately 5.5 ns), and narrow (<or=20 nm) absorption and fluorescence bands. The latter will facilitate selective excitation/detection and multiprobe applications using both intensity- and lifetime-imaging techniques.

Comparison of electron-transfer rates for metal- versus ring-centered redox processes of porphyrins in monolayers on Au(111)

The standard electron-transfer rate constants ( k ( 0 )) are measured for redox processes of Fe versus Zn porphyrins in monolayers on Au(111); the former undergoes a metal-centered redox process (conversion between Fe (III) and Fe (II) oxidation states) whereas the latter undergoes a ring-centered redox process (conversion between the neutral porphyrin and the pi-cation radical). Each porphyrin contains three meso-mesityl groups and a benzyl thiol for surface attachment. Under identical solvent (propylene carbonate)/electrolyte (1.0 M Bu 4NCl) conditions, the Zn (II) center has a coordinated Cl (-) ion when the porphyrin is in either the neutral or oxidized state. In the case of the Fe porphyrin, two species are observed a low-potential form ( E l (0) approximately -0.6 V) wherein the metal center has a coordinated Cl (-) ion when it is in either the Fe (II) or Fe (III) state and a high-potential form ( E h (0) approximately +0.2 V) wherein the metal center undergoes ligand exchange upon conversion from the Fe (III) to Fe (II) states. The k ( 0 ) values observed for all of the porphyrins depend on surface concentration, with higher concentrations resulting in slower rates, consistent with previous studies on porphyrin monolayers. The k ( 0 ) values for the ring-centered redox process (Zn chelate) are 10-40 times larger than those for the metal-centered process (Fe chelate); the k ( 0 ) values for the two forms of the Fe porphyrin differ by a factor of 2-4 (depending on surface concentration), the Cl (-) exchanging form generally exhibiting a faster rate. The faster rates for the ring- versus metal-centered redox process are attributed to the participating molecular orbitals and their proximity to the surface (given that the porphyrins are relatively upright on the surface): a pi molecular orbital that has significant electron density at the meso-carbon atoms (one of which is the site of attachment of the linker to the surface anchoring thiol) versus a d-orbital that is relatively well localized on the metal center.

A bipodal-tethered porphyrin for attachment to silicon surfaces in studies of molecular information storage

An approach for molecular information storage entails use of redox-active molecules attached to an electroactive surface. Understanding the structural features that give rise to robust molecular monolayers with high charge density is an essential objective. Toward this goal, a zinc-porphyrin bearing an all-carbon bipodal tether, 5-(1,6-heptadien-4-yl)-15-phenyl-10,20-di-p-tolylporphinatozinc(II), has been synthesized by the reaction of a dipyrromethane and a dipyrromethane-1,9-dicarbinol. The porphyrin was attached to Si(100) via a high temperature procedure. The resulting molecular monolayers exhibited surface coverages, adsorption geometries, electron-transfer rates, and stabilities under repeated electrochemical cycling that are similar to those obtained with monopodal carbon tethers. The observation that the bipodal anchor does not provide enhanced stability is surprising given that attachment is achieved via two versus one covalent linkage.

Polyvalent display of heme on hepatitis B virus capsid protein through coordination to hexahistidine tags

The addition of a hexahistidine tag to the N terminus of the hepatitis B capsid protein gives rise to a self-assembled particle with 80 sites of high local density of histidine side chains. Iron protoporphyrin IX has been found to bind tightly at each of these sites, making a polyvalent system of well-defined spacing between metalloporphyrin complexes. The spectroscopic and redox properties of the resulting particle are consistent with the presence of 80 site-isolated bis(histidine)-bound heme centers, comprising a polyvalent b-type cytochrome mimic.

Metal-molecule interactions upon deposition of copper overlayers on reactively functionalized porphyrin monolayers on Si(100)

The interaction of evaporated Cu deposited on a series of porphyrins in monolayers covalently attached to Si(100) substrates was investigated using cyclic voltammetry and FTIR spectroscopy. Each porphyrin contains a triallyl tripod attached to the porphyrin via a p-phenylene unit. The tripod anchors the porphyrin to the Si(100) substrate via hydrosilylation of the allyl groups. Two of the porphyrins are Zn chelates that possess meso p-cyanophenyl substituentsone, ZnP-CND, contains a single group opposite (distal) to the tripodal surface anchor, whereas the other, ZnP-CNL, contains two groups orthogonal (lateral) to the surface anchor. A third Zn porphyrin, ZnP, containing nonreactive p-tolyl groups at all three nonanchoring meso positions, was examined for comparison. The fourth porphyrin, FbP-HD, is a metal-free species (free base) that contains nonreactive phenyl (distal) and p-tolyl groups (lateral) at the three nonanchoring meso positions. The fifth porphyrin, CuP-HD, is the Cu chelate of FbP-HD, and serves as a reference complex for evaluating the effects of Cu metal deposition onto FbP-HD. The studies indicate that all of the porphyrin monolayers are robust under the conditions of Cu deposition, experiencing no noticeable degradation. In addition, the Cu metal does not penetrate through the monolayer to form electrically conductive filaments. For the ZnP-CND monolayers, the deposited Cu quantitatively reacts/complexes with the distal cyano group. In contrast, for the ZnP-CNL monolayers no reaction/complexation of the lateral cyano groups is observed. For the FbP-HD monolayers, Cu deposition results in quantitative insertion of Cu into the free base porphyrin. Collectively, the studies demonstrate that porphyrin monolayers are amenable to direct deposition of Cu overlayers and that functionalization of the porphyrins can be used to mediate the attributes of the metal-molecule junction.

Effects of substituents on synthetic analogs of chlorophylls. Part 2: Redox properties, optical spectra and electronic structure

The optical absorption spectra and redox properties are presented for 24 synthetic zinc chlorins and 18 free base analogs bearing a variety of 3,13 (beta) and 5,10,15 (meso) substituents. Results are also given for a zinc and free base oxophorbine, which contain the keto-bearing isocyclic ring present in the natural photosynthetic pigments such as chlorophyll a. Density functional theory calculations were carried out to probe the effects of the types and positions of substituents on the characteristics (energies, electron distributions) of the frontier molecular orbitals. A general finding is that the 3,13 positions are more sensitive to the effects of auxochromes than the 5,10,15 positions. The auxochromes investigated (acetyl>ethynyl>vinyl>aryl) cause a significant redshift and intensification of the Qy band upon placement at the 3,13 positions, whereas groups at the 5,10,15 positions result in much smaller redshifts that are accompanied by a decrease in relative Qy intensity. In addition, the substituent-induced shifts in first oxidation and reduction potentials faithfully track the energies of the highest occupied and lowest unoccupied molecular orbitals (HOMO and LUMO), respectively. The calculations show that the LUMO is shifted more by substituents than the HOMO, which derives from the differences in the electron densities of the two orbitals at the substituent sites. The trends in the substituent-induced effects on the wavelengths and relative intensities of the major features (By, Bx, Qx, Qy) in the near-UV to near-IR absorption bands are well accounted for using Gouterman's four-orbital model, which incorporates the effects of the substituents on the HOMO-1 and LUMO+1 in addition to the HOMO and LUMO. Collectively, the results and analysis presented herein and in the companion paper provide insights into the effects of substituents on the optical absorption, redox and other photophysical properties of the chlorins. These insights form a framework that underpins the rational design of chlorins for applications encompassing photomedicine and solar-energy conversion.