The Influence of pH on Long-Range Electron Transfer and Proton-Coupled Electron Transfer in Ruthenium-Modified Azurin

Area of Science:

• Biochemistry
• Bioinorganic Chemistry
• Physical Chemistry

Background:

• Long-range electron transfer (ET) is fundamental to biological processes, particularly in metalloproteins.
• Protein charge ladders demonstrate how metal ion redox states affect surface amino acid ionization.
• Understanding pH-dependent redox mechanisms is crucial for biological ET.

Purpose of the Study:

• Investigate intramolecular ET in a ruthenium-modified azurin system across a range of pH.
• Determine the redox behavior of a ruthenium model complex using Pourbaix diagrams.
• Propose a mechanistic model for ET in azurin based on pH-dependent observations.

Main Methods:

• Utilized flash-quench methods to generate ruthenium (Ru) 3+ oxidants.
• Performed variable pH studies on intramolecular ET reactions in ruthenium-modified azurin.
• Generated a Pourbaix diagram for a Ru model complex: Ru(2,2'-bipyridyl)2(imidazole)2(PF6)2.

Main Results:

• Intramolecular ET rate constants in azurin did not align with standard ET models using fixed parameters.
• A pH-dependent shift in the ET mechanism was observed, transitioning at pH 7.5.
• The Ru3+/2+ redox couple exhibited distinct behavior at acidic (≤ 7) and alkaline (≥ 7.5) pH values.

Conclusions:

• Proposed a model where electron transfer is purely electronic at acidic pH and becomes proton-coupled at alkaline pH.
• The findings suggest a pH-dependent mechanistic switch in biological redox reactions.
• This study provides new insights into intramolecular ET mechanisms in metalloproteins.