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PubMed2007

Self-assembly of double-stranded DNA molecules at nanomolar concentrations.

Some proteins have the property of self-assembly, known to be an important mechanism in constructing supramolecular architectures for cellular functions. However, as yet, the ability of double-stranded (ds) DNA molecules to self-assemble has not been established. Here we report that dsDNA molecules also have a property of self-assembly in aqueous solutions containing physiological concentrations of Mg2+. We show that DNA molecules preferentially interact with molecules with an identical sequence and length even in a solution composed of heterogeneous DNA species. Curved DNA and DNA with an unusual conformation and property also exhibit this phenomenon, indicating that it is not specific to usual B-form DNA. Atomic force microscopy (AFM) directly reveals the assembled DNA molecules formed at concentrations of 10 nM but rarely at 1 nM. The self-assembly is concentration-dependent. We suggest that the attractive force causing DNA self-assembly may function in biological processes such as folding of repetitive DNA, recombination between homologous sequences, and synapsis in meiosis.

Base Sequence↗

PubMed1996

Continuous monitoring of Pi release following nucleotide hydrolysis in actin or tubulin assembly using 2-amino-6-mercapto-7-methylpurine ribonucleoside and purine-nucleoside phosphorylase as an enzyme-linked assay.

ATP and GTP are hydrolyzed during self-assembly of actin and tubulin, respectively. It is known that nucleotide is hydrolyzed on the polymer in two consecutive steps, chemical cleavage of the gamma-phosphate followed by the slower release of Pi. This last step has been shown to play a crucial role in the dynamics of actin filaments and microtubules. Thus far, evidence for a transient GDP-Pi state in microtubule assembly has been obtained using a glass fiber filter assay that had a poor time resolution [Melki, R., Carlier, M.-F., & Pantaloni, D. (1990) Biochemistry 29, 8921-8932]. We have used a new Pi assay [Webb, M. R. (1992) Proc. natl. Acad. Sci. U.S.A. 89, 4884-4887], in which the purine phosphorylase catalyzes the phosphorolysis of 2-amino-6-mercapto-7-methylpurine ribonucleoside (MESG) into mercaptopurine and ribose phosphate, which is accompanied by an increase in absorbance. This enzyme-linked assay has been used to follow the release of Pi during polymerization of Mg-actin. A value of 350 s was found for the half-time for Pi release on F-actin, in good agreement with previous determinations. The release of Pi following GTP hydrolysis in microtubule assembly was followed using a stopped-flow apparatus. Rapid microtubule assembly was achieved using taxol. The use of a stopped-flow apparatus permitted the continuous recording, with a dead time of 0.8 ms, of both time courses of microtubule assembly and Pi release with greatly improved time resolution. The release of Pi developed with a short lag (35 and 2 s for G-actin and tubulin, respectively) following assembly and appeared 50-fold faster on microtubules than on actin filaments.

Actins↗

PubMed1997

Potent inhibition of terminal complement assembly by clusterin: characterization of its impact on C9 polymerization.

The interactions of the heterodimeric apolipoprotein and complement inhibitor, clusterin (CL, 80 kDa), with actively assembling terminal complement proteins were characterized. Clusterin inhibited at three sites and by two modes of action. Clusterin inhibited C9 assembly on C5b-8 and C5b-9 and also bound to C5b-7 to prevent membrane attachment. The impact on C5b-9 assembly was the most potent. C9 assembly was monitored by assembly-induced fluorescence changes of C9 labeled with fluorescein isothiocyanate (FITC-C9). Assembly of monomeric FITC-C9 with C5b-8 or C5b-9(1) produced a substantial decrease in fluorescence intensity due to changes in the environment of the probe. Addition of the next subunit of unlabeled C9 produced a further small change. One equivalent of FITC-C9 bound to C5b-8 at low temperatures, but the fluorescence change and addition of more C9 did not occur until the temperaure was increased. Kinetic analysis of the fluorescence change suggested an irreversible, first-order process with an activation energy of 29 kcal/mol (k = 0.12 s(-1) at 25 degrees C). The kinetic properties differed for C9 addition to C5b-9(1) (0.27 s(-1) at 25 degrees C, 21 kcal/mol), indicating that C9 activation occurred at a different or altered site. Clusterin binding to C5b-8-(FITC-C9)1 caused fluorescence quenching similar to that of unlabeled C9, indicating that it bound to the C9 binding site. Clusterin binding to C5b-8 and C5b-9(1) was reversible with affinities that were 2 and 15 times that of C9 for the C5b-8 and C5b-9(1) complexes, respectively. The results suggested that the presence of <10% of the circulating clusterin in its heterodimeric, active form could reduce the rate of complement cytolysis of nucleated cells by 10-fold, and under some conditions by 100-fold or more. This would provide a high level of protection for certain cells and may allow time for action by other inhibitors of complement.

Clusterin↗

PubMed1997

Mechanism of myosin subfragment-1-induced assembly of CaG-actin and MgG-actin into F-actin-S1-decorated filaments.

The kinetics and mechanism of myosin subfragment-1-induced polymerization of G-actin into F-actin-S1-decorated filaments have been investigated in low ionic strength buffer and in the absence of free ATP. The mechanism of assembly of F-actin-S1 differs from salt-induced assembly of F-actin. Initial condensation of G-actin and S1 into oligomers in reversible equilibrium is a prerequisite step in the formation of F-actin-S1 . Oligomers have a relatively low stability (10(6) M-1) and contain S1 in a molar ratio to actin close to 0.5. Increased binding of S1 up to a 1:1 molar ratio to actin is associated with further irreversible condensation of oligomers into large F-actin-S1 structures of very high stability. In contrast to salt-induced assembly of F-actin, no monomer-polymer equilibrium, characterized by a critical concentration, can be defined for F-actin-S1 assembly, and end-to-end annealing of oligomers is predominant over growth from nuclei in the kinetics. Simultaneous recordings of the changes in light scattering, pyrenyl- and NBD-actin fluorescence, ATP hydrolysis, and release of Pi during the polymerization process have been analyzed to propose a minimum kinetic scheme for assembly, within which several elementary steps, following oligomer formation, are required for assembly of F-actin-S1. ATP hydrolysis occurs before polymerization of MgATP-G-actin but not of CaATP-G-actin. The release of inorganic phosphate occurs on F-actin-S1 at the same rate as on F-actin.

Actin Cytoskeleton↗

PubMed1997

Linkage between operator binding and dimer to octamer self-assembly of bacteriophage lambda cI repressor.

Cooperative binding of the bacteriophage lambda cI repressor dimer to specific sites of the phage operators OR and OL controls the developmental state of the phage. Cooperativity has long been thought to be mediated by self-assembly of repressor dimers to form tetramers which can bind simultaneously to adjacent operators. More recently, we demonstrated that when free repressor dimers self-associate in solution, tetramer is an intermediate in a concerted assembly reaction leading to octamer as the predominant higher order species [Senear, D. F., et al. (1993) Biochemistry 32, 6179-6189]. Even as a minority component in the assembly reaction, tetramer can account for pairwise cooperativity. In a similar manner, were it able to bind all three operators simultaneously, octamer could account for three-way cooperativity. In fact, based solely on repressor self-assembly, the naive prediction is that the repressor-OR interactions should be substantially more cooperative than they are. Evidently, there are unfavorable contributions to cooperativity from processes other than repressor self-assembly. Here, we focus on coupling between repressor self-association and operator binding as one possible unfavorable contribution to cooperativity. Sedimentation equilibrium analysis was used to compare the dimer-octamer association reactions of a repressor dimer-OR1 complex and free repressor dimer. Fluorescence anisotropy was used to investigate OR1 binding to free dimers and dimers assembled as higher order species. The results of these experiments indicate a significant and salt-dependent unfavorable contribution generated by such coupling. Since the oligonucleotides used in these experiments are the size of single operator sites, this coupling is mediated by the protein, not by the DNA. This mechanism does not account for an additional, salt-independent, unfavorable contribution which we presume is transmitted via the DNA. Thus, unfavorable contributions generated by structural transitions in both macromolecules serve to moderate the effect of self-association alone. We speculate that this is a general feature of cooperative protein-DNA interactions.

Bacteriophage lambda↗

PubMed1998

Identification of a minimal core of the synaptic SNARE complex sufficient for reversible assembly and disassembly.

Assembly of the three neuronal membrane proteins synaptobrevin, syntaxin, and SNAP-25 is thought to be one of the key steps in mediating exocytosis of synaptic vesicles. In vivo and in vitro, these proteins form a tight complex. Assembly is associated with a large increase in alpha-helical content, suggesting that major structural and conformational changes are associated with the assembly reaction. Limited proteolysis by trypsin, chymotrypsin, and proteinase K of the ternary complex formed from recombinant proteins lacking their membrane anchors revealed a SDS-resistant minimal core. The components of this core complex were purified and characterized by N-terminal sequencing and mass spectrometry. They include a slightly shortened synaptobrevin fragment, C- and N-terminal fragments of SNAP-25, and a C-terminal fragment of syntaxin that is slightly larger than the previously characterized H3 domain. Recombinant proteins corresponding to these fragments are sufficient for assembly and disassembly. In addition, each of the two SNAP-25 fragments can individually form complexes with syntaxin and synaptobrevin, suggesting that they both contribute to the assembly of the SNARE complex. Upon complex assembly, a large increase in alpha-helical content is observed along with a significantly increased melting temperature (Tm). Like the full-length complex, the minimal complex tends to form an oligomeric species; global analysis of equilibrium ultracentrifugation data suggests a monomer-trimer equilibrium exists. These conserved biophysical properties may thus be of fundamental importance in the mechanism of membrane fusion.

Amino Acid Sequence↗

PubMedDate not supplied

Experimental evidence for multiple assembled states of Sc3 from Schizophyllum commune.

The hydrophobin Sc3 from the fungus Schizophyllum commune assembles from the aqueous phase into ordered structures with substantially different characteristics depending upon experimental conditions. Under the first condition, a vortexing procedure widely reported in the literature, interfacial assembly yields highly ordered, stacked beta-sheets. We have also observed a previously unreported assembly of Sc3 under a second condition, which occurs in a time-dependent manner from quiescent solution. The resulting types of assembled states have been compared utilizing fluorescence techniques, sodium dodecyl sulfate polyacrylamide gel electrophoresis, immunoblotting, density gradient centrifugation, and phase contrast and atomic force microscopy. A model based on this study and previous literature is proposed that suggests three distinct states of Sc3: (1) soluble Sc3 consisting of unimers or multimers in micelle-like association, (2) interfacially assembled I-Sc3 with highly ordered, stacked beta-sheets, presumably formed in a templated manner at the air/water interface of microscopic bubbles generated by vortexing, and (3) solution-assembled S-Sc3, a less-ordered structure formed in a time-dependent manner in the absence of an interface.

Benzothiazoles↗

PubMedDate not supplied

Self-assembling of polymer-enzyme conjugates at oil/water interfaces.

Interface-binding enzymes are desirable for biphasic reactions in that they offer simultaneous access to substrates dissolved in both phases across the interface. It has been shown that conjugating water-soluble enzymes with hydrophobic polymers facilitated the assembling of enzymes at oil/water interfaces. In this work, the interfacial assembling of alpha-chymotrypsin conjugated with polystyrene, poly(methyl methacrylate), and poly(l-lactic acid) was examined using the pendant drop method. The interface-assembling process of the conjugates from the organic phase followed a similar pattern of that of native alpha-chymotrypsin from the aqueous buffer phase, i.e., the interfacial tension decreased gradually with time. However, when the conjugates were dispersed in the form of particulates in the aqueous phase, in which the conjugate was insoluble, the assembling occurred faster and the interfacial tension quickly approached zero. It was suspected that the assembling in this case involved two steps, i.e., the adsorption of the particulates and the subsequent rearrangement, dissociation, and redispersion of the conjugates at the interface. The effect of other factors, including the polarity of organic solvent and pH and ionic strength of the aqueous phase, was evaluated. It was found that the polar solvent slightly facilitated the assembling, whereas pH and ionic strength showed minimal effects.

Biochemistry↗

PubMed2001

Self-assembly of 10-microm-sized objects into ordered three-dimensional arrays.

This paper describes the self-assembly of small objects--polyhedral metal plates with largest dimensions of 10 to 30 microm--into highly ordered, three-dimensional arrays. The plates were fabricated using photolithography and electrodeposition techniques, and the faces of the plates were functionalized to be hydrophobic or hydrophilic using self-assembled monolayers (SAMs). Self-assembly occurs in water through capillary interactions between thin films of a hydrophobic liquid (a liquid prepolymer adhesive) coated onto the hydrophobic faces of the plates; coalescence of the adhesive films reduces the interfacial free energy of the system and drives self-assembly. By altering the size and surface-patterning of the plates, the external morphologies of the aggregates were varied. Curing the adhesive furnished mechanically stable aggregates that were characterized by scanning electron microscopy (SEM). For assemblies formed by plates partially composed of a sacrificial material, a subsequent etching step furnished fully open, three-dimensional microstructures. This work validates the use of capillary interactions for three-dimensional mesoscale self-assembly in the 10-microm-size regime and opens new avenues for the fabrication of complex, three-dimensional microscructures.

Electroplating↗

PubMed2002

Template-directed self-assembly of 10-microm-sized hexagonal plates.

This article presents a strategy for the fabrication of ordered microstructures using concepts of design inspired by molecular self-assembly and template-directed synthesis. The self-assembling components are 4-microm-thick hexagonal metal plates having sides 10 microm in length ("hexagons"), and each template consists of a 4-microm-thick circular metal plate surrounding a central cavity, the perimeter of which is complementary in shape to the external edges of a two-dimensional, close-packed array of hexagons. The hexagons and templates (collectively, "pieces") were fabricated via standard procedures and patterned into hydrophobic and hydrophilic regions using self-assembled monolayers (SAMs). Templated self-assembly occurs in water through capillary interactions between thin films of a nonpolar liquid adhesive coating the hydrophobic faces of the pieces. The hexagons tile the cavities enclosed by the templates, and the boundaries of the cavities determine the sizes and shapes of the assemblies. Curing the adhesive with ultraviolet light furnishes mechanically stable arrays having well-defined morphologies. By allowing control over the structures of the resulting aggregates, this work represents a step toward the development of practical methods for microfabrication based on self-assembly.

Chemistry, Organic↗

PubMed2002

Enantioselective formation of a dynamic hydrogen-bonded assembly based on the chiral memory concept.

In this paper, we report the enantioselective formation of a dynamic noncovalent double rosette assembly 1a(3).(CYA)(6) composed of three 2-pyridylcalix[4]arene dimelamines (1a) and six butylcyanuric acid molecules (BuCYA). The six 2-pyridyl functionalities of the assembly interact stereoselectively with chiral dicarboxylic acids 3a-e via two-point hydrogen-bonding interactions. One of the two enantiomeric assemblies (P- or M-) 1a(3).(CYA)(6) is formed in excess as the result of the complexation of the chiral diacids, resulting in formation of optically active assemblies. The complexations with dibenzoly tartaric acids D-3a and L-3a (3 equivalent), respectively, leading to the formation of diastereomeric assemblies (P)-1a(3).(BuCYA)(6).(D-3a)(3) and (M)-1a(3).(BuCYA)(6).(L-3a)(3) with 90% diastereomeric excess. The diastereomeric excess in (M)-1a(3).(BuCYA)(6).(L-3a)(3) is "memorized" when L-3a is removed by precipitation with ethlylenediamine (EDA). The assembly (M)-1a(3).(BuCYA)(6) is still optically active (90% enantiomeric excess), although none of its individual components are chiral. (M)-1a(3).(BuCYA)(6) has a high kinetic stability toward racemization (E(a) = 119 kJ mol(-)(1), half-life of (M)-1a(3).(BuCYA)(6) is ca. 1 week at 20 degrees C).

Journal Article↗

PubMed2003

Iron-sulfur cluster biosynthesis. Characterization of frataxin as an iron donor for assembly of [2Fe-2S] clusters in ISU-type proteins.

ISU (eukaryotes) and IscU (prokaryotes) are a homologous family of proteins that appear to provide a platform for assembly of [2Fe-2S] centers prior to delivery to an apo target protein. The intermediate [2Fe-2S] ISU-bound cluster is formed by delivery of iron and sulfur to the apo ISU, with the latter delivered through an IscS-mediated reaction. The identity of the iron donor has thus far not been established. In this paper we demonstrate human frataxin to bind from six to seven iron ions. Iron binding to frataxin has been quantitated by iron-dependent fluorescence measurements [K(D)(Fe(3+)) approximately 11.7 microM; (K(D)(Fe(2+)) approximately 55.0 microM] and isothermal titration calorimetry (ITC) [K(D)(Fe(3+)) approximately 10.2 microM]. Enthalpies and entropies for ferric ion binding were determined from calorimetric measurements. Both fluorescence (K(D) 0.45 microM) and ITC measurements (K(D) 0.15 microM) demonstrate holo frataxin to form a complex with ISU with sub-micromolar binding affinities. Significantly, apo frataxin does not bind to ISU, suggesting an important role for iron in cross-linking the two proteins and/or stabilizing the structure of frataxin that is recognized by ISU. Holo frataxin is also shown to mediate the transfer of iron from holo frataxin to nucleation sites for [2Fe-2S] cluster formation on ISU. We have demonstrated elsewhere [J. Am. Chem. Soc. 2002, 124, 8774-8775] that this iron-bound form of ISU is viable for assembly of holo ISU, either by subsequent addition of sulfide or by NifS-mediated sulfur delivery. Provision of holo frataxin and inorganic sulfide is sufficient for cluster assembly in up to 70% yield. With NifS as a sulfur donor, yields in excess of 70% of holo ISU were obtained. Both UV-vis and CD spectroscopic characteristics were found to be consistent with those of previously characterized ISU proteins. The time course for cluster assembly was monitored from the 456 nm absorbance of holo ISU formed during the [2Fe-2S] cluster assembly reaction. A kinetic rate constant k(obs) approximately 0.075 min(-)(1) was determined with 100 microM ISU, 2.4 mM Na(2)S, and 40 microM holo frataxin in 50 mM Tris-HCl (pH 7.5) with 4.3 mM DTT. Similar rates were obtained for NifS-mediated sulfur delivery, consistent with iron release from frataxin as a rate-limiting step in the cluster assembly reaction.

Escherichia coli↗

PubMed2003

Crystal and molecular structures of ionophore-siderophore host-guest supramolecular assemblies relevant to molecular recognition.

Ionophore-siderophore host-guest assemblies composed of 18-crown-6 and ferrioxamine B, benzo-18-crown-6 and ferrioxamine B, and cis-syn-cis-dicyclohexano-18-crown-6 and ferrioxamine B were successfully crystallized, and their structures were determined by single-crystal X-ray diffraction. All three crystal lattices also include solvated Mg(II) and perchlorate ions. The ionophore-siderophore host-guest assembly is noncovalently held together by a hydrogen bonding interaction between the pendant protonated amine in the second coordination sphere of ferrioxamine B and the hydrogen bond acceptor oxygen atoms in the crown ether. The crystals of 18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 19.8327(11) A, b = 20.4111(11) A, c = 15.1698(8) A, and beta = 96.435(1) degrees. The crystals of benzo-18-crown-6:ferrioxamine B host-guest assembly are triclinic, with space group P(-)1, and two molecules per unit cell with dimensions a = 11.1747(10) A, b = 16.0580(15) A, c = 18.4175(17) A, alpha = 80.469(3) degrees, beta = 81.481(3) degrees and gamma = 70.212(2) degrees. The crystals of cis-syn-cis-dicyclohexano-18-crown-6:ferrioxamine B host-guest assembly are monoclinic, with space group P2(1)/c, and four molecules per unit cell with dimensions a = 20.1473(13) A, b = 21.5778(15) A, c = 14.8013(10) A, and beta = 94.586(2) degrees. The crystal structures of all three host-guest assemblies contain a racemic mixture of Lambda-N-cis, cis and Delta-N-cis, cis coordination isomers of ferrioxamine B. The crystal structures indicate that the steric rigidity of the benzo-18-crown-6 and cis-syn-cis-dicyclohexano-18-crown-6 cavity has a pronounced effect on the conformation of the crown ring and ultimately on the hydrogen bonding interactions between the crown ethers and ferrioxamine B. The structural parameters and the conformational features of the ferrioxamine B guests compare very well with each other and with those of the ferrioxamine B structure obtained in the absence of a host. Structural features relevant to siderophore molecular recognition are discussed.

Crown Ethers↗

PubMed2003

Assessment of cooperativity in self-assembly.

A method has been proposed to assess cooperativity in self-assembly processes. The method is based on a clear distinction between intermolecular and intramolecular processes which are compared with the corresponding reference reactions. It has been applied to two classical cases, namely the self-assembly of helicates and of porphyrin ladders, by using data previously published by the groups of Lehn and Anderson, respectively. Contrarily to the conclusions of the authors, pointing out self-assembly processes driven by positive cooperativity, the method here presented indicates in both cases the absence of cooperative effects. The methods previously used to assess cooperativity, in particular Scatchard plot and/or Hill plot, are criticized as being inappropriate for self-assembly, because they are pertinent to a specific case only, namely the intermolecular binding of a monovalent ligand L to a multivalent receptor M, a case very different from self-assembly which involves both inter- and intramolecular interactions. The present method underscores the fact that positive cooperativity in artificial self-assembling systems is probably much more rare than it was previously thought.

Journal Article↗

PubMed2006

Surface-functionalized CdSe nanorods for assembly in diblock copolymer templates.

Poly(ethylene oxide)-covered CdSe nanorods were prepared and assembled in diblock copolymer templates by floating the block copolymer templates onto aqueous nanorod solutions. The assembly was enabled by consideration of the surface ligand coverage of the nanorods. Alkane-covered CdSe nanorods prepared by state-of-the-art techniques are not compatible with this assembly process. However, poly(ethylene oxide) (PEO)-functionalized CdSe nanorods were successfully used to assemble the nanorods into the channels and pores of diblock copolymer templates. Other water-dispersible CdSe nanorods, such as those covered with 11-mercaptoundecanoic acid (MUA), did not give the desired assemblies. These results are understood by considering the surface energies of the PEO-covered CdSe nanorods in this interfacial assembly process.

Journal Article↗

PubMed2006

Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET).

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.

Journal Article↗

PubMed2006

Effect of side-chain substituents on self-assembly of perylene diimide molecules: morphology control.

Effect of side-chain substitutions on the morphology of self-assembly of perylene diimide molecules has been studied with two derivatives modified with distinctly different side-chains, N,N'-di(dodecyl)-perylene-3,4,9,10-tetracarboxylic diimide (DD-PTCDI) and N,N'-di(nonyldecyl)-perylene-3,4,9,10-tetracarboxylic diimide (ND-PTCDI). Due to the different side-chain interference, the self-assembly of the two molecules results in totally different morphologies in aggregate: one-dimensional (1D) nanobelt vs zero-dimensional (0D) nanoparticle. The size, shape, and topography of the self-assemblies were extensively characterized by a variety of microscopies including SEM, TEM, AFM, and fluorescence microscopy. The distinct morphologies of self-assembly have been obtained from both the solution-based processing and surface-supported solvent-vapor annealing. The nanobelts of DD-PTCDI fabricated in solution can feasibly be transferred to both polar (e.g., glass) and nonpolar (e.g., carbon) surfaces, implying the high stability of the molecular assembly (due to the strong pi-pi stacking). The side-chain-dependent molecular interaction was comparatively investigated using various spectrometries including UV-vis absorption, fluorescence, X-ray diffraction, and differential scanning calorimetry. Compared to the emission of ND-PTCDI aggregate, the emission of DD-PTCDI aggregate was significantly red-shifted (ca. 30 nm) and the emission quantum yield decreased about three times, primarily due to the more favorable molecular stacking for DD-PTCID. Moreover, the aggregate of DD-PTCDI shows a pronounced absorption band at the longer wavelength, whereas the absorption of ND-PTCDI aggregate is not significant in the same wavelength region. These optical spectral observations are reminiscent of the previous theoretical investigation on the side-chain-modulated electronic properties of PTCDI assembly.

Journal Article↗

PubMed2003

A novel type of hydrogen-bonded assemblies based on the melamine.cyanuric acid motif.

This paper reports the formation of novel hydrogen-bonded assemblies 1(3).CA obtained upon mixing cyanuric acid (CA) with melamine derivatives 1, in which two of the three possible H-bonding arrays have been blocked. The four components are held together by 9 hydrogen bonds and form a rigid planar structure in which a central CA (three ADA motifs: A = acceptor, D = donor) is hydrogen bonded to three peripheral melamine derivatives (DAD motif). Furthermore, the synthesis and assembly studies are described of hydrogen-bonded assemblies 2-4.CA, comprised of three melamine derivatives that are covalently connected, and CA. The overall thermodynamic stability of assemblies 2-4.CA is superior to 1(3).CA (I(Tm) = 9 vs 3.6). The presence of the 2.CA complex in chloroform was confirmed by (1)H NMR spectroscopy and MALDI-TOF mass spectrometry. Substitution of the trimelamines with chiral or fluorescent groups (R(3)) enabled the study of the assemblies by CD and fluorescence spectroscopy. Titration experiments revealed strongly enhanced stabilities even in the presence of polar solvents, such as THF and CH(3)OH. Depending on the polarity of the solvent, stacking between the planar assembly units was observed.

Journal Article↗