Time-resolved fluorescence and anisotropy of covalently coupled 1-pyrenebutyric acid for monitoring the crystallization conditions of lysozyme

Time-resolved fluorescence and anisotropy measurements of trace amounts of 1-pyrenebutyric acid labeled hen egg-white lysozyme (PBA-HEL) were used to characterize hen egg-white lysozyme (HEL) crystallization conditions. The effects of sodium chloride and protein concentrations on the fluorescence lifetimes and rotational correlation times of the labeled protein were examined. These results were compared with the effects of the salts ammonium acetate and ammonium sulfate. Addition of protein precipitants caused increases in the rotational correlation times which were attributed to a combination of steric, hydrodynamic, general electrostatic and specific ionic interactions. This decrease in the rotational mobility of HEL appears to be a necessary but not sufficient condition to allow the formation of specific interactions leading to crystallization. The results demonstrated that fluorescence measurements are effective in characterizing and monitoring protein crystallization processes prior to the appearance of macroscopic crystals.