DFT Investigation of 4-Mercaptoimidazole-Copper(I/II) Redox Chemistry and C2-Substituent Effects: A Minimal Scaffold Study of Ovothiol-Type Redox Cycling.

4-Mercaptoimidazole (4MC) is the minimal parent scaffold of the ovothiol family of redox-active mercaptohistidines. We present a DFT study of the tautomerism, copper coordination, redox chemistry, and electronic structure of 4MC and its C2-substituted derivatives in aqueous solution. Twelve parent species (neutral tautomers, anions, disulfides, and Cu(I)/Cu(II) complexes) were optimised at BP86/def2-SVP with single-point energies at BP86/def2-TZVP/IEFPCM(water). The thione tautomer (N3-H) and its anion dominate (ΔG=-2.56 kJmol-1 and -14.63 kJmol-1), leaving N1 free for copper coordination; the cis-(N3) Cu(II) isomer is the global minimum (81%). The disulfide/thiol and Boltzmann-weighted Cu(II)/Cu(I) reduction potentials are E∘=0.286 V and 0.828 V vs. SHE, giving a positive EMF of 0.542 V comparable to the natural ovothiols. An M06-2X functional benchmark quantifies the method contribution, raising the copper potential to 1.063 V and confirming that the residual offset to the ovothiol literature values reflects the minimal scaffold. Three C2 derivatives (CH3, Cl, NO2; σp from -0.17 to +0.78) show weak Hammett correlations for both redox couples, yet the EMF remains positive for all of them (0.302 to 0.738 V), indicating that C2 substitution tunes the thermodynamic coupling between the thiol-disulfide and copper centres without disrupting it. Electronic structure analysis (NBO, QTAIM) reveals polar covalent Cu-S bonding, spin delocalisation onto sulfur in Cu(II) complexes, and Cu-S dissociation with a switch to Cu-N coordination in the NO2 derivative. The core mercaptoimidazole scaffold thus retains the essential ovothiol redox chemistry, and C2 modification offers a genuine handle for modulating the driving force of the redox cycle.