INTERACTIVE DESIGN AND SYNTHESIS OF A NOVEL ANTIBACTERIAL AGENT

beta-Lactam compounds act on penicillin-recognizing enzymes via acylat ion of the hydroxyl group of an active site serine. When the resulting acyl enzyme is kinetically stable, as in the case of a penicillin-bin ding protein (PBP), the biosynthesis of a bacterial cell wall is inhib ited, and death of the organism-results. The de novo design of an anti bacterial agent targeted to a PBP might be possible if the three-dimen sional structural requirements of the equilibrium (i.e., fit) and cata lytic (i.e. reactivity) steps of the aforementioned enzymatic process could be determined. For a model of the active site of a PBP from Stre ptomyces R61, the use of molecular mechanics calculations to treat ''f it,'' and ab initio molecular orbital calculations to treat ''reactivi ty,'' leads to the idea that the carboxyl group (G(1)) and the amide N -H (G(2)) of the antibiotic are hydrogen bonded to a lysine amino grou p and a valine carbonyl group in the enzyme-substrate complex. These t wo hydrogen bonds place the serine hydroxyl group on the convex face o f the antibiotic, in position for attack on the beta-lactam ring by a neutral reaction, catalyzed by water, that involves a direct proton tr ansfer to the beta-lactam nitrogen. Molecular orbital calculations of structure-reactivity relations associated with this mechanism suggest that C=N is bioisosteric to the beta-lactam N-C(=O), comparable to a b eta-lactam in its reactivity with an alcohol, and that the product RO( C-N)H is formed essentially irreversibly (-Delta E > 10 kcal/mol). Acc ordingly, structures containing a G(1) and a G(2) separated by a C=N, and positioned in different ways with respect to this functional group , have been synthesized computationally and examined for their ability to fit to the PBP model. This strategy identified a 2H-5,6-dihydro-1, 4-thiazine substituted by hydroxyl and carboxyl groups as a target for chemical synthesis. However, exploratory experiments suggested that t he C=N of this compound equilibrates with endocyclic and exocyclic ena mine tautomers. This required that the C2 position be substituted, and that the hydroxyl group not be attached to the carbon atom adjacent t o the C=N. These conditions are met in a 2,2-dimethyl-3-(2-hydroxyprop yl)- 1,4-thiazine, which also exhibits the necessary fit to the PBP mo del. Two epimers of this compound have been synthesized, from D- and L -serine. The compound derived from L-serine is not active. The compoun d derived from D-serine exhibits antibacterial activity, but is unstab le, and binding studies with PBP's have not been performed. It is hope d that these studies can be carried out if modification of the lead st ructure leads to compounds with improved chemical stability.