Metallocenyl 7‐ACA Conjugates: Antibacterial Activity Studies and Atomic‐Resolution X‐ray Crystal Structure with CTX‐M β‐Lactamase

Here, we report the antibacterial properties of two metallocenyl (ferrocenyl and ruthenocenyl) 7-aminocephalosporanic acid (7-ACA) antibiotic bioorganometallic conjugates. Continuing a trend we found in our previous studies, the ruthenocenyl conjugate showed greater antibacterial activity than its ferrocenyl counterpart. Compared with the previously published 7-aminodesacetoxycephalosporanic acid (7-ADCA) conjugates, the 3-acetyloxymethyl group significantly improved the compounds' activity. Furthermore, the Rc-7-ACA compound was more active against clinical S. aureus isolates than the ampicillin reference. Antibacterial activity of the two metallocenyl 7-ACA derivatives was further confirmed by scanning electron microscopy (SEM). Using a CTX-M-14 beta-lactamase competition assay based on nitrocefin hydrolysis, we showed that the Rc-7-ACA bound more favorably to CTX-M-14 than its ferrocenyl counterpart, again confirming the superiority of the ruthenocenyl moiety over the ferrocenyl one in interacting with proteins. We also report a 1.47 A resolution crystal structure of Rc-7-ACA in complex with CTX-M-14 E166A mutant, an enzyme sharing a similar active site configuration with penicillin-binding proteins, the molecular target of beta-lactam antibiotics. These results strengthen the case for the antibacterial utility of the Rc and Fc groups.