Triple helix formation in amphiphilic discotics: demystifying solvent effects in supramolecular self-assembly.

A set of chiral, amphiphilic, self-assembling discotic molecules based on the 3,3'-bis(acylamino)-2,2'-bipyridine-substituted benzene-1,3,5-tricarboxamide motif (BiPy-BTA) was prepared. Amphiphilicity was induced into the discotic molecules by an asymmetrical distribution of alkyl and oligo(ethylene oxide) groups in the periphery of the molecules. Small-angle X-ray scattering, cryogenic transmission electron microscopy, and circular dichroism spectroscopy measurements were performed on the discotic amphiphiles in mixtures of water and alcohol at temperatures between 0 °C an 90 °C. The combined results show that these amphiphilic discotic molecules self-assemble into supramolecular fibers consisting of either one or three discotic molecules in the fiber cross-section and that the presence of water induces the bundling of the supramolecular fibers. The rich phase behavior observed for these molecules proves to be intimately connected to the mixing thermodynamics of the water-alcohol mixtures.

Helical self-assembly and co-assembly of fluorinated, preorganized discotics.

The synthesis and self-assembly properties of a fluorinated C(3)-symmetrical 3,3'-bis(acylamino)-2,2'-bipyridine discotic (1) in the mesophase and in solution are described. First, 3,4,5-tris-(1H,1H,2H,2H,3H,3H-perfluoroundecyl-1-oxy)benzoyl chloride was coupled to mono-t-BOC protected 2,2'-bipyridine-3,3'-diamine to afford after deprotection the corresponding fluorinated aromatic amine on a multigram scale. Then, three-fold reaction of this amine with trimesyl chloride yielded the target C(3)-symmetrical fluorinated disc. The latter displayed columnar liquid crystallinity over a temperature range of more than 350 K in which helical rectangular and hexagonal columnar mesophases were detected by X-ray diffraction measurements. (1)H-NMR spectroscopy showed a preorganized structure due to strong intramolecular hydrogen bonding between the amide N-H's and bipyridine nitrogen atoms, even in the presence of a large excess of hexafluoroisopropanol. This preorganized structure allows the formation of helical self-assemblies in fluorinated solvents, as was established using UV-Vis spectroscopy. The fluorinated disc and two chiral hydrocarbon analogues (a C(3)-symmetrical and a desymmetrized disc) were mixed in a 1:10 v:v mixture of methoxynonafluorobutane (MNFB) and 1,1,2-trichloro-1,2,2-trifluoroethane (Freon 113). Importantly, the C(3)-symmetrical hydrocarbon disc dissolves only in the presence of fluorinated disc in the latter solvent mixture, proving a mutual interaction. CD spectroscopy performed on these mixtures points to a preference for alternating self-assemblies of fluorinated and chiral hydrocarbon discotics.

Controlled supramolecular oligomerization of C3-symmetrical molecules in water: the impact of hydrophobic shielding.

The supramolecular oligomerization of three water-soluble C(3)-symmetrical discotic molecules is reported. The compounds all possess benzene-1,3,5-tricarboxamide cores and peripheral Gd(III)-DTPA (diethylene triamine pentaacetic acid) moieties, but differ in their linker units and thus in their propensity to undergo secondary interactions in H(2)O. The self-assembly behavior of these molecules was studied in solution using circular dichroism, UV/Vis spectroscopy, nuclear magnetic resonance, and cryogenic transmission electron microscopy. The aggregation concentration of these molecules depends on the number of secondary interactions and on the solvophobic character of the polymerizing moieties. Hydrophobic shielding of the hydrogen-bonding motif in the core of the discotic is of paramount importance for yielding stable, helical aggregates that are designed to be restricted in size through anti-cooperative, electrostatic, repulsive interactions.

Desymmetrization of 3,3'-bis(acylamino)-2,2'-bipyridine-based discotics: the high fidelity of their self-assembly behavior in the liquid-crystalline state and in solution.

Two novel nonsymmetrical disc-shaped molecules 1 and 2 based on 3,3'-bis(acylamino)-2,2'-bipyridine units were synthesized by means of a statistical approach. Discotic 1 possesses six chiral dihydrocitronellyl tails and one peripheral phenyl group, whereas discotic 2 possesses six linear dodecyloxy tails and one peripheral pyridyl group. Preorganization by strong intramolecular hydrogen bonding and subsequent aromatic interactions induce self-assembly of the discotics. Liquid crystallinity of 1 and 2 was determined with the aid of polarized optical microscopy, differential scanning calorimetry, and X-ray diffraction. Two columnar rectangular mesophases (Col(r)) have been identified, whereas for C(3)-symmetrical derivatives only one Col(r) mesophase has been found. In solution, the molecularly dissolved state in chloroform was studied with (1)H NMR spectroscopy, whereas the self-assembled state in apolar solution was examined with optical spectroscopy. Remarkably, these desymmetrized discotics, which lack one aliphatic wedge, behave similar to the symmetric parent compound. To prove that the stacking behavior of discotics 1 and 2 is similar to that of reported C(3)-symmetrical derivatives, a mixing experiment of chiral 1 with C(3)-symmetrical 13 has been undertaken; it has shown that they indeed belong to one type of self-assembly. This helical J-type self-assembly was further confirmed with UV/Vis and photoluminescence (PL) spectroscopy. Eventually, disc 2, functionalized with a hydrogen-bonding acceptor moiety, might perform secondary interactions with molecules such as acids.

Microwave-induced electrostatic etching: generation of highly reactive magnesium for application in Grignard reagent formation.

A detailed study regarding the influence of microwave irradiation on the formation of a series of Grignard reagents in terms of rates and selectivities has revealed that these heterogeneous reactions may display a beneficial microwave effect. The interaction between microwaves and magnesium turnings generates violent electrostatic discharges. These discharges on magnesium lead to melting of the magnesium surface, thus generating highly active magnesium particles. As compared to conventional operation the microwave-induced discharges on the magnesium surface lead to considerably shorter initiation times for the insertion of magnesium in selected substrates (i.e. halothiophenes, halopyridines, octyl halides, and halobenzenes). Thermographic imaging and surface characterization by scanning electron microscopy showed that neither selective heating nor a "specific" microwave effect was causing the reduction in initiation times. This novel and straightforward initiation method eliminates the use of toxic and environmentally adverse initiators. Thus, this initiation method limits the formation of by-products. We clearly demonstrated that microwave irradiation enables fast Grignard reagent formation. Therefore, microwave technology is promising for process intensification of Grignard based coupling reactions.

Matrix suppression and analyte suppression effects of quaternary ammonium salts in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry: an investigation of suppression mechanism.

In the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) analysis of some quaternary ammonium salts (QASs), very clean spectra of the quaternary ammonium ions were recorded with a strong matrix suppression effect (MSE). The QASs also showed a considerable analyte suppression effect (ASE). It was demonstrated that the MSE and ASE of the QASs can be explained well by the cluster ionization model. According to this model, MALDI ions are formed from charged matrix/analyte clusters. Various analyte ions and matrix ions might coexist in the cluster, and they will compete for the limited number of net charges available. If enough quaternary ammonium ions are present in the cluster, they will take away the net charges, thus resulting in the MSE and ASE. Our results also suggest that 'the cluster ionization model' is not in conflict with 'the theory of ionization via secondary gas-phase reactions'. The initial MALDI ions produced from charged matrix/analyte clusters will collide with other molecules or ions in the MALDI plume. Depending on the properties of the initial ions and the composition of the MALDI plume, secondary gas-phase reactions might result from these collisions. The final ions observed are the combined results of 'cluster ionization' and 'ionization via secondary gas-phase reactions'.

Dehydrogenation of tertiary amines in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

In the matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS) analysis of various compounds synthesized in our laboratory, strong [M - H]+ ion peaks were often observed for the molecules with tertiary amino groups. In this work, the MALDI TOF MS behavior of two groups of compounds that incorporate tertiary amino moieties was investigated. One group is bisurea dimethylanilines (BUDMAs) prepared for the study of molecular recognition in thermoplastic elastomers, and the other group is the poly(propylene imine) diaminobutane dendrimers. The results clearly demonstrate the appearance of the [M - H]+ ions. In order to understand the possible mechanisms for the generation of these ions, a series of model compounds, ranging from primary to tertiary amines, were investigated. Unlike the tertiary amines, no [M - H]+ ion peaks were recorded for the primary amines, and only barely detectable ones, if any, for some secondary amines. It appears that the tertiary amino groups play an important role in the formation of these ions. In addition to MALDI TOF MS analysis, these samples were also applied to electrospray ionization (ESI) MS where no [M - H]+ ions were observed. The results indicate that the generation of [M - H]+ ion is due to the unique MALDI conditions and is likely to be formed via dehydrogenation of a protonated tertiary amine resulting in an N=C double bond. The absence of [M - H]+ ion peaks for the primary and secondary amines is probably because upon their formation these ions could easily transfer one proton to the corresponding amines in the MALDI gas-phase plume, yielding neutral imines that cannot be detected by MS.

Double cation adduction in matrix-assisted laser desorption/ionization time-of-flight mass spectrometry of electron deficient anthraquinone derivatives.

Six anthraquinone derivatives were analyzed using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). Clear (pseudo) molecular ions were observed for all the compounds. Interestingly, for some derivatives, strong ions with double cation adduction were also recorded in the positive mode. It is remarkable that all these ions are singly charged. In this work, possible mechanisms for the double cation adduction were investigated and discussed. It appears that the double cation adduction was due to the electron deficient nature of the derivatives, and formed by taking up two singly charged cations and one electron. Substituents on the anthraquinone ring were found to have a significant effect on the double cation adduction. In contrast, no considerable influence of the acidity of MALDI matrix/solution was observed, even on the double proton adduction. Furthermore, it was demonstrated that double cation adduction might occur in the MALDI gas-phase plume. In addition to the anthraquinones, three more electron deficient compounds of different types, i.e. a perylene bisimide derivative (PB), 3,7-decanoylamino-4,8-dihydrobenzo[1,2-b:4,5-b']dithiophene-4,8-dione (TQ) and 6,6-phenyl C61-butyric acid methyl ester (PCBM), were also analyzed with MALDI TOF MS. The results indicate that the 'abnormal' double cation adduction might be a 'normal' phenomenon in the MALDI TOF MS analysis of many electron deficient compounds.

Synthesis of 3,6-diaminophthalimides for ureidophthalimide-based foldamers.

[reaction: see text]. Herein, we report an improved methodology for the synthesis of a variety of 3,6-diaminophthalimides in high yields. This enables decoration of the periphery of foldamers with a wide range of functionalities.

Radical cation formation in characterization of novel C3-symmetric disks and their precursors by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry.

Four C3-symmetrical tris(dipeptide) disks and their precursors were characterized using matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI TOF MS). The C3-symmetrical disks were based on a benzene-1,3,5-triscarboxamide core extended by oligopeptides with trialkoxyanilide tails. The results indicate that MALDI TOF MS is a powerful and straightforward analytical technique for characterizing C3-symmetrical disks and their precursors. Clear (pseudo)-molecular ion peaks could readily be identified. It is remarkable that strong radical ion signals were observed for all the compounds, including the anilines that were expected to be protonated prior to laser irradiation using acidic MALDI matrixes. Possible mechanisms for radical ion formation were investigated with the employment of radical scavengers, with various matrixes and with direct laser desorption/ionization (LDI). Most likely the radicals are formed by losing one electron from the aniline nitrogen and stabilized by conjugation through the phenyl ring. It appears that direct photo/thermal ionization of analytes is an important route for the radical ion formation of the compounds with trialkoxy aniline/anilide groups.

Chiral polymers by iterative tandem catalysis.

Racemic omega-substituted caprolactones can be completely converted into chiral polyesters of remarkable MW and high ee by combining lipase-catalyzed ring-opening polymerization with Ru-catalyzed racemization.

Facile synthesis of a chiral polymeric helix; folding by intramolecular hydrogen bonding.

In a single condensation step, a poly-ureidophthalimide is synthesized, which folds into a chiral, helical architecture according to circular dichroism spectroscopy.

Hydrogen-Bonded Complexes of Diaminopyridines and Diaminotriazines: Opposite Effect of Acylation on Complex Stabilities.

The association behavior of several 2,4-diamino-s-triazines, 2,6-diaminopyridines, and their acylated derivatives with uracil derivatives was studied. In solution (1)H-NMR and IR spectroscopy were used, and in the solid state as (co)crystals X-ray diffraction was used. Acylation of 2,6-diaminopyridine leads to an increase of the association constant in CDCl(3) of the complexes with N-propylthymine from 84 to 440-920 M(-)(1), whereas acylation of diamino-s-triazines leads to a dramatic fall in the association constant of the complexes with N-propylthymine from 890 to ca. 6 M(-)(1). This phenomenon is related to different conformational preferences of these compounds. The amide groups in bis(acylamino)pyridines prefer a trans conformation, with the carbonyl group anti with respect to the ring nitrogen and coplanar with the aromatic ring. The amides of bis(acylamino)triazines, however, reside predominantly in a cis conformation. Repulsive secondary electrostatic interactions between the cis-amide and uracil carbonyl groups are thought to be responsible for the low association constant of complexes of bis(acylamino)triazines with uracils. The relatively high dimerization constants of bis(acylamino)triazines have been rationalized by the strong tendency to dimerize via quadruple hydrogen bonding.

False results caused by solvent impurity in tetrahydrofuran for MALDI TOF MS analysis of amines.

Tetrahydrofuran (THF) is one of the most frequently used solvents in the MALDI TOF MS analysis of synthetic compounds. However, it should be used with caution because a trace amount of 4-hydroxybutanal (HBA) might be generated and accumulated in THF during storage. Since only a tiny amount of analytes is required in MALDI MS measurements, a trace amount of HBA might have a significant effect on the MS results. It was found that HBA will quickly react with primary and secondary amino compounds, leading to false results about the sample composition with an extra series of ions with additional mass of 70 Da in between. The formation of HBA can be inhibited by butylated hydroxytoluene (BHT) antioxidant. Therefore, when THF is required as the solvent for sample preparation, it is strongly recommended to use a BHT-stabilized one, at least for the analysis of compounds with amino groups.

Small sized perylene-bisimide assemblies controlled by both cooperative and anti-cooperative assembly processes.

Cooperative aggregation of monomers into one-dimensional nanostructures typically results in elongated objects. Here we analyse in depth the self-assembly of an N-monoarylated perylene bisimide which shows characteristics of a cooperative growth mechanism but unexpectedly yields objects of small size, due to anti-cooperativity by attenuated growth.

A pitfall of using 2-[(2E)-3-(4-tert-butylphenyl)-2-methylprop-2-enylidene]malononitrile as a matrix in MALDI TOF MS: chemical adduction of matrix to analyte amino groups.

2-[(2E)-3-(4-tert-Butylphenyl)-2-methylprop-2-enylidene]malononitrile (DCTB) has been considered as an excellent matrix for matrix-assisted laser desorption/ionization (MALDI) of many types of synthetic compounds. However, it might provide troublesome results for compounds containing aliphatic primary or secondary amino groups. For these compounds, strong extra ion peaks with a mass difference of 184.1 Da were usually observed, which might falsely indicate the presence of some unknown impurities that were not detected by other matrices. On the basis of the possible mechanisms proposed, these extra ions are the products of nucleophilic reactions between analyte amino groups and DCTB molecules or radical cations. In these reactions, an amino group replaces the dicyanomethylene group of DCTB forming a matrix adduct via a -C=N-bond. An aliphatic primary amine could react easily with DCTB and the reaction could start once they are mixed in a MALDI solution. For an aliphatic secondary amine, on the other hand, the reaction most likely occurs in the gas phase. Protonation of amino groups by adding acid seems to be a useful way to stop DCTB adduction for compounds with one single amino group, but not for compounds with multiple amino groups. Unlike aliphatic primary or secondary amines, aliphatic tertiary amines and aromatic amines do not yield DCTB adducts. This is because tertiary amines do not have the required transferrable H-(N) atom to form an extra -C=N-bond, while aromatic amines are not sufficiently nucleophilic to attack DCTB. In view of the possible matrix adduction, care should be taken in MALDI time-of-flight mass spectrometry (TOF MS) when DCTB is used as the matrix for compounds containing amino group(s).

Tuning the stacking properties of C3-symmetrical molecules by modifying a dipeptide motif.

C3-symmetrical molecules are described which consists of a 1,3,5-benzenetricarboxamide core extended with dipeptide fragments bearing peripheral mesogenic groups. Small structural modifications in the dipeptide fragment have been performed to demonstrate their influence on the stability of the stacks and on the order within the self-assemblies formed. Seven C3-symmetrical discs have been investigated, all with different combinations of glycine, L- and/or D-phenylalanine in the dipeptide fragments. Characterization of these discotics in the neat state using differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM) and in solution with circular dichroism (CD), UV-visible spectroscopy, low-concentration proton nuclear magnetic resonance and IR spectroscopy reveals that there is a clear trend in the stack stability, going from the glycine-phenylalanine motifs to the phenylalanine-phenylalanine ones. The combination of a larger hydrophobic core, more confinement of space and the possibility of additional pi-pi interactions leads to more stable stacks. Surprisingly, the weakest stacks consist of discotics of which the center is extended with L-phenylalanyl-glycines and not of discotics of which the center is extended with the glycyl-L-phenylalanine sequences. Furthermore, the XRD investigations show that it is difficult to form well-ordered self-assemblies in the neat state. And, CD measurements point out that some of the discs have a very complex energy landscape in solution. These observations suggest that small differences in the balance between the secondary interactions originating from the benzenetricarboxamide core and the dipeptide fragments, have a strong influence on the order within the stack. From these results it can be concluded that subtle modifications in the peptide fragments of the discs cause significant changes in the stacking properties, stressing the importance of understanding the self-assembly mechanism of each discotic in order to clarify its self-assembly behavior.

Switching from S- to R-selectivity in the Candida antarctica lipase B-catalyzed ring-opening of omega-methylated lactones: tuning polymerizations by ring size.

Novozym 435-catalyzed ring-opening of a range of omega-methylated lactones demonstrates fascinating differences in rate of reaction and enantioselectivity. A switch from S- to R-selectivity was observed upon going from small (ring sizes or=8). This was attributed to the transition from a cisoid to a transoid conformational preference of the ester bond on going from small to large lactones. The S-selectivity of the ring-opening of the small, cisoid lactones was low to moderate, while the R-selectivity of the ring-opening of the large transoid lactones was surprisingly high. The S-selectivity of the ring-opening of the small, cisoid lactones combined with the established R-selectivity of the transesterification of (aliphatic) secondary alcohols prevented polymerization from taking place. Ring-opening of the large, transoid lactones was R-selective with high enantioselectivity. As a result, these lactones could be polymerized, without exception, by straightforward kinetic resolution polymerization, yielding the enantiopure R-polyester with excellent enantiomeric excess (>99%).

Chiral poly(ureidophthalimide) foldamers in water.

Poly(ureidophthalimide)s decorated with hydrophilic side chains, that ensure solubility in aqueous media, have been synthesized and characterized by UV/Vis and circular dichroism (CD) spectroscopy. Temperature and concentration dependent CD measurements in water have revealed an almost temperature and concentration independent Cotton effect, indicative for a strong intramolecular organization. Similar studies in THF demonstrate the dynamic nature of the secondary architecture, a characteristic of foldamers. In addition, the bisignated Cotton effect in water is opposite in sign to that in THF, suggestive for a solvent-dependent preference for one helical handedness. Mixing experiments prove the dominance of water in determining the handedness of the helical architecture. The solvent allows for control over the helical architecture and thus governs the supramolecular synthesis.

Chiral alignment of OPV chromophores: exploitation of the ureidophthalimide-based foldamer.

The ability of foldamers to adopt a secondary structure in solution has been exploited to organize peripheral functionality. Our previously reported poly(ureidophthalimide) foldamer proved to be an excellent scaffold for the chiral organization of peripherally positioned oligo(p-phenylenevinylene) (OPV) chromophores. Facile high-yielding synthesis gave access to the required OPV-decorated building blocks. A condensation polymerization provided polymers of sufficient length to allow construction of a helical architecture comprising several turns. Short and long chains were separated by chromatography. Circular dichroism studies in THF of the longer chains indicate the presence of helically arranged OPVs. However, such an effect is not observed in CHCl3. Remarkable are the measurements of the OPV foldamers in heptane. A bisignate Cotton effect is observed in heptane of a sample with a THF history. No Cotton effect is observed in heptane of a sample with a CHCl3 history. In this example of supramolecular synthesis, the solvent dictates the expression of supramolecular chirality in a secondary structure. The short-chain oligomeric fractions that are unable to create a full turn revealed on scanning tunneling microscopy analysis the presence of circular architectures at the graphite/1-phenyloctane interface. This is in full agreement with the proposed conformation of the decorated foldamers.