Design, Synthesis, and Study of Lactam and Ring-Expanded Analogues of Teixobactin

Abstract

This paper describes the chemical synthesis, X-ray crystallographic structure, and antibiotic activity assay of lactam analogues of teixobactin and explores ring-expanded analogues of teixobactin with β³-homo amino acids. Lactam analogues of teixobactin containing all four stereoisomers of aza-threonine at position 8 were synthesized on a solid support from commercially available stereoisomeric threonine derivatives. The threonine stereoisomers are converted to the diastereomeric aza-threonines by mesylation, azide displacement, and reduction during the synthesis. d-Aza-Thr₈,Arg₁₀-teixobactin exhibits 2-8-fold greater antibiotic activity than the corresponding macrolactone Arg₁₀-teixobactin. Azateixobactin analogues containing other stereoisomers of aza-threonine are inactive. A dramatic 16-128-fold increase in the activity of teixobactin and teixobactin analogues is observed with the inclusion of 0.002% of the mild detergent polysorbate 80 in the MIC assay. The X-ray crystallographic structure of N-Me-d-Gln₄,d-aza-Thr₈,Arg₁₀-teixobactin reveals an amphipathic hydrogen-bonded antiparallel β-sheet dimer that binds chloride anions. In the binding site, the macrolactam amide NH groups of residues 8, 10, and 11, as well as the extra amide NH group of the lactam ring, hydrogen bond to the chloride anion. The teixobactin pharmacophore tolerates ring expansion of the 13-membered ring to 14-,15-, and 16-membered rings containing β³-homo amino acids with retention of partial or full antibiotic activity.

Figure 1.

(A) Antiparallel β-sheet dimer formed by teixobactin. (B) Coordination of an anion by the teixobactin macrolactone ring. (C) Hypothesized coordination of an anion by an azateixobactin macrolactam ring.

Figure 2.

Synthesis of d-aza-Thr₈,Arg₁₀-teixobactin (2a).

Figure 3.

Diastereomeric azateixobactin analogues 2a–2d.

Figure 4.

X-ray crystallographic structure of N-Me-d-Gln₄, d-aza-Thr₈,Arg₁₀-teixobactin (3a) binding chloride anions (PDB 6PSL). (A and B) Monomer side and top views. (C and D) Dimer side and top views with two bridging water molecules shown as non-bonded spheres. (E) Alignment of the dimer assembly.

Figure 5.

Arg₁₀-teixobactin and ring-expanded Arg₁₀-teixobactin analogues containing β³-homo amino acids at positions 9, 10, and 11. The ring-expanded analogues contain 14-, 15-, and 16-membered macrolactone rings.