HypA, a nickel accessory protein from H. pylori, binds a zinc ion in it's structural site, a loop with two conserved CXXC motifs (Ac-ELECKDCSHVFKPNALDYGVCEKCHS-NH(2)). There are at least three hypotheses on the binding mode of this ion. In this paper, we try to understand how Zn(2+) binds to this fragment and why Ni(2+), a metal with quite a high affinity towards thiolic sites, doesn't compete with zinc in the binding to this motif. Potentiometric titrations, mass spectrometry, NMR, UV-Vis and CD spectroscopy help us to compare the coordination modes in both metal complexes and discuss their thermodynamic stabilities
