Thermodynamic and Structural Determinants of Ni(II), Co(II), and Zn(II) Binding to an XHXXHXH Peptide Motif.

Metal-binding peptides serve as compact platforms for molecular recognition, catalysis, and responsive biomaterials, yet the sequence rules that govern their metal-bound structures remain to be fully elucidated. Clarifying whether short histidine (H)-rich motifs converge to preferred conformations upon metal binding is therefore important for rational design of metallopeptides. To address this, we prepared 30 peptides consisting of the XHXXHXH motif and examined their interactions with Zn(II), Ni(II), and Co(II). The X residues were systematically replaced with Gly, L-Ala, D-Ala, Pro, or Val to modulate backbone flexibility and dihedral-angle space. Circular dichroism and isothermal titration calorimetry showed that the fully glycine-containing peptide GHGGHGH, although flexible in the apo state, adopted a characteristic Zn(II)-bound conformation with a CD spectrum closely resembling that of AHAGHAH, indicating convergence to a preferred metal-induced structure. Across the 30 peptides, binding enthalpy and entropy displayed compensation, linking thermodynamic properties to the structures formed upon metal binding. Comparable analyses with Ni(II) and Co(II) further suggested that multiple conformers coexist in complexes of GHGGHGH despite modest differences in affinity. These results show that the XHXXHXH motif encodes intrinsic preferences for metal-bound conformations and might offer valuable insights for designing metallopeptides with tunable structure and metal-ion recognition.