Erythrosin B phosphorescence as a probe of oxygen diffusion in amorphous gelatin films

Gelatin, a multifunctional food protein with an extended fibrous structure, was labeled at lysine residues with the triplet probe erythrosin-5-isothiocyanate. Measurements of erythrosin phosphorescence in gelatin films cast below the melting temperature of the triple helix provided information about both oxygen diffusion and molecular mobility on the sub-millisecond time scale within the amorphous, triple-helically cross-linked gelatin matrix. A decrease in the phosphorescence lifetime in air compared with nitrogen indicated that diffusion of oxygen through the gelatin matrix was detectable only in intermediate and high moisture environments at 23 °C. Diffusion coefficients (D) and permeabilities (P) of oxygen in hydrated gelatin films were determined from measurements of the decrease in phosphorescence quenching (increase in intensity) with outgassing of oxygen. Values were on the order of 10−8 cm2 s−1 and 10−13 mol (cm s atm)−1 for D and P, respectively, and increased with relative humidity from 58% RH to 84% RH. Oxygen diffusion in films equilibrated at RH>84% could not be adequately described by a uniform diffusion model with a single diffusion coefficient; this complex diffusive behavior correlated with a dramatic increase in the mobility of the protein matrix as revealed by a decrease in the erythrosin lifetime under nitrogen at high humidity.