A-level Chemistry/OCR (Salters)/Penicillins

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The penicillin nucleus[edit]

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[edit]

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 inhibitors[edit]

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[edit]

Name R group in side-chain Natural/semi-synthetic Uses/properties Skeletal formula 3D model
benzylpenicillin
penicillin G
natural general infections, gonorrhoea and syphilis Benzylpenicillin-2D-skeletal.png Benzylpenicillin-3D-balls.png
flucloxacillin semi-synthetic controlling resistant Staphylococcus Flucloxacillin structure.svg Flucloxacillin-3D-balls.png
penicillin F natural not used commercially Penicillin-F-2D-skeletal.png
penicillin X natural not used commercially Penicillin-X-2D-skeletal.png
penicillin K natural not used commercially Penicillin-K-2D-skeletal.png
penicillin V natural and semi-synthetic general infections, ear, nose and throat Penicillin-V-2D-skeletal.png
methicillin semi-synthetic controlling resistant Staphylococcus Methicillin.png
ampicillin semi-synthetic lung and wound infections Ampicillin structure.svg
amoxycillin semi-synthetic lung and urinary tract infections Amoxicillin-2D-skeletal.png Amoxicillin-3D-balls.png
carbenicillin semi-synthetic pneumonia, burns Carbenicillin.svg