Fourier Transform Infra Red Spectroscopy (FTIR)
FTIR difference spectroscopy can be extremely useful for protein and DNA/RNA characterisation, ligand interactions and for analysing protein dynamics particularly in membrane environments. The technique generally involves monitoring spectral contributions from intrinsic probes e.g. the amide I bond C=O stretching associated with the protein backbone, the C=O stretch of carboxylic acids involved in proton transfer reactions as well as vibrations associated with solvents and ligands. Different lipid compositions give rise to distinct spectral profiles in the C−H stretching region. The technique is particularly useful for monitoring changes in protein secondary structure since different secondary structural contributions occur in distinct regions of the spectrum (by contrast in the case of Circular Dichroism spectroscopy the beta-sheet contributions are often swamped by the larger intensities of alpha-helical contributions). FTIR spectroscopy can be enhanced by the introduction of site-specific probes allowing the examination of a particular region of a protein. Recent studies involving the incorporation of azido probes into the G-protein coupled receptor (GPCR) Rhodopsin have enabled the study of receptor activation (Ye et al. 2009).
FTIR has certain advantages over other spectroscopic techniques in that it can analyse, solid, aggregated and fibrillar protein compositions. During the process of fibril formation (as in the case of proteins involved in amyloidogenic diseases) FTIR studies show characteristic shifts in the amide I peak corresponding to the formation of cross beta-structure.