Noah Lotan
242
PseudopolyeWec1:rolytePolyamino Acids’ Noah Latan LJepartment of Biophysics, Weizmann lnstitute of Science, Rehovot, israel (Received September 6, 1972) publication costs assisted by U. S. Department of Health, Education, and Welfare
The conformational changes of poly-N5-(4-hydroxybutyl)-~-glutamine have been investigated in the IXl-methanol solvent system. Optical rotatory dispersion (ORD) measurements indicate that, with increasing LiCl concentration, an a-helix-to-random coil transition takes place, its midpoint being at about 3 M LX1. Intrinsic viscosity measurements, on the other hand, reveal two transitions. The first, in the range 2-4 M salt, is associated with a decrease in viscosity, as expected for the rod-like to disordered chain transformation suggested by the ORD data. The second, in the range 4-6 M salt, is characterized by an increase of viscosity, Le., a change in the opposite direction. It is proposed that the polypeptide itinds Ili+ions, thus acquiring polyelectrolyte character.
The effects of sahs on the conformation of polyamino acids and proteins in aqueous solutions have been extensively investigated.* Nonaqueous solvents, on the other hand, have received only limited attention for such studi e ~ . ~ -In 6 particular, lithium chloride (2-5 N ) has been shown6 to affect the uv and ir spectra, as well as the optical rotatory properties of poly-L-tyrosine in methanol; these changes have been interpreted as indicating an ahelix-to-random coil transition, but no evidence has been presented for the actual formation of the random coil state. Poly-NS-(4-hydroxybutyl)-L-glutamine(PHBG) has been shown previousl\l7J to assume in methanol the ahelical conformation, and we report here two conformational transitions of PHBG in the LiC1-methanol solvent system. The polymer sample used during the present investigations has a molecular weight of 120,000, as estimated from its intrinisic viscosity in methanol ([a]MeOH = 2 dl/g), and using a relationship establishedg for a structurally related polymer, in the same solvent. The conformational changes of PHBG have been followed by optical rotatory dispersion (ORD) ana viscosity measurements. Prior to these measurements, ,311 the solutions were passed through a Teflon filter (type lIJSWPQ-1300, Millipore Corp.). In the clear solutions thus obtained, the polymer concentration was determined by micro-Kjeldahl nitrogen analysis; the LiCl was titrated with Ng(ClO&, following essentially a procedure described elsewhere.1° ORD spectra have been obtained with a Cary 60 spectropolarimeter, and the Moffitt-Yangll parameter bo was calculated from data in the range 270-370 m p . Viscosity measurements were performed with calibrated Ubbelohde semimicro dilution type viscometerf (Cannon Instruments Co.), using polymer solutions of 2-10 mg/ml; the intrinsic viscosity values reported, [a], were obtained by extrapolation to infinite polymer dilution. The experimental results are summarized in Figure 1. When LiCl is added to a solution of PHBG in methanol, in the range 2-4 salt the bo parameter varies from about -700 to zero (henceforth transition I); such a change is characteristi(+ for a right-handed ar-helix-torandom coil transition. No change in bo is produced upon additional increasing of the salt concentration up to 6.1 M . The hydrodynamic properties, however, behave differently. In the range 0-4 M LiC1, a large decrease of the inThe Journal of Ph:ysical Chornistry, Vol. 77, No. 2, 7973
trinsic viscosity (from 2 to about 0.6 dl/g) is observed, indicating that transition I is a conformational change of the high molecular weight polymer from a rod-like shape to a disordered chain, as suggested also by the ORD data. However, on further increase of LiCl concentration, an additional transition (henceforth transition II) i s observed, as indicated by viscosity changes in the opposite direction, i.e., increase; the increase in the intrinsic viscosity from [9]3.7 M = 0.66 dl/g to [q]5.2 M = 1.12 dl/g, followed by a leveling-off region ( [ q ] 6 . 1 M = 1.12 dl/g) are indicative of a conformational change from a hydrodynamically compact structure to a more expanded one. A biphasic character of the viscosity changes for polyamino acids undergoing conformational transitions, similar to the one described above, has been observed previously only for aqueous solutions of polyelectrolytes, namely, poly-L-glutamic acid12 and poly-~-Eysine.l3In these cases, transition I1 was induced upon fully ionizing the side chain groups, and it led to an extended conformation, stabilized by electrostatic repulsion. By analogy, we conclude that transition I1 of PHBG in the EiCl-methanol solvent system also leads to formation of a polyelectrolyte. In principle, such a structure will be obtained if one of the ions binds to the peptide group (eq I), while the counter-
The support of this research by a grant from the U. S . Department of Health, Education, and Welfare (Grant No. 06-003) is gratefully acknowledged. P. H. von Hippei and Th. Schleich, “Structure and Stability of Biological Macromolecules,” Voi. II, s. N. Timasheff and G. D. Fasman, Ed., Marcel Dekker, New York. N.Y., 1969, p417. J. S. Frazen, C. Bobik, and J. €3. Harry, Biopolymers, 4, 637 (1966). N. Lotan, M. Bixon, and A. Berger, Biopolymsrs, S, 69 (1967). J. H. Bradbury and M. D. Fenn, J. Mol. Biol., 36,231 (1968). M. Shiraki and K. Imahori, Sci. Pap. Coll. Gen. Educ., Univ. Tokyo, 16,215 (1966). N. Lotan, A. Yaron, and A. Berger, Biopolymers, 4, 365 (1966). F. J. Joubert, N. Lotan, and H. A. Scheraga, BiochfSmistry, 9, 2197 (1970). N. Lotan. A. Yaron, A. Beraer, and M. Sela, Biopolymers, 3, 625 (1965). F. W. Cheng, Microchem. J., 3 , 537 (1959). W. Moffitt and J. T. Yang, Roc. Natl. Acad. Sci. U. S., 42, 596 (1956). P. Doty, A. Wada, J. T. Y a w , and E. R . @lout,J. Polymer Sci., 23, 851 (1957). J. Applequist and P. Doty, “Polyamino Acids, Polypeptides. and Proteins.” M. A. Stahmann, Ed., University Wisconsin Press, Madison, Wisc., 1962, p 161.
ton Exchange in Crystalline Zirconium Phosphates
243
-200 a,
B
also, a solid adduct poly-L-proline-LiBr has been isolat- . ed.19 On the other hand, studies of the nmr21,22 and thermodynamic proper tie^^^-^^ of solutions of various electrolytes in methanol have indicated that, under these conditions, the C1- anion is extensively solvated.
+
-CO-NH-
LiCl F=
OI
-C(O--Li)=NH-
P
+-
4- C1-
(1)
-600 Po
C10
2 4 LiCl conc. ( M I
8
Figure 1. Conformational changes of PHBG in the LiCI-methanol
solvent system, as foliawed by (0)ORD and ( 0 )viscosity.
ion is strongly solvated (eq 2). Indeed, experimental evidence has accumulated to indicate that both requirements are met. Thus, complex formation between Li+ and the amide group has been inferred from measurements of viscosity,14 heats of r e a c t i ~ n , ' ~rate of proton exchange,16 and 7Li spin-lattice relaxation tirne.l? Moreover, a crystalline N-methylacetamide-LiC1 complex has been obtained and its structure determined by X-ray analysis;18
4- nMeOH
.--L
CI-(MeOB),
(2)
(14) J. Bello and H. R . Bello, Nature (London), 194, 681 (1962). (15) J. Bello, D. J. Haas, and H. R. Bello, Biochemistry, 5, 2539 (1966). (16) T. Schleich, B. Rollefson, and P. H. von Hippel, J. Amer. Chem. SOC.,93, 7070 (1971) (17) T. Schleich, R. Gentzier. and P. H. von Hippel, J , Amer. Chem. SOC.,90, 5954 (1968). (18) D. J. Haas, Nature (London), 201, 64 (1964). (19) W. F. Harrington and J. Kurtz, J. Mol. Biol., 17, 440 (1966). (20) J. Engel, J. Kurtz, E. Katchalski, and A. Berger, J. Mol. Biol., 17, 255 (1966). (21) R. D. Green, J. S. Martin, W. B. McG.Cassie, and J. 8. Hayne, Can. J. Chem., 47, 1639 (1969). (22) R. N. Butler and M. C. R. Symons, Chem. Commun., 71 (1969). (23) C . V. Krishnan and H. L. Friedman, J. Phys Chem., 75, 3606 (1971). (24) I. M. Kolthoff and M, K. Chantooni, Jr., J. Phys. Chem., 76, 2024 (1972). (25) T. Kenjo, S. Brown, E. Held, and R . M. Diamond, J. Phys. Chem., 76, 1775 (1972).
On the Wlechariisrn of Ion Exchange in Crystalline Zirconium Phosphates. V I I. The Crystal Structure mi Bis(arnmonium orthophosphate) Monohydrate' A. Clearfield" and J. M. Troup Contribution from the Clippinger Graduate Research Laboratories, Department of Chemistry, Ohio University, Athens, Ohio 45701 (Received June 28, 7972) Publication costs assisted by the Ohio University Research Institute
The crystal structure of the ammonium ion exchanged phase of a-zirconium phosphate, Zr(NH4POdz. H20, has been determined. The space group is P21/c with a = 9.131(5) A, b = 5.417(5) i\, c = 19.19(1) A, and @ = 102.7(1)". There are four molecules per unit cell. The structure is essentially that of a-zirconium phosphate with the layers spread apart to accommodate the ammonium ions. Each ammonium ion is surrounded by four P-0--type oxygen atoms. Each such oxygen atom has in turn four ammonium ion near neighbors. The water molecule resides between the ammonium ions and is sufficiently close to hydrogen bond to them.
The crystalline compound a-zirconium bis( monohydrogen orthophosphate) monohydrate, Zr(HPO&HzO, hereafter referred to as a-ZrPbehaves as an ion exchanger.2 Both hydrogen ions of the orthophosphate groups are exchangeable. wit]? sodium, potassium, and ammonium ions the prckons are replaced by the cations in two stagThe first exchange reaction leads to the formation of the half-exchanged phase. This is followed by conversion of
the half-exchanged phase to a phase containing 2 mol of cation per formula weight. Since the crystal structure of a-ZrP fs known,? a knowledge of the structures of the exchanged phases would greatly aid in clarifying the nature of the (1) Acknowledgment for support of this work is made to the National Science Foundation under Grants No. GP-10150 and GP-26050. Portions of this paper were taken from the M.S. thesis of J. M. Troup presented to the Chemistry Department, Ohio University, August 1971.
The Journal of Physical Chemistry, Vo!. 77, No. 2, 1973
