Summary
Formol diphtheria toxoid was purified by a combination of multi-membrane electro-decantation, ammonium sulphate fractionation and acid precipitation, yielding a product with a potency of 2950 Lf/mg protein nitrogen without impairing the antigenieity in guinea-pigs.
Physieochemical investigation showed that the purified toxoid was homogeneous by gel diffusion, electrophoresis, diffusion and sedimentation measurements. Using sedimentation and diffusion data a molecular weight of 73,300 and a frictional ratio of 1.34 were calculated.
The iso-electric point of formol diphtheria toxoid, as determined by a new method using a modified form of M.M.E.D. apparatus, was found to be pH 4.15 and pH 4.25 in acetate buffers of ionic strengths 0.05 and 0.025 respectively.
