THE CONFORMATION OF XANTHENE DYES IN THE MYOSIN SULFHYDRYL ONE BINDING-SITE .1. METHODS AND MODEL SYSTEMS

Derivatives of the fluorescent probes fluorescein and rhodamine specif ically and covalently modify the highly reactive thiol (SH1) of myosin subfragment 1 (S1). Both probes develop circular dichroism (CD) upon modification of SH1 at the visible absorption band of the chromophore. A model system of chiral complexing agents (aromatic chiral amines) i nteracting with fluorescein in solvent develops a CD signal that mimic s that produced by S1. The model system suggests that a specific inter action of the probe with an aromatic chiral residue in the SH1 binding pocket induces the CD signal. Several other spectroscopic signals, in cluding absorption and fluorescence intensity and anisotropy, characte rize the fluorescein or rhodamine binding to SH1. A coupled dipole met hod is adapted to interpret these spectroscopic signals in terms of th e probe-S1 complex conformation. The computation of the orientation of the principal hydrodynamic frame (PHF) of S1 from its crystallographi c alpha-carbon backbone structure permits the known orientation of the probe in the PHF of S1 to further constrain the conformation of the p robe-S1 complex. The coupled dipole interpretation of spectroscopic da ta combined with constraints relating the probe dipole orientation to the PHF of S1 determines the conformation of the probe-S1 complex. The methods developed here are applied to the spectroscopic signals from fluorescein or rhodamine in the SH1 binding site of S1 to obtain an at omic resolution model of the probe-Si conformation [Ajtai and Burghard t, Biochemistry, 34 (1995) 15943-15952].