Spontaneous gelation of a novel histamine H4 receptor antagonist in aqueous solution.

PURPOSE: Low molecular weight hydrogelators typically require a stimulus such as heat, antisolvent, or pH adjustment to produce a gel. This study examines gelation of a novel histamine H4 receptor antagonist that forms hydrogels spontaneously at room temperature. METHODS: To elucidate the mechanism and structural moieties responsible for this unusual gelation, hydrogels were characterized by rheology, optical microscopy, and XRD. SEM was performed on xerogels; NMR measurements were conducted in gelator solutions in the presence of a gel-breaker. The influence of temperature, concentration, pH, and ionic strength on elastic and viscous moduli of the hydrogels was evaluated; gel points were established via thorough rheological criteria. RESULTS: The observed are "true" gels with a fibrillar texture and lamellar microstructure. On a molecular level, the gels are composed of aggregates of partially ionized species stabilized by hydrophobic interactions of aromatic moieties. The gel-to-sol transition occurs at physiologically relevant temperatures and is concentration-, pH-, and ionic strength-dependent. CONCLUSIONS: We hypothesize that this spontaneous gelation is due to the so-called "spring" effect, a high energy salt form that transiently increases aqueous solubility above its equilibrium limit. Upon equilibration, this supersaturated system undergoes aggregation that avoids crystallization and produces a hydrogel.

Dipole tuning of charge transport in molecular junctions.

Molecules with different magnitude and direction of dipole moments are sandwiched between mercury (Hg) and p(+)-Si to form Hg-molecules-(p(+)) Si junctions. The importance of the dipole moment of molecules in controlling the symmetry of current-voltage curves is shown by carrying out charge transport measurements at these molecular junctions. Junction parameters are obtained by considering charge transport across the junction as a combination of tunneling and Schottky emission. An interesting effect due to dipole reversal resulting in rectification of junction curves in the opposite quadrants of the current-voltage axis is observed and analyzed. The junction curves also exhibit a different degree of rectification with increasing magnitude of the dipole moment of the sandwiched molecules.

Controlling molecular crystal polymorphism with self-assembled monolayer templates.

The control of crystal polymorphism is a long-standing issue in solid-state chemistry, which has many practical implications for a variety of commercial applications. At least four different crystalline forms of 1,3-bis(m-nitrophenyl) urea (MNPU), a classic molecular crystal system, are known to crystallize from solution in various concomitant combinations. Herein we demonstrate that the introduction of gold-thiol self-assembled monolayers (SAMs) of substituted 4'-X-mercaptobiphenyls (X = H, I, and Br) into the crystallization solution can serve as an effective means to selectively template the nucleation and growth of alpha-, beta-, and gamma-MNPU phases, respectively. Polymorph control in the presence of SAM surfaces persists under a variety of solution conditions and consistently results in crystalline materials with high phase purity. The observed selectivity is rationalized on the basis of long-range two-dimensional geometric lattice matching and local complementary chemical interactions at the SAM/crystal interfaces.

Selective growth of a less stable polymorph of 2-iodo-4-nitroaniline on a self-assembled monolayer template.

Orthorhombic and triclinic crystals of 2-iodo-4-nitroaniline (INA) grow concomitantly from supersaturated ethanol solutions, but the less stable orthorhombic phase can be selectively grown on 3'-X-4-mercaptobiphenyl (X = NO(2), I) self-assembled monolayer templates.