Effects of sodium tetraborate and boric acid on nonisothermal mannitol crystallization in frozen solutions and freeze-dried solids.

The purpose of the present study was to elucidate the effects of sodium tetraborate (borax) and boric acid on the crystallization of mannitol in frozen aqueous solutions and freeze-dried solids. Thermal analysis of frozen solutions showed that sodium tetraborate inhibits mannitol crystallization at sodium tetraborate/mannitol molar concentration ratios of approximately 0.05, which is much lower than the other co-solutes studied (boric acid, sucrose, sodium phosphate buffer). Inhibition of the mannitol crystallization in frozen solutions resulted in highly amorphous mannitol in the freeze-dried solids. Mannitol remained in an amorphous state in some of the combination freeze-dried solids, even at elevated temperatures. Changes in the thermal transition temperatures (glass transition temperature of maximally freeze-concentrated solute ( T'g) and glass transition temperature of freeze-dried solid (Tg)) suggested reduced mannitol molecular mobility with increases in the sodium tetraborate ratio. Fourier-transform infrared spectroscopy (FT-IR) analysis of the bovine serum albumin secondary structure showed apparent protein structure-stabilizing effects of the amorphous mannitol and sodium tetraborate combination during the freeze-drying process. The mannitol and sodium tetraborate combination also protected lactate dehydrogenase (LDH) from inactivation during freeze-drying. We conclude that the complex formation and the accompanying reduction in molecular mobility make sodium tetraborate an effective mannitol crystallization inhibitor in frozen solutions and freeze-dried solids.