To probe the extent of protein conformational changes in apo-SsTroA induced by Zn 2+ /Mn 2+ binding, we recorded NMR spectroscopy at 25uC. Full 1 H-15 N spectra of SsTroA were acquired in the absence of metal and loaded with Zn 2+ or Mn 2+ . A superposition of the results is presented in Figure 6 . All of the cross-peaks represent backbone and side chain amide group. The spectral region encompassed is commonly referred to as the ''fingerprint'' for the protein conformation, and it is a known probe of minor conformational changes. As shown in Figure 6 , the apoprotein (purple) maintains a similar tertiary structure compared with the metal bound proteins, suggesting that the apo-SsTroA is not randomly unfolded, but seems to fluctuate among several stable states on the intermediate time scale. Such a result is consistent with the observation of T. pallidum, as suggested by Lee et al [22] .