Effect of Solvents on NMR Spectra of Penicillins

High-resolution NMR spectra of penicillin G and ampicillin in deuterium oxide, water, deuterated dimethyl sulfokide, and deuterated dimethyl sulfoxide-deuterium oxide were examined. The α-amino group of ampicillin could not be observed in these solvents but was observed using the pivaloyl derivative of ampicillin in deuterochloroform. No evidence for intramolecular hydrogen bonding between the protons of the α-amino group and the oxygen of the β-lactam carbonyl group was obtained. The amino group does not restrict the rotation of the side chain, which has been shown to be important in interaction with phospholipids.