A control on hydrophobic and hydrophilic interactions between HEWL and metal Schiff-base complexes comprising of different metal ions and ligands

The structural effects of different copper(II) and nickel(II) Schiff base complexes on hen egg white lysozyme (HEWL) have been investigated through steady state and time resolved absorption and fluorescence, and circular dichroism spectroscopy. The Schiff base ligands with N₄ donor atoms show both hydrophobic and hydrophilic interactions, however hydrophilic interaction prevails with ligands having N₂O₂ donor atoms. Variation of metal ions from Cu²⁺ to Ni²⁺ with each type of Schiff base ligand increases the probability of hydrophilic over hydrophobic interactions, which supports their significance in regulating the binding affinity between HEWL and metal complexes. On photo-excitation the complexes comprising of Cu²⁺ ion instead of Ni²⁺ ion and ligands with N₄ donor system rather than N₂O₂ donor system, increases the probability of intersystem crossing to populate the corresponding triplet state as observed from laser flash photolysis study. The better binding affinity of nickel complexes with different selectivities compared to copper complexes towards HEWL emphasizes the potentiality of less explored nickel complexes in drug–protein interactions. - Highlights: • Ni^II and Cu^II -Schiff base complexes bind hen egg white lysozyme spontaneously. • Both hydrophobic and hydrophilic interactions are effective for N₄ ligands. • For N₂O₂ ligands the hydrophilic is predominant over hydrophobic interaction. • Binding affinity and selectivity of Ni^II-complexes are better than Cu^II-complexes. • Replacement of Cu²⁺ by Ni²⁺ in a ligand enhances chance of hydrophilic interaction