Quasi-elastic scattering of laser light is now becoming a powerful tool for the study of dynamic properties of both biological and non-biological macromolecules. In solutions of biological macromolecules, aggregation-disaggregation of molecules under study usually depends on solvent conditions. Therefore, special attention must be paid in light-beating spectroscopy. By use of both homodyne and heterodyne methods, the spectral densities of solutions of F-actin and of a complex of F-actin and heavy meromyosin were measured. The half-width of the heterodyne spectrum was much wider than that of the homodyne spectrum. This was probably due to polydispersity of F-actin. The results showed that laser light scattering gave information about the dynamics of free filaments, although low frequency rheometry has suggested the rubber-like elasticity of the solution of F-actin and heavy meromyosin. The interaction between F-actin and heavy meromyosin was studied in the presence of pyrophosphate (PPi). At PPi concentrations between 10 and 100 μM, the spontaneous bending motion of F-actin occurred as if heavy meromyosin were not present, although turbidity of the solution indicated that heavy meromyosin was, in fact, bound to F-actin. This meant that there were two types of binding state of the F-actin-heavy meromyosin complex. A plausible model is that, in the absence of PPi, the two heads of one heavy meromyosin molecule simultaneously interact with two neighbouring monomers in F-actin, whereas a single head binding occurs in the presence of PPi. Above 100 μM PPi, heavy meromyosin seemed to dissociate from F-actin. However, it was not clear whether or not this was due to a direct effect of PPi on heavy meromyosin alone, because the dynamic properties of F-actin also changed. Assuming that actin filaments are cross-linked by myosin, a simple model is proposed to explain qualitatively the rubber-like elasticity of the solution of an F-actin-heavy meromyosin complex.
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