Two-dimensional infrared correlation spectroscopy studies on secondary structures and hydrogen bondings of side chains of proteins

This review paper reports usefulness of two-dimensional (2D) correlation spectroscopy in analyzing infrared (IR) spectra of proteins in aqueous solutions. In the 2D approach, spectral peaks are spread over the second dimension, thereby simplifying the visualization of complex spectra consisting of many overlapped bands, and enhancing spectral resolution. 2D correlation spectroscopy has a powerful deconvolution ability for highly overlapped amide I, amide II, and amide III bands of proteins, enabling these bands to be assigned to various secondary structures. It also provides the specific order of the spectral intensity changes taking place during the measurement on the value of controlling variable affecting the spectra. Therefore, one can monitor the order of secondary structure variations in proteins by using 2D IR correlation spectroscopy. 2D correlation spectroscopy also provides new insight into the hydrogen bondings of side chains of proteins. In this review the principles and advantages of 2D correlation spectroscopy are outlined first and then three examples of the applications of 2D IR spectroscopy to protein research are presented.