Tryptophan photophysics in rabbit skeletal myosin rod

The fibrous region of myosin (myosin rod) is an α-helical, two-stranded coiled-coil made up of identical chains of nearly 1000 residues. Myosin from rabbit skeletal muscle has two tryptophans per chain located at identical hydrophobic d sites in the heptad repeat that forms the basis for hydrophobic dimerization. The fluorescence excitation and emission spectra of rod in high salt buffer (where the rod exists as a coiled-coil monomer) at 20°C are red- and blue-shifted, respectively, from the comparable spectra of N-acetyl-tryptophanamide or l-tryptophan. These spectral shifts, as well as red-shifts in the emission spectra induced by excitation on the red edge of the absorption or by increases in temperature, indicate that (on average) the tryptophans are partially exposed to aqueous solvent yet in contact with the protein matrix. The tryptophan intensity decays show an unusual bimodal distribution; the major species has a discrete lifetime of about 5.2 ns while the minor species exhibits a complex decay with a broad (3.4 ns full width at half maximum) Gaussian distribution of lifetimes centered around 1.3 ns. The long lifetime species has a blue-shifted excitation and red-shifted emission characteristic of the indole chromophore in a polar (probably aqueous) environment while the short lifetime species has the spectral parameters characteristic of indole in a non-polar environment. Although assignment of these lifetime species to particular tryptophans in the rod is problematic, this study indicates that the coiled-coil interface presents a complex heterogeneous environment that may undergo rapid conformational mobility.