Temperature scanning FTIR analysis of secondary structures of proteins embedded in amorphous sugar matrix

Heat-induced changes in secondary structures of five proteins (bovine serum albumin, BSA; human serum albumin, HSA; myoglobin; ribonuclease A, RNase A; and, beta-lactoglobulin, beta-Lg) in an amorphous sugar matrix were analyzed by temperature-scanning Fourier transform infrared spectroscopy to elucidate the mechanism of heat-induced conformational change of solid-phase proteins. Three sugars, trehalose, maltose, and dextran (MW 6000), were used. Loss of alpha-helices due to increasing temperature was observed for BSA, HSA, and myoglobin, which are rich in alpha-helices. RNase A showed a marked decrease in predominant secondary structural components (beta-sheet) with increasing temperature. However, no noticeable changes in the content of secondary structures, except for a slight loss of alpha-helices, were observed for beta-Lg, which is also beta-sheet-rich. These heat-induced conformational changes were significant at temperatures above the glass transition temperature. The heat-induced conformational change in BSA dried with sugar appeared time-independent and was clearly different from that due to dehydration and from the thermal conformational change for a solution of BSA. In particular, differences in secondary structural components that increased due to loss of alpha-helices were noted.