The combined use of amide I bands in polarized Raman, IR, and vibrational dichroism spectra for the structure analysis of peptide fibrils and disordered peptides and proteins

Raman spectroscopy is generally a versatile tool to explore the structure of proteins and peptides in solution. However, in spite of the development of ultraviolet (UV) resonance Raman spectroscopy as a tool that allows for the investigation of samples with lower concentrations, Raman has not played the same role as infrared (IR) spectroscopy with regard to secondary structure analysis. Reported UV Raman studies have recently focused on the ψ‐dependence of amide III as a useful tool for the structural analysis even of disordered peptides. Amide I based structural analysis generally uses visible excitation. Rather than describing the traditional secondary structure analysis of proteins, this review focusses on work that used Raman in conjunction with other vibrational spectroscopies such as IR and vibrational circular dichroism to probe the conformational distribution of amino acid residues in unfolded peptides and the self‐assembling of peptides into fibrils. We argue against the use of spectral decomposition into Gaussian bands particularly if this type of analysis is exclusively applied to either IR or Raman band profiles. The article emphasizes the delocalized excitonic character of amide I wavefunctions which has to be accounted for in any attempt to use the respective bands for structure analysis. Excitonic coupling between amide I modes in polypeptides produces inhomogeneous and asymmetric distribution of band intensities in both polarized Raman and FTIR spectra of oligo/polypeptide and proteins. In the spectra of short oligopeptides the non‐coincidence between Raman and IR profiles of amide I is a measure of conformational preferences of amino acid residues. Combining IR, Raman, and vibrational circular dichroism yields valuable information about the strand arrangements and the chirality of β‐sheet fibrils.