In an effort to minimize enzymatic removal of unnatural modifications, we sought a strategy that installed epitopes onto the fourth position within the stem peptide via the MurF dipeptide pathway. Dipeptide 2 was synthesized to mimic the lysine side chain in the single amino acid metabolic labeling strategy. Treatment of B. subtilis cells with dipeptide 2 led to an insignificant increase over untreated cells. From these results, it became apparent that side-chain size may have contributed to drastically reduced labeling levels. We proceeded to synthesize a similar dipeptide using d-2,3-diaminopropionic acid with the aim of minimizing the overall size of the amino acid side chain. However, flow cytometry analysis of B. subtilis bacteria treated with dipeptides 2 and 3 revealed minimal increases in labeling. Interestingly, it is apparent from these results that various levels of tolerance for unnatural cell wall analogues exists depending on the entry point into the peptidoglycan biosynthesis pathway. We recently demonstrated that side-chain structural features control incorporation efficiency of single d-amino acids. Finally, we synthesized a dipeptide conjugated with a DNP hapten analogous to dipeptides 2 and 3 and observed no discernible anti-DNP antibody recruitment to the cell surface (data not shown). With these early results in hand, we set out to redesign a dipeptide-based labeling strategy that exploited the reduced removal at the fourth position on the stem peptide but improved incorporation efficiency.

Fura, J. M., Pidgeon, S. E., Birabaharan, M., & Pires, M. M. (2016). Dipeptide-based metabolic labeling of bacterial cells for endogenous antibody recruitment. ACS infectious diseases, 2(4), 302-309.

Synthesis of a series of N-(2-acetylaminothiazole-5-sulphonyl) amino acids (II-VI) and some of their corresponding methyl esters (VII-XI) is described. Coupling of N-tosyl-or N-phthalylamino acid with 2-amino-2-thiazoline using the DCC method furnishes 2-(N-tosyl- or N-phthalylamino acyl) amino-2-thiazoline (XII-XXI). Hydrazinolysis of 2-(N-Pht-L-Phe or N-Pht-L-Ala) amino-2-thiazoline in ethanol afforded 2-(L-Phe or L-Ala) amino-2-thiazoline (XXII and XXIII) respectively. Synthesis of the dipeptide 2-(N-Tos-L-Val-L-Leu) amino-2-thiazoline (XXIV) has been achieved employing the azide method. Sixteen thiazole- and thiazoline-amino acid derivatives were found to be active against a number of microorganisms.

EL-NAGGAR, A. M., Ahmed, F. S. M., EL-SALAM, A. A., Haroun, B. M., & Latif, M. S. A. (1982). Synthesis of some biologically active substituted thiazole and thiazoline‐amino acid derivatives. Chemical Biology & Drug Design, 19(4), 408-412.