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, Na(3)Citrate, 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 degrees 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, Na(3)Citrate, 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 degrees 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. (C) 2008 Elsevier Ltd. All rights reserved.