Excitation wavelength dependent fluorescence anisotropy of eosin-myosin adducts. Evidence for anisotropic rotations.

Steady-state and time-resolved fluorescence anisotropy measurements of eosin in solution and eosin-5-maleimide bound to purified myosin were made to study localized motions of the "head region" of this protein. The lifetime and apparent Debye rotational relaxation times of eosin in aqueous solution are essentially invariant with changes in excitation wavelength. In more viscous solvents, such as propylene glycol/water mixtures, the apparent Debye rotational relaxation times of eosin differ upon excitation in the regions of positive and negative anisotropy. Using eosin attached to the SH-1 thiol of the myosin head differing rotational modes of the bound probe were detected, dependent upon excitation wavelength. The main features of the anisotropy data for eosin-myosin are consistent with the existence of a 'crevice' or 'pocket' in the myosin head. A model is presented which allows estimation of the ratio of distinct rotational diffusion terms (selected by different excitation wavelengths) that produce both the observed steady-state anisotropy and differential phase results.