In nature, thiolate-based systems are the primary targets of divalent mercury (Hg-II) toxicity. The formation of Hg(Cys)(x) cores in catalytic and structural protein centers mediates mercury's toxic effects and ultimately leads to cellular damage. Multiple studies have revealed distinct Hg-II-thiolate coordination preferences, among which linear Hg-II complexes are the most commonly observed in solution at physiological pH. Trigonal or tetrahedral geometries are formed at basic pH or in tight intraprotein Cys-rich metal sites. So far, no interprotein tetrahedral Hg-II complex formed at neutral pH has been reported. Rad50 protein is a part of the multiprotein MRN complex, a major player in DNA damage-repair processes. Its central region consists of a conserved CXXC motif that enables dimerization of two Rad50 molecules by coordinating Zn-II. Dimerized motifs form a unique interprotein zinc hook domain (Hk) that is critical for the biological activity of the MRN. Using a series of length-differentiated peptide models of the Pyrococcus furiosus zinc hook domain, we investigated its interaction with Hg-II. Using UV-Vis, CD, PAC, and Hg-199 NMR spectroscopies as well as anisotropy decay, we discovered that all Rad50 fragments preferentially form homodimeric Hg(Hk)(2) species with a distorted tetrahedral HgS4 coordination environment at physiological pH; this is the first example of an interprotein mercury site displaying tetrahedral geometry in solution. At higher Hg-II content, monomeric HgHk complexes with linear geometry are formed. The Hg(Cys)(4) core of Rad50 is extremely stable and does not compete with cyanides, NAC, or DTT. Applying ITC, we found that the stability constant of the Rad50 Hg(Hk)(2) complex is approximately three orders of magnitude higher than those of the strongest Hg-II complexes known to date.