The complexation of calcium and zinc cations by pyrroglutamate analogs has been studied in the gas phase by means of electrospray ionization mass spectrometry (ESI–MS). Complexes were obtained from the solutions of calcium perchlorate and zinc perchlorate in acetonitrile. The complexes with calcium are singly and doubly charged with various stoichiometries while zinc complexes are singly charged except for one ligand. Solvation with acetonitrile and presence of perchlorate counter-ions are observed when the complexes are in the gas phase. The complexes formed with both metals are mainly L2M and LM species. All tested compounds are better complexing agents for calcium than for zinc.

Coordinating abilities of 4R-1,2,4-triazole derivatives (R=glycine ethyl ester (L1), glycine (L2), diethylamino malonate (L3), methionine (L4) and diethyl aminomethylphosphonate (L5)) towards ZnII ions have been studied in solution, in solid state and versus three zinc-β-lactamases. The crystal structure of [Zn3(L4)6(H2O)6] (6) is described; it is the first crystal structure involving a 1,2,4-triazole functionalized methionine. It forms a trinuclear complex with central zinc octahedrally coordinated by only L4, whereas terminal zinc ions coordination sphere is completed by three water molecules. L4 exhibits a dual functionality of a bridging bidentate ligand as well as an anion. A dense hydrogen bonding network connects these trinuclear entity into a 3D supramolecular network. The ZnII ions in 6 are held at equidistance (3.848Å) which coincidently matches with the corresponding Zn⋯Zn distance in the binuclear zinc enzyme from Bacillus cereus (3.848 and 4.365Å). Among L1–L5 screened for β-lactamase assay, L4 shows modest inhibition for BcII enzyme.

Aminophosphonic acids and aminobis(phosphonic acids) have been prepared by the alkylation of Schiff bases with methyl bromoacetate or ethyl acrylate. Other pathways, like the modified Pudovik reaction and Kabachnik-Fields reaction, have been considered for the synthesis of the a-phosphonic bioisoster of aminocitrate. Partial or complete deprotection of the phosphonate ester have been realised by either acidic hydrolysis or by treatment with trimethylsilyl bromide. Evaluation against penicillin-binding proteins has shown that our compounds are modest inhibitors of class A beta-lactamases, but have an interesting activity against R39 (D,D-peptidase/carboxypeptidase). ((C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009)

Aminocitrate (and homolog) derivatives have been prepared by bis-alkylation of glycinate Schiff bases with bromoacetates (and ethyl acrylate), followed by N-acylation and esters (partial or complete) deprotection. Aminoisocitrate was similarly obtained by mono-alkylation with diethyl fumarate. Evaluation against representative beta-lactamases revealed that the free acid derivatives are modest inhibitors of class A enzymes, whilst their benzyl esters showed a good inhibition of OXA-10 (class D enzyme). A docking experiment featured hydrophobic interactions in the active site.

The title compound 4 has been prepared in four steps from ethylglycinate in 63% overall yield. This amino analog of citric acid has been co-crystallized with the class A beta-lactamase BS3 of Bacillus licheniformis and the structure of the complex fully analyzed by X-ray diffraction. Tris-ethyl aminocitrate 3 and the free tris-acid 4 have been tested against a member b- lactamase from all distinct subgroups. They are novel inhibitors of class A beta-lactamases, still modest but more potent than citrate and isocitrate. (C) 2008 Elsevier Ltd. All rights reserved.