Modeling Biological Macromolecules in Solution

The use of the general (tri-axial) ellipsoid as a model for the conformation of biological macromolecules in solution is discussed. The recent derivation of an expression for the viscosity increment of dispersions of tri-axial ellipsoids [S. E. Harding et al. (1979) IRCS Med. Sci. 7,33; and (1981) J. Colloid Interface Sci. 79,7-131 makes the fitting of such a model possible by the derivation of appropriate volume-independent functions. We now derive these volume-independent functions for tri-axial ellipsoids and investigate by exhaustive computer simulation the possibility of deriving the two axial ratios (a lb , b /c ) from data of various types, in every case containing plausible experimental error. It is shown that transport properties alone cannot be used to yield estimates for the axial ratios, given current experimental precision. However, a combination of transport and rotational diffusion properties is more promising, and an algorithm is developed and tested that will reliably yield estimates from simulated data obtained by the methods of sedimentation, viscosity, and electric birefringence.