The molar friction constant of proteins determined in ultracentrifugal analysis may be modified in the presence of water by variations in the hydrogen-ion, electrolyte, and protein concentration of the solution, by the addition both of amides, amino acids, and other chemicals and of other proteins, by heat, by ultraviolet light, by ultrasonic waves, etc.It is important to understand the cause of such modifications in the sedimentation behavior of the dissolved unit. In some cases the effect is due to a real dissociation, but it must be recognized that in other instances it may arise from a change in the shape of the molecule, or even from a change in the degree of solvation. In extreme cases sedimentation velocity may be modified by orientations of the molecular kinetic unit.In this study of protein stability in solution we shall (1) make the attempt to analyze the result of solvent change in influencing sedimentation and diffusion constants by setting up simple relationships between molecular weight, sedimentation constant (s), diffusion constant (D), and the Svedberg dissymmetry factor (f/fo), (8) give representative experimental data to show how these constants are modified by the dissociation of dissolved units, and (8) consider other systems for which changes in molecular form rather than actual dissociation may be responsible in part for observed differences in molar friction constant. I. MATHEMATICAL RELATIONSHIPSIt will be convenient for the discussion following to have at hand certain relationships between molecular weight, M, sedimentation constant, s, diffusion constant, D, and dissymmetry number, ///0.The frictional resistance to sedimentation of a spherical molecule is
The toxin-antitoxin flocculation reaction has long been regarded as an atypical example of the precipitin reaction. Marrack (1) states, "The behavior of the diphtheria toxin-antitoxin system is altogether abnormal. The range within which precipitation occurs is very narrow and no precipitate forms when the ratio of antibody to antigen exceeds about twice the ratio at the equivalence point." In addition, antitoxin is associated with the pseudoglobulin fraction of immune horse serum in contrast to antibacterial antibodies which are associated with the water-insoluble or euglobulin fraction. It must be remembered, however, that practically all the work on protein-antiprotein systems has utilized antibody obtained from the rabbit. The only systems using horse antibody which have been extensively studied, excepting toxin-antitoxin reactions, have been polysaccharide-antipolysaccharide systems. In other words, the exceptional character of the toxin-antitoxin reaction may be merely a species difference in the antibody used and the reaction may well be a typical example of horse antiprotein systems. Evidence to suggest that this is indeed the case has been furnished by immunizing a horse against crystalline ovalbumin. As will be reported subsequently, the ovalbumin-antiovalbumin system in the horse gives rise to a reaction characteristic of diphtheria toxin-antitoxin flocculation. Moreover, diphtheria antitoxin produced in the rabbit gives a typical precipitin reaction.During the last ten years considerable advance has been made in our knowledge of the nature of the reaction between antigen and antibody in immune systems. In particular, the introduction of absolute quantitative methods by Heidelberger and Kendall for determining the quantity of antibody entering into the precipitin reaction and the composition of the specific precipitates (2), has made it possible to develop a theory of antigenantibody union on the basis of the law of mass action, assuming a series of 247 on
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