Ammonium-containing receptors

The chiral cavitands 3.109 have been developed by combining the amino acid residue L-alanine with macrocyclic cavitands (calixarenes - Section 3.14).51 These ammonium ion receptors are able to complex a range of amino acids and their methylester hydrochloride salts, all of which contain an -NH3+ functionality capable of interaction with the carboxylate residues of the host. In general amino acids are bound only very weakly in aqueous solution, while association constants with the chiral methyl esters range from 620 M-1 for L-tryptophan methylester to 110 M-1 for L-alanine methylester. The methylester of glycine is not bound at all. Receptors related to 3.109 with variable four peptide loops arrayed around a central calixarene core have been used to bind to the surfaces of proteins. The... 

New diester/diamide-calix[4]aienediquinones and a diamide-benzo-15-crown-5-calix[4]arene receptor molecule have been prepared and shown to complex Group 1 and 2 metal, ammonium and alkylammonium cations with association constants up to 4.8 X 10 with Ba + and (3) in acetonitrile solution. The redox-active quinone containing receptors electrochemically recognise these cationic guest species, including for the first time, the amperometric detection of an alkyl ammonium cationic guest species by a redox-active ionophore. 

Polytopic macrocyclic receptors 1, 2 (Figure 10.1) are able to complex the zwitterionic form of the amino acids by a double non-covalent charge interaction [28,29]. The unsymmetrical benzocrown sulfonamide derivative, 2 which contains benzo-18-crown-6 and benzo-15-crown-5 moieties was used as a ditopic receptor for multiple molecular recognition of the amino acids, by combining two non-covalent interactions ammonium-crown hydrogen bonding and carboxylate- complexed Na+-benzo-15-crown-5 charge interactions [28,33]. 

Obtained in acetonitrile solution containing 0.2 mol dm-3 Bu NBF4 as supporting electrolyte. Solutions were 1 x 10 3 mol dm-3 in receptor and potentials were determined with reference to an Ag/Ag+ electrode. Two-electron process. Four-electron process. Cathodic shift in redox wave produced by the presence of anions (up to 5.0 equiv) added as their tetrabutyl-ammonium salts. 

Tlie neurotransmitter acetylcholine is both a quaternary ammonium compound (see Box 6.7) and an ester. After interaction with its receptor, acetylcholine is normally degraded by hydrolysis in a reaction catalysed by the enzyme acetylcholinesterase. This enzyme contains a serine residue that acts as the nucleophile, hydrolysing the ester linkage in acetylcholine (see Box 13.4). This effectively acetylates the serine hydroxyl, and is an example of transesterification (see Section 7.9.1). For continuation of acetylcholine degradation, the original form of the enzyme must be regenerated by a further ester hydrolysis reaction. 

Spherical cryptand (13) contains four nitrogen atoms in a tetrahedral arrangement this is the topologically ideal receptor for the ammonium ion (NH4+) which has a tetrahedral... 

Lehn and coworkers have profitably employed tartaric acid-containing crown ethers as enzyme models. The rate of proton transfer to an ammonium-substituted pyridinium substrate from a tetra-l,4-dihydropyridine-substituted crown ether was considerably enhanced compared to that for a simple 1,4-dihydropyridine. The reaction showed first order kinetic data and was inhibited by potassium ions. Intramolecular proton transfer from receptor to substrate was thus inferred via the hydrogen bonded receptor-substrate complex shown in Figure 16a (78CC143). 

Tobey, S. L., Anslyn, E. V., Energetics of phosphate binding to ammonium and guanidinium containing metallo-receptors in water. J. Am. Chem. Soc. 2003, 125, 14807-14815. 

Anslyn and coworkers [56,57] synthesized receptors containing Cu(II) binding sites with appended ammonium 69 and guanidinium 70 groups. These receptors showed high affinities for tetrahedral anions such as HPO " and... 

A number of other receptors structurally relating to 36 were described that possess overall similar binding properties although some characteristic differences were noted. Compound 37, for example, containing a cyclotriveratrylene-derived cap instead of a TREN moiety, also binds imidazolidin-2-one or ammonium halides, but since this compound lacks the tertiary amino group it does not allow binding properties to be controlled by protonation [84],... 

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