RESUMEN
Tetraaryloxy-substituted diazadibenzoperylene bridging ligands 1a,b were employed in transition metal-directed self-assembly with Pd(II) and Pt(II) phosphane triflates 2a,b which resulted in complex dynamic equilibria between molecular triangles 3a-d and molecular squares 4a-d in solution. Characterization of the equilibria and assignment of the metallacycles was accomplished by (1)H and (31)P[(1)H] NMR spectroscopy in combination with electrospray ionization Fourier transform ion cyclotron mass spectrometry (ESI-FTICR-MS). It was found that the equilibria depend on several factors, such as the metal ion (Pd(2+) or Pt(2+)), the solvent, and the steric demand of the phenoxy substituents of the diazadibenzoperylene ligands 1a,b. Introduction of bulky tert-butyl groups in 1b shifts the equilibrium significantly in the direction of the molecular squares. Molecular dynamics simulations of the triangle and square structures revealed critical steric effects and restricted conformational flexibilities of the phosphane and diazadibenzoperylene ligands that help explain the distinct dynamic behavior observed in variable-temperature NMR studies. Concentration-dependent UV/vis and fluorescence spectroscopy revealed the limited stability of the assemblies and confirmed the reversible nature of the dynamic equilibria.
RESUMEN
Tetraaryloxy-substituted perylene tetracarboxylic acid bisimides with one or two 4-pyridyl receptor substituents at the imide functionality were synthesized and employed in transition metal directed self-assembly with Pd(II) and Pt(II) phosphane triflates. Upon mixing of the components, quantitative formation of functional molecular square-type complexes containing four dye molecules and model complexes of a 2:1 (perylene bisimide ligand:transition metal ion) stoichiometry was observed. The isolated metallosupramolecular squares were characterized by 1H and 31P [1H] NMR spectroscopy as well as conventional electrospray ionization (ESI) and ESI-FTICR mass spectrometry, which gave evidence for the structure and the high stability of these giant cyclic dye assemblies (molecular weight (3a) 8172, Pt-Pt corner diagonal ca. 3.4 nm). Studies of the optical absorption and fluorescence properties and the electrochemistry and spectroelectrochemistry of both the perylene bisimide ligands and the perylene bisimide metal complexes show that Pt(II) coordination does not interfere with the optical and electrochemical properties of the perylene bisimide ligands; this gives squares with high fluorescence quantum yields (phiF (3a)=0.88) and three fully reversible redox couples. The latter could be unambiguously related to quantitative formation of perylene bisimide radical cations (E1/2 = +0.93 V vs. Fc/Fc+), radical anions (E1/2= - 1.01 V vs. Fc/Fc+), and dianions (E1/2 = -1.14 V vs. Fc/Fc+); these redox reactions change the charge state of the cyclic assembly from +12 to zero. In contrast, Pd(II) coordination influenced the electrochemical properties of the assembly because of an irreversible palladium reduction at E1/2= -1.15 V versus Fc/Fc+. Finally, dynamic ligand exchange processes between different metallosupramolecular assemblies were investigated by multinuclear NMR and electrospray mass spectrometry. These studies confirmed the reversible nature of the pyridine-Pt(II)/Pd(II) coordination process.
RESUMEN
A series of three perylene tetracarboxylic acid bisimide dyes 3a-c bearing phenoxy substituents at the four bay positions of the perylene core were synthesized and their complexation behavior to complementary ditopic dialkyl melamines 8a-c was investigated. Binding constants and Gibbs binding energies for the hydrogen bonds between the imide and the complementary melamine moiety have been determined in several solvents by NMR and UV/Vis titration experiments with monotopic model compounds 5 and 9. The effects of the solvent polarity and specific solvent-solute interactions on the degree of polymerization of (3 x 8)n are discussed, and a general formula to estimate the chain length of [AA-BB]n nylon-type supramolecular polymers is derived. In addition to the formation of a hydrogen-bonded supramolecular chain. pi-pi interactions were observed for perylene bisimide-melamine assemblies 3b x 8b and 3b x 8c in aliphatic solvents. The orthogonal nature of hydrogen bonding and pi-pi interactions leads to three-dimensional growth yielding large-sized aggregates already in dilute solution. On suitable substrates, densely intertwined networks of nano- to mesoscopic strands are formed which have been investigated by electron microscopy, confocal fluorescence microscopy and optical polarization microscopy. The high fluorescence and excellent photostability of these superstructures is promising for future studies on energy migration and artificial light harvesting at the nano- and mesoscopic length scale.
Asunto(s)
Clorofila/química , Imidas/química , Perileno/análogos & derivados , Perileno/química , Triazinas/química , Fluorescencia , Luz , Espectroscopía de Resonancia Magnética , Microscopía Confocal , Microscopía Electrónica , Conformación Molecular , Polímeros , Soluciones , Espectrofotometría UltravioletaRESUMEN
Cyclic nucleotide-gated (CNG) channels represent one of the three known cellular receptor classes for cGMP. Activation of CNG channels by binding of cyclic nucleotides to a site in the C-terminus results in opening of the channel pore, entry of Ca2+ into the cell and subsequent induction of Ca2+-dependent processes. In this review we will summarize new data on the complex molecular structure and the activation mechanism of CNG channels. In addition, we will discuss the role of CNG channels as mediators of NO:cGMP-dependent cellular processes.
Asunto(s)
AMP Cíclico/fisiología , GMP Cíclico/fisiología , Activación del Canal Iónico , Óxido Nítrico/fisiología , Animales , Humanos , Canales Iónicos/químicaRESUMEN
Sensory transduction in olfactory neurons involves the activation of a cyclic nucleotide-gated (CNG) channel by cAMP. Previous studies identified a CNG channel alpha subunit (CNG2) and a beta subunit (CNG5), which when heterologously expressed form a channel with properties similar but not identical to those of native olfactory neurons. We have cloned a new type of CNG channel beta subunit (CNG4. 3) from rat olfactory epithelium. CNG4.3 derives from the same gene as the rod photoreceptor beta subunit (CNG4.1) but lacks the long, glutamic acid-rich domain found in the N terminus of CNG4.1. Northern blot and in situ hybridization revealed that CNG4.3 is expressed specifically in olfactory neurons. Expression of CNG4.3 in human embryonic kidney 293 cells did not lead to detectable currents. Coexpression of CNG4.3 with CNG2 induced a current with significantly increased sensitivity for cAMP whereas cGMP affinity was not altered. Additionally, CNG4.3 weakened the outward rectification of the current in the presence of extracellular Ca2+, decreased the relative permeability for Ca2+, and enhanced the sensitivity for L-cis diltiazem. Upon coexpression of CNG2, CNG4.3, and CNG5, a conductance with a cAMP sensitivity greater than that of either the CNG2/CNG4.3 or the CNG2/CNG5 channel and near that of native olfactory channel was observed. Our data suggest that CNG4.3 forms a subunit of the native olfactory CNG channel. The expression of various CNG4 isoforms in retina and olfactory epithelium indicates that the CNG4 subunit may be necessary for normal function of both photoreceptor and olfactory CNG channels.
Asunto(s)
Canales Iónicos/genética , Canales Iónicos/fisiología , Neuronas Receptoras Olfatorias/metabolismo , Células Fotorreceptoras Retinianas Bastones/metabolismo , Secuencia de Aminoácidos , Animales , Secuencia de Bases , Calcio/metabolismo , Línea Celular , Membrana Celular/fisiología , Permeabilidad de la Membrana Celular , AMP Cíclico/farmacología , Canales Catiónicos Regulados por Nucleótidos Cíclicos , Exones , Humanos , Intrones , Canales Iónicos/química , Riñón , Sustancias Macromoleculares , Potenciales de la Membrana/efectos de los fármacos , Potenciales de la Membrana/fisiología , Datos de Secuencia Molecular , Técnicas de Placa-Clamp , Ratas , Proteínas Recombinantes/biosíntesis , Proteínas Recombinantes/química , Segmento Externo de la Célula en Bastón/embriología , TransfecciónRESUMEN
We have investigated the subunit composition and the molecular structure of the CNG channel expressed in rat pineal gland. Three types of subunits have been cloned: an alpha-subunit (CNG1), two beta-subunit splice variants (rCNG4.1 and rCNG4.2) and a hydrophilic glutamic acid-rich protein (rGARP). In situ hybridization with sections of rat brain revealed the co-expression of CNG1, CNG4 and GARP in pinealocytes. In addition, CNG4-specific transcripts were detected in the arcuate, periventricular and supraoptic nuclei of the hypothalamus.
Asunto(s)
Encéfalo/metabolismo , Canales Iónicos/biosíntesis , Proteínas de la Membrana/biosíntesis , Glándula Pineal/metabolismo , Transcripción Genética , Empalme Alternativo , Secuencia de Aminoácidos , Animales , Clonación Molecular , Codón , Canales Catiónicos Regulados por Nucleótidos Cíclicos , Humanos , Canales Iónicos/química , Sustancias Macromoleculares , Proteínas de la Membrana/química , Datos de Secuencia Molecular , Especificidad de Órganos , Ratas , Alineación de Secuencia , Homología de Secuencia de AminoácidoRESUMEN
The cDNA of three variants of a cyclic nucleotide-gated (CNG) channel modulatory subunit (CNG4c-CNG4e) has been cloned. CNG4c, CNG4d, and CNG4e differ slightly from each other within an amino-terminal sequence that was originally reported as part of the bovine retinal glutamic acid-rich protein (GARP). The core region of CNG4 is homologous to the second subunit of the human rod photoreceptor channel (hRCNC2b), suggesting that both proteins are alternatively spliced products of the bovine and human homologue of the same gene. CNG4 transcripts are present in retina, testis, kidney, heart, and brain. Expression of CNG4 in HEK293 cells did not lead to detectable currents. Coexpression of CNG4 with the principal subunit of the bovine testis CNG channel (CNG3) resulted in currents which differed in several aspects from that induced by CNG3 alone. The heterooligomeric CNG3/CNG4 and the homooligomeric CNG3 channels were modified by Ca2+-calmodulin and some calmodulin antagonists. The results suggest that CNG4 forms functional heterooligomeric channels with CNG3 in vitro and probably also in intact tissues.