Isotopically substituted ( 15 N) N TAILHN and N TAILD3 were prepared by growing transformed bacteria in minimal M9 medium supplemented with 15 NH 4 Cl (0.8 g/L). A 50-mL preculture grown overnight to saturation in LB medium containing 100 mg/mL ampicillin and 17 mg/mL chloramphenicol, was harvested, washed in minimal M9 medium, and inoculated into 1 L of minimal M9 medium supplemented with ampicillin and chloramphenicol. The culture was grown at 37 C. At OD 600 of 0.5, IPTG was added to a final concentration of 0.2 mM and the cells were grown first at 37 C for 3 h, and then over night at 28 C. The induced cells were harvested, washed, and collected by centrifugation. The resulting pellets were frozen at À20 C.