Effects of types of sugar on the stabilization of protein in the dried state

The effects of various sugars on the structural stabilization of protein during freeze-drying were investigated. The degree of native structure of protein that was freeze-dried and rehumidified at constant relative humidities (RHs) was evaluated by measurement of the alpha-helix content by Fourier-transform infrared spectroscopy. Bovine serum albumin (BSA) and several types of sugars, including sucrose, trehalose, and dextrans, were used as a model protein and sugars, respectively. The glass transition temperature, T(g), for the amorphous sugar samples was measured by differential scanning calorimetry (DSC) to characterize the structural stability of sugars. The dependence of the alpha-helix content (C(alpha-helix)) of BSA on the sugar content (c(sugar)) could, in most cases, be represented by a Langmuir-type equation: C(alpha-helix) = K x (C(alpha-helix) (max) - C(alpha-helix) (0)) x c(sugar)/(1 + K x c(sugar)) + C(alpha-helix) (0), where K is a constant, indicating the ability of amorphous sugar matrix to embed protein, and C(alpha-helix) (0) and C(alpha-helix) (max) indicate the alpha-helix content in the absence of sugar and saturating levels of sugar, respectively. The preservation effects of the sugars could be characterized by K and C(alpha-helix) (max). Both K and C(alpha-helix) (max) values tended to be higher with decreasing T(g) values for the amorphous sugar, probably because an amorphous sugar matrix with lower T(g) values is structurally more flexible. The rehumidification of protein that was freeze-dried in the presence of sugar induced the refolding of protein structure, whereas the protein dried alone did not show any recovery of its native structure.