The optical spectra measured for a training set of 23 globular proteins using equilibrium hydrogen-deuterium exchange difference Fourier transform IR (FTIR) spectroscopy method presented in the previous chapter were further tested for structural information. The systematic variation of the band intensity with the solvent deuterium content was assessed for quantitative correlation with protein tertiary structure using factor analysis and restricted multiple regression scheme. For this purpose, a protein tertiary structure descriptor was developed that represents the interactions between well-defined secondary structural elements such as helices and sheets. It accounts for the packing of the secondary structures as well as their distribution in space. The contact profiles were calculated from the contact maps as distributions of the different interactions between these structural elements as functions of distance. They were reconstructed using the H/D exchange data and yielded decent qualitative agreement.
The contact descriptor developed here represents an additional quantitative information on protein structure. Previously, a well-established secondary structure description provided by the fractional components of secondary structure (FC values) was used to characterize proteins from their optical spectra through the percentage amounts of the different secondary structure elements. Determination of the number of segments of these elements differentiates the structures that contain relatively the same FC values as shown by the work on ribonuclease A and T1 (16). Finally, the current tertiary descriptor offers a means to further describe the possible arrangement in space of these segments and rule out those that are inconsistent with the rest of the experimental data.