Surface-Altered Protonation Studied by Photoelectron Spectroscopy and Reactive Dynamics Simulations

Area of Science:

• Physical Chemistry
• Biochemistry
• Surface Science

Background:

• Protonation states of functional groups at aqueous interfaces are critical for chemical and biological processes.
• Understanding these interfacial protonation differences is essential but has been debated.
• Previous studies lacked molecular-scale insights into bulk versus surface protonation.

Discussion:

• X-ray photoelectron spectroscopy (XPS) and reactive molecular dynamics (MD) simulations were employed as complementary techniques.
• Both methods probed the molecular-scale distribution of protonated cysteine species in aqueous solution.
• A significant divergence in species distribution was observed between the aqueous bulk and the interface.

Key Insights:

• The distribution of protonated cysteine species at the aqueous surface differs markedly from the bulk.
• This difference cannot be solely explained by altered proton sharing between conjugate acid/base pairs.
• Novel protonated species unique to the surface environment were identified by simulations.

Outlook:

• Further investigation into the specific nature and implications of surface-unique species.
• Exploring the broader applicability of these findings to other amino acids and biomolecules.
• Refining theoretical models to better capture interfacial phenomena in aqueous systems.