Copper-Ligand Interactions and Physiological Free Radical Processes. pH-Dependent Influence of Cu²⁺ Ions on Fe²⁺-Driven OH^- Generation

Abstract

Prior to comparative studies on the reactivity of various copper complexes with respect to OH radicals, the influence of free Cu²⁺ ions on the superoxide-independent generation of OH radicals through Fenton assays and water gamma radiolysis has been tested in the present work.

Cu²⁺ ions have been shown to behave in a distinct manner towards each of these two production systems. As was logically expected from the noninvolvement of copper in OH^- radical production through gamma radioiysis, no influence of Cu²⁺ ions has been observed on the amount of radicals detected in that case. In contrast, Cu²⁺ ions do influence OH^- radical generation through iron-driven Fenton reactions, but differently depending on copper concentration.

When present in high concentrations, Cu²⁺ ions significantly contribute to OH^- radical production, which confirms previous observations on the reactivity of these in the presence of hydrogen peroxide. At lower levels corresponding to copper/iron ratios below unity on the contrary, Cu²⁺ ions behave as inhibitors of the OH^- production in a pH-dependent manner over the 1–6 range investigated: the lower the pH, the greater the inhibition.

The possible origin of this previously unreported inhibitory effect is discussed.

Key Words:

• Hydroxyl radical
• Fenton reaction
• gamma radiolysis
• Fe²⁺ ions
• Cu²⁺ ions
• ESR