Electrochemically induced far-infrared difference spectroscopy on metalloproteins using advanced synchrotron...

Area of Science:

• Biophysics
• Spectroscopy
• Protein Chemistry

Background:

• The far-infrared (far-IR) terahertz domain (600-3 cm(-1)) provides unique information on protein structure, hydrogen bonds, and metal-ligand interactions.
• Fourier transform infrared (FT-IR) spectroscopy is a powerful tool for studying proteins in solution.

Purpose of the Study:

• To compare the performance of thermal sources with synchrotron far-IR for capturing reaction-induced FT-IR difference signals in proteins.
• To demonstrate the utility of synchrotron far-IR spectroscopy for detecting subtle spectral shifts in proteins, such as those induced by metal isotope labeling or temperature changes.

Main Methods:

• Utilized the AILES far-IR beamline at the synchrotron SOLEIL.
• Employed a model protein, Cu-azurin, in a short path length electrochemical cell adapted for transmission spectroscopy in vacuum-purged optics.
• Recorded reaction-induced Fourier transform infrared (FT-IR) difference signals.

Main Results:

• Successfully detected minute spectral shifts in Cu-azurin induced by metal isotope labeling and temperature changes.
• Identified vibrational modes involving copper-ligand interactions, enabling the analysis of metal sites and redox states in proteins.
• Extracted hydrogen-bonding signatures relevant to protein function from complex spectral backgrounds by analyzing temperature-induced band shifts and intensity changes.
• Observed a temperature-sensitive IR mode involving Cu(II)-Histidine vibrations, suggesting a role for hydrogen bonding with water in modulating Cu(II)-histidine bond properties.

Conclusions:

• Synchrotron far-IR spectroscopy is highly effective for analyzing protein structure, metal sites, and hydrogen bonding interactions.
• This technique provides valuable insights into protein function and dynamics, complementing other spectroscopic methods like resonance Raman spectroscopy.
• Experimental data support theoretical proposals regarding the role of specific histidine-water interactions in the electron-transfer activity of Cu-azurin.