Studies of reaction kinetics in relation to the Tg' of polymers in frozen model systems

The rates of diffusion-controlled processes in a frozen system can be influenced by the presence of glassy states. One characteristic of cryostabilization by this mechanism is a change in the temperature dependence of reaction rates at the Tg' of the system. The cryostabilization behavior of solutes such as maltodextrin, carboxymethylcellulose (CMC), and sucrose was studied. Three different model reaction systems (enzyme hydrolysis, protein aggregation, and non-enzymatic oxidation) were used. Maltodextrin had a consistent pattern of cryostabilization behavior at temperatures ranging from -3 degrees C to -20 degrees C for all three model systems. Significant retardation effects were evident in the temperature range corresponding to its glassy states. Sucrose did not show a stabilizing effect in the non-proteinaceous model system (the non-enzymatic oxidation reaction). This could partly be due to the absence of the glassy state, since the storage temperatures were above its Tg'. However, in the protein aggregation model system, sucrose was an excellent stabilizer in protecting actomyosin from aggregation. This may be explained by a "solute exclusion" mechanism. CMC did not show any stabilizing effect in the protein aggregation and non-enzymatic oxidation model systems studied, even though it has a Tg' as high as that of maltodextrin. These results demonstrated that although the presence of a glassy state may well have a retarding effect on the rates of diffusion processes, just knowing the Tg' of a polymer is not sufficient for prediction of its stabilization effect in a frozen system.