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Vol. 62
Acknowledgments.-The support of this research by the National Science Foundation, Washington, D. C., is gratefully acknowledged. Distillations of the solvents were carried out by Miss Dolores Sicilia.
mentioned definition of the effective hydrodynamic volume is comparable with the value obtained from self-diffusion experiments. Hence, we re-affirm the statement3 that “it is impossible to relate the effective hydrodynamic volume of a dissolved protein molecule to its partial specific volume.” Tanford agrees that two hydrodynamic properON T H E INTERPRETATION OF ties have to be measured in order to deduce both HYDRODYNAMIC DATA FOR DILUTE the size and the shape of the effective hydrodynamic PROTEIN SOLUTIONS particle. However, he tries t o imply a greater senhitivity in the interpretation of hydrodynamic BYHAROLD A. SCHERAUA AND LEOMANDELKERN data than actually exists by stating that our 0 (and Department of Chemistyy, Cornell University, Ithaca, N e w Y o r k , and presumably also our 6) function represents a poor Polumer Structure Sectzon, Natzonal Bureau of Standards, Washington choice of measurements. As emphasized previ36,D. C. 0us1y,~Jwhereas a single property (such as viscosRereived September SO, 1967 ity) varies considerably with axial ratio for constant Several questions have recently been raised’ volume, one doesn’t know in advance what the volabout our method of interpretation of hydrody- ume is. Hence, one must use a function (e.g., our namic data on dilute protein solutions,2and about a or 6 function, or any other equivalent one) which recent application of this method to measurements pdepends on a pair of hydrodynamic measurements. on bovine serum a l b ~ m i n . We ~ believe that the A few calculations will show that any such funcargument developed by Tanford,’ in this connec- tion, which combines two hydrodynamic measuretion, is misleading and thus necessitates further ments, is very insensitive to changes in axial ratio. clarification. As stated in his equation 1, Tanford chooses to express the effective hydrodynamic volume of a dis- EXTRACTION OF INORGANIC SALTS BY solved protein molecule in terms of its partial 2-OCTANOL. 111. ZINC AND CADMIUM specific volume and a quantity 61,which he defines4 as “the number of grams of solvent incorporated CHLORIDES. AQUEOUS PHASE ACTIVITIES‘ in the hydrodynamic particle per gram of dry protein.” The arbitrariness of this assumption and its BY T. E. MOORE,NORMAN G. RHODEAND ROBERTE. disregard of physical reality have already been disWILLIAMS cussed in great detail both by us2 and by Sadron15 The Department of Chemistry, Oklahoma State University, Stallwater, Oklahoma whose treatment is essentially equivalent to ours. Received October 10, 1967 In addition to this arbitrary division of the effective hydrodynamic volume into two terms ( M / Preliminary experiments in these laboratories N ) G and 6I(M/N)vlo,Tanford’s procedure is also have shown that the extraction of both Zn(C104)z very misleading since one is thereby tempted to and Cd(C104)2from aqueous solutions (4 m) ocattach reality to 6’ as the mass of water actually curs readily. When solubilities of ZnC12and CdClz bound to one gram of protein. Tanford, in fact, is in 2-octanol are compared? however, a large difinconsistent on this point since he makes this lat- ference is found, ZnCL being over 1000 times as ter identity when he asserts’ that the validity of his soluble at 25”. This suggested that effective sepaequation 1 is confirmed by Wanq’s considerationsB ration might be achieved through the 2-octanol of the self-diffusion of water in dilute aqueous pro- extraction of aqueous mixtures of the chlorides. tein solutions. It is incorrect to obtain 61 from This is in general agreement with earlier observaself-diffusion since it is clearly stated by Wang and tions regarding the non-specificity of 2-octanol as quite apparent in his theoretical development that an extraction solvent for metal perchlorates conthe hydrodynamic behavior of the dissolved pro- trasted to the much more specific behavior of the tein molecule does not enter into his calculation. corresponding chlorides.2 Thus, it is not surprising that in Wang’s treatment To test this theory, six series of solutions were the appropriate and correct volume to be consid- extracted with the octanol at 25”. Figure 1 preered is that which describes the domain of the mole- sents the variation of the distribution coefficients, cule, with &, in this instance, being the specific kd, of ZnClz and CdClz in the different series. The solvation of the actual protein molecule. However, distrlbution coefficient is here defined as the ratio this latter conclusion is limited to problems involv- of the molal concentration in the non-aqueous phase ing the self-diffusion of water and has no applica- to the molal concentration in the aqueous phase. tion to the present matter concerned with the in- The equilibrium mixtures of octanol and water are terpretation of the hydrodynamic data of protein considered as solvents in each phase. solutions. A similar error is committed by Tanford It is evident from the figure that separation and Buzzell,4 who assume that the value of 61 ob- factors, s, of the order of 50-60 (s = kd(ZnClz)/ tained from intrinsic viscosity data and the afore- kd(CdClz)) are obtained with the ZnClz-CdCln (1) C. Tanford, THISJOURNAL, 61, 1023 (1957). mixtures investigated. These values, however, (2) H.A. Scheraga and L. Mandelkern, J . Am. Chem. Soc., 76, 179 are only about 50y0 of the values calculated from (1953). (3) G. I. Loeb and H. A. Scheraga, THISJOURNAL, 60, 1633 (1956). (4) C. Tanford and J. G. Buzsell, THISJOURNAL, 60, 225 (1956). (5) C. Sadron, Prou. in Biophys.. 3, 237 (1953). (6) J. H. Wan& J . Am. Chem. Soc., ‘76,4755 (1954).
(1) Supported under Contract AT(11-1)-71 No. 1 with the U. 8. Atomic Energy Commission. (2) T.E. Moore, Roy J. Laran and Paul C. Yates, THISJOURNAL, 19, 90 (1955).
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