Dehydration-induced conformational changes of poly-L-lysine as influenced by drying rate and carbohydrates

The conformation of hydrated and air-dried poly-L-lysine in thin films was studied using Fourier transform IR spectroscopy in the amide-I region. Hydrated poly-L-lysine has a random coil conformation. Upon slow drying of small droplets of the polypeptide solution over a period of several hours, an extended beta-sheet conformation is adopted. This conformational transition can be prevented by fast air-drying within 2-3 min. Slow air-drying in the presence of sucrose also preserves the aqueous conformation and results in the formation of a glassy state. Comparison of shifts of the OH band with temperature indicates that sucrose/poly-L-lysine mixtures form a molecularly more densely packed glassy matrix, having a higher glass transition temperature (Tg), than sucrose alone. Whether direct interaction of sugar and polypeptide or glass formation is involved in the stabilization during slow air-drying was studied by drying in the presence of glucose or dextran. Compared with dextran (and sucrose to a lesser extent), glucose gives superior protection. Dried glucose has the lowest Tg and the best interacting properties. We conclude that either immobilization by fast air-drying or sufficient interaction with a protectant through hydrogen bonding (slow drying) plays the leading role in the preservation of the aqueous protein structure.