Conformations of penicillin G: crystal structure of procaine penicillin G monohydrate and a refinement of the structure of potassium penicillin G

The geometric details and mode of puckering of the five-membered (thiazolidine) ring have been found to be different in two penicillin G salts. In procaine penicillin G monohydrate (aqueous procaine penicillin G or APPG) the penicillin molecule is in an 'extended' form whereas it is 'coiled' in potassium penicillin G. The solid-state conformation of penicillin G therefore depends more on local environment than on nature of substitution in the side chain. The procaine molecules in APPG adopt an enlarged (91°) torsion angle in the O–C–C–N group and exhibit little quinonoid character in the p-aminobenzoate group.