In the presence of Mn 2+ , the complex solution leads to selective proton line broadening (data not shown). Two dimensional Heteronuclear Single Quantum Coherence (HSQC) NMR spectra of Mn 2+ -SsTroA are shown in Figure 6B . The number of peaks was less than the expected number of amide groups, presumably due to the paramagnetic broadening effects (Mn 2+ is a paramagnetic agent). The cross peaks, still visible after incubating with Mn 2+ , belong to protons distal from the Mn 2+ binding site. By contrast, the broadening and disappearance of the cross-peak strongly indicates the involvement of the amide proton close to the Mn 2+ coordination sphere. It should be noted that almost all of the peaks in the presence of Zn 2+ and Mn 2+ have similar chemical shifts (Fig. 6C) , indicating that the protein regions adopt identical structures when bound to Zn or Mn. This suggests that Zn 2+ and Mn 2+ could bind to the SsTroA in similar positions, perhaps even the same site. A similar result is observed in N. gonorrhoeae MntC protein, where the Zn and Mn ions compete for the same site [33] .