Stereoselective Synthesis of Peptidoglycan Fragments On Solid Support

Growing of bacteria goes along with the insertion of peptidoglycan strands inside the cell wall. Enzymes catalyzing the biosynthesis of peptidoglycan are the targets of various antibiotics like penicillin. Inhibition of peptidoglycan biosynthesis leads to lysis of bacterial cells. However, bacteria have developed resistance mechanisms. Many research groups are still studying the different mechanisms of resistance in order to develop new drugs. During lysis, peptidoglycan fragments are released. Nucleotide-binding Oligomerization Domains of mammalian proteins, NOD1 and NOD2 are able to recognize these peptidoglycan motifs from Grampositive and Gram-negative bacteria and activation of immune responses is induced. Therefore, NOD1 and NOD2, are studied as new targets to control the anti-inflammatory response. The γ-D-glutamylmeso-diaminopimelic acid 1 is detected by NOD1 and muramyl dipeptide 2 by NOD2 (Figure 1).1 Figure 1: γ-D-glutamyl-meso-diaminopimelic acid dipeptide 1 and muramyl dipeptide 2 Dipeptide 1 contains the meso-2,6-diaminopimelic acid (m-A2pm), a non-proteogenic amino acid mostly found in Gram-negative bacteria. The first amino acid coupling with the resin for the synthesis of dipeptide 1 by Fmoc-Solid-Phase Peptide Synthesis (Fmoc-SPPS), is diastereoselectively protected m-A2pm. With the introduction of SPPS by Merrifield, peptide synthesis was greatly improved. Thus, purification became easier, and conversion higher. Today, SPPS is highly automated. However, m-A2pm is not commercially available. Chowdhury2 described diastereoselective synthesis of protected m-A2pm by homogeneous phase Cross Metathesis (CM). For an easy handling and rapid peptide production, we reported here a synthesis of peptidoglycan fragments entirely synthesized on solid support. For the synthesis of desired molecules, Fmoc-SPPS, heterogeneous phase CM and sugar chemistry are three methods used to anchor amino acids, m-A2pm and sugars derivatives, respectively, on a resin as solid support. As cited above, Chowdhury2 described diastereoselective synthesis of m-A2pm by homogeneous phase CM. In this study, we adapted the CM on solid support. The protected (S)-allylglycine ((S)-Agl) 1 previously anchored on the resin acted as the first olefin for the CM, the second one was the protected (R)-vinylglycine ((R)-Vgl) 2 in solution. Amino Acid (AA),protecting groups (PG) and enantiomers were selected according to the synthesis of desired peptides. Concentration of olefin 2, choice of protecting groups, catalyst, temperature and solvent were parameters to optimize the CM conversion. Deanchoring from the resin and hydrogenation led to desired peptides (Figure 2). Figure 2: CM synthesis of anchored m-A2pm, R= -Oor AA and R’= H or AA In conclusion, peptides with m-A2pm in different positions were synthesized by heterogeneous phase CM on solid support. Furthermore, this method allowed the specific binding to the R or S site of m-A2pm by correct choice of enantioselectively synthesized Agl and Vgl with adequate protecting groups. Heterogeneous phase CM showed an easy handling and a rapid peptide production. In the future, this strategy will be applied to the synthesis of various muropeptides. [1] Boudreau, M. A.; Fisher, J. F.; Mobashery, S. Messenger Functions of the Bacterial Cell Wallderived Muropeptides. Biochemistry, 51(14), 2974–2990, 2012. [2] Chowdhury, A.R.; Boons, G-J. The synthesis of diaminopimelic acid containing peptidoglycan fragments using metathesis cross coupling. Tetrahedron Letters, 46(10), 1675-1678, 2005.