Helical Aggregates Research Articles

Self‐Assembly of m‐Diethynylbenzene Macrocycles Containing Exoannular Chiral Side Chains

AbstractInduced circular dichroism (CD) spectra of the m‐diethynylbenzene macrocycles (S)‐2 and (R)‐2 that have exoannular chiral side chains are observed in a methanol/chloroform (8:2) solution, indicating the formation of chiral, helical aggregates in solution. Solid films prepared on the surface of quartz substrates by spin‐coating solutions of (S)‐2 also exhibit CD signals that are remarkably dependent on the solvent used for the spin‐coating. The relationship between the CD spectra and the morphology of the solid films observed by atomic force microscopy is discussed.

The reinfocement of gellan gel network by the immersion into salt solution

The effect of immersion into salt solutions on rheological properties of gellan gels was investigated. The storage Young's modulus of gellan gels increased with time during the immersion into salt solutions. The increase of the storage Young's modulus can not be explained solely by change in the concentration of gellan. The ellipticity at 202 nm decreased by the immersion, suggesting the formation and aggregation of gellan helices. It was considered that during immersion cations penetrated into gellan gels to induce the formation and aggregation of gellan helices in gels, resulting in reinforcement of the gel network.

Formation of Helical J-Aggregate of Chiral Thioether-Derivatized Phthalocyanine Bound by Palladium(II) at the Toluene/Water Interface

A chiral thioether-substituted phthalocyanine ((2,3,9,10,16,17,23,24-octakis-1-phenylethylthiophthalocyaninato)magnesium(II) [MgPc(SEtPh)8]) has been synthesized, which can be bound by soft-metal ions such as palladium(II) ion. Aggregates formed from MgPc(SEtPh)8 and Pd(II) in toluene and at the toluene/water interface were characterized by means of UV-vis absorption and circular dichroism (CD) spectrometries using centrifugal liquid-membrane (CLM) cell. Matrix-assisted laser desorption ionization time-of-flight mass spectrometry (MALDI-TOF/MS) and scanning electron microscope (SEM) were used as complementary techniques. The toluene solution of MgPc(SEtPh)8 had no optical chirality. However, the addition of PdCl2 into the toluene solution of MgPc(SEtPh)8 induced optical chirality, which indicated the formation of a twisted H-type dimer (face-to-face fashion) of MgPc(SEtPh)8 bound by four PdCl2 in the bulk toluene solution. On the other hand, in the toluene/water interface, helical J-aggregate (head-to-tail fashion) of MgPc(SEtPh)8 bound by PdCl2 was formed, in which one J-aggregate unit contained five 1:2 complexes of MgPc(SEtPh)8-Pd(II) on average. On the basis of the experimental results and the exciton theory for optical chirality, a possible mechanism for the chiral aggregation of MgPc(SEtPh)8-Pd(II) complexes at the interface was proposed. In the present study, we demonstrated a novel strategy for the design of helical phthalocyanine aggregates using the liquid/liquid interface as a template.

An Enantiomeric Nanoscale Architecture Obtained from a Pseudoenantiomeric Aggregate: Covalent Fixation of Helical Chirality Formed in Self‐Assembled Discotic Triazine Triamides by Chiral Amplification

Covalent fixation of a chiral helical structure which is created in a self-assembling system by a chiral-amplification method based on the sergeants/soldiers principle is reported. Disk-shaped triazine triamides self-assembled to form columnar-type helical aggregates through pi-stacking interactions among the central triphenyltriazine moieties, hydrogen-bonding interactions among the amide groups, and van der Waals interactions among the alkyl groups in nonpolar solvents such as hexane, octane, toluene, and p-xylene. When the achiral triazine triamide soldier component is mixed with a tiny amount of the chiral triazine triamide sergeant component, control of the intrinsic supramolecular helicity of the self-assembled soldier component by the sergeant component leads to chiral amplification and formation of a pseudoenantiomeric aggregate with only one handedness of the helix. The helicity can be preserved by ring-closing olefin metathesis polymerization mediated by Grubbs catalyst when an achiral component with terminal olefinic groups forms the pseudoenantiomeric aggregate in the presence of a tiny amount of the chiral component without olefinic groups. After polymerization and removal of the chiral component, the polymeric architecture obtained from the achiral soldier component is optically active and thus can be regarded as an enantiomeric object in which the chiral information transferred from the chiral sergeant component is preserved. The nanoscale chiral structure is fixed perfectly, as indicated by CD spectroscopic evidence obtained in a polar THF medium at high temperature and low concentration. AFM and TEM observations show a nanoscale fibrous structure with a diameter of 2-4 nm, which corresponds to the molecular size of the triazine triamide monomer.

A structural model for cyanine dyes templated into the minor groove of DNA

3,3-Diethylthiadicarbocyanine (DiSC 2(5)) is a monocationic dye which forms cofacial dimers that insert into the minor groove of DNA [J. Seifert, R. Conner, S. Kushon, M. Wang, B. Armitage, J. Am. Chem. Soc. 121 (1999) 2987]. These dyes self-assemble into long helical aggregates in AT-rich regions with the dimers aligned in an end-to-end fashion. A model is presented that allows for the construction of large helical aggregates with continuously variable structural parameters. The spectra or excited states are computed using a direct intermediate neglect of differential overlap (INDO) single configuration interaction (SCI) method. Results are reported for both H- and J-type aggregates ranging in size from 2 to 6 dimers. A more approximate model based on transition charge densities enables calculations of larger aggregates. These models are used to derive structural parameters of both H- and J-type aggregates from the available spectral data, resulting in a new structural model for J-type aggregation in these systems.

Femtosecond Fluorescence Studies of Self-Assembled Helical Aggregates in Solution

For the diamino-bipyridine based C(3)-symmetrical disk molecule, TAB, (sub)picosecond fluorescence transients have been observed by means of femtosecond fluorescence upconversion and picosecond time-correlated photon counting techniques. The dodecyl peripheral side chains of the synthetic compound are large enough to allow, in apolar solvents, self-assembling of the discotic molecules to helical aggregates. In polar solvents, the hydrogen bonding and pi-pi interactions pertaining to the chiral aggregation are compensated by solvation and self-assembling of the disklike molecules is disrupted. For comparison, time-resolved fluorescence measurements have been performed for the subgroup molecule, DAC, which is the (asymmetric) building block for TAB. It is concluded that, after pulsed photoexcitation, TAB and DAC exhibit excited-state intramolecular double proton transfer (ESIDPT) with a typical time of approximately 200-300 fs, irrespective of the degree of aggregation. Picosecond components in the fluorescence of TAB and DAC, ranging from 3 to 25 ps, are representative of vibrational cooling effects in the excited product state. Only aggregated TAB shows a rapid ( approximately 1 ps) decay of its fluorescence anisotropy. This component is attributed to excited-state energy transfer within the aggregate. Finally, the excited-state lifetime of TAB in the aggregated form is found to be an order of magnitude longer than that for TAB in its nonaggregated form. It is inferred that aggregation diminishes the influence of low-frequency twisting motions in the radiationless decay of the excited state.

Sites of Tau Important for Aggregation Populate β-Structure and Bind to Microtubules and Polyanions

The aggregation of the microtubule-associated tau protein and formation of "neurofibrillary tangles" is one of the hallmarks of Alzheimer disease. The mechanisms underlying the structural transition of innocuous, natively unfolded tau to neurotoxic forms and the detailed mechanisms of binding to microtubules are largely unknown. Here we report the high-resolution characterization of the repeat domain of soluble tau using multidimensional NMR spectroscopy. NMR secondary chemical shifts detect residual beta-structure for 8-10 residues at the beginning of repeats R2-R4. These regions correspond to sequence motifs known to form the core of the cross-beta-structure of tau-paired helical filaments. Chemical shift perturbation studies show that polyanions, which promote paired helical filament aggregation, as well as microtubules interact with tau through positive charges near the ends of the repeats and through the beta-forming motifs at the beginning of repeats 2 and 3. The high degree of similarity between the binding of polyanions and microtubules supports the hypothesis that stable microtubules prevent paired helical filament formation by blocking the tau-polyanion interaction sites, which are crucial for paired helical filament formation.

Role Reversal in a Supramolecular Assembly: a Chiral Cyanine Dye Controls the Helicity of a Peptide−Nucleic Acid Duplex

A new dicarbocyanine dye bearing branched, chiral N-alkyl substituents was synthesized and its ability to form helical aggregates on peptide nucleic acid (PNA) double-helical templates was studied. The dye aggregates less effectively than an analogous dye bearing linear, achiral substituents, presumably due to steric problems with packing the branched substituents compared with the linear substituents. When the PNA duplex has a left-handed helicity, addition of the achiral dye leads to formation of a left-handed dye aggregate. However, when the chiral dye aggregates in the presence of this duplex, a right-handed structure is formed, suggesting that the dye alters the helicity of the underlying template. When a racemic PNA duplex (i.e., equal amounts of right- and left-handed helices) is used, no chirality is observed for the dye aggregate formed by the achiral dye but a right-handed helical aggregate is once again formed by the chiral dye. These results indicate that chirality is transferred from the dye to the PNA, as opposed to other examples of polymer-templated dye aggregation where chirality is transferred from the template to the dye.

Effect of PNA Backbone Modifications on Cyanine Dye Binding to PNA−DNA Duplexes Investigated by Optical Spectroscopy and Molecular Dynamics Simulations

Optical spectroscopy and molecular dynamics simulations have been used to study the interaction between a cationic cyanine dye and peptide nucleic acid (PNA)-DNA duplexes. This recognition event is important because it leads to a visible color change, signaling successful hybridization of PNA with a complementary DNA strand. We previously proposed that the dye recognized the minor groove of the duplex, using it as a template for the assembly of a helical aggregate. Consistent with this, we now report that addition of isobutyl groups to the PNA backbone hinders aggregation of the dye when the substituents project into the minor groove but have a weaker effect if directed out of the groove. UV-Visible and circular dichroic spectroscopy were used to compare aggregation on the different PNA-DNA duplexes, while molecular dynamics simulations were used to confirm that the substituents block the minor groove to varying degrees, depending on the configuration of the starting amino acid. In addition to a simple steric blockage effect of the substituent, the simulations suggest that directing the isobutyl group into the minor groove causes the groove to narrow and the duplex to become more rigid, structural perturbations that are relevant to the growing interest in backbone-modified PNA for applications in the biological and materials sciences.

Self-assembling of biocompatible BAB amphiphilic triblock copolymers PLL(Z)–PEG–PLL(Z) in aqueous medium

A novel biocompatible BAB amphiphilic triblock copolymers which consist of poly(ethylene glycol) (PEG) and poly( ε-benzyloxycarbonyl l-Lysine) (PLL(Z)) were synthesized by anion ring opening polymerization of N-carboxyanhydride of ε-benzyloxycarbonyl- l-Lysine ( l-Lys(z)-NCA) using α-amino- ω-amino-poly(ethylene glycol) as initiator in DMF. The block copolymers were characterized by IR, 1H-NMR, GPC, DSC. The results showed that the block copolymers were of narrow molecular distribution and well defined structure. The self-assembled behaviors of block copolymers in aqueous medium were investigated. The effects of various factors, such as cosolvents, initial concentration, temperature, annealing time and times of frozen-thaw cycle etc., on the aggregate morphologies were investigated by transmission electron microscopy (TEM). Different morphological aggregates such as sphere, rod and vesicle etc. could be obtained at controlled conditions, at the same time, a novel helical aggregate was observed. These regular nanometer structures have potential applications in biomedicine due to the biocompatibility of two blocks.

Helical Aggregate of Oleic Acid and Its Dynamics in Water at pH 8

Abstract It is found that helical structures can be formed by a simple achiral fatty acid, oleic acid, in the pH range of 8.0–8.1 at room temperature. Moreover, we succeeded in the first observation of spontaneous dynamics of these helical aggregates in water by optical microscopy.

Kappa-carrageenan interactions in systems containing casein micelles and polysaccharide stabilizers

When polysaccharide food gums are added to milk-based fluid systems, phase separation of casein micelles inevitably occurs, due to biopolymer incompatibility. κ-Carrageenan can prevent bulk phase separation, although stabilized systems are still microscopically phase-separated. The mechanism by which κ-carrageenan stabilizes casein micelles from phase separation was studied in this research. Images from field emission scanning electron microscopy suggested that κ-carrageenan adhered to the casein micelle surface. Dynamic light scattering data showed casein micelle diameter increasing linearly ( r 2=0.978) as κ-carrageenan concentration increased, within the concentration range of 0–0.03% (w/w) and 3.65% casein. Phase separation experiments in 13% milk solids-not-fat (3.65% casein) −0.14% locust bean gum mixed systems with κ-carrageenan in the coil state (60 °C), κ-carrageenan in the helical state, but with helix aggregation blocked (addition of NaI), λ-carrageenan, guar (all at 0.01–0.03%) and agarose (0.01–0.05%) were used to understand the conditions in which stabilization of phase separation occurred. Our work suggests that both κ-carrageenan adsorption to casein micelle surfaces and κ-carrageenan helix aggregation are required for it to be effective at preventing casein micelle macroscopic phase separation from polysaccharides.

Optical spectra and localization of excitons in inhomogeneous helical cylindrical aggregates

We study the linear optical properties of helical cylindrical molecular aggregates accounting for the effects of static diagonal disorder. Absorption, linear dichroism, and circular dichroism spectra are presented, calculated using brute force numerical simulations and a modified version of the coherent potential approximation that accounts for finite size effects by using the appropriate open boundary conditions. Excellent agreement between both approaches is found. It is also shown that the inclusion of disorder results in a better agreement between calculated and measured spectra for the chlorosomes of green bacteria as compared to our previous report, where we restricted ourselves to homogeneous cylinders [Didraga, Klugkist, and Knoester, J. Phys. Chem. B 106, 11474 (2002)]. For the excitons that govern the optical response, we also investigate the disorder-induced localization properties. By analyzing an autocorrelation function of the exciton wave function, we find a strongly anisotropic localization behavior, closely following the properties of chiral wave functions which previously have been found for homogenoeus helical cylinders [Didraga and Knoester, J. Chem. Phys. 121, 946 (2004)]. It is shown that the circular dichroism spectrum may still show a strong dependence on the cylinder length, even when the exciton wave function is localized in a region small compared to the cylinder's size.

The effect of the linear charge density of carrageenan on the ion binding investigated by differential scanning calorimetry, dc conductivity, and kHz dielectric relaxation

The effect of the linear charge density of natural polyelectrolyte, carrageenan, on the ion binding to carrageenan molecules in relation to the gelation was investigated by using the dielectric relaxation spectroscopy, dc conductivity, optical rotation, and differential scanning calorimetry (DSC). Although carrageenan is an anionic polysaccharide, carrageenan molecules in the helix state at low temperatures can bind not only cation, such as potassium and cesium, but also anion, such as iodide. The dc conductivity steeply decreases just below the coil–helix transition temperature, which indicates the binding of ion to the carrageenan molecules in the helix state due to the increase of the linear charge density compared with that in the coil state. The addition of NaI promotes the helix formation, and prevents from aggregation of helices, which was suggested by the results of the dynamic shear modulus and the DSC, and resulted in an increase of the relaxation amplitude of the lowest frequency relaxation (∼kHz) attributed to the fluctuation of the tightly bound counter ions along the high charge density region (helix). It is concluded that binding of iodide induces (1) the increase in the amount of tightly bound counterions to carrageenan molecules and (2) the formation of non-aggregated helix.

Thermoreversible gelation strongly coupled to coil-to-helix transition of polymers

This paper theoretically studies thermoreversible gelation driven by aggregation of helices formed on the polymer chains. Two fundamentally different cases of (i) multiple association of single helices and (ii) association by multiple helices with multiplicity k (such as double helices ( k = 2 ), triple helices ( k = 3 ), etc.) are treated on the basis of different equations. The helix length distribution on a polymer chain (or assemble of chains for multiple helices) is derived as a function of polymer concentration and temperature. Theoretical calculation of the total helix content in the solution is compared with experimental data of optical rotation in ι -carrageenan solutions at different polymer concentrations. It is shown that at low temperature there is a sharp transition from network to bundle state (pair, triplet, etc.). To confirm such a network/pairing transition, we carried out Monte Carlo simulation of polymer solution in which hydrogen-bonded zipper-like cross-links are formed.

Shape Selection in Chiral Self-Assembly

Many biological and synthetic materials self-assemble into helical or twisted aggregates. The shape is determined by a complex interplay between elastic forces and the orientation and chirality of the constituent molecules. We study this interplay through Monte Carlo simulations, with an accelerated algorithm motivated by the growth of an aggregate out of solution. The simulations show that the curvature changes smoothly from cylindrical to saddlelike as the elastic moduli are varied. Remarkably, aggregates of either handedness form from molecules of a single handedness, depending on the molecular orientation.

Gelation of gellan gum aqueous solutions studied by polarization modulation spectroscopy.

Circular birefringence (CB, or optical rotation) and linear birefringence (LB) were measured for gellan gum aqueous solutions with and without salt to examine the gelling system in the helical structure as well as in the orientation. It was found that gelling samples with salt show nonzero LB values, whereas LB is zero for the samples without salt even in the gel state. This difference can be explained by the thermal deformation of the system containing anisotropic aggregations of helices formed with the shielding effect of the added salt on the intramolecular and intermolecular electrostatic repulsions. Considering that the presence of LB in the system affects the estimation of CB, we developed an original procedure of the CB measurement to eliminate the contribution of LB. It was shown that our methods for eliminating the contribution of LB can improve the CB measurement for the gellan gum gel. The temperature dependence of [alpha] for the samples with salt in the gel state is quite different from that for the samples without salt, suggesting that the aggregates of helices in the samples containing a high concentration of salt form a supramolecular structure that contributes to CB.

Green Self‐Assembling Porphyrins and Chlorins as Mimics of the Natural Bacteriochlorophylls c, d, and e

AbstractNovel porphyrins and chlorins that self‐assemble in nonpolar solvents in a manner similar to that of the bacteriochlorophylls c, d, and e have been synthesized by a common protective group approach. The supramolecular assemblies have broad and red‐shifted absorption spectra in comparison to those of the monomeric building blocks. The presence of a carbonyl group in conjugation with the tetrapyrrolic macrocycle produces green colors both in the free bases and in their zinc complexes, which, after self‐assembly, are thus perfect artificial mimics of the chlorosomal antennas encountered in green photosynthetic bacteria. Enantiopure building blocks produce large helical aggregates with M or P helicity determined by the chirality of the 1‐hydroxyethyl substituent. It is demonstrated that the groups essential for self‐organization to occur − namely the hydroxy group, the zinc metal atom and the carbonyl group − do not have to be collinear, as has been presumed until now. Surprisingly, the chlorin derivative does not show hyperchromicity relative to similarly substituted porphyrins. This fact allows us to conclude that the more readily available porphyrins may be used for efficient artificial antennas of potential use in solar devices; otherwise it is necessary to increase the number of synthetic steps in order to incorporate into chlorins the annulated cyclopentanone ring, a structural feature carefully optimized by evolution in all (bacterio)chlorophylls. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004)

Local chain mobility of gellan in aqueous systems studied by fluorescence depolarization.

The local chain mobility of a gellan, an electrolyte polysaccharide, in aqueous systems was examined with respect to the effect of the temperature, the concentration of gellan (c(G)), and the concentration of added salt (c(S)). The relaxation time of local motion was estimated for fluorescein isothiocyanate (FITC)-labeled-gellan by the fluorescence depolarization technique, and the chain mobility was discussed. The relaxation time increased with decreasing temperature, in particular when accompanying the coil-helix transition due to the great difference in chain mobility between the coil and the helical conformations. The effect of c(G) was observed for gellan solutions even below the critical concentration of chain entanglement (2 wt.-%) for well-expanded nonelectrolyte polymers with size similar to that of the gellan. This suggests that the actual excluded volume of gellan is larger than that of nonelectrolyte polymers due to the electrostatic repulsion between segments. The relaxation time for 0.2 wt.-% systems of gellan in coil conformation is independent of c(S), whereas a c(S) dependence of the relaxation time is clearly observed for 0.5 wt.-% systems. The degree of expansion of the gellan chain is independent of the shielding effect of cations on the electrostatic repulsion between gellan segments due to the stiffness of gellan chain. On the other hand, the c(G) as well as the c(S) dependence of the chain mobility is clearly observed for gellan in the helical conformation, examined over the concentration range, probably due to the partial aggregation of helices induced by the attractive interaction between gellan segments.

Self-assembling of helical poly(phenylacetylene) carrying L-valine pendants in solution, on mica substrate, and on water surface.

In the present work, we investigated self-assembling of a poly(phenylacetylene) carrying L-valine pendants (PPA-Val) in a water/methanol solution, upon evaporation of the solution on mica, and on the water surface. With intercalation of a fluorescence probe of Ru(phen)2(dppx)2+ (phen = 1,10-phenanthroline, dppx=7,8-dimethyldipyridophenazine) into the hydrophobic cavities associated by the PPA-Val chains, their helical structures were directly detected in solution with an in situ fluorescence microscope. Helical aggregates were observed with AFM upon evaporation of the solvents, suggesting that the helical structures in the solution are the building blocks of the helical aggregates. Self-assembling structures of PPA-Val on the water surface were, however, very different from that formed upon evaporation of its THF solution on the mica surface. The polymer chains associated into a monolayer of extended fibers on the water surface, whereas superhelical fibers formed on the mica surface. Water molecules play a critical role in inducing the polymer to form diverse morphological structures in its bulk solution and on its surface. In solution, the isotropic hydrophobic effect drove the polymer chains to form superhelical aggregates, while on the water surface, the hydrophobic effect concentrated mainly on the lateral part of the polymer, thus giving a monolayer of extended fibers.