Novel Use of a Static Modification of Two-Dimensional Correlation Analysis. Part I: Comparison of the Secondary Structure Sensitivity of Electronic Circular Dichroism,FT-IR,and Raman Spectra of Proteins

A modification of Noda's algorithm that allows for calculation of two-dimensional (2D) correlation maps is presented as a method for analysis of a series of (static) spectra of proteins. In this approach, fractional secondary structure was used as the perturbation to generate the 2D correlation. The functional dependence of the spectral intensities on secondary structure is approximated by an even-order polynomial fit to the protein spectra at each spectral frequency. These functions are used to calculate the 2D correlation and disrelation maps, and their regression coefficients are used to weight the results to minimize artifacts. Electronic circular dichroism (ECD), Fourier transform infrared (FT-IR) (amide I and II regions), and Raman spectra of up to 22 proteins are used in the study. Spectral regions identified by the α-helix- and β-sheet-based 2D correlation maps are in agreement with established interpretation of ECD and FT-IR spectra in terms of secondary structure and provide insight into secondary structure assignment for a broad range of Raman bands. Comparison of our functional fit method, specifically designed to identify synchronous correlations, with Noda's Fourier transform-based method, which generates asynchronous maps as well, is discussed.