A new series of phosphonyl derivatives has been prepared and tested for inhibition of serine (classes A and C) beta-lactamases. The results were compared with those previously acquired with aryl phosphonate monoesters and with alkaline hydrolysis rates. A methyl p-nitrophenyl phosphate monoanion was markedly poorer as an inhibitor of the class C beta-lactamase of Enterobacter cloacae P99 than a comparable p-nitrophenyl phosphonate. Phosphonyl fluorides, thiophenyl esters, N-phenylphosphonamidates and a p-nitrophenyl thionophosphonate were, in general, comparable with p-nitrophenyl phosphonates in inhibitory power. The incorporation of a specific amino side chain led to an increase in the rates of inhibition of around 10(4)-fold. Apparently unresponsive to the addition of the side chain to the enzyme was N-phenyl methylphosphonamidate, where binding of the side chain may interfere with access of the leaving group to a proton which is necessary to active-site phosphonylation and inhibition. Typical class A beta-lactamases were significantly more refractory than the class C enzyme to all of these reagents.

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