Nanosecond spectroscopy of retinol

Nanosecond fluorescence spectroscopy was used to study the unique binding site of the retinol-binding protein (RBP) from human serum. At pH 7.4, the binding of retinol to RBP caused the following spectroscopic changes in the ligand: (a) an enhancement of the fluorescence decay time (gamma = 8 ns); and (b) an increase in the emission anisotropy (A = 0.29). Retinol in hexane has a fluorescent decay time of 4.2 ns and a low emission anisotropy (A = 0.02). The increase in the fluorescence decay time of bound retinol is not due to dielectric relaxation effects of polar groups, since nanosecond time-resolved emission spectra of either retinol in glycerol or retinol bound to RBP, failed to show any time-dependent shifts in emission maxima during the time period investigated 0 to 30 ns. The degree of rotational mobility of bound retinol was investigated by time emission anisotropy measurements. The observed rotational correlation time (theta = 7.2 ns) is consistent with a rigid compact macromolecule of 21,000 molecular weight.