Synthesis and Biological Studies of Lanthionine Derivatives
The widespread use of antibiotics has caused bacteria to become drug resistant. Efforts have been done by many research groups to identify new molecules as inhibitors or inactivators of resistant proteins like Penicillin Binding Proteins (PBPs) or to find new targets which allow putting peptidoglycan (PG) biosynthesis under control. The aim of this work is to develop a new labeling method for PG allowing to identify novel antibiotics and worthy target for inhibition. In Escherichia coli, meso-diaminopimelic acid (A2pm) is a key molecule for reticulation of PG. During this work, we synthesized meso-lanthionine, the monosulfur analogue of meso A2pm, and some of its derivatives. As these compounds are susceptible to enter PG biosynthesis via MurE enzymatic addition onto UDP (S) Ala γ (R) Glu, their use in a new labeling method has been investigated. Another approach was the introduction of labeled lanthionines in PG via the recycling pathway by replacement of the natural tripeptide (S) Ala γ (R)-Glu-meso-A2pm with tripeptide analogues containing modified lanthionines. We have developed a new stereoselective synthesis of the three lanthionine diastereomers (meso-Lan, (R,R)-Lan and (S,S)-Lan) and of an α-benzylated analogue. Good yields at the gram scale and excellent diastereomeric excesses (>99%), without any enrichment step, were obtained in aqueous solution. Biological experiments showed the incorporation of meso-Lan and (R,R)-Lan into PG. Upon this results, [35S]lanthionine diastereomers could be used to study the biosynthesis of PG and its turnover in relation to cell growth and division. Unfortunately, the α-benzylated lanthionine was not incorporated. This result indicates that the introduction of lanthionines with a substituted aromatic group at this position is not feasible via the reaction catalysed by MurE. We have also developed a synthesis of protected lanthionines bearing an α-alkyl group useful for the preparation of PG building block analogues. An enantiomeric excess of 97% was obtained for the alkylation of an oxazoline precursor. An SN2 reaction with a cysteine on the β carbon of cyclic sulfamidate precursors was highly successful, despite the sterically crowded α-carbon. In this way, we have obtained several protected α-alkyl lanthionines. These products are promising building blocks for peptide synthesis. In this report, we also describe a diastereoselective synthesis of α-substituted tripeptides. These compounds, containing a fluorescent tag or a photoactivatable group situated on an α carbon, could potentially be used to study PG biosynthesis. We have obtained chiral α alkyl sulfamidates with enantiomeric excess of 89 97%. Those sulfamidates are excellent electrophiles with cysteine to provide lanthionines. The lanthionines were then readily converted into tripeptides (S)-Ala-γ-(R)-Glu-(R,S/R,R)-α-benzyl-lanthionine and phenylazido analogues in a one pot reaction. Those tripeptides were substrates of Mpl in vitro. We did not observe any incorporation of these tripeptides in the PG of E. coli.
chiral, enantioelective synthesis, sulfamidate, amino acid, peptide, serine, peptidoglycan, synthèse énantiosélective, acide aminé, sérine, peptidoglycane, Physical, chemical, mathematical & earth Sciences, Chemistry, Physique, chimie, mathématiques & sciences de la terre, Chimie
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