Secondary structures of proteins adsorbed onto aluminum hydroxide: Infrared spectroscopic analysis of proteins from low solution concentrations

Comparative studies of the secondary structures of six model proteins, adsorbed onto aluminum hydroxide gel (Alhydrogel) or in aqueous solution, were carried out by Fourier transform infrared (FTIR) spectroscopy. The analysis of high-quality spectra of all six model proteins, with a broad range of secondary structure compositions, obtained at 15 mg/ml by the conventional method and at 0.5 and 1.0 mg/ml adsorbed to Alhydrogel revealed that adsorption onto hydrophilic surfaces of aluminum hydroxide particles did not alter the secondary structures of the proteins. The results of this study suggest that adsorbing proteins to Alhydrogel provides a means of obtaining FTIR spectra to study secondary structure and conformational changes of proteins in aqueous solution at very low concentrations. The new procedure effectively lowers the concentration requirement for FTIR studies of proteins in aqueous solutions by at least 40-fold, as compared with the conventional FTIR method. It permits FTIR study of proteins to be carried out in the same concentration range as is used for circular dichroism and fluorescence, thereby making it possible to compare structural information obtained by three commonly used techniques in protein biophysical characterization.