Molecular mobility of amorphous disaccharides films by phosphorescence of various probes with indole chromophore group

Amorphous solids lack the long-range order of crystalline solids. Their molecular mobility properties not only modulate the biological properties of seeds, spores and some organisms during anhydrobiosis, but also control the stability and quality of pharmaceuticals, dried and frozen foods. Tryptophan phosphorescence in proteins provides a sensitive long-lived signal of molecular mobility of the local environment in amorphous sugar matrix. However, there are many factors that need to be studied for a better understanding of the relationship between tryptophan triplet-state lifetimes in proteins and molecular mobility of the local environment. Those factors include the molecular structure of probes and matrix, intermolecular and intramolecular reaction, water content in the matrix, temperature and so on. The object of this research is to investigate how different groups on phosphorescent probes as well as two different disaccharide matrix influence on non-radiative phosphorescence decay rate at a wide temperature range, and to discuss the possible inter- or/and intra- molecular interaction among probes, water, and disaccharide molecules. Sucrose and trehalose are utilized here as amorphous disaccharide to evaluate how the probes differ in their response to matrix dynamics. And we here used 14 different probes including indole, tryptophan, tryptophanamide, N-acetyltryptophan, N-acetyltryptophanamide, 5-hydoxyltryptophan, 5-methyltryptophan, 4-fluorotryptophan, 6-fluorotryptophan, 5-bromotryptophan, the dipeptides Trp-Gly and Gly-Trp, and 20-residue Trp-cage protein. Measurements of phosphorescence lifetime for all those triplet state probes in amorphous sucrose and trehalose films as a function of temperature provide data implying significant mobility in the amorphous sugars in the glassy state as well as at the glass-to-melt transition. Generally, all those probes except 5-Br-Trp have an extreme sensitivity of the triplet excited state decay processes to the local environment. Since amorphous trehalose have more packed structure with higher Tg than amorphous sucrose, the molecular mobility in trehalose film are less influenced by temperature and more restricted than one in sucrose over the temperature range from -10°C to 120°C, which is observed by using all different probes in these two films. Most probes provide similar trends of the mobility in amorphous sugar film over a wide temperature range, while there are subtle differences which could be separately caused by intramolecular quenching in Trp-Gly, heavy atom effect in 5-bromotryptophan and 4-fluoro-, 6-fluoro-tryptophan, intermolecular hydrogen-bonding effect in 7-azatryptophan and 5-hydoxyl-tryptophan.M.S.Includes bibliographical referencesIncludes vitaby Ping Wan