Conformational analysis of josamycin, a 16-membered macrolide free in solution and bound to bacterial ribosomes
The potent 14-membered macrolide antibiotics displayed a strong NMR response in the TRNOESY experiments whereas their metabolites which do not retain antimicrobial activity gave essentially blank TRNOESY spectra. These TRNOESY experiments are here extended to a macrolide (josamycin) belonging to the 16-atom macrolide class in order to compare responses according to the macrocycle size. Analysis of transferred nuclear Overhauser effect (TRNOE) experiments resulted in a set of constraints for all proton pairs. These constraints were used in structure determination procedures based on molecular modelling to obtain a bound structure compatible with the experimental NMR data. This study allowed us to identify the binding structure at the ribosome for the active 16-membered macrolide. The different conformations existing in solution for this antibiotic in the free state will be compared and correlated with the binding conformation at the ribosome obtained with TRNOE experiments so as to establish a structure–activity relationship. The comparative results indicate that one conformation (called S3) pre-existing in the conformational equilibrium of macrocycle in solution and close to the major one S5, is preferred in the bound state.