Fig.1

Structural formula of tetracycline hydrochloride derivatives. Metacycline hydrochloride (1A): R1=OH, R2/R3=CH2; Oxytetracycline hydrochloride (1B): R1=R3=OH, R2=Me; Tetracycline hydrochloride (1C): R1=H, R2=Me, R3=OH

Fig.2

1H NMR spectra of tetracycline hydrochloride and its derivatives. (a) Metacycline hydrochloride; (b) Oxytetracycline hydrochloride; (c) Tetracycline hydrochloride; (d) Tetracycline hydrochloride (0.02 mmol) in DMSO-d6 after addition of 5.5 mmol D2O; (e) Tetracycline hydrochloride plus one equivalent cobalt ion (CoCl2·6H2O). Some minor signals in Fig. (e) are due to protons hyperfine-coupled with the bound paramagnetic Co2+; The two signals at δH 1.06 and 3.44 in Fig. (a) are ethanol residuals from the manufacturer. S represents the solvent peak, W represents the water peak, and * represents exchangeable hydrogen

Table 1

NMR signal assignments of tetracycline hydrochloride and its derivatives

Fig.3

1H-13C HMQC spectrum of tetracycline hydrochloride in DMSO-d6

Fig.4

1H-13C HMBC spectrum of tetracycline hydrochloride in DMSO-d6 with a pulse-delay time of 1.5 s to emphasize medium-strong coupling. * indicates that the strong C-H coupling doublets are not fully suppressed

Fig.5

Relaxed-eye stereo view of superimposed tetracycline structures from tetracycline-bound RNA (purple; PDB ID: 1HNW) and elongation factor Tu (cyan; PDB ID: 2HDN). The left and right pictures are for relaxed-eye stereo view[34, 35, 38]. The 2(C)-CONH2 bond in the RNA-bound structure was rotated by 180° to be consistent with that in the Tu-bound structure. The thin green lines are potential H-bonds with proton donors adjusted to proper positions in the H-bond donor/acceptor pairs