Light Scattering by Very Large Molecules: Application of Transmission Method to Actomyosin

Abstract

IN estimating the size of very large colourless molecules by light scattering, according to the transmission method¹, the following relationships are relevant: where τ is the turbidity of the solution to be tested, l is the optical path-length, J and J ₀ are the intensities of incident and transmitted light, c is the solute concentration, λ is the wave-length of light, M is the weight average molecular weight of the solute, N is Avogadro's number, µ, and µ₀ are the refractive indices of solution and solvent, Q _exp. and Q _theor. are the particle dissipation factors, the former to be determined from the wave-length dependence of turbidity and refractivity and the latter from analytical expressions due to Doty and Steiner¹ and Bueche, Debye and Cashin². The dependence of β′ on molecular dimensions is shown in Fig. 1; the limiting values, β′_∞, are respectively 2.0, 1.7, 1.45 and 1.0 for spheres, monodisperse coils, polydisperse coils and thin rods. It will be seen that a comparison between β and β′_∞ gives information regarding the shape of sufficiently large scattering particles. The approximate value of M for such solutes can be obtained from (3), in conjunction with an extrapolation method proposed by Cashin and Debye³, in which (Hc/τ)^λ _{c → 0} is plotted against (µ₀/λ)², thereby enabling an estimation of the intercept for (µ₀/λ)² = 0. Inserting the reciprocal of this intercept in (2) leads to Q ^λ _exp.; and comparison of this with Q ^λ _theor. enables one to compute the order of magnitude of the molecular dimension.