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Photoinduced proton transfer in differently structured water: an EPR approach to solving a classic problem
Antonio Barbon1, Anton Savitsky2, Igor A Grigoriev3
1Department of Chemical Sciences, University of Padova, Padova, Italy. antonio.barbon@unipd.it.
Scientific Reports
|February 9, 2026
Summary
Proton transfer (PT) rates significantly slow in solutions with chaotropic compounds like guanidine hydrochloride, indicating water structure impacts chemical reactions. This study uses electron paramagnetic resonance to probe these effects.
Area of Science:
- Physical Chemistry
- Chemical Kinetics
- Biophysical Chemistry
Background:
- Proton transfer (PT) is fundamental to chemical and biological processes.
- The Grotthuss model for PT in water, proposed over 200 years ago, remains a subject of active research.
- Understanding how water structure influences PT is crucial.
Purpose of the Study:
- To investigate the mechanism of proton transfer (PT) in aqueous solutions.
- To determine the impact of water structure, altered by chaotropic agents, on PT rates.
- To develop and utilize a novel method for studying PT kinetics.
Main Methods:
- Utilized electron paramagnetic resonance (EPR) spectroscopy.
- Employed pH-sensitive stable nitroxyl radicals.
- Generated protons on the sub-nanosecond timescale via photolysis of 2-nitrobenzaldehyde.
- Studied PT rates in aqueous solutions containing urea, guanidine hydrochloride (Gdn·HCl), and potassium chloride (KCl).
Main Results:
- Chaotropic compounds significantly impact PT rates.
- In 6 M Gdn·HCl, PT occurred 40-fold slower compared to pure water.
- Demonstrated the method's sensitivity to changes in water structure.
Conclusions:
- Water structure plays a critical role in modulating proton transfer kinetics.
- The developed EPR method is effective for monitoring PT in various environments.
- The technique holds potential for studying PT in complex systems like ice and proteins.
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