Raman spectroscopic study of globulin from Phaseolus angularis (red bean)

Fourier-transform Raman spectroscopy was used to study the conformation of red bean globulin (RBG) in various buffered environments and on heat treatment. The Raman spectrum of 20% (w/v) freeze-dried RBG in 0.01 M phosphate buffer at pH 7.4 suggests that its secondary structure composition is dominated by β-sheets and random coils. Extreme pH values caused the amide I and amide III bands to shift, suggesting a transition from an ordered structure (α-helix or β-sheet) to a random coil conformation. The decreases of the intensity of the band near 760 cm−1, at acidic and basic pHs, indicate an unfolding of the protein, leading to exposure of hydrophobic residues. Increasing concentrations of added NaCl led to progressive increases of the band intensity near 760 cm−1, suggesting an increased “buriedness” of tryptophan residues. Protein structure perturbants, including sodium dodecyl sulfate (SDS), β-mercaptoethanol (β-ME), dithiothreitol (DTT), N-ethylmaleimide (NEM) and ethylene glycol (EG) were also found to have a marked influence on the conformation of RBG. The Raman data indicate progressive denaturation of RBG during heat treatment at 90C. Heat-induced aggregation caused changes in the protein backbone vibrations, and increases in the tryptophan band intensity, suggesting that hydrophobic interactions may play an important role in RBG aggregation. The shifts in amide III bands indicate that β-sheet is the major secondary structure in the heat-induced aggregates.