Effect of plasticizer on dynamic site heterogeneity in cold-cast gelatin films

We have used phosphorescence from the triplet probe erythrosin-5-isothiocyanate covalently linked to lysine residues in gelatin to monitor the molecular mobility in cold-cast gelatin films plasticized by water and glycerol. The phosphorescence intensity decay was well modeled by a stretched exponential in which the lifetime τ and the stretching factor β (which reflects heterogeneity) were the adjustable parameters. Both τ and β varied with emission wavelength in dry gelatin, with longer lifetime and broader distribution (smaller β) at short wavelength (650 nm) and shorter lifetime and narrower distribution (larger β) at long wavelength (730 nm). Such behavior indicates that erythrosin probes are distributed among dynamically distinct sites within the gelatin matrix that differ in their intrinsic molecular mobility. Analysis of the variation of lifetimes with temperature indicates that the less mobile, blue-emitting sites have also higher activation energy for collisional quenching of the triplet state than the more mobile, red-emitting sites. The presence of water (hydration against 75% RH) reduced, while glycerol (at 50 wt%) virtually eliminated the spectral heterogeneity, perhaps reflecting an increase in the rate of matrix relaxation around the triplet state. Each plasticizer also decreased the apparent activation energy for collisional quenching to undetectably low levels, suggesting further that the plasticizers decrease the size of the cooperative units whose motions led to collisional quenching. These data provide novel insight into the local structure and dynamics of the amorphous gelatin matrix and the molecular mechanism(s) underlying the effects of plasticizers.