Size and density of fibrin fibers from turbidity

In agreement with earlier observations that the angular dependence of light scattering by fibrin gels obeys the theory for light scattering by very long and thin rigid rodlike particles (intensity proportional to the square of half the scattering angle), we find that the turbidity, tau, of the less opaque gels varies as the inverse third power of the wavelength, lambda. Mass-length ratios of the fibers calculated from these two measurements closely agree. For fibrin gels containing fibers with a very high mass-length ratio (of which we had not been able to obtain interpretable scattering data), the turbidity is found not quite to vary as 1/lambda3. For these opaque gels, the fiber diameter is no longer negligible with respect to the wavelength. It is shown how the radius of gyration of the fiber cross section (and therefore the radius of cylindrical fibers) can be obtained from the ratio of slope and intercept of a plot of 1/tau lambda3 vs. 1/lambra2. The square of the radius of the fibers is found to be proportional to the mass-length ratio. The density of the fibers is calculated to be 0.28. This corresponds to a ratio of fiber volume to volume of protein contained in the fiber of 5.0.