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Nanometer-sized cavities are present in oligonuclear coordination compounds formed in molecular self-assembly processes from C(3)-symmetric ligands and appropriate metal complex fragments. The structures obtained can be described as basic polyhedra such as tetrahedron, hexahedron, or octahedron (see picture).
Membrane filters prepared from porous aluminum oxide (Anopore) were investigated for their potential use as a durable support for enzymes. Alliinase (EC 4.4.1.4) was chosen as a model enzyme for immobilization experiments. To allow for smooth fixation, the enzyme was immobilized indirectly by sugar-lectin binding. Monomolecular layers of the lectin concanavalin A and alliinase were applied by self-assembling processes. As an anchor for these layers, the sugar, mannan, was covalently coupled to the membrane surface. This procedure exhibits several advantages: (i) enzyme immobilization can be carried out under smooth conditions; (ii) immobilization needs little time; and (iii) protein layers may be renewed.
Three [3]catenanes with cavities large enough to accommodate aromatic guests have been designed and synthesized (yields = 5-20 %) by means of kinetically controlled self-assembly processes. The X-ray structural analysis of one of three [3]catenanes confirmed the presence of a rectangular cavity (dimensions = 7 x 11 A) lined by pi-electron-rich recognition sites and hydrogen-bond acceptor groups. In spite of their apparently ideal recognition features, none of these [3]catenanes bind guests incorporating a pi-electron-deficient bipyridinium unit. However, the template-directed syntheses of the [3]catenanes also produce, in yields of 2-23%, [2]catenanes incorporating a 1,5-dioxynaphtho[38]crown-10 interlocked with a bipyridinium-based tetracationic cyclophane. The X-ray structural analyses of two of these [2]catenanes revealed that a combination of [pi...pi] and [C-H...pi] interactions is responsible for the formation of supramolecular homodimers in the solid state. 1H NMR spectroscopic investigations of the four [2]catenanes demonstrated that supramolecular homodimers are also formed (Ka= 17-31M(-1), T= 185 K) in (CD3)2CO solutions. Dynamic 1H NMR spectroscopy revealed that the 1,5-dioxynaphtho[38]crown-10 and tetracationic cyclophane components in the four [2]catenanes and in the three [3]catenanes circumrotate (deltaGc(not equal to) = 9-14 kcal mol(-1)) through each other's cavity in (CD3)2CO. Similarly, the 1,5-dioxynaphthalene and the bipyridinium ring systems rotate (deltaGc(not equal to) =10-14 kcal mol(-1)) about their [O...O] and [N...N] axes, respectively, in solution.
Dinuclear triple-stranded helicates or meso-helicates are formed by self-assembly of linear ligand strands with appropriate metal ions. Very often this self-assembly proceeds with high diastereoselectivity leading either to the helicate or the meso-helicate. The stereoselectivity of the self-assembly process can be influenced by different factors such as the rigidity of the ligand (-->helicate), the number of methylene units in the alkyl spacers (even number of CH2 units --> helicate; odd number --> meso-helicate), template effects, or chiral substituents.
Attractive pi-pi interactions between two of the four outside cavity faces of 1,3-bis-pyridylmethylcalix[4]arene (1) and both faces of 1,4-diiodotetrafluorobenzene (2a) form infinite one-dimensional non-covalent ribbons where the two modules alternate. These ribbons are cross-linked by electron donor-acceptor interactions between picolyl nitrogen atoms of calixarene 1 in one chain and iodine atoms of perfluoroarene 2a in another chain and the two-dimensional supramolecular network 3a is formed. A similar behaviour is also shown by 1,4-dibromotetrafluorobenzene (2b). The halogen bonding and the attractive pi-pi interactions occur in directions which are nearly orthogonal each other. Diiodotetrafluorobenzene, being involved in both these interactions, appears to be a particularly interesting tecton. The ability of electron-poor arenes to elicit the exo-receptor potential of calixarene module by connecting their outside faces through pi-pi interactions may be developed as a new and general binding protocol in calixarene self-assembly processes.
The synthesis and X-ray structures of three metal complexes with terpyridine-derived ligands that contain amino-pyrimidine and amino-pyrazine moieties are presented. They have been designed in view of directing their self-assembly into specific supramolecular arrays through molecular recognition interactions. The solid-state structures indeed reveal extensive hydrogen-bonded networks. The Co complex 4a with PF6- counterions builds a two-dimensional infinite interwoven grid through strong double hydrogen bonds (d(N-H-N) =2.918-3.018 A) between the amino groups and the N atoms of the rings, with all H-bonding sites saturated. Changing the anions to BF4- in 4b leads to a similar infinite but partially broken grid with a quarter of the H-bonding sites unsaturated (d(N-H-N)=2.984-3.206 A). In the case of the Zn complex 12 with triflate anions, half of the hydrogen bonds are formed. Only one of the two orthogonal ligands has hydrogen bonds (d(N-H-N) = 3.082, 3.096 A) to the neighbouring complexes and thus builds linear, supramolecular, polymeric chains. These structural differences are mainly attributed to crystal-packing effects caused by the different anions. The data presented here may also be regarded as a prototype for the generation of organised arrays through sequential self-assembly processes.
The complexation reactions between Ag- and a series of enantiopure ligands belonging to the CHIRAGEN (from CHIRAlity GENerator) family (L1, L2, L3, based on (-)-5,6-pinene bipyridine) have been studied in solution. It has been shown that the length of the bridge plays a fundamental role in the self-assembly processes leading to different compounds: mononuclear complexes (with L3), mixtures of polynuclear complexes (with L2) and circular helicates (with L 1). Although the absolute configuration of the chiral centres in all three ligands is the same, the metal-centred chirality of L3 (delta) is inverted with respect to that in the other two complexes with L1 and L2 (delta). The metal configuration is thus opposite in the mononuclear complex with respect to the polynuclear species. Detailed thermodynamic studies were carried out for the Ag+ and L1 ligand system by 1H and 109Ag NMR spectroscopy (as a function of concentration, temperature and pressure). At low temperature and high pressure, the [Ag6L1(6)]6+ hexanuclear circular helicate forms a tetranuclear circular helicate [Ag4L1(4)]4+: 2[Ag6L1(6)]6+ <=> 3 [Ag4L1(4)]4+. The thermodynamics parameters, obtained by temperature and pressure variation, have the following values: K298 = (8.7 +/- 0.7) x 10(-5) mol x kg(-1), deltaHo = -15.65 +/- 0.8 kJ x mol(-1), deltaSo = -130.2 +/- 3 J x mol(-1) x K(-1) and deltaVo(256 K)= -160 +/- 12 cm3 x mol(-1). The reaction volume calculated according to Connolly's method indicates that the calculated structure of [Ag4L1(4)]4+ is plausible. Both the signs and large magnitudes of deltaSo and deltaVo are counterintuitive, yet can be understood by modelling methods.
A simple template-mediated route, starting from triethalolamine 1, sodium hydride or caesium carbonate, and iron(III) chloride led to the six- and eight-membered iron coronates [Na c [Fe6[N(CH2CH2O)3]6]]+ (2) and [Cs c (Fe8[N(CH2CH2O)3]8]]+ (3). In the reaction of N-methyldiethanolamine 4 (H2L1) or N-(2,5-dimethylbenzyl)iminodiethanol 6 (H2L2) with calcium hydride followed by addition of a solution of iron(III) chloride, the neutral unoccupied coronands [Fe6Cl6(L1)6] (5) and [Fe6Cl6(L2)6] (7) were formed. Subsequent exchange of the chloride ions of 7 by bromide or thiocyanate ions afforded the ferric wheels [Fe6Br6(L2)6] (8) or [Fe6(NCS)6(L2)6] (9), respectively. Titration experiments of solutions of dianion (L1)2- with iron(III) chloride in THF revealed interesting mechanistic details about the self-assembling process leading to 5. At an iron/ligand ratio of 1:1.5 star-shaped tetranuclear [Fe[Fe(L1)2]3] (11) was isolated. However, at an iron/ligand ratio of 1:2, complex 11 was transformed into the ferric wheel 5. It was shown, that the interconversion of 5 and 11 is reversible. Based on the mechanistic studies, a procedure was developed which works for both the synthesis of homonuclear 11 and the star-shaped heteronuclear clusters [Cr[Fe(L1)2]3] (12) and [Al[Fe(L1)2]3] (13). The structures of all new compounds were determined unequivocally by single-crystal X-ray analyses.
The reaction of [(arene)RuCl2]2 (arene = C6H6, cymene, C6H3Et3, or C6Me6) or [Cp*RhCl2]2 with 3-hydroxy-2-pyridone in the presence of Cs2CO3 gives trinuclear metallamacrocyclic complexes. The self-assembly process was shown to be completely diastereoselective, and a racemic mixture of complexes with M(R)M(R)M(R) or MsMsMs (M=Ru, Rh) configuration was obtained. Plausible mononuclear intermediates of the formula [(arene)RuCl(C5H4NO2)] (arene = cymene, C6Me6) have been isolated and characterized. A structurally related trimer was synthesized by using [(cymene)RuCl2]2 and 3-acetamido-2-pyridone instead of 3-hydroxy-2-pyridone. The macrocycles were shown to be highly potent ionophores for Na+ and/or Li+ with negligible affinities for the larger cation K+. The selectivities of the receptors depend on the pi-ligand present: whereas the (C6H6)Ru- and (cymene)Ru complexes bind both Li+ and Na+, the (C6Me6)Ru-, (C6H3Et3)Ru-, and Cp*Rh complexes bind exclusively Li+. For all receptors, the presence of alkali metal ions can be detected electrochemically: the peak potential is shifted by > 300 mV toward anionic potential upon binding. This behavior was utilized to detect Li+ and Na+ colorimetrically. Single crystal X-ray analyses have been carried out on eight complexes, four of which are bound to an alkali metal halide ion pair. Structural parameters, which affect the affinity and selectivity are discussed. A computational study on [[MX][12]crown-3] complexes (M =Li, Na; X=Cl, Br, I) was performed in order to compare relevant bond lengths and angles of the energy-minimized structures with those of the organometallic receptors.
The synthesis of ten benzyl ether based self-assembling monodendrons containing benzo[15]crown-5 at their focal point is presented. These dendritic building blocks self-assemble either directly or via complexation with NaOTf in two-dimensional smectic B, smectic A, and p6mm hexagonal columnar (Phi(h)) and three-dimensional Pm3n cubic lattices. Retrostructural analysis of these lattices and of the lattices generated from the same monodendrons containing various other functional groups at their focal point by X-ray diffraction experiments provided for the first time a correlation between the molecular structure and the shape of the monodendron, the shape of the supramolecular dendrimer and the symmetry of the lattice. It has been shown that complexation with NaOTf provides the following five different trends: a) stabilization of the three-dimensional Pm3n cubic lattice self-organized from spherical dendrimers that are self-assembled from conic monodendrons; b) stabilization of the two-dimensional S(A) phase generated from parallel-piped monodendrons; c) no effect on the stability of the two-dimensional S(B) phase generated from parallel-piped monodendrons; d) stabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers that are self-assembled from tapered monodendrons; and e) destabilization of the two-dimensional p6mm hexagonal columnar phase self-organized from cylindrical supramolecular dendrimers self-assembled from half-disc monodendrons. Mechanisms of NaOTf mediated self-assembly processes were suggested. These monodendritic crown ethers and their NaOTf complexes provide the largest diversity of liquid crystalline phases encountered so far in any library of supramolecular dendrimers.
The new quadridentate bischelating ligands 2 and 3 display in solution the typical absorption and emission properties expected for naphthalenediimide derivatives. Spectrophotometric studies show that systems 2 and 3 interact with Zn(II), Cd(II) and Cu(I) in CHCl3 or MeCN according to an apparent 1:1 stoichiometry. Molecular modelling, ESI-MS and 1H NMR experiments indicate that the complex species formed in the presence of stoichiometric amounts of metal ion are nonhelical [2 + 2] adducts. The metal-induced self-assembling process is signalled by an intense excimer-type emission caused by the intramolecular interaction of two naphthalenediimide subunits that face each other in the [2 + 2] adduct, as shown by molecular modelling studies. In the presence of excess metal ion, a disassembling process takes place, leading to a dinuclear complex with a 2:1 metal/ligand stoichiometry, in which the intramolecular excimer is no longer allowed to form because the interaction between the naphthalenediimide subunits has been lost. Thus, the overall metal-induced assembling/disassembling process is signalled by the appearance and disappearance of the excimer band in the emission spectrum.
We demonstrate that it is possible to separate a broad band of DNA on a solid substrate without topological obstacles. The mobility was found to scale with molecular size (N) as N(-0.25), while the resolution scaled as N(0.75) indicating that diffusivity on this substrate was minimal. By varying the buffer concentration we were able to show that the mobility for a given chain length scaled with the persistent length (p) as p(1/2). This could be shown to be related to the Gaussian conformation of the chains adsorbed on the surface. A two-dimensional corrugated surface of nonporous silica beads was produced using a self-assembling process at the air/water interface. Even though the surface corrugations were comparable to persistence length we show that they do not affect the mobility, indicating that surface friction rather than topological constraints are the predominant mechanism of separation on a surface.
Owing to the presence of the recurring sequence XPGX' (where X and X' are hydrophobic residues), the molecular structure of the sequences between cross-links in elastin is viewed primarily as a series of beta-turns which become helically ordered by hydrophobic folding into beta-spirals, which in turn assemble hydrophobically into twisted filaments. Both hydrophobic folding and assembly occur when the temperature is raised above Tt, the onset of an inverse temperature transition. Using poly[fv(VPGVG),fx(VPGXG)] (where fv and fx are mole fractions with fv + fx = 1 and X is now any of the naturally occurring amino acid residues), plots of fx versus Tt result in a new hydrophobicity scale based directly on the hydrophobic folding and assembly processes of interest. With the reference values chosen at fx = 1, the most hydrophobic residues of elastin, Tyr (Y) and Phe (F), have low values of Tt, -55 and -30 degrees C, respectively, and the most hydrophilic residues, Glu (E-), Asp (D-) and Lys (K+), have high values of 250, 170 and 120 degrees C, respectively. Raising the average value of Tt for a chain or chain segment from below to above physiological temperature drives hydrophobic unfolding and disassembly; lowering Tt does the reverse. This delta Tt mechanism has been used reversibly to interconvert many energy forms and is used here to explain initiating events of elastogenesis, pulmonary emphysema, solar elastosis and the paucity of elastic fibres in scar tissue. In general, oxidation and/or photolysis convert(s) hydrophobic residues into polar residues with the consequences of irreversibly raising Tt to above 37 degrees C, hydrophobic unfolding and disassembly (fibre swelling), and greater susceptibility to proteolysis.
Tobacco mosaic virus (TMV) particles are rod-like, 300 nm long and 18 nm in diameter. TMV consists of 2140 protein subunits, each with a relative molecular mass of 17420 (158 residues), arranged on a helix of pitch 2.3 nm with 16 1/3 subunits per turn. Winding through this helix is a single strand of RNA 6400 nucleotides long. Three bases are bound to each protein subunit. TMV has a central hole of diameter 4.0 nm. Assembly of TMV occurs by the threading of the RNA through the central hole of the growing rodlet of viral coat protein and involves a preassembled double disk as intermediate. Given the structure of the subunit, such a mechanism requires that the segment of polypeptide chain which separates the nucleic acid binding site from the lumen of the cylinder should be able to move out of the way during the assembly process. Evidence from diffraction studies and from proton nuclear magnetic resonance spectroscopy points to a segment of about 20 amino acid residues being very flexible in the disk. In the helical virus these residues take on a well-defined conformation which completely shields the nucleic acid from the central channel.
In the present work the layer-by-layer nano-assembly technique was used for the development of complex catalytic microparticles on the basis of firefly luciferase (FL). FL films containing 1, 2, or 3 monolayers were assembled on silver electrode QCM-resonators and on 520-nm diameter sulfonated polystyrene latex by alternate adsorption of FL and polycations using electrostatic interactions for the interlayer interaction. The assembly process was studied with quartz crystal microbalance, UV-vis spectroscopy, and microelectrophoresis (surface potential). Structural studies of the resulting multilayers confirmed stepwise deposition of FL and cationic poly(dimethyldiallyl ammonium chloride) with a bilayer thickness of 14 nm; a systematic shift of the surface potential from +28 mV for poly(dimethyldiallyl ammonium chloride) to -14 mV for luciferase outermost layer was established. The functionality and stability of the biocolloids were demonstrated by monitoring the intensity of the light emission. Factors influencing the light emitted upon catalytic activity of FL such as the number of luciferase layers in the film and polyion layer at the outermost layer were studied.
In eukaryotes, secretory proteins are folded and assembled in the endoplasmic reticulum (ER). Many heterologous proteins are retained in the ER due to suboptimal folding conditions. We previously reported that heterologous secretion of Pyrococcus furiosus beta-glucosidase in Saccharomyces cerevisiae resulted in the accumulation of a large fraction of inactive beta-glucosidase in the ER. In this work, we determine the effect of introducing additional genes of ER-resident yeast proteins, Kar2p (binding protein [BiP]) and protein disulfide isomerase (PDI), on relieving this bottleneck. Single-copy expression of BiP and PDI worked synergistically to improve secretion by reverse similar 60%. In an effort to optimize BiP and PDI interactions, we created a library of beta-glucosidase expression strains that incorporated four combinations of constitutively or inducibly-expressed BiP and PDI genes integrated to random gene copynumbers in the yeast chromosome. Approximately 15% of the transformants screened had secretion level improvements higher than that seen with single BiP/PDI gene overexpression, and the highest secreting strain had threefold higher beta-glucosidase levels than the control. Nineteen of the improved strains were re-examined for beta-glucosidase secretion as well as BiP and PDI levels. Within the improved transformants BiP and PDI levels ranged sevenfold and tenfold over the control, respectively. Interestingly, increasing BiP levels decreased beta-glucosidase secretion, whereas increasing PDI levels increased beta-glucosidase secretion. The action of PDI was unexpected because beta-glucosidase is not a disulfide-bonded protein. We suggest that PDI may be acting in a chaperone-like capacity or possibly creating mixed disulfides with the beta-glucosidase's lone cysteine residue during the folding and assembly process.
A set of enantiomerically pure cyclic multinuclear complexes with the formula cis-[a(2)PdL](n) (n+) [a(2)=(R,R)-1,2-diaminocyclohexane (R,R-dach), (S,S)-1,2-diaminocyclohexane (S,S-dach); n=4, 6; LH=2-hydroxypyrimidine (2-Hpymo), 4,6-dimethyl-2-hydroxypyrimidine (2-Hdmpymo) and 4-hydroxypyrimidine (4-Hpymo)] were obtained by reaction of cis-[a(2)Pd(H(2)O)(2)](2+) and LH in aqueous media. The polynuclear complexes were studied by (1)H NMR spectroscopy and X-ray crystallography. These studies revealed that the N1,N3-bridging mode exhibited by the pyrimidine moieties is ideally suited for formation of inorganic analogues of calixarenes (metallacalixarenes) in a self-assembly process. The most stable species are the tetranuclear metallacalix[4]arenes, which are obtained in all cases. Hexanuclear species, namely, [a(2)Pd(2-dmpymo)](6) (6+), were also isolated and fully characterised. (1)H NMR experiments show conversion of [a(2)Pd(2-dmpymo)](6) (6+) to [a(2)Pd(2-dmpymo)](4) (4+) on heating. Analogously to organic calixarenes, these systems are also capable of incorporating hard metal ions at the oxo surface. Additionally, investigations on the receptor properties of these metallacalixarenes towards mononucleotides showed that enantioselective recognition processes occur in aqueous media.
New ferrocenyl Schiff-base polychlorotriphenylmethyl radicals have been synthesized and characterized. The imino group of one such radical undergoes an irreversible trans to cis structural isomerization induced by light. Such photoinduced isomerization has been monitored by UV/Vis and ESR spectroscopy and also monitored by HPLC. ESR frozen solution experiments at low temperature revealed that the cis isomer dimerizes, showing a strong antiferromagnetic interaction. Although numerous photochromic supramolecular systems have been described, such a photoinduced self-assembly process represents the first example of a one-way photoswitchable magnetic system in which a conversion between a doublet and a singlet ground state species is promoted by a photoinduced dimerization process driven by the formation of hydrogen bonds. DFT calculations on the minimized structure and on the rotational barriers have been performed to establish the origin of such behavior. The effect of the substituents and the media polarity on the photoisomerization of this imine chromophore have also been studied. It has been observed that the efficiency of the process is markedly dependent on the presence and characteristics of electron-donor and electron-acceptor substituents of the ferrocenyl Schiff-base polychlorotriphenylmethyl radicals as well as on the polarity of the solvent.