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Phosphorescence of erythrosin B as a robust probe of molecular mobility in amorphous solid sucrose.

Luminescence from the triplet probe erythrosin B (tetra-iodo fluorescein, Ery B) provides spectroscopic characteristics such as lifetime and emission energy that are sensitive to molecular mobility of the local environment in amorphous solids. This study investigated how variations in the local concentration of Ery B free acid as well as the presence of the dispersing solvent affect the spectroscopic measurements of solid matrix properties (the free acid of Ery B is poorly soluble in water and thus must be introduced via an organic solvent). The emission energy of Ery B from 5 to 100 °C in thin films of amorphous sucrose at various probe and solvent (N,N-dimethyl formamide, DMF) concentrations was determined using excitation at 500 nm and emission over the range 520–750 nm. The emission lifetime was determined over the same temperature range using a stretched exponential analysis of intensity decays collected using excitation at 530 nm and emission at 680 nm. Variations in the probe/sucrose mole ratio (concentration) over the range from 0.5 to 10 × 10−4 and 10-fold variations in the amount of DMF used to disperse the probe did not affect the emission energy, the shape of the emission spectra, or the measured lifetimes of Ery B in amorphous sucrose. These results thus indicate that erythrosin B introduced into amorphous solids can provide a robust measure of the intrinsic mobility of the solid matrix that is relatively insensitive to final probe concentration or presence of residual solvent.

Effect of xanthan on the molecular mobility of amorphous sucrose detected by erythrosin B phosphorescence.

Molecular mobility in amorphous solids is modulated by composition and environmental conditions such as temperature. Phosphorescence of erythrosin B was used to generate a mobility map of amorphous sucrose film doped with xanthan gum at weight ratios of xanthan/sucrose ranging from 0.0001 to 0.01. On the basis of analysis of the emission energy and lifetime of erythrosin B in pure sucrose and sucrose-xanthan films over the temperature range from 5 to 100 degrees C, we conclude that xanthan influences the molecular mobility as well as the dynamic site heterogeneity of amorphous sucrose in a dose-dependent fashion. At xanthan/sucrose weight ratios below approximately 0.0005, both emission energy and lifetime decreased and k(TS0) (the nonradiative decay rate of the triplet state) increased, indicating that xanthan increased the matrix molecular mobility. At weight ratios above 0.001, both emission energy and lifetime increased and k(TS0) decreased, indicating that xanthan decreased matrix mobility, reaching a plateau at weight ratios between 0.005 and 0.01. The concentration at which the effect of xanthan switched from increasing to decreasing mobility was similar to the concentration at which polymer chains overlapped in solution, suggesting that the dynamic changeover reflected the onset of chain overlap in the amorphous solid. Systematic trends in the emission bandwidth and lifetime heterogeneity and variations in the emission lifetime vs wavelength indicated that xanthan reduced the matrix dynamic site heterogeneity except at a weight ratio of 0.01. These data illustrate the complex effects of a polymer with a rigid structure and large side chains on the mobility of an amorphous, hydrogen-bonded sugar matrix.

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 P(2)O(5)) containing the salts NaCl, MgCl(2), CaCl(2), NaAcetate, Na(3)Citrate, NaH(2)PO(4), or Na(2)HPO(4) at a mole ratio of 0.2:1 (salt/sucrose). All the salts examined, except NaH(2)PO(4), 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 Na(2)HPO(4) decreased mobility more than NaCl, CaCl(2), or MgCl(2), 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 MgCl(2) and CaCl(2). These results suggest that salts may play a significant role in modulating the mobility, and thus the long-term stability, of amorphous biological matrixes.

Effect of gelatin on molecular mobility in amorphous sucrose detected by erythrosin B phosphorescence

We have used phosphorescence from the triplet probe erythrosin B (Ery B) to evaluate the effect of gelatin on the molecular mobility of the amorphous sucrose matrix as a function of temperature. Ery B was dispersed in amorphous sucrose and sucrose-gelatin films at ratios of approximately 1:10(4) (probe/sucrose), and delayed emission spectra and emission decay transients were measured over the temperature range from 5 to 100 degrees C. Analysis of spectra using a lognormal function provided the peak energy and bandwidth of the emission. The emission peak frequency decreased at low (0.00022-0.0007) gelatin concentrations and increased at high (above 0.0022) gelatin concentrations, indicating that gelatin increased the extent, and thus the rate, of dipolar relaxation at low gelatin content and decreased the extent at higher gelatin content. Decay transients were well fit to a stretched exponential function at all gelatin contents and temperatures. Analysis of the emission lifetimes provided a measure of the rate of non-radiative decay to the ground state, an indicator of matrix molecular mobility. This rate increased at low (0.00022-0.0022) and decreased at high (>0.0073) gelatin wt ratios. Analysis of the effect of gelatin on the emission bandwidth, the stretching exponent beta, and the variation of lifetime across the emission band indicated that matrix dynamic site heterogeneity increased at low and decreased at high gelatin wt ratios. These results provide a novel insight into the complex dynamic effects of the gelatin polymer on the molecular mobility of the amorphous sucrose matrix.

Effect of starch on the molecular mobility of amorphous sucrose.

Molecular mobility in amorphous solid biomaterials is modulated by the composition and environment (primarily temperature). Phosphorescence of the triplet probe erythrosin B was used to generate a mobility map within amorphous sucrose films doped with starch ranging from 0.001 to 0.1 g starch/g sucrose. Data on the emission energy and lifetime of erythrosin B in sucrose and sucrose-starch films over the temperature range from 5 to 100 °C indicates that starch influences the molecular mobility as well as dynamic site heterogeneity of amorphous sucrose in a dose-dependent manner. At a starch/sucrose weight (wt) ratio below 0.005, both emission energy and lifetime decreased, and both the dipolar relaxation rate and nonradiative quenching rate k(TS0) increased, indicating that starch increased the matrix molecular mobility. At a ratio above 0.005, both emission energy and lifetime increased, and the dipolar relaxation rate and nonradiative quenching rate decreased, indicating that starch decreased the matrix mobility both in the glass and in the melt. The mobility showed a minimum value at a ratio of 0.01. The interactions existing in the sucrose-starch matrix are considered as the determining factor to influence the molecular mobility of sucrose-starch mixtures. Changes in the distribution of emission energies (emission bandwidth) and lifetimes indicated that starch increased the spectral heterogeneity at high contents while showing insignificant change or a slight decrease in the heterogeneity at low starch contents. These data illustrate the complex effects of a polymer with mainly linear structure and flexible conformation on the mobility of an amorphous, hydrogen bonded sugar matrix.