X-ray Structures of Three Penem Antibiotics: Molecular Mechanical and Dynamic Aspects

The structures of three -lactam penem antibiotics—i.e. the sodium5R-5,6(R*)]]-6-(1-hydroxyethyl)-7-oxo-3-(1-pyrrolidinylthioxomethyl)thio]methyl]-4-thia-1-azabicyclo3.2.0] hept-2-ene-2-carboxylate (compound 1), the 5R-3(S*),5,6(R*)]]-3-2-(aminocarbonyl)-1-pyrrolidinyl] methyl]-6-(1-hydroxyethyl)-7-oxo-4-thia-1-azabicyclo3.2.0]hept-2-ene-2-carboxylic acid (compound 2), and the 5R-5,6(R*)]]-3-(2-amino-2-oxoethyl) methylamino]methyl]-6-(1-hydroxyethyl)-7-oxo-4-thia-1-azabicyclo3.2.0]hept-2-ene-2-carboxylic acid (compound 3)—have been determined by X-ray analyses. In the crystal lattice two conformational isomers of 1 are present, which differ from each other in the spatial arrangement of the dithiocarbamate chain. Compounds 2 and 3 are in zwitterionic form, being the hydrogen of the carboxylic acid moved to the amino nitrogen of the chain at C2. This hydrogen atom, in both molecules, forms intramolecular hydrogen bonds with an oxygen atom of the carboxylate moiety and with the oxygen atom of the amido group of the side chain. The 3D structures of 1, 2, and 3 have been compared with those of previously reported -lactam penem antibiotics. Particularly, the Woodward parameter and the Cohen distance, which are considered important in determining the antibiotic activity, have been discussed. Least-squares minimizations (RMS) of the distances between nuclei of selected pairs of atoms defining the pharmacological pattern have been performed, comparing five common antibiotics (imipenem, ritipenem, cephaloridine, amoxycillin, and benzylpenicillin) with our compounds. Finally, molecular dynamics calculations have been carried out on the three penem antibiotics at different temperatures. The conformational behavior of the hydroxyethyl chain, the carboxylate group, and the chain at C2 is discussed by considering the variation of some selected dihedral angles.