Effect of dextran molecular weight on protein stabilization during freeze-drying and storage

The effect of dextran molecular weight on structural stability of freeze-dried products and protein stability in amorphous matrices was investigated during storage at elevated temperatures. Glucose-6-phosphate dehydrogenase (G6PDH) was freeze-dried in 10% (w/v) dextrans of five molecular weights (12 kD, 42 kD, 71 kD, 512 kD and 2000 kD) to residual water content of 0.027 ( 0.004 g/g dry mass. The molecular weight of dextrans affected the glass transition temperature (Tg) of freeze-dried products and the recovery of enzyme activity after freeze-drying. As the molecular weight of dextrans increased from 12 kD to 2000 kD, the Tg increased from 100 degrees C to 120 degrees C, whereas the recovery of protein activity decreased from 85 +/- 4 % to 70 +/- 5%. The inactivation of freeze-dried protein during storage followed a bi-phasic first-order kinetics. The stability of amorphous matrices and protein increased significantly as the molecular weight increased from 12 kD to 512 kD. However, at a higher molecular weight (2000 kD), the stability was reduced. In a separate experiment, the stability of dried dextran/protein samples was studied during heating from 30 degrees C to 99 degrees C at 0.2 degrees C/min and subsequent incubation at 99 degrees C. Dextran with an average molecular weight of 512 kD was again found to provide the best protection. Mechanisms that cause the differences in protein stability among different molecular weight dextrans remain unclear.