Dependence of amide vibrations on hydrogen bonding
Nataliya S Myshakina1, Zeeshan Ahmed, Sanford A Asher
1Department of Chemistry, University of Pittsburgh, Pennsylvania.
The Journal of Physical Chemistry. B
|August 30, 2008
Summary
Hydrogen bonding impacts amide vibrations differently than previously thought. DFT calculations show AmI and AmII frequencies depend on C=O and N-H groups, respectively, challenging the resonance model for peptide bonds.
Area of Science:
- Spectroscopy
- Computational Chemistry
- Biophysics
Background:
- The resonance model traditionally explains hydrogen bonding effects on amide vibrational spectra.
- This model suggests hydrogen bonding stabilizes a zwitterionic amide structure.
- Previous studies cast doubt on this model's predictions for carbonyl hydrogen bonding.
Purpose of the Study:
- To investigate the impact of hydrogen bonding on N-methylacetamide (NMA) functional groups using DFT calculations.
- To clarify the contributions of C=O and N-H groups to amide vibrational frequencies under hydrogen bonding.
- To provide a more accurate interpretation of amide vibrational spectra in relation to hydrogen bonding.
Main Methods:
- Density Functional Theory (DFT) calculations were employed.
- The study focused on N-methylacetamide (NMA) as a model for peptide bonds.
- Analysis of vibrational spectra, specifically AmI, AmII, and AmIII bands, was performed.
Main Results:
- Hydrogen bonding's influence on the AmI vibration primarily stems from the C=O group.
- The AmII vibration's frequency dependence on hydrogen bonding is mainly due to the N-H group.
- The AmIII band's response to hydrogen bonding is equally influenced by both C=O and N-H groups.
Conclusions:
- The resonance model's explanation for hydrogen bonding effects on amide spectra requires revision.
- Understanding normal mode composition is critical for interpreting amide vibrational frequencies.
- Hydrogen bonding affects different amide functional groups distinctly, influencing various vibrational modes.
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