A Circular Dichroism and Fluorescence Spectrometric Assessment of Effects of Selected Chemical Denaturants on Soybean (Glycine max L.) Storage Proteins Glycinin (11S) and β-Conglycinin (7S)

Soybean glycinin (11S) and β-conglycinin (7S) were subjected to select chemical treatments at various concentrations and resulting changes in protein structures were investigated by circular dichroism (CD) and fluorescence spectrometry. Fluorescence quenching results indicated that urea ≥3 M caused significant unfolding of 11S, but not that of 7S. GuHCl was more effective than urea in denaturation of 11S. A two-step transition in 11S structure was observed with a possible existence of a folding intermediate at 2.5 M GuHCl. Sodium dodecyl sulfate (SDS) measurably altered secondary and tertiary structures of 11S and 7S below SDS critical micellar concentration (CMC), possibly due to formation of mixed peptide-SDS micelles. SDS treatment increased α-helical and unordered structures of both proteins at the expense of β-sheet structure. NaCl and CaCl2 caused a significant decrease in fluorescence intensity without shifting emission λmax. Exposure of 7S and 11S to NaSCN respectively at ≥0.3 and ≥0.6 M NaSCN caused a significant increase in fluorescence intensity measured at the corresponding λmax of the protein. β-Mercaptoethanol (β-ME), N-ethylmaleimide (NEM), and phytic acid caused variable red shifts, 2.5–4 nm, in the emission λmax.