Chemical Structures of Local Anesthetics
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The Local Anesthetics, unlike the general anesthetics, have a definite structure/ function relationship. The amines are molecules based on nitrogen. Each can be protonated to get a positive “+” charge (except the quaternary form, which has a permanent “+” charge), making them weak bases.
BH+ B + H+ -NH+ -N + H+
The carboxyl groups are weak acids.
AHA- + H+ -COOH -COO- + H+
Para-amino-benzoic acid (PABA) absorbs UV light because of its ring structure (it is an aromatic carboxylic acid).
An ester is made up of a carboxylic acid and an alcohol (R-COOH + R-OH). You must be able to recognize an ester from its structure: R-COO-R’. Also important, 2 ester bonds make up succinylcholine.
For LA’s (both esters and amides), the R group is an aromatic ring, and the R’ group is a short hydrocarbon chain plus a tertiary amine (see examples on handout). Thus, the LA molecule is an amphiphile (usually linear), with one hydrophobic side and one hydrophilic side.
Category: Pharmacology Notes
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