Vancomycin Dynamic Combinatorial Chemistry

Vancomycin is a glycopeptidic antibiotic enlisted as the last line of defense against methicilin-resistant Gram-positive infections. Vancomycin exerts its antibacterial action by binding to the terminal D-Ala-D-Ala fragment of the growing peptidoglycan biosynthetic precursor through an intricate network of five hydrogen bonds, thereby inhibiting cell wall growth and cross-linking. However, excessive usage of vancomycin has clinically mutanted the D-Ala-D-Ala of bacteria cell wall into the D-Ala-D-Lac which makes binding ability of vancomycin drop down 1000 times.Dynamic combinatorial chemistry (DCC) makes use of reversible bond-forming reactions to create thermodynamically controlled library. Upon addition of a target molecule or a probe, the molar fractions of individual library member are perturbed as a function of their affinity to that target or probe. Solid-phase screening with the merits of easy separation and operation is benefit from the potential screening of large number compounds and quick determination of the interesting molecular structures and the interactions between them. This thesis aimed to develop a highly throughput resin-bound probe screening method by using a dynamic library of N-demethylvanco -mycin to find vancomycin analogues with better binding ability to the D-Ala-D-Ala in the DCL of amino group of N-demethylvancomycin's leucin residue.1) Through the action of succinic anhydride the -NH₂ function of PEGA resin was converted into -COOH. The resin-bound probe PEGA-L-Lys(-Suc-D-Ala-D-Ala -OH)₂ then was obtained with 100% yield by t-butyl esters protective stratege of D-Alanine. The hydrogen-bound interactions between PEGA-L-Lys(-Suc-D-Ala-D-Ala-OH)₂ and N-demethylvancomycin were studied by high resolution magic angle spinning NMR (HR/MAS NMR).2) Through the screening actions of PEGA-L-Lys(-Suc-D-Ala-D-Ala-OH)₂ a novel of vancomycin analog has been identified which is stronger binding to D-Ala-D-Ala as two times as N-demethylvancomycin by the U.V. difference spectroscopy evaluation.3) In order to further validate and compare the solid-phase screening method, corresponding liquid-phase DCL was designed and screened in this thesis with free (Ac)₂-L-Lys-D-Ala-D-Ala-OH as probe. Same analog of vancomycin was also identified, however, with the lower selectivity.In summary, 1) PEGA-L-Lys(-Suc-D-Ala-D-Ala-OH)₂ is able to be used as probe to screening DCL of N-demethylvancomycine. 2) The resin-bound probe method is compatible with the study of the interactions between small molecules and middle molecules. 3) The solid-phase screening approach is more specific than liquid-phase method.Based on our previous results, the vancosamine Fmoc derivatives of the new vancomycin analogues were also synthesized. Their antibacterial activity against various vancomycin sensitive and resistant strains is under testing.Meaning while, screening the same vancomycin dynamic library by using free (Ac)₂-L-Lys-D-Ala-D-Lac-OH probe was also performed in this thesis. However, none of potential compounds was found. This result indicates that looking for the vancomycin analog with strong binding to D-Ala-D-Lac will be more difficult and lower chance. Increasing the library size could be necessary.