Vibrational Circular Dichroism

Abstract

The use of electronic circular dichroism (ECD) of transitions in the ultraviolet for biomolecular structural studies has been one of the dominant applications of the technique as amply demonstrated in the preceding chapters of this book. In retrospect, it is amazing that so much useful structural information has been gleaned from ECD spectra which typically provide only a few independent, poorly resolved spectral features in these molecules. The compilation of such a body of structural insight stands as a tribute to the exquisite structural sensitivity of this chiroptical technique. In proteins, the most useful transitions that yield insight into secondary structure have proven to be the n−π* and π−π* of the amide groups, both of which lie in the region from 220 to 190 nm. The π−π* transitions of aromatic residues are useful for monitoring tertiary structure but offer limited interpretability. For nucleic acids, the π−π* transitions of the bases are spectrally more spread out but are still severely overlapped. Interactions among these transitions in the respective polymers yield information about the peptide backbone conformation or the nucleotide base stacking. On the other hand, information about other structural aspects of these biomolecules is more difficult to obtain because of the difficulty of accessing spectral transitions centered on other parts of the molecule. Furthermore, since the accessible electronic excitations are relatively delocalized and involve changes in the π−bonding electron configurations, the resulting transitions are susceptible to significant frequency shifts and intensity variations caused by environmental or local perturbations. Spectrally, these transitions are broad and overlapping.