Fig. 2. Characterisation of the teixobactin–lipid II complex. (a–d) Results of solution NMR titrations of teixobactin with a water soluble C₁₅ variant of Gram-positive lipid II in aqueous solution (a, b) and with native C₅₅ Gram-negative lipid II in DPC micelles (c, d). Panels b and d show the measured apparent K_a values and plotted onto the chemical structure of teixobactin in a and c. Dark blue indicates sites with apparent K_a values higher than the average plus one standard deviation and light blue indicates sites with apparent K_a values higher than the average. Dark grey indicates sites with a below average apparent K_a (b and d). In a and c grey circles indicate sites for which data are not available. (e) 2D ¹H-¹⁵N solid state NMR correlation spectrum of sedimented [U¹³C-¹⁵N]teixobactin in complex with natural abundance Gram-negative lipid II in DPC micelles acquired at 600 MHz ¹H Larmor frequency and 90 kHz magic angle spinning frequency. In panel e star indicates peaks folded in the ¹⁵N dimension. The two resonances for residue 1 are due to the zwitterionic form of N-Me-d-Phe₁ being the major form at pH 6.5. (f) ¹H-¹⁵N (light grey) and ¹H^α-13C^α (dark grey) chemical shift perturbations between TXB_DPC and TXB_{lipid II}. Comparisons of secondary ¹³C^α (g), ¹H^α (h) and ¹³C^β (i) chemical shifts between TXB_DPC (black squares) and TXB_{lipid II} (red circles).