Bond Cleavage Reactions in Solid Aqueous Carbohydrate Solutions
AbstractPurpose. To investigate chemical reactivity in water soluble glasses.
Methods. Rates of bond cleavage reactions in freeze-dried and freeze-concentrated aqueous carbohydrate solutions were measured above and below the glass transition temperatures (Tg). The kinetics of two reactions have been determined in formulations containing di- and polysaccharides: (1) fission of the Asp-Pro peptide bond in Physalaemin and Hamburger peptide by following the release of proline, using a ninhydrin based reaction and (2) the unimolecular dissociation of 2-(4-nitrophenoxy) tetrahydropyran by following the release of the 4-nitrophenoxy anion.
Results. The results show clearly that reaction occurs below the glass transition temperature, albeit at very reduced rates. No significant enhancement of the temperature dependence of the rate constant was observed near Tg. Different water soluble glasses provide different degrees of stability. The order of stabilisation was sucrose> Ficoll (low mol. weight)> Byco A ≅ Ficoll (high mol. weight)> dextran. The density of the matrix, and therefore the degrees of freedom of mobility of the reactant, is thought to be responsible for these differences.
Conclusions. The storage of therapeutic agents, such as proteins, in glassy matrices below Tg does not confer indefinite stability. When formulating products, notice should be taken of the differing stabilisation properties of excipients.