Ammonium Group.The ammonium group of ACh can be replaced by other “-onium”
compounds (phosphonium, arsonium, or sulfonium), but only with the loss of 90% of
the activity. One of the methyl groups on the ammonium can be exchanged for larger
alkyl residues: for instance, the dimethylethyl derivative is about 25% active. However,
the insertion of larger groups or the replacement of more than one methyl leads to an
almost complete loss of activity. This finding implies that the size of the quaternary
ammonium group and its charge distribution are important to the activity of ACh, since
the hydrophobic auxiliary binding site next to the anionic site of the receptor is opti-
mized for two methyl groups, strengthening the ionic interaction. The uncharged carbon
analog 3,3–dimethyl-butyl acetate has only 0.003% activity. It is interesting to note that
many muscarinic agonists are tertiary amines—for example, pilocarpine (2.2), arecol-
ine (4.10), and oxotremorine (4.11). At physiological pH, however, these amines are
likely to be protonated and to occur in rigid ring structures. In this way, hydrogen bond-
ing between the protonated amino group and the -COO- group of the anionic site of the
receptor is not prevented by the intramolecular interaction of the -C=O and -NH(CH 3 ) 2
groups in the ligand. Such a bond would completely distort the ligand conformation and
prevent normal binding to the AChR.
Ethylene Chain.The ethylene bridge of ACh ensures the proper distance between
the ammonium group and the ester group, and is therefore critical in binding to the
receptor. Although it is rather dangerous to assign a definite distance between the -onium
and ester groups (estimated at about 0.6 nm), the “rule of five” states that there should
be no more than four atoms between the N+and the terminal methyl group. Lengthening
of the chain results in rapidly decreasing activity; interestingly, however, the 2-butyne
analog has 50% ACh activity. If the ethylene is branched, only methyl groups are
allowed, as shown in the muscarinic agonist methacholine (4.6). The α-methyl analog
of ACh has more nicotinic activity.
Ester Group.The ester group does not lend itself to much modification either. Large
aromatic acid moieties in the ester produce ACh antagonists rather than agonists, some of
which are useful as anticholinergic agents. If ethers and ketones replace the ester, some
activity is retained. The only useful replacement for the acetate has been a carbamate
group, resulting in carbachol (4.7), which is highly active because of its slow hydrolysis.
Cyclic Analogues of ACh.Cyclic ACh analogues include the naturally occurring
agonists muscarine (4.3), pilocarpine (2.2), and arecoline (4.10), all of which are mus-
carinic compounds. Dioxolanes such as (4.12) are muscarinic analogs of very high
potency. Cyclization is a good drug design strategy in that in constrains conformational
flexibility, thereby increasing receptor specificity.
NEUROTRANSMITTERS AND THEIR RECEPTORS 211