Selectivity in the binding and detection of charge diffuse ions

Abstract

The selective binding of charge diffuse alkyl and arylammonium ions relies upon multiple weak interactions with a complementary synthetic receptor. Using appropriately sized lipophilic cyclodextrin derivatives, the chemoselective binding of alkylammonium ions such as dopamine, acetyl choline, guanidine, and long chain cationic surfactants may be achieved allowing their selective detection by either potentiometric or amperometric methods of analysis. Enantioselectivity in the binding of chiral phydroxyarylammonium ions, such as propranolol, allows chiral sensors to be developed. The selective detection of various clinically important analytes, such as imipramine, lignocaine and creatinine has also been studied.The selective binding of charge dense metal ions is a relatively mature area of research that relies upon matching the donor atom, coordination number and geometric preferences of the cation with the structure of the ligand. For 'onium ions, such as R-NH3+, NMe4+ and the guanidinium cation, (NH2)3C+, the positive charge is delocalised over several atoms, giving a charge diffuse species. With the tetramethylammonium ion, for example, an approximate MNDO calculation indicates that 72% of the positive charge is distributed over the 12 peripheral hydrogen atoms. In seeking to bind such charge diffuse cations, therefore, particular attention must be paid to size complementarity and binding interactions to a synthetic receptor must involve multiple hydrogen bonding. Most of the published work relating to the complexation of tetraalkylammonium ions has tended to focus upon anionic cavitands in which electrostatic attraction probably dominates the binding (1,2,3). With the exception of certain nelectron rich macrocyclic ligands (4,5), most of the neutral ionophores that have been developed to bind ammonium ions only complex primary ammonium ions. Chiral crown ethers, for example, bind certain arylammonium ions enantioselectively (6,7,8), but the chemoselectivity is modest (severe interference from Na+ or K+) and N-alkylation compromises the N-H ... 0 hydrogen bonding. LiDoDhilic CyclodextrinsWe were attracted by the properties of lipophilic cyclodextrin derivatives (9,lO) for the electroanalysis of a broad spectrum of 'onium ions. Neutral, lipophilic ionophores are preferred for potentiometric analysis so that cation and pH interference may be minimised. Such compounds are finding widespread application in chiral GC and HPLC analysis, and the parent cyclodextrins form well defined 1 : 1 inclusion complexes with a variety of size-matched aryl guest species. Our initial work focused on the evaluation of peralkylated cyclodextrin derivatives such as 1,2,3 and 4 as enantioselective ionophores (1 1,12). These derivatives were characterised -paying particular attention to the precise degree of alkylation -by NMR, electrospray mass spectrometric and reductive depolymerisation methods. It is interesting to note that per-0-ethyl-P-cyclodextrin is almost totally insoluble in water but is perfectly soluble in organic ...

Synthetic receptors

“…Such modifications can be used to control the selectivity of CD towards guests. Parker et al have developed CD systems for creatinine, 33 Kean et al have synthesised receptors for carboxylic acids, 34 Yoshida and Ichikawa have prepared molecules selective for anilinonaphthalenesulfonate anions, 35 and Smith and Cotner have constructed CDs able to discriminate phosphotyrosine. 36 The inherent asymmetry of the CD allows for chiral recognition.…”

Selective complexation and sensitive analysis of charge diffuse cationic species using lipophilic cyclodextrins

“…The catecholamine oxidation of dopamine 12 occurs at +200 mV (vs. Ag/AgCl) on a screen-printed carbon electrode that has been coated with 2,3,6-tri-O-ethyl-b-cyclodextrin 3. 20 Dopamine concentrations as low as 10 211 mol dm 23 may be detected in this way, comparing favourably to the levels found in samples of cerebrospinal fluid (10 28 mol dm 23 ). The more sterically bulky cation, imipramine-an anti-depressive drug with a toxic level in blood of 5 mg per mlis more effectively analysed using a sensor incorporating a poly-O-octyl-g-cyclodextrin ionophore.…”

Solubilization of Hexafluorobenzene by the Micellar Aromatic Core Formed from Aggregation of Amphiphilic (2,3-O-Dibenzyl-6-O-sulfobutyl) Cyclodextrins

“…Per-Oalkylated cyclodextrins, with the exception of per-O-methylated, are practically insoluble in water and are considered lipophilic. 65 The V H of DBSBA and DBSBB was taken as the van der Waals volume as computed using the QSAR module in HyperChem 8.01. l c was taken as the distance from the para hydrogen of the benzyl group to the alpha methylene carbon (proximal to sulfonate). The value of a o was 59 Å 2 , as determined from sodium dodecyl sulfonate monolayer data at an air/water interface.…”

“…The entire molecule, with the exception of the sulfonate moieties and sodium counterions, was taken as the hydrophobic component. Per- O -alkylated cyclodextrins, with the exception of per- O -methylated, are practically insoluble in water and are considered lipophilic . The V H of DBSBA and DBSBB was taken as the van der Waals volume as computed using the QSAR module in HyperChem 8.01. l c was taken as the distance from the para hydrogen of the benzyl group to the alpha methylene carbon (proximal to sulfonate).…”