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Proton percolation on hydrated lysozyme powders.
G Careri1, A Giansanti, J A Rupley
1Dipartimento di Fisica, Universita di Roma I, Roma 00185, Italy.
Summary
Protein hydration capacitance reveals a critical water threshold (0.15 g/g) for protonic conduction, independent of pH and solvent. This suggests percolative proton transfer is key for biological processes.
Area of Science:
- Biophysics
- Physical Chemistry
- Materials Science
Background:
- Protein hydration is crucial for biological function.
- Protonic conduction in proteins is essential for processes like enzyme catalysis and membrane transport.
- Percolation theory provides a framework for understanding phase transitions and connectivity in disordered systems.
Purpose of the Study:
- To analyze the hydration dependence of protein capacitance using percolation theory.
- To identify critical hydration levels for protonic conduction.
- To investigate the influence of pH and solvent on proton transfer in proteins.
Main Methods:
- Capacitance measurements of protein samples across a range of pH (3-10) and frequencies (10 kHz to 4 MHz).
- Application of percolation theory to analyze the relationship between hydration level and capacitance.
- Investigation of solvent deuterium isotope effects and pH dependency.
Main Results:
- A critical hydration threshold (h(c) = 0.15 g water/g protein) was identified, marked by a sharp increase in capacitance.
- This threshold was independent of pH (below 9) and showed no solvent deuterium isotope effect.
- Observed fractional surface coverage at h(c) aligns with surface percolation theory predictions.
Conclusions:
- Protonic conduction in proteins occurs via percolative proton transfer along hydrogen-bonded water networks.
- The invariance of the critical threshold suggests the sudden emergence of long-range connectivity at h(c).
- Percolative processes are fundamental to enzyme catalysis and membrane transport, highlighting the importance of water structure.