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Streaming potential experiments were performed to determine the zeta potentials for 0.09% fibrinogen solutions (97% clottable) in contact with Pyrex capillaries as a function of pH and ionic strength. It is observed that the slopes of the f vs. pH lines decrease as the ionic strength increases and that there are no discontinuities in these lines. Within the limits of the ionic strength range investigated, 0.004-0.013 (phosphate and acetate buffers), it is found that the zero f for fibrinogen solutions varied from pH 5.85 to 5.95, respectively. These pH values are the isoelectric points for fibrinogen at these ionic strength levels. The pH of the isoelectric point extrapolated to zero ionic strength occurs at 5.80. It is found that the thickness and tenacity of fibrinogen layers adsorbed on the capillary wall will depend on the pH of the initial coating and the jpHsequence of subsequent buffer streaming. The zeta potentials observed for the fibrinogen-solid interface are due to the thickness and electrical surface properties of the adsorbed protein layer and to the concentration of electrolyte in solution.(1) This is paper number 6 in the series "Electrostatic Forces Involved in Blood Coagulation." This work was supported, in part, by the Office of Naval Research, U. S. Navy under contract N6onr-264, T. O. 10 and also by grants from the Kress, Lasker, Hyde and Hampil Foundations.

A study of trypsin, soybean trypsin inhibitor and the inhibitor-trypsin compound with the ultracentrifuge is presented in which the stoichiometry of the reaction between trypsin and the inhibitor, the stability of the compound and the nature of the bonding between the components were investigated. It has been found that mixtures of trypsin and the inhibitor exhibit compound formation at PH 6.0 but not at pH 2.8 in 0.1 M phosphate buffer. Maximum compound formation occurs a t pH 6.0 with mixtures containing approximately equal parts by weight of trypsin and inhibitor. The composition of the compound is shown to be constant. Sedimentation constants for trypsin and inhibitor are presented. The crystalline soybean trypsin inhibitor-trypsin compound was prepared and its sedimentation behavior was studied as a function of protein concentration, pH and ionic strength. The compound dissociates into its components below PH 3. The sedimentation constant, ~p ( ,~, has an average value of 3.9 S from PH 3.6 to 10.4. Below pH 3.0, the ssWlw equals 2.5 S, a value comparable to that for the trypsin and inhibitor. There is no evidence of dimerization or concentration dependence of the sedimentation constant for the three proteins in the concentration and PH range studied. The compound shows no evidence of dissociation in salt solutions as concentrated as 3 M even after standing 72 hours. It is also shown that the compound will form in the presence of the tryptic inhibitor benzoyl-L-arginine. The molecular weights of the crystalline inhibitor-trypsin compound, trypsin and inhibitor are discussed.