engleski

Bis(Amino Acid) Oxalyl Amides as Ambidextrous Gelators of Water and Organic Solvents: Supramolecular Gels with Temperature Dependent Assembly/Dissolution Equilibrium

Bis(LeuOH) (1a), bis(ValOH) (2a) and bis(PhgOH) (5a) (Phg denotes (R)-phenviglycine) oxalyl amides are efficient low molecular weight organic gelators of various organic solvents and their mixtures as well as water, water/DMSO, and water/DMF mixtures. The organisational motifs in aqueous gels are dominated primarily by lipophilic interactions while those in organic solvents are formed by intermolecular hydrogen bonding. Most of the gels are thermoreversible and stable for many months. However, 2a forms unstable gels with organic solvents which upon ageing transform into variety of crystalline shapes. For some 1a/alcohol gets, a linear correlation between alcohol dielectric constants (epsilon) and gel melting temperatures (T-g) was found. The H-1 NMR and FTIR spectroscopic investigations of selected gels reveal the existence of temperature dependent network assembly/dissolution equilibrium. In the H-1 NMR spectra of gels only the molecules dissolved in entrapped solvent could be observed. By using an internal standard. the concentration of dissolved gelator molecules could be determined. In FTIR spectra, the bands corresponding to network assembled and dissolved gelator molecules are simultaneously present. This enabled determination of the K- gel values by using both methods. From the plots of ln K-gel versus 1/T, the DeltaH(gel) values of selected gels have been determined (- DeltaH(gel) 10-36 kJ mol(-1) range) and found to be strongly solvent dependent. The DeltaH(gel) values determined by H-1 NMR and FTIR spectroscopy are in excellent agreement. Crystal structures of 2a and rac-5a show the presence of organisational motifs and intermolecular interactions in agreement with those in get fibres elucidated by spectroscopic methods.

Amino acids ; Amides ; Gels ; Nmr spectroscopy ; Supramolecular chemistry

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