Near-infrared analysis of protein secondary structure in aqueous solutions and freeze-dried solids

Near-infrared spectroscopy (NIR) of various proteins (bovine serum albumin, lysozyme, ovalbumin, γ-globulin, β-lactoglobulin, myoglobin, cytochrome-c) was investigated as a possible analytical method of the protein secondary structure in various physical states. The spectra of proteins in aqueous solutions (transmission mode, solvent-compensated) and those in freeze-dried solids (nondestructive diffuse reflection mode) showed several bands at similar frequencies in the combination (4000-5000 cm−1) and first overtone (5600-6600 cm−1) spectral regions. The normalized second-derivative near-infrared spectra of proteins in aqueous solutions suggested that some bands indicated α-helix (4090, 4365-4370, 4615, and 5755 cm−1) and β-sheet (4060, 4405, 4525-4540, 4865, and 5915-5925 cm−1) structures. The proteins mostly maintained spectra characteristic of their native structure after freeze-drying, although some reductions in α-helical structure and increase in unordered or β-sheet structures were observed. The near-infrared analysis also showed β-sheet formation of heat-treated BSA in aqueous solutions and in subsequently freeze-dried solids. The present results thus indicated that the nondestructive near-infrared analysis can be used for the investigation of dehydration-induced changes in protein secondary structures.