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The use of analytical ultracentrifugation and freeze-fracture electron microscopy in solution allowed us to observe the monomeric and dimeric forms of Mg.71. This subfragment of the myosin molecule contains the LC2 light chain and is comparable to a "native" myosin head. Sedimentation-diffusion equilibrium ultracentrifugation shows that it is necessary to use slightly different conditions in order to obtain a pure Mg.S1 dimer, as compared to the case of chymotryptic S1 (LC2-free S1). For example, in a buffer leading to a complete dimerization of chymotryptic S1, Mg.S1 is only in the form of a monomer-dimer mixture, with comparable proportions of monomer and dimer. The freeze-fracture technique, applied to solutions containing Mg.S1 or chymotryptic S1, revealed that the monomeric species both have the same maximum chord (about 120 A) and that both dimeric species also have the same maximum chord (about 250 A). The maximum chord of the monomer is comparable to the surface-to-surface spacing between the myosin and actin filaments, in a fiber at the slack length. In sharp contrast this chord is higher than this spacing in a stretched fiber. The consequences of this fact are discussed, with particular reference to the sarcomere length-tension relationship.

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