Proton-coupled electron transfers: pH-dependent driving forces? Fundamentals and artifacts

Area of Science:

• Photochemistry
• Chemical Kinetics
• Biophysical Chemistry

Background:

• Concerted proton-electron transfer (CPET) to water has been proposed to explain certain reactions, like tryptophan oxidation by Ru complexes.
• This proposed mechanism implies a pH-dependent driving force, which contradicts fundamental chemical physics principles.

Purpose of the Study:

• To investigate the mechanism of tryptophan oxidation by photogenerated Ru complexes.
• To determine if unconventional H2O-CPET with a pH-dependent driving force occurs.
• To clarify reaction pathways and refine models of CPET reactions.

Main Methods:

• Experimental examination of tryptophan derivatives (ethyl ester with NH3(+)/NH2 group and ethyl ester-methyl amide).
• Analysis of reaction products and kinetics under varying conditions.
• Comparison with model substrates like phenol.

Main Results:

• No evidence of unconventional H2O-CPET with a pH-dependent driving force was found for tryptophan derivatives.
• The reaction mechanism involves a rate-determining electron-transfer step followed by deprotonation.
• Similar findings were observed with phenol, suggesting broader applicability.

Conclusions:

• The proposed unconventional H2O-CPET mechanism is not supported by experimental data for accessible substrates.
• The reaction proceeds through a conventional electron transfer followed by proton transfer.
• These findings help refine CPET models and understand water's role in natural systems.