1561
Foaminess of Binary and Ternary Solutions
value of pKlO relative to that of the methyl or ethyl copolymer. The evidence so far, while not conclusive, shows no such variation in pKlo v a l ~ e s . ~ J ~ In the 0.5 M NaCl solutions the butyl copolymer exhibits insolubility in the region of a where the maximum in the pK curves would be expected. However, the conformational transition occurs a t higher a than it does in water, and the initial separation of the pKla and pKd curves and their mutual convergence as the end of the transition region is approached resemble these features observed in the absence of excess electrolyte and may be explained similarly. We have seen that in the compact conformation of the butyl copolymer the electrostatic forces due to the negative carboxylate groups and the environmental influence of the nonpolar butyl groups exerted opposing effects on the (CH&NH+ groups of the probe, the former being acid weakening and the latter acid strengthening. One may speculate that if we were dealing with a compact polycation instead of a polyanion the electrostatic and nonpolar environmental effects would reinforce one another, both acting to increase the acid strength of the probe. Such an effect has indeed been reported for dansylated bovine serum albumin for which the value of p& of the conjugated probe was found to be 2.3 units lower than that of the free probe.16 Moreover, denaturation with 8 M urea reduced the pKd lowering to 0.9 units, a result which could also be predicted from our findings with the butyl copolymer by assuming that the denaturation changes the protein molecule from its native compact to a random coil form, thereby both weakening the electrostatic effects and destroying the nonpolar environmental influence of the hydrophobic groups. It is noteworthy that these protein results were originally interpreted quite differently. These conclusions demonstrate the utility of the synthetic hydrophobic polyacids both as model compounds for
proteins and as a means for gauging the response of conjugated optical probes to environmental changes. References and Notes (1) Support of this research by the United States Public Health Service (Grant No. GM 12307) is gratefully acknowledged. (2) Author to whom correspondence should be addressed. (3) 0. S. Mannlng, Annu. Rev. Phys. Chem., 23, 117 (1972). (4) R. W. Armstrong and U. P. Strauss, Encycl. Polym. Sci. Techno/., 10, 781 (6969). (5) L. M. Gross and U. P. Strauss in ”Chemical Physics of Ionic Solutions”, B. E. Conway and R. G. Barradas, Ed., Wiley, New York, N.Y., 1966, pp 361-389. (6) M. Nagasawa and A. Holtzer, J. Am. Chem. SOC.,86, 538 (1964). (7) G. S. Manning and A. Holtzer, J. Phys. Chem., 77, 2206 (1973). (8) U. P. Strauss in “Polyelectrolytes”, E. Selegny, Ed., Ridel, Dordrecht. Holland, 1974, pp 79-85. (9) A. W. Schultz and U. P. Strauss, J. Phys. Chem., 76, 1767 (1972). (10) P. L. Dubin and U. P. Strauss, J. Phys. Chem., 74, 2842 (1970). (11) S. Lifson, J. Chem. Phys., 26, 727(1957). (12) R. A. Markus, J. Phys. Chem., 58, 621 (1954). (13) G. Weber, Blochem. J., 51, 155 (1952). (14) I. M. Klotzand J. Ayers, J. Am. Chem. SOC., 79, 4078 (1957). (15) I. M. Klotz and H. F. Fiess, Biochlm. Biophys. Acta, 38, 57 (1960). (16) R. F. Steiner and H. Edelhoch, Chem. Rev., 62, 457 (1962). (17) G. M. Edelman and W. 0. McClure, Acc. Cbem. Research, 1, 65 (1968). (18) A. J. Begala and U. P. Strauss, J. Phys. Cbem., 76, 254 (1972). (19) N. Seller and M. Wiechmann, Prog. Thin-Layer Chromatogr. Relat. Methods, 1970, 1, 95-144 (1970). (20) H. T. S. Brltton, “Hydrogen Ions”, van Nostrand, New York, N.Y., 1932, p 225. (21) D. Lagunoff and P. Ottolenghi, C. R. Trav. Lab. Carlsberg, 35, 63 (i965). (22) At wavelengths below 300 mp there are small deviations from the spectra of the free probe probably due to light scattering of the copolymer. Since the calculations of y were based on the absorbances at 340 mp where the spectra of free and bound probe were identical, these devlations should not affect our results. (23) In the absence of excess salt the values of pKla and PKd overlapped and therefore only one curve was drawn through both sets of data. This ovorlap is probably fortuitous and results from the near equality of the intrlnslc pKvalues. (24) U. P. Strauss and A. Rosengart, unpublished results. (25) Previously presented evidence indicates that factors other than the electrostatic potential are involved for this polyacid over the range 1 01 2,’ but a probe with a PKd about three units higher than the one used here would be needed to study this CY region.
