Penicillin G
Properties
| State | Solid (white crystalline powder) |
| Color | White |
| Solubility | Freely soluble as the sodium or potassium salt; the free acid is poorly water-soluble |
| Melting Point | 214 °C (decomposes) |
| Boiling Point | Decomposes before boiling |
About Penicillin G
Penicillin G (benzylpenicillin) is the original beta-lactam antibiotic, formula C16H18N2O4S, molar mass 334.39 g/mol. The structure is a fused bicyclic system: a five-membered thiazolidine ring sharing an edge with the strained four-membered beta-lactam, with a phenylacetamide side chain off the alpha carbon. Alexander Fleming noticed it in 1928 when a Penicillium notatum spore landed on a Staphylococcus plate at St Mary's Hospital and cleared a zone around itself. He published, the work sat dormant for a decade, then Howard Florey, Ernst Chain, and Norman Heatley at Oxford industrialized the production during 1940-43 — the deep-tank fermentation process that scaled up in Peoria, Illinois turned penicillin from a laboratory curiosity into the antibiotic that saved Allied troops from wound sepsis at Normandy. Mechanism: penicillin G mimics the D-Ala-D-Ala terminal dipeptide of the bacterial peptidoglycan precursor and acylates the active-site serine of the transpeptidase enzymes (penicillin-binding proteins, PBPs) that cross-link the cell wall. The acyl-enzyme is essentially irreversible because the strained beta-lactam opens to a stable amide. Without cross-linked peptidoglycan, the cell wall fails under turgor pressure and the bacterium lyses. Pen G is most active against Gram-positive cocci (Streptococcus pyogenes, S. pneumoniae, viridans streptococci), Treponema pallidum (syphilis), and Neisseria meningitidis. Resistance is overwhelmingly through bacterial beta-lactamases that hydrolyze the beta-lactam ring before it can reach the PBP — which is why we now have beta-lactamase-stable derivatives (methicillin, oxacillin) and beta-lactamase inhibitor combinations (amoxicillin/clavulanate, piperacillin/tazobactam).
Where you'll encounter it
If you've ever been diagnosed with strep throat in a clinic and gotten an IM shot in the gluteus, that thick white suspension was benzathine penicillin G — a poorly soluble depot form that releases active drug over 2-4 weeks, which is also the WHO-recommended single-dose treatment for primary syphilis worldwide. Microbiology students still grow Penicillium chrysogenum (the high-yielding strain that replaced notatum) in surface culture to demonstrate the historical fermentation. Industrial penicillin G production now runs around 70,000 tonnes per year from optimized P. chrysogenum strains in 200,000-liter fed-batch bioreactors, with most of it converted to 6-aminopenicillanic acid (6-APA) by penicillin acylase, then re-acylated to make ampicillin, amoxicillin, and the rest of the semi-synthetic penicillin family. Pharmaceutical chemists use 6-APA the way organic chemists use cyclopentadiene — a versatile platform.
Common Uses
- Treatment of choice for syphilis (single IM dose of benzathine penicillin G)
- First-line therapy for streptococcal pharyngitis, scarlet fever, and rheumatic fever prophylaxis
- IV penicillin G for bacterial endocarditis, meningococcal meningitis, and gas gangrene
- Industrial substrate for enzymatic conversion to 6-APA, the precursor of all semi-synthetic penicillins
- Teaching example for the discovery of antibiotics and bacterial resistance mechanisms
Safety Information
GHS: H317 (skin sensitizer Category 1), H334 (may cause respiratory sensitization). True IgE-mediated penicillin allergy occurs in about 0.5-2 percent of patients exposed; anaphylaxis is rare (about 1 in 10,000 doses) but can be fatal. Allergy history must be verified before every administration. Self-reported penicillin allergy is the most common drug allergy in medical records but is incorrect about 90 percent of the time, which has driven the current push for formal penicillin allergy de-labeling programs. Side effects of therapeutic dosing include rash, diarrhea, and Jarisch-Herxheimer reactions when treating syphilis. Massive IV doses can cause seizures, particularly in renal impairment.
This safety summary is for educational reference only and may not be complete. It is not a substitute for Safety Data Sheets (SDS), medical advice, or professional chemical safety guidance. Always consult appropriate SDS and qualified professionals before handling chemicals.