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Jan van Esch

Publications and source records attributed to Jan van Esch.

13 recordsLinked to original sources
PubMed2006

Carbosilane dendrimeric carbodiimides: site isolation as a lactamization tool.

The convergent syntheses of three generations of carbosilane dendrimeric carbodiimides are described. The wedge-type building blocks were synthesized in a divergent way, starting from allyl chloride and a repetitive sequence of hydrosilylation with HSiCl3 and a Grignard reaction with allylmagnesium bromide. Hydrogenation of the terminal double bonds led to inert and stable wedges. The chloride substituent at the focal point was transformed into several functional groups that eventually led to dendrimeric structures with a carbodiimide core. The extent of the site isolation effect of the dendrimers was studied with dilution experiments monitored by FT-IR spectroscopy on the corresponding dendrimeric ureas. These studies showed that only the first generation self-aggregates via hydrogen bonding, while the second and the third do not, implying isolation of core-bound moieties. The dendrimeric carbodiimides mediated lactamization reactions to obtain homodiketopiperazines.

Journal Article↗
PubMed2005

Cyclohexane-based low molecular weight hydrogelators: a chirality investigation.

Seven new 1,3,5-cyclohexyltricarboxamide-phenylalanine derivatives were synthesized in order to investigate the effect of the amino acid chirality on the gelating properties of these small molecules in water. Gelation tests have shown that enantiomerically pure homochiral 1,3,5-cyclohexyltricarboxamide-L-phenylalanine is a non-hydrogelator as it crystallizes from water, whereas the heterochiral derivatives with either two L-phenylalanine moieties and one D-phenylalanine (LLD), or vice versa (DDL), are very good hydrogelators. Concentration-dependent gel-to-sol transition-temperature (T(gs)) curves for LLD or DDL gels show a sigmoidal behaviour, which is in contrast to the logarithmic curves generally observed for gels derived from low molecular weight gelators (LMWGs). Such sigmoidal behaviour can be related to interactions between fibre bundles, which give rise to intertwined bundles of fibres. Transmission electron microscopy (TEM) images of LLD and DDL gels show a network of thin, unbranched, fibre bundles with diameters of 20 nm. Right-handed twisted fibre bundles are present in the LLD gel, whereas left-handed structures can be found in the DDL gel. Each bundle of fibres consists of a finite number of primary fibres. Gels consisting of mixtures of gelators, LLD and DDL, and nongelators (LLL or DDD) were investigated by means of T(gs) measurements, CD spectroscopy and TEM. Results show that the incorporation of nongelator molecules into gel fibres occurs; this leads to higher T(gs) values and to changes in the helicity of the fibre bundles. Furthermore, it was found that peripheral functionalization of the homochiral derivatives LLL or DDD by means of a second amino acid or a hydrophilic moiety can overcome the effect of chirality; this process in turn leads to good hydrogelators.

Journal Article↗
PubMed2005

Two-stage enzyme mediated drug release from LMWG hydrogels.

An enzymatically cleavable low molecular weight gelator-(model) drug conjugate system can be employed to effect a two-step enzyme mediated drug release, demonstrating the potential of LMWG systems for the development of drug delivery devices.

Chymotrypsin↗
PubMed2005

Cyclohexane bis-urea compounds for the gelation of water and aqueous solutions.

A new class of efficient hydrogelators has been developed by a simple modification of the peripheral substituents of cyclohexane bis-urea organogelators with hydrophilic hydroxy or amino functionalities. These bis-urea hydrogelators were synthesised in two or three steps using an alternative procedure to the common isocyanate method. Gelation was obtained with organic solvents, water and strongly basic aqueous solutions like 25% ammonia. Hydrogelation was found to depend on a delicate balance between the hydrophobicity of the alkyl chains, hydrophilicity of the terminal substituents and the enantiomeric purity of the compound. The hydrogels consisted of a network of fibers, in which all urea groups are involved in intermolecular hydrogen bonding. Most likely, gelation is driven by hydrophobic interactions of the methylene units, whereas hydrogen bond formation between the urea groups provides the necessary anisotropy of the aggregation and the high thermal stability of the gels.

Cyclohexanes↗
PubMed2004

Entrapment and release of quinoline derivatives using a hydrogel of a low molecular weight gelator.

Gels of low molecular weight gelators (LMWGs) are self-assembled, thermoreversible, viscoelastic materials which can also be rendered sensitive to light, pH or chemical substances by simple chemical modifications. In addition, the ability of some of these LMWGs to gelate water (hydrogelators) makes these gels interesting, new materials for drug delivery applications. In this paper, for the first time, a release study from LMWG gels is presented. This study concerns the release of small (model) drug molecules: 8-aminoquinoline (AQ) and 2-hydroxyquinoline (HQ), from gels of N,N'-dibenzoyl-L-cystine (DBC). DBC forms stable, clear gels in water, 150 mM NaCl solution and PBS (phosphate-buffered saline at pH 7.4) with gel-to-sol transition temperatures (Tgs) of 40-120 degrees C, depending on the gelator concentration (from 2.23 to 22.3 mM). The release of HQ from DBC gels was found to be approximately seven times faster than that of AQ and the initial release of the latter follows the kinetics of gel degradation. These observations indicate that AQ is preferentially retained in the gel, presumably as a result of stronger interactions with the gelator molecules (i.e. DBC-COO-+ H3N-AQ). These results indicate the potential of LMWG gels as delivery vehicles for small drug molecules and also show that the release profiles for such systems can be fine-tuned by the correct choice of gelator-drug combination.

Aminoquinolines↗
PubMed2003

Orthogonal self-assembly of low molecular weight hydrogelators and surfactants.

The concurrent self-assembly of new 1,3,5-trisamide-cyclohexane-based low molecular weight hydrogelators and various surfactants in water leads to the formation of self-assembled fibrillar networks with encapsulated micelles. This prototype system presents an example of orthogonal self-assembly, that is, the independent formation of two different supramolecular structures, each with their own characteristics that coexist within a single system.

Amides↗
PubMed2003

Supramolecular control of two-dimensional phase behavior.

We have used directed two-component self-assembly to "pattern" organic monolayers on the nanometer scale at the liquid/solid interface. The ability of the scanning tunneling microscope to investigate structural details in these adlayers was used to gain insight into the two-component two-dimensional phase behavior. The components are symmetrically alkylated bisurea derivatives (R1-urea-spacer-urea-R2; R1, R2=alkyl, spacer=alkyl or bisthiophene). The bisthiophene unit acts as a marker and its bisurea derivative (T2) is a component in all the mixtures investigated. By varying the position of the hydrogen-bond forming urea groups along the molecule and the length of the alkyl chains of the other components, the effect of 1) hydrogen bonding, 2) molecule length, 3) odd-even effects, and 4) shape complementarity on the two-dimensional phase behavior was investigated. Insight into the effect of these parameters leads to the control of the two-dimensional patterning: from randomly intermixed systems to phase separation.

Journal Article↗