The contribution of charge to affinity at functional (M₃) muscarinic receptors in guinea‐pig ileum assessed from the effects of the carbon analogue of 4‐DAMP methiodide

Abstract

1 4-Diphenylacetoxy-1:1-dimethyl cyclohexane (carbo-4-DAMP) is the carbon analogue of 4-diphenylacetoxy-N-methylpiperidine (4-DAMP) methiodide. The compounds differ only in that the quarternary nitrogen atom in 4-DAMP methiodide is replaced by a quaternary carbon atom, which is uncharged. 2 Carbo-4-DAMP appears to act competitively at functional (M3) muscarinic receptors in guinea-pig ileum. Estimates of log affinity constant are 6.0 at 30°C and 5.9 at 37'C, i.e. the compound has 0.1% of the affinity of 4-DAMP methobromide. 3 The absence of charge makes little difference to the conformation as determined by X-ray crystallography. The bond lengths and angles are very similar, though the bonds in the cyclohexane ring of carbo-4-DAMP are consistently slightly longer than those in the piperidinium ring of 4-DAMP methiodide, and the presence of the charge slightly reduces the space between molecules. 4 The difference between the affinities of 4-DAMP methobromide and carbo-4-DAMP indicates that the contribution of coulombic forces to the binding between 4-DAMP methiodide and muscarinic (M3) receptors is at least 17 kJ mol' (4.1 kcal mol-') at 37°C. How much this is an underestimate depends upon how much hydrophobic binding is greater with the uncharged compound.

Influence of monovalent cations on the binding of a charged and an uncharged (‘carbo’‐)muscarinic antagonist to muscarinic receptors

“…However, several uncharged 'carbo'-compounds, including BS-6181, have been shown previously to possess considerable affinities (10 to 1000 fold lower than those of the isosteric charged molecules) for muscarinic receptors (Banister & Whittaker, 1951;Barlow & Tubby, 1974;Barlow et al, 1992;Waelbroeck et al, unpublished). It might be argued that these uncharged compounds are highly hydrophobic, and therefore might be able to interact non-specifically with the plasma membrane.…”

“…methods have been used to test this hypothesis: (1) Investigation of the pharmacological properties of isosteric uncharged drugs (containing quaternary carbon instead of quaternary ammonium residues) (Burgen, 1965;Banister & Whittaker, 1951;Barlow & Tubby, 1974;Barlow et al, 1992) or of isosteric 'uncharged' receptors (replacement of the charged aspartate residue of the muscarinic binding site located in the third transmembrane domain by an uncharged asparagine residue) (Fraser et al, 1989;Hulme, 1995;Spencer et al, 1995).…”

“…Unprotonated scopolamine and unprotonated pirenzepine can bind, although weakly to muscarinic receptors (Barlow & Winter, 1981;Barlow & Chan, 1982;Asselin et al, 1983;Birdsall et al, 1989;Ehlert & Delen, 1990). The pharmacological properties of carbon analogues of classical muscarinic agonists and antagonists (Banister & Whittaker, 1951;Barlow & Tubby, 1974;Barlow et al, 1992;Waelbroeck et al, un-published observation) in fact suggest that the contribution of the ionic bond to binding is highly variable. In this study, we analysed the binding properties of an uncharged muscarinic antagonist, the 3,3-dimethylbutan-1-ol ester of diphenylglycolic acid (BS-6181, Figure 1), previously studied by Barlow & Tubby (1974) …”

Flavonoids with M1 Muscarinic Acetylcholine Receptor Binding Activity

“…Most mAChR active ligands contain a positively-charged ammonium group under physiological conditions, but the fact that there are examples of active compounds that lack this feature [12][13][14][15] suggests that this may not be an absolute requirement. In our laboratory, we have previously identified a group of six polyoxygenated flavones from the bark extract of Melicope subunifoliolata (Stapf) T.G.…”