Featuring amyloids with Fourier transform infrared and circular dichroism spectroscopies

Amyloids are fibrillar aggregates of proteins characterized by a basic scaffold consisting of cross β-sheet structure that can exert physiological or pathological effects. Both far-UV circular dichroism and Fourier transform infrared (FTIR) spectroscopies are techniques used for the fast analysis of protein secondary structure. Both techniques are complementary and preferentially used depending on the physical state of the analyte, the major secondary structure element and the relative abundance of given amino acids. Although there are special setups for working with films, circular dichroism is best suited for ideal diluted solutions of polypeptides exhibiting α-helix as major structural element and low content of aromatic residues. During the last decade, a related technique, linear dichroism, has been applied to study the orientation of protein subunits within amyloid oligomers or fibrils in solution. Alternatively, FTIR works best with concentrated solutions, solids and films, and resolves with accuracy the β-sheet composition, but it is affected by contributions of amide groups. The advent of new infrared techniques based on correlation analysis of time-dependent variations induced by external perturbations that generates two-dimensional IR maps has enabled to greatly increase spectral resolution and to extend its applicability to protein secondary structure characterization in a variety of physical environments. Within the amyloid field, conjunction of both spectroscopies has provided the first filter step for amyloid detection and has contributed to decipher the structural aspects of the amyloid formation mechanism.