Intermolecular interaction effects in the amide I vibrations of polypeptides

Area of Science:

• Spectroscopy
• Computational Chemistry
• Biophysics

Background:

• Previous studies on beta polypeptide Amide I vibrations yielded inconsistent results.
• Normal coordinate analysis of crystalline polyglycine I indicated a near-zero D(10) interaction constant, contradicting earlier perturbation theory applications.

Purpose of the Study:

• To reconcile discrepancies between normal coordinate analysis and perturbation treatments of beta polypeptide Amide I vibrations.
• To investigate the role of transition dipole coupling in explaining spectral splittings.

Main Methods:

• Detailed normal coordinate analysis of crystalline polyglycine I.
• Application of perturbation theory, including the previously neglected D(11) term.
• Investigation of transition dipole coupling as a physical mechanism.

Main Results:

• The D(10) interaction constant was found to be essentially zero, not the large value previously assumed.
• Inclusion of the D(11) term in perturbation expressions was suggested.
• Transition dipole coupling was identified as the physical origin for the D(11) term.
• This mechanism successfully explained spectral splittings in hydrogen-bonded carboxylic acid dimers.

Conclusions:

• The D(11) term, arising from transition dipole coupling, provides a satisfactory explanation for Amide I mode splittings in beta polypeptides.
• This revised theoretical approach corrects inconsistencies in previous treatments of vibrational spectroscopy in beta-sheet structures.