A-level Chemistry/OCR (Salters)/Penicillins
The penicillin nucleus
The pharmacophore (part of a molecule that confers pharmacological activity) of penicillins is called the penicillin nucleus. It contains a β-lactam ring.
The β-lactam ring
The penicillin nucleus found in all penicillins contains a so-called β-lactam ring. This is a four-membered cyclic amide that is unstable in acidic or alkaline conditions. The small ring us under a great deal of strain, making the normally unreactive amide group more susceptible to hydrolysis, resulting in ring-opening to form open-chain compounds.
β-Lactamase is an enzyme produced by certain penicillin-resistant bacteria. It inactivates penicillin antibiotics by disrupting their β-lactam ring. Once way to protect penicillins against β-lactamase is to modify their side chains − methicillin and flucloxacillin are examples of penicillins whose side chains make them β-lactamase resistant. Another tactic is to protect vulnerable penicillins with a β-lactamase inhibitor. These compounds, such as clavulanic acid, are not antibiotics, but they allow penicillin antibiotics to do their job by inhibiting β-lactamase.
Table of penicillins
|Name||R group in side-chain||Natural/semi-synthetic||Uses/properties||Skeletal formula||3D model|
|natural||general infections, gonorrhoea and syphilis|
|flucloxacillin||semi-synthetic||controlling resistant Staphylococcus|
|penicillin F||natural||not used commercially|
|penicillin X||natural||not used commercially|
|penicillin K||natural||not used commercially|
|penicillin V||natural and semi-synthetic||general infections, ear, nose and throat|
|methicillin||semi-synthetic||controlling resistant Staphylococcus|
|ampicillin||semi-synthetic||lung and wound infections|
|amoxycillin||semi-synthetic||lung and urinary tract infections|