Assigning flavin's difference-FTIR spectral bands in solution: frequency and intensity shifts in flavin's 1-electron
D P Ngan Le1, Gary Hastings1,2, Samer Gozem1
1Department of Chemistry, Georgia State University, Atlanta, Georgia 30303, USA. ghastings@gsu.edu.
Abstract:
Flavins are versatile cofactors that undergo different redox, chemical, and/or photophysical transformations depending on the protein they are bound to. A powerful tool available for studying these transformations is Fourier transform infrared (FTIR) difference spectroscopy, where changes in the FTIR absorption bands relate to specific changes in flavin's bonding or interactions with its neighboring environment. While the infrared (IR) spectra of oxidized flavins are well-characterized, fewer computational and experimental studies have focused on characterizing the IR spectra of flavins in their reduced (radical semiquinone or hydroquinone) states. Here, we employ hybrid quantum mechanical/molecular mechanical (QM/MM) models with implicit solvation to compute vibrational frequencies and IR intensities for a model flavin (lumiflavin) in its oxidized, anionic semiquinone, anionic hydroquinone, and neutral hydroquinone states. The water solvent configurations around the flavin are sampled with molecular dynamics for each state. These simulations, applied with semi-empirically determined broadening and frequency-scaling factors, are used to assign the main features of experimental FTIR difference spectra in the diagnostic 1350-1750 cm-1 range from a variety of sources. The calculations show distinct, redox-state-dependent frequency shifts, especially for CO stretching bands and CN stretching bands, consistent with changing formal bond orders in flavin's pteridine rings upon reduction. These shifts can serve as spectral fingerprints for specific radical and 2-electron reduced forms, which will aid in interpreting these bands in FTIR difference spectroscopy measurements of flavoproteins.
More Related Videos
Related Concept Videos
UV–Vis Spectroscopy: Molecular Electronic Transitions
¹H NMR: Interpreting Distorted and Overlapping Signals
As Δν decreases and the signals move closer, the doublets appear increasingly distorted. The intensities of the inner lines increase at the cost of those of the outer lines as the signals are...
IR Frequency Region: Fingerprint Region
NMR Spectroscopy: Chemical Shift Overview
For instance, the proton...
IR Spectroscopy: Molecular Vibration Overview
Stretching vibrations are vibrational motions that occur along the bond line, changing the bond length or distance between two bonded atoms. They are further distinguished as symmetric or asymmetric. In symmetric stretching, the...
IR and UV–Vis Spectroscopy of Aldehydes and Ketones


