Amide I'-II' 2D IR spectroscopy provides enhanced protein secondary structural sensitivity

Area of Science:

• Biophysics
• Spectroscopy
• Protein Science

Background:

• Protein secondary structure determination is crucial for understanding protein function.
• Traditional methods like amide I' infrared spectroscopy have limitations in distinguishing subtle structural differences.

Purpose of the Study:

• To demonstrate the utility of multimode 2D IR spectroscopy for distinguishing protein secondary structures.
• To investigate the underlying vibrational dynamics responsible for enhanced sensitivity.

Main Methods:

• Polarization-dependent amide I'-II' 2D IR spectroscopy was performed on poly-l-lysine in beta-sheet, alpha-helix, and random coil conformations.
• An excitonic model was developed and parametrized using 2D IR surfaces to predict amide I'-II' spectra.

Main Results:

• Multimode 2D IR spectroscopy, utilizing both amide I' and II' diagonal and cross peaks, effectively differentiated between beta-sheet, alpha-helix, and random coil structures.
• Enhanced sensitivity was attributed to frequency and amplitude correlations between amide II' and I' spectra, reflecting secondary structure symmetry.
• The excitonic model identified dominant vibrational interactions, including negative amide II'-II' through-bond coupling and amide I'-II' coupling within the peptide unit.

Conclusions:

• Multimode 2D IR spectroscopy offers superior capability for protein secondary structure identification compared to amide I' spectroscopy alone.
• Vibrational coupling within the peptide unit plays a key role in the sensitivity of this technique.
• The developed excitonic model provides a framework for predicting and interpreting amide I'-II' 2D IR spectra of proteins.