DIFFERENTIAL SCANN1NG CALORIMETRIC STUDY ON THE NON-FREEZING WATER IN LYSOZYME
溶菌酶内不冻水的差示扫描量热研究

The low temperature thermodynamic behaviour of the water-lysozyme system was investigated in the water content range of 0.03 to 2.70 g (water)/g (Protein) before and after denaturation using differential scanning calorimery (DSC) and a difference DSC method. At the low hydration the measured heat values of fusion of water show a strong dependence on the keeping-time, tk, at the low temperature and the rate of ice crystallization always determines the measured results. The melting peaks of ice were observed at the water contents of about 0.28 and 0.26 g/g after keeping the samples at 210 K for 15 min and 15 hrs, respectively. At the same keeping-times the endothermal effects started to be measured at about 0.18 and 0.14 g/g by the difference DSC method. At the water contents of 0.32, 0.44, and 1.03 g/g the heat values measured at tk of 15 hrs are 1.68, 1.18 and 1.03 times larger than those at 15 min, respectively. For the denatured protein the measured values show a much stronger time-dependence than those for the native. The results suggest that at the low hydration the water-protein system is a metastable disperse one, for which the low temperature measurement is dominated by kinetics and thus has uncertainties to a great extent, and that the behaviour of the 'so-called" non-freezing fraction" of water is more complex than has hitherto been thought the case. The interconversion with time may occur and no definite boundary exist between different classes of water.