MOLECULAR ASSEMBLIES IN GEL AND FIBRE OR CRYSTAL OF BIS-(AMINO ACID) OXALYL DIAMIDE DERIVATIVES

Milan Jokic, Janja Makarevic, Mladen Zinic, Berislav Peric, and Biserka Kojic-Prodic

Rudjer Boškovic Institute, Bijenicka c. 54, P.O.B. 1016, 10001 Zagreb, Croatia, kojic@rudjer.irb.hr

Keywords: molecular assemblies, hydrogen bonds, gelators, fibres

The research of supramolecular organization of organic molecules into nano- or micro- dimensional assemblies is of high relevance for design of new materials and for understanding of the organization of matter in general. A variety of low molecular mass organic compounds can gelate various organic solvents and water. One of macroscopic manifestation of self-aggregation is gel formation, the processes related to biotic world. Thus, we selected to study peptide-based materials of biological importance. Basic units selected for research are chemically 'tailored' to eliminate intramolecular hydrogen bonds but to produce intermolecular ones required for the aggregation. Our experiments revealed that oxalyl amide (HN-CO-CO-NH) unit is responsible for gelling whereas analogous malonyl (HN-CO-CH₂-CO-NH) and urea (NH-CO-NH) units did not. Bis(amino acid) oxalyl amides gelate polar solvents (DMSO, DMSO+H₂O, DMF, DMF+H₂O) in contrast to bis(amino aciod ester) derivatives which do not exhibit gelling ability. For this type of compounds we found that stereochemistryof molecule and the structure of the substituent on the chiral centres strongly influence on gel occurence. Current investigations of carbamoyl derivatives of various amino acid derivatives and solvent used, revealed the formation of gels, fibres or tubular aggregates. For (CO-Leu-NH₂)₂ and some urea derivatives, fibres (10-20 nm in diameter) were detected by light and electron microscopy. For some samples X-ray diffraction patterns were recorded which confirmed a certain degree of crystallinity. The sample of (R,S)-bis(phenylglycine) oxalyl amide in DMF+H₂O forms fine needles suitable for the crystal structure determination. The three-dimensional aggregation via hydrogen bonds also involves disordered water molecules. The unit NH-CO-CO-NH exhibits intramolecular hydrogen bonds of pseudo C5-H type which also participates in intermolecular ones forming sheets.