Franck–Condon-like Progressions in Infrared Spectra of Biological Molecules

Infrared spectra in the NH stretch region are often used for structure determination of gas-phase biological molecules. Vibrational couplings complicate the structure determination process by giving rise to additional vibrational bands along with the expected fundamental transitions. We present an example of a strong anharmonic coupling in a biological molecule, Ac-Phe-Ala-LysH+, which causes the appearance of long vibrational progressions in the infrared spectrum. By analyzing the spectra of the ground and the electronically excited state, we determined that the coupling occurs between the NH stretch (ωNH) and a low-frequency torsion of the phenyl ring (ωτ). We describe the vibrational progressions using a Born–Oppenheimer-like separation of the high-frequency stretch and low-frequency torsion with a quartic Taylor expansion for the potential energy surface that accounts for the equilibrium distance and frequency change of the torsional vibration upon the NH stretch excitation. We also demonstrate that small conformational changes in the peptide are sufficient to break this coupling.


Published in:
Journal of Physical Chemistry A, 119
Year:
2015
Publisher:
Washington, American Chemical Society
ISSN:
1089-5639
Keywords:
Laboratories:




 Record created 2015-10-13, last modified 2018-03-17


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