The effect of salts on molecular mobility in amorphous sucrose monitored by erythrosin B phosphorescence

Salts are present in most amorphous biomaterials such as dried or frozen solid foods, plant seeds, and bacterial spores, and in some pharmaceutical formulations. However, knowledge of how salts modulate the physical properties of amorphous solid sugars, a major component in these systems, is lacking. We have used phosphorescence of the triplet probe erythrosin B (Ery B) to monitor molecular mobility in amorphous sucrose films (dried against P₂O₅) containing the salts NaCl, MgCl₂, CaCl₂, NaAcetate, Na₃Citrate, NaH₂PO₄, or Na₂HPO₄ at a mole ratio of 0.2:1 (salt/sucrose). All the salts examined, except NaH₂PO₄, significantly increased the phosphorescence lifetime of Ery B over the temperature range from 5 to 100 °C. This increase is due to a reduction in the rate of collisional quenching of the triplet state due to interactions with the matrix, indicating that these salts decreased the matrix molecular mobility. NaAcetate, Na₃Citrate, and Na₂HPO₄ decreased mobility more than NaCl, CaCl₂, or MgCl₂, perhaps due to specific hydrogen bonding interactions between the anion and sucrose. Systematic variations in the probe emission lifetime across the excitation and emission bands at 25 °C indicate that there are sites of different mobilities within amorphous solid sucrose; this dynamic site heterogeneity was enhanced in the presence of the divalent cationic salts MgCl₂ and CaCl₂. These results suggest that salts may play a significant role in modulating the mobility, and thus the long-term stability, of amorphous biological matrixes.