Study of the secondary structure of proteins in aqueous solutions by attenuated total reflection Fourier transform infrared spectrometry

Abstract An approach to the determination of secondary structure content in proteins in aqueous solutions based on attenuated total reflection (ATR) Fourier transform infrared (FT-IR) spectrometry is proposed. ATR-FT-IR spectra of eleven proteins with known crystal structures were recorded. An algorithm for careful subtraction of the solvent background was developed and the reproducibility of the spectra was established for a wide range of protein concentrations in aqueous solutions. Two techniques were compared for the determination of secondary structure content [classical least-squares analysis (CLS) and partial least-squares analysis (PLS)] and optimum conditions for their utilization were suggested. The best correlation between the ATR-FT-AIR approach and X-ray diffraction data was obtained with the PLS analysis and the distinction of four types of secondary structures (ordered and disordered α-helix, β-sheet and undefined conformation). The averages of the differences in the percentage content between X-ray and IR secondary structures predicted in the ATR mode are 7.1% and 2.8% for the ordered and disordered α-helix, respectively, 6.5% for the β-sheet and 4.7% for the undefined structure.