Amino acid functionalised calixarenes: crystal growth modifiers and low molecular weight gelators
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A selection of amino acid functionalised calixarenes was studied. Acidic amino acid functionalised calixarenes were investigated as crystal growth modifiers. The self-assembly behaviour of proline functionalised calixarenes were also examined.Calixarenes functionalised at the wide-rim with acidic amino acids were investigated as potential crystal growth modifiers on model mineral systems and were compared with previously investigated narrow-rim analogues. This study found that the wide-rim aspartic acid and glutamic acid functionalised calixarenes had an impact on the morphology of calcium carbonate and barium sulfate. The iminodiacetic acid calixarene was found to have minor impact on the morphology of calcium carbonate.In comparison with the narrow-rim analogues, the wide-rim and narrow-rim aspartic acid calixarenes had similar potency on calcium carbonate while the wide-rim glutamic acid calixarene appeared to be more potent than the narrow-rim calixarene. Calcium carbonate crystallised in the presence of wide-rim aspartic acid and glutamic acid calixarenes and their narrow-rim analogues had stepped edges which resembled calcium carbonate found in biomineral systems (e.g. mollusc shells). Desupersaturation experiments showed the wide-rim aspartic acid calixarene to be a more potent inhibitor of barium sulfate crystallisation than the wide-rim glutamic acid calixarene.An aqueous solution of L-proline calixarene exhibited surfactant-like properties. The self-assembly behaviour of L-proline calixarene in solution was characterised by UV-visible and NMR spectroscopy. The critical aggregation concentration of L-proline calixarene was determined to be 4–9 mM by monitoring a selection of dye compounds using UV-visible spectroscopy. The interaction of L-proline calixarene with small molecules and in the presence of electrolytes was also characterised by NMR spectroscopy. The 1H NMR spectra of L-proline calixarene in the presence of small molecules (tetrahydrofuran, 1-methyl-2-pyrrolidinone, and tert-butyl alcohol) suggested that the calixarene was either acting as a host or selfassembling into larger aggregates. Variable temperature NMR experiments showed that these interactions were thermoreversible.The formation of a hydrogel by proline calixarenes in the presence of electrolytes was a serendipitous discovery. Investigations with selected electrolytes showed that the anion was the dominant factor in determining if a hydrogel formed, with lesser influence from the cation. The thermostability of the calixarene-electrolyte hydrogels tended to follow the Hofmeister series, where more stable hydrogels formed in the presence of anions towards the chaotropic end of the series. A racemic mixture of the proline calixarenes gave a less stable hydrogel compared with hydrogels of the enantiopure compounds. The calixarene-electrolyte hydrogels had a fibrous appearance, as characterised by atomic force microscopy. Some hydrogels collapsed slowly and deposited crystals of sufficient quality for single crystal x-ray crystallography. These structures gave some insights into how intermolecular forces may give rise to the fibrous gel structures; in particular hydrogen bonds between proline moieties, and the inclusion of a proline group into the cavity of a neighbouring calixarene, leading to spiral-like structures. A sarcosine functionalised calixarene also formed hydrogels upon addition of some electrolytes, whereas the proline-functionalised calixarene analogue did not.Since a range of hydrogels have been used as matrices to control crystal growth, a proline calixarene-lithium nitrate hydrogel was used as a potential template for barium sulfate crystallisation. The results suggested that lithium nitrate was the main factor in modification of the barium sulfate morphology. The hydrogel did have a minor impact by slowing the diffusion of sulfate anions into the barium-containing gel phase resulting in smaller particles.The versatility of amino acid functionalised calixarenes has been highlighted here. Wide-rim functionalised acidic amino acid calixarenes were found to be effective crystal growth modifiers; having an impact on the morphology and growth kinetics of model crystallisation systems, calcium carbonate and barium sulfate. Interesting supramolecular chemistry was observed with proline functionalised calixarenes; the proline calixarene interacts with small molecules in solution and either act as a host or self-assembles into large aggregates. The formation of proline calixarene aggregates is interesting as they give rise to hydrogels in the presence of selected electrolytes.
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