Effect of temperature on molecular mobility, oxygen permeability, and dynamic site heterogeneity in amorphous α-lactalbumin films

We investigated molecular mobility and oxygen diffusion in amorphous solid films of α-lactalbumin (α-La) using phosphorescence from the xanthene probe erythrosin B in order to elucidate the molecular mechanism(s) that control oxygen permeability in amorphous solid proteins. Emission peak energy and bandwidth were determined by fitting spectra to a lognormal bandwidth function; intensity decays were fit to a stretched exponential function to determine the lifetime and the stretching exponent. Peak emission energy decreased gradually over the range from −20 to over 120 °C, indicating a gradual increase in the matrix dipolar relaxation rate. Bandwidths were constant at low temperature but increased dramatically above ∼50 °C, indicating that the dynamic heterogeneity of the protein matrix increased at high temperature. Emission lifetimes decrease gradually at low and more steeply at high temperature, indicating that the rate of matrix collisional quenching increased with temperature. Arrhenius analysis of the rate constant for non-radiative decay showed a gradual increase in quenching indicative of matrix softening. Comparison of lifetimes in air and N2 (±oxygen) monitored oxygen permeability. Oxygen permeability became detectable at about 0 °C and appeared to correlate with matrix mobility. The emission spectrum shifted to higher energy as a function of time following excitation, whereas the phosphorescence lifetime decreased with increasing emission wavelength; both behaviors provided strong evidence for distinct sites within the protein matrix varying in molecular mobility. Phosphorescence spectroscopy thus provides a simple tractable tool for monitoring conditions that activate oxygen diffusion in amorphous solid foods.

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► Solid matrix of dry α-lactalbumin softens with temperature over range 0–100 °C.
► Activated protein motions increase in complexity at higher temperature.
► Protein matrix has onset temperature of ∼0 °C for oxygen permeability.
► Rate of oxygen permeability scales with matrix mobility in protein matrix.
► Dry protein is dynamically heterogeneous.