Photogating of ionic currents across lipid bilayers. Hydrophobic ion conductance by an ion chain mechanism

Carbon-1 versus Carbon-3 Linkage of d-Galactose to Porphyrins: Synthesis, Uptake, and Photodynamic Efficiency

The use of glycosylated compounds is actively pursued as a therapeutic strategy for cancer due to the overexpression of various types of sugar receptors and transporters on most cancer cells. Conjugation of saccharides to photosensitizers such as porphyrins provides a promising strategy to improve the selectivity and cell uptake of the photosensitizers, enhancing the overall photosensitizing efficacy. Most porphyrin-carbohydrate conjugates are linked via the carbon-1 position of the carbohydrate because this is the most synthetically accessible approach. Previous studies suggest that carbon-1 galactose derivatives show diminished binding since the hydroxyl group in the carbon-1 position of the sugar is a hydrogen bond acceptor in the galectin-1 sugar binding site. We therefore synthesized two isomeric porphyrin-galactose conjugates using click chemistry: one linked via the carbon-1 of the galactose and one linked via carbon-3. Free base and zinc analogs of both conjugates were synthesized. We assessed the uptake and photodynamic therapeutic (PDT) activity of the two conjugates in both monolayer and spheroidal cell cultures of four different cell lines. For both the monolayer and spheroid models, we observe that the uptake of both conjugates is proportional to the protein levels of galectin-1 and the uptake is suppressed after preincubation with an excess of thiogalactose, as measured by fluorescence spectroscopy. Compared to that of the carbon-1 conjugate, the uptake of the carbon-3 conjugate was greater in cell lines containing high expression levels of galectin-1. After photodynamic activation, MTT and lactate dehydrogenase assays demonstrated that the conjugates induce phototoxicity in both monolayers and spheroids of cancer cells.

Responsive porphyrinoid nanoparticles: development and applications

The economy of space and materials and the continuously increasing demand for advanced functionalities for diverse technologies requires the development of new synthetic methods. Many nanomaterials have enhanced photophysical and photochemical properties in solutions and/or on surfaces, while others have enhanced chemical properties, compared to the atomic, molecular, or bulk phases. Nanomaterials have a wide range of applications in catalysis, sensors, photonic devices, drug delivery, and as therapeutics for treatment of a variety of diseases. Inorganic nanoparticles are widely studied, but the formation of organic nanomaterials via supramolecular chemistry is more recent, and porphyrinoids are at the forefront of this research because of their optical, chemical, and structural properties. The formation of nanoscaled materials via self-assembly and/or self-organization of molecular subunits is an attractive approach because of reduced energy requirements, simpler molecular subunits, and the material can be adaptive to environmental changes. The presence of biocompatible groups such as peptides, carbohydrates, polyglycols and mixtures of these on the periphery of the porphyrin macrocycle may make nanoparticles suitable for therapeutics. This perspective focuses on responsive, non-crystalline porphyrinoid nanomaterials that are less than about 100 nm in all dimensions and used for catalytic or therapeutic applications.

Porphyrins as Molecular Electronic Components of Functional Devices

The proposal that molecules can perform electronic functions in devices such as diodes, rectifiers, wires, capacitors, or serve as functional materials for electronic or magnetic memory, has stimulated intense research across physics, chemistry, and engineering for over 35 years. Because biology uses porphyrins and metalloporphyrins as catalysts, small molecule transporters, electrical conduits, and energy transducers in photosynthesis, porphyrins are an obvious class of molecules to investigate for molecular electronic functions. Of the numerous kinds of molecules under investigation for molecular electronics applications, porphyrins and their related macrocycles are of particular interest because they are robust and their electronic properties can be tuned by chelation of a metal ion and substitution on the macrocycle. The other porphyrinoids have equally variable and adjustable photophysical properties, thus photonic applications are potentiated. At least in the near term, realistic architectures for molecular electronics will require self-organization or nanoprinting on surfaces. This review concentrates on self-organized porphyrinoids as components of working electronic devices on electronically active substrates with particular emphasis on the effect of surface, molecular design, molecular orientation and matrix on the detailed electronic properties of single molecules.

Synthesis of novel asymmetric zinc (II) phthalocyanines bearing octadecyloxyl and glucosyl groups

A novel asymmetric zinc (II) phthalocyanine substituted by one lipophilic octadecyloxyl group and three hydrophilic glucosyl groups was synthesized. Using Q-TOF MS, the major byproducts formed during the deprotection processes were identified. An improved procedure was worked out to convert these byproducts to the desired product in quantitative yield.

Self-organized porphyrinic materials

The self-assembly and self-organization of porphyrins and related macrocycles enables the bottom-up fabrication of photonic materials for fundamental studies of the photophysics of these materials and for diverse applications. This rapidly developing field encompasses a broad range of disciplines including molecular design and synthesis, materials formation and characterization, and the design and evaluation of devices. Since the self-assembly of porphyrins by electrostatic interactions in the late 1980s to the present, there has been an ever increasing degree of sophistication in the design of porphyrins that self-assemble into discrete arrays or self-organize into polymeric systems. These strategies exploit ionic interactions, hydrogen bonding, coordination chemistry, and dispersion forces to form supramolecular systems with varying degrees of hierarchical order. This review concentrates on the methods to form supramolecular porphyrinic systems by intermolecular interactions other than coordination chemistry, the characterization and properties of these photonic materials, and the prospects for using these in devices. The review is heuristically organized by the predominant intermolecular interactions used and emphasizes how the organization affects properties and potential performance in devices.

Electrostatic self-organization of robust porphyrin-polyoxometalate films

Strategies to create thin films using layer-by-layer methods use oppositely charged polymeric polyelectrolytes for both or at least one component to beneficially exploit multitopic electrostatic interactions between the deposited layers with opposite charges. In contrast, the electrostatic deposition of tetracationic 5,10,15,20-tetrakis(1'-methyl-4'-pyridinio)porphyrin tetra(p-toluenesulfonate) (TMPyP(4+)) with tetraanionic polyoxometalates such as EuPW(11)O(39)(4-) or SiW(12)O(40)(4-) onto charged substrates, such as mica, or polar substrates, such as glass and indium-tin oxide (ITO), demonstrates that the use of polymeric components is not a priori necessary. The use of molecules in sequential dipping approaches requires a careful balance in the interaction energies between the oppositely charged molecules, as demonstrated by the observation that a tetraanionic porphyrin such as 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin does not form layers with TMPyP(4+). In the present case, these systems require several rounds of dipping to obtain films of uniform coverage and durability. The thin films deposited onto glass, quartz, ITO, and mica are surprisingly robust, since they are not removed by sonication in either organic solvents or 100 mM NaCl.

meso-Tetra(pentafluorophenyl)porphyrin as an efficient platform for combinatorial synthesis and the selection of new photodynamic therapeutics using a cancer cell line

The four para fluoro groups on 5,10,15,20-tetrakis-(2,3,4,5,6-pentafluorophenyl)-porphyrin (TPPF20) are known to react with a variety of nucleophiles, but the reaction conditions for this substitution reaction depend on the nature of the nucleophiles, e.g. primary amines versus thiols. Glycosylated derivatives of this core porphyrin have been shown to be effective photodynamic agents in the induction of necrosis or apoptosis in several cancer cell lines. The present report demonstrates that TPPF20 can be used as a core platform to efficiently generate a variety of solution-phase combinatorial libraries. The focused combinatorial libraries have substituents that are chosen from a set of motifs known to bind biopolymers such as DNA, be taken up by cancer cells, or to render the compounds amphipathic. Incubation of a breast cancer cell line with these solution-phase libraries, followed by cell lyses and extraction, affords a selection assay. Matrix-assisted laser desorption ionization (MALDI) mass spectrometry of the extracts allows identification of the molecules taken up by the cells. Cell binding assays of the winning compounds synthesized directly indicate that both glycosylation and amphipathicity are key properties since neither tetraglycosylated porphyrins nor those with four polar groups are selected to the same extent. In addition, photodynamic efficacy was evaluated.

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.

Self-organization of self-assembled tetrameric porphyrin arrays on surfaces

The incorporation of designed self-assembled supramolecular structures into devices requires deposition onto surfaces with retention of both structure and function. This remains a challenge and can present a significant barrier to developing devices using self-organizing materials. To examine the role of peripheral groups in the self-organization of self-assembled multiporphyrinic arrays on surfaces, Pd(II)-linked square and Pt(II)-linked trapezoidal tetrameric porphyrin arrays with peripheral tert-butylphenyl or dodecyloxyphenyl functionalities were investigated using various spectroscopies and atomic force microscopy. The Pd(II) assembled squares disassemble upon deposition on glass surfaces, while the Pt(II) assembled trapezoids are more robust and can be routinely cast on these surfaces. The orientation and length of the peripheral alkyl substituents influence the resultant structures on surfaces. The tert-butylphenyl-substituted porphyrin array forms discrete columnar stacks, which assemble in a vertical direction via pi-stacking interactions among the macrocycles. The tetrameric porphyrin array with dodecyloxyphenyl groups forms a continuous film via van der Waals interactions among the peripheral hydrocarbon chains. The super-molecules with liquid crystal-forming moieties also form three-dimensional crystalline structures at higher deposition concentrations. These observations clearly demonstrate that the number, position, and nature of the peripheral groups and the supramolecular structure and dynamics, as well as the energetics of interactions with the surface, are of key importance to the two-dimensional and three-dimensional self-organization of assemblies such as porphyrin arrays on surfaces.

Porphyrins linked directly to the 5,5' positions of 2,2'-bipyridine: a new supramolecular building block and switch

The synthesis and coordination chemistry of two porphyrin dimers linked either at the 5,5' or the 4,4' positions of 2,2'-bipyridine are described. These compounds, which may serve a molecular tectons for the constructions of a variety of supramolecular arrays of diverse function, reveal that the ground- and excited-state electronic communication between the chromophores is only moderately affected by the complexation state of the bipyridyl moiety. The nature of the metal ion chelated by the bipyridine only slightly perturbs the ground-state spectra, and differences observed in the excited state are largely ascribed to the heavy atom effect. This work also shows that conformational changes in structural subunits, in this case induced by bipyridyl complexation of various metal ions, do not necessarily require reorganization of supramolecular systems.

Designing supramolecular porphyrin arrays that self-organize into nanoscale optical and magnetic materials

Tessellation of nine free-base porphyrins into a 3 x 3 array is accomplished by the self-assembly of 21 molecular entities of four different kinds, one central, four corner, and four side porphyrins with 12 trans Pd(II) complexes, by specifically designed and targeted intermolecular interactions. Strikingly, the self-assembly of 30 components into a metalloporphyrin nonamer results from the addition of nine equivalents of a first-row transition metal to the above milieu. In this case each porphyrin in the nonameric array coordinates the same metal such as Mn(II), Ni(II), Co(II), or Zn(II). This feat is accomplished by taking advantage of the highly selective porphyrin complexation kinetics and thermodynamics for different metals. In a second, hierarchical self-assembly process, nonspecific intermolecular interactions can be exploited to form nanoscaled three-dimensional aggregates of the supramolecular porphyrin arrays. In solution, the size of the nanoscaled aggregate can be directed by fine-tuning the properties of the component macrocycles, by choice of metalloporphyrin, and the kinetics of the secondary self-assembly process. As precursors to device formation, nanoscale structures of the porphyrin arrays and aggregates of controlled size may be deposited on surfaces. Atomic force microscopy and scanning tunneling microscopy of these materials show that the choice of surface (gold, mica, glass, etc.) may be used to modulate the aggregate size and thus its photophysical properties. Once on the surface the materials are extremely robust.

Self-organization of self-assembled photonic materials into functional devices: photo-switched conductors

Linear porphyrin arrays self-assembled by either hydrogen bonding or metal ion coordination self-organize into lipid bilayer membranes. The length of the transmembrane assemblies is determined both by the thermodynamics of the intermolecular interactions in the supermolecule and by the dimension and physical chemical properties of the bilayer. Thus, the size of the porphyrin assembly can self-adjust to the thickness of the bilayer. An aqueous electron acceptor is placed on one side of the membrane and an electron donor is placed on the opposite side. When illuminated with white light, substantial photocurrents are observed. Only the assembled structures give rise to the photocurrent, as no current is observed from any of the component molecules. The fabrication of this photogated molecular electronic conductor from simple molecular components exploits several levels of self-assembly and self-organization.

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.

Evidence for ion chain mechanism of the nonlinear charge transport of hydrophobic ions across lipid bilayers

The conductivity across a lipid bilayer by tetraphenylborate anion is increased 10-fold on the photoformation of lipophilic porphyrin cations. The cations alone have negligible conductivity. This nonlinear photogenerated increase of ion conductivity is termed the photogating effect. Substitution of H by Cl in the para position of tetraphenylborate leads to a 100-fold enhancement of conductivity, whereas the dark conductivities for this and other substituted borates are the same. Moreover, the halo-substituted borates show a large enhancement of conductivity in the low concentration range (10(-8) M), whereas that of tetraphenylborate is small and space charge is negligible. The enhanced ion conductivity has great structural sensitivity to the structure of the anion, the cation, and the lipid, whereas the partition coefficient of all the borates and the concentration of photoformed cations are only slightly affected. The photogated ion transport has a twofold larger activation energy than transport in the dark. Time-resolved photocurrents and voltages demonstrate that the translocation rate of the porphyrin cation is also enhanced 100-fold by the Cl-borate anion but only 10-fold by the H-borate anion. For these reasons the nonlinear gating effect cannot be explained by electrostatics alone, but requires an ion chain or ion aggregate mechanism. Kinetic modeling of the photoinduced current with a mixed cation-anion ion chain can fit the data well. The photogating effect allows the direct study of ion interactions within the bilayer.

Charge transfer across a single lipid-water interface causes ion pumping across the bilayer

The photoformation of magnesium-porphyrin cations (P+) at a single lipid bilayer-water interface can pump lipophilic borate anions completely across the lipid bilayer and causes an actual reversal of the photovoltage. The system consists of a lipid bilayer containing magnesium octaethylporphyrin, an aqueous or interfacial electron acceptor on one side, and chloro- or fluoro-substituted tetraphenylborate in both aqueous electrolyte solutions. With 1-micros pulsed illumination, an immediate positive photovoltage is observed, which decreases on the microsecond and millisecond time scales. On the time scale of seconds, as the P+ cation concentration decays in reverse electron transfer, the voltage swings negative to a value almost equal to its initial value and finally decays with a half-time (approximately 20 s) longer than the time constant of the system (approximately 5 s). Thus, an ion gradient across the membrane is formed, trapped by the nonlinear relation between ion mobility and ion concentration. Continuous light illumination confirms that negative charge moves in the direction opposite that of the initial photoinduced electron transfer. Steady-state measurements indicate an ion pumping efficiency of approximately 30%. This simple mechanism may be a progenitor of photobiological ion pumps.

Photoinduced electron transport across a lipid bilayer mediated by C70

Electron transport across a membrane is central to photosynthesis, to mitochondrial respiration and to the design of molecular systems for solar energy conversion. Relatively few synthetic molecules, however, have been shown to facilitate transport of electrons across a lipid bilayer. We report here that C70 can act as both a photosensitizer for electron transfer from a donor molecule and a mediator for electron transport across a lipid bilayer membrane. The steady-state photocurrent density obtained from the C70-bilayer system is about 40 times higher, at comparable light intensities, than that of the carotene-porphyrinquinone system, previously the most efficient artificial system. The C70-bilayer system has a quantum yield of about 0.04, while the stability (tens of minutes) and turnover number (electrons transported per C70 before decay) of 10(3) are one to three orders of magnitude greater than those of other systems. We anticipate that other higher fullerenes may also provide the basis for efficient transmembrane electron-transport systems.

Photogating of ionic currents across lipid bilayers. Electrostatics of ions and dipoles inside the membrane

The conductances of the lipophilic ions tetraphenylboride and tetraphenylphosphonium across a lipid bilayer can be increased or decreased, i.e., gated, by the photoformation of closed-shell metalloporphyrin cations within the bilayer. The gating can be effected by pulsed or continuous light or by chemical oxidants. At high concentrations of lipophilic anions where the dark conductance is saturated due to space charge in the bilayer, the photogated conductance can increase 15-fold. The formation of porphyrin cations allows the conductance to increase to its nonspace charge limited value. Conversely, the decrease of conductance in the light of phosphonium cations diminishes toward zero as the dark conductance becomes space charge limited. We present electrostatic models of the space charge limited conductance that accurately fit the data. One model includes an exponentially varying dielectric constant for the polar regions of the bilayer that allows an analytical solution to the electrostatic problem. The exponential variation of the dielectric constant effectively screens the potential and implies that the inside and outside of real dielectric interfaces can be electrically isolated from one another. The charge density, the distance into the membrane of the ions, about one-quarter of its thickness, and the dielectric constant at that position are determined by these models. These calculations indicate that there is insufficient porphyrin charge density to cancel the boride ion space charge and the following article proposes a novel ion chain mechanism to explain these effects. These models indicate that the positive potential arising from oriented carbonyl ester groups, previously used to explain the 10(3)-fold larger conductance of hydrophobic anions over cations, is smaller than previously estimated. However, the synergistic movement of the positive choline group into the membrane can account for the large positive potential.