A solid-state 13C nuclear magnetic resonance study of the conformational states of penicillins

Abstract

¹³ C Cross polarization–magic angle spinning spectra have been obtained for crystalline pencillins in which essentially all the resonances have been resolved and assigned. Comparison of the spectra of a series of penicillins of known structure has enabled separation of the effects of intermolecular and intramolecular interactions to be achieved. Two aspects of particular interest have emerged from the work. First, dynamic behaviour of the sidechain is evident in the spectra and this has been shown to depend critically on the crystalline environment of the penicillin molecules. Second, by correlating chemical shifts in the series of molecules, it has proved possible to predict features in the solid-state conformations of penicillins of unknown crystal structure. Further, on comparison with solution spectra, the existence of rapidly interconverting thiazolidine ring puckers in solution is suggested and their relative populations estimated for the different penicillins studied.