All NMR experiments were performed at 25uC on a Bruker DMX 600MHz spectrometer with a triple resonance cryo-probe. Uniformly 15 N-labeled SsTroA was prepared as described above. Sample of apo-SsTroA were prepared at 0.4 mM protein concentration in 20 mM sodium acetate buffer (pH 6.5). Samples of metal bound SsTroA were prepared by the addition of 100 mM zinc acetate or manganese chloride stock to a final concentration of 1 mM metal ions in 0.4 mM protein, and the samples were then subjected to three rounds of concentration with an Amicon Ultra 15 centrifugal filters (Millipore) followed by dilution to remove unbound metal. Samples for NMR measurements contained 0.4 mM 15 N labeled SsTroA, 95% H 2 O/5% 2 H 2 O in 20 mM sodium acetate buffer (pH 6.5). All NMR spectra were processed and analyzed using Felix software (Accelrys Inc.).