Hydration and protein dynamics: Frequency domain fluorescence spectroscopy on proteins in reverse micelles

Lysozyme, human serum albumin (HSA), and liver alcohol dehydrogenase (LADH) have been studied in reverse micelles by frequency domain fluorescence spectroscopy. The emission of the tryptophanyl residues of the proteins was monitored. Fluorescence and anisotropy decays were measured from 2 to 350 MHz for each protein in reverse micelles and in aqueous solutions. The wide range of modulation frequencies available allowed direct monitoring of the internal motions of tryptophan residues, occurring in the subnanosecond time range, together with the whole protein rotational dynamics in the micelles. The results indicate that the rotational correlation times for the internal motions and the overall protein rotation in reverse micelles decrease with increasing water concentration. Lysozymes showed peculiar rotational dynamics which reflect denaturation occurring as the protein increases its water content in the reverse micelle. This effect was not observed for the other proteins. Dynamic measurements appear useful in understanding structural changes arising from the interactions between proteins and micellar systems. © 1991 American Chemical Society