Comparison of calculated and experimental FTIR spectra of specifically labeled ubiquinones

Density functional theory has been used to calculate harmonic vibrational frequencies and intensities of unlabeled and labeled ubiquinone-1 in both the gas phase and in solution (CCl₄). Calculations were undertaken using the B3LYP functional and the 6-31G+(d) basis set. Calculations using larger basis sets did not significantly alter the calculated spectra. Calculations for ubiquinone-2, -3, -6 or -8 gave similar results.Calculations show that eight ubiquinone-1 conformations, which differ only in the orientation of their methoxy groups, are likely to coexist in solution at room temperature. The calculated infrared spectra for these eight conformations vary considerably. However, by averaging the Boltzmann weighted spectra for the eight conformers, composite spectra are calculated that successfully model both the frequency and intensity information inherent in the experimental FTIR spectra obtained for both unlabeled and labeled ubiquinones in solution.The calculated spectra presented here provide a robust foundation from which to consider and assess future modeling of the vibrational properties of ubiquinones embedded in protein complexes.