Oct., 1957
CHELATE EQUILIBRIA OF Cu(I1) AND POLYMER WITH MINO NO SIDE-CHAINS
1357
A SPECTROSCOPIC STUDY OF CHELATE EQUILIBRIA OF COPPER(I1) WITH A POLYMER CONTAINING U-AMINO ACID SIDE CHAINS' BY H. MORAWETZ AND E. SAMMAK~ Polymer Research Institute, Polytechnic Institute of Brooklyn, Brooklyn, N . Y . Received February 96, 106Y
The Cu( 11) chelation equilibria with poly-( e-methacr lyl L-lysine) were studied spectroscopically, using the spectra of Cu(I1) valinate and Cu(II)(valinate)z to calculate the cXelAe distribution with the polymer. The dichelate formation is strongly favored by the high concentration of chelating groups in the macromolecular coil. The monochelate also forms more easily with the polymer than with valine, even when the polymer carries a positive charge.
Introduction Previous investigations3m4of the Cu(I1) chelation with polymeric acids have shown that the nature of the complexes formed is determined by the high local density of ligand groups within the swollen polymer coils. An attempt was made to identify the complex by comparison of the Cu(I1) spectra in polymeric acid and in acetate solutions, but the existence of four different copper-carboxylate complexes and the poor precision in the determination of the third and fourth acetate complex formation constants6 introduce large uncertainties into such interpretations. I n the present investigation the Cu(I1) complexation with poly-( e-methacrylyl-L-lysine) (I) was investigated. It was assumed that only two types of complexes would be formed in which (-CHr
x"' -In
A0 NH (CHZ)4
I +
booCu(I1) is coordinated with one or two amino acid groups. Since the spectra of these complexes may be obtained from measurements on solutions of simple amino acids, it is possible t o calculate from spectral data the distribution of the chelates present in the polymer solution. Experimental e-Methacrylyl-L-lyshe (MAL).-A modification of the procedure described by Kurtze was used for the acylation of the e-amino group of lysine. A hot solution of 91.4 g. (0.50 mole) of L-lysine hydrochloride (Nutritional Biochemicals Company) in 750 ml. of water was stirred for 30 min. with 30.5 g. (0.138 mole) of basic Cu(I1) carbonate ( CuzCOa(OH)z) and filtered. The solution of Cu( L-lysinate)z was cooled and 55 g. (0.55 mole) of methacrylyl chloride ( Monomer-Polymer Co.) in 500 ml. of anhydrous ether were added dropwise with stirring maintaining the temperature a t 5" and keeping the pH close to 8 by gradual addition of sodium hydroxide. After stirring for one hour a t room temperature, the precipitated Cu(e-methacryly1-L(1) Financial support of this study by the U. S. Army Office of Ordnance Research is gratefully acknowledged. (2) Abstracted from a thesis to be submitted to the Graduate School of the Polytechnic Institute of Brooklyn, in partial fulfillment of the requirements for a Ph.D. degree. (3) A. M. Kotliar and H. Moraweta, J . A m . Chem. &c., 77, 3692 (1955). (4) H. Morawetn, J. Polymer Sei., 17, 442 (1955). (5) 6. Fronaeus, Acta Chem. Scond., 6, 139, 859 (1951). (6) A. C. Kurta, J. B i d . Cham., 140, 705 (1941).
lysinate)2was filtered and washed with water and ethanol. Hydrogen sulfide was passed through a suspension of the copper complex in 50% aqueous methanol, the copper sulfide was filtered off and MAL recovered by evaporating the filtrate. The crude product had to be purified by chromatography before it could be crystallized. Best results were obtained in passing 2.5 g. of crude MAL in one liter of wet butanol through a column (41 mm. diameter) filled with a slurry of 200 g. of cellulose powder in wet butanol. Elution with the same solvent and vacuum evaporation of one-half of the fraction collected between 700 and 2000 ml. yielded 1.66 g. of crystalline MAL. The crystals were soluble in water to the extent of about 5%, slightly soluble in methanol but insoluble in other organic solvents. The potentiometric titration with 0.1 N sodium hydroxide of 0.1 g. of MAL in 10 ml. of water containing 18% formaldehyde gave an equivalent weight of 214.6 i 1.4 (theoretical 214.3). Anal. Calcd. for CloH18NpOa: C, 56.06; H, 8.47; N, 13.08. Found: C, 56.20; H, 8.59; N, 13.24. &Valine.-This amino acid (Nutrional Biochemicals Co.), specified to have a nitrogen content within 1% and a specific rotation within 2% of theory, was used as received. Polymerization of MAL.-A carefully degassed water solution containing 0.1 M MAL and 5 X 10-6 M azo-bisisobutyronitrile initiator was heated to 65" for 6 days. After thorough dialysis the solution was freeze-dried and the poly-(e-methacrylyl-L-lysine)(PMAL) was obtained in 60% yield. The polymer was highly hygroscopic and elemental analysis indicated that 6.2% water was retained after vacuum drying for 12 hours at 60" over phosphorus pentoxide: The C/N ratio was 4.15 (calcd. 4.28) showing that no lysine residues had been lost by hydrolysis during the golymeriration and the dialysis of the polymer. A 10-8 ase molar solution of PMAL had a specific resistance of 2.2 X 106 ohm-cm. a t 22", pointing to the absence of electrolyte impurities. Molecular Weight of PMAL.-The 45, 90 and 135" light scattering of PMAL solutions containing 0.1 M sodium perchlorate was determined for polymer concentrations of 0.84, 1.85, 2.88 and 4.16 gJ1. at 24.6" using a Brice-Phoenix Photometer. The refractive index increment was measured with a Raleigh Interferometer. A t the wave length8 of 436 and 546 mp dn/dc = 0.258, 0.240; ( H c / T ) ~= 4.09 X 10-6, 4.22 X 10-8; dissymmetry = 1.33, 1.24. Treating the polymer as polydisperse coils, these data lead to7 M , = 282,000 f 13,008 and a root mean square end. to-end distance of 660 i20 A. Standardization of Stock Solutions.-L-Valine was standardized by the formol titration as described above. Solutions of C.P. cupric nitrate were standardized iodometrically.8 The concentration of PMAL solutions was determined by drying to constant weight at 110". The result was expressed as moles of amino acid residues per liter (Cp). The value found by Van Slyke NH2determination (Schwartrkopf Microanal. Laboratory, Woodside, N. Y.) was 4% higher, while potentiometric titration with base in the presence of 18% formaldehyde or of two moles of copper per mole of amino acid0 gave values lower by 8%. Spectral Measurements.-Light absorption at 600 and 700 mfi was measured with a Beckman D U spectropho(7) P. Doty and R. F. Steiner, J . Chem. Phys., 18, 1211 (1950). (8) I. M. Koltboff and E. B. Sandell. "Textbook of Quantitative Inorganic Analysis," The Maomillan Co., New York, N. Y., 1946, p. 634. (9) H. Flood and V. Loran, Tids. Kjemi, Berguesm Me#.,4, 35 (1944); C. A., 41, 6488d (1947).
H. MORAWETZ AND E. SAMMAK
1358
Vol. 61
TABLE I EXTINCTION COEFFICIENT INCREMENT OF Cu(I1) IN VALINESOLUTIONS I 0.1 .0025
Soln. no.
Valine CWOa)z NaC10, PH
I1
Ae600
Awoo
0.02 .002
...
...
3.01 16.2 25.0
A e A = ecuval
E
+
LYAAEA o r 2 ~ A e z ~
- ecu
and
&A
= ecuvsl,
(1)
- ecU
The values of AEA and AEZAcan thus be calculated from the Ae values of any two solutions. This leads to AEA = 21, AQA = 58 a t 600 mp and A ~ A= 39, AEZA= 30 a t 700 mp. (2) At relatively low p H and with an excess of copper over valine, the amount of dichelate formed by is negligible and KA is related to A ~ A KA f(H+)
A~/A~A
(CC,
- A ~ / A ~ A ) ( C V-, ~ a e / A e ~ ) f ( H + ) KiKz
+ Kl(H+) + (H+)2
KIKa
...
3.98f0.04 20.7 26.6
Results Spectra of Cu(I1) Chelates with L-Valine.Although e-isobutyryl-L-lysine (BL) resembles the monomer unit in PMAL more closely, L-valine was used as the low molecular weight analog t o determine the spectra of its copper chelates. This choice was dictated by the low solubility of BL and its Cu(I1) complexes and was justified by the observation that the spectra of BL and valine chelates were indistinguishable from each other. Table I lists the compositions of seven solutions with beeo0 and AETOO, the increments in the molar extinction coefficient of Cu(I1) a t 600 and 700 mp. Three methods were used in analyzing the data. (1) For solutions I-IV the concentrations of the complexes (CuVal) + and (CuValz)' were calculated using for the valine ionization equilibria pK1 = 2.32, p K z = 9.62 and for the first and second chelate formation constants log K A = 7.93 and log K2.4 = 5.57.1° At a given wave length Ae is related to CYA = (CuVal) +/Ccu and CYZA= (CuValJ o / CC" by Ae
0.01 .OOl
e . .
3.05i0.02 11.1 19.4
tometer. For solutions with stoichiometric Cu(I1) concentrations (Cc,) above 0.01 M , 1 cm. Corex cells were used, the blank containing an equivalent copper concentration in the absence of complex forming species. For lower CcU values, 10 cm. cells were employed, the blank containing all ingredients except for the copper. I n either case, a correction was applied to the reading for the absorption of the species absent in the blank, to obtain the molar extinction coefficient increment Ae = [log( lo/l)/XCcu] 7 ecU where 10/l is the ratio of incident and transmitted light intensities, X the path length in cm. and ec, the molar extinction coefficient of Cu++ (0.90 at 600 mp and 6.56 a t 700 mp). Test solutions were adjusted to the desired pH with 1 M perchloric acid or sodium hydroxide and were equilibrated at 25' before being placed into the cell compartment maintained a t 24-26'. The variation in the optical density of a typical test solution over this temperature range was less than the uncertainty in the scale reading. A Cambridge Research pH meter was used to measure pH both before and after the spectral determinations. Any variation exceeding 0.01 pH unit was reported as a range of PH.
where
IV
I11
0.05 .003
(2)
(10) L. E. Maley and D. P. Mellor, AustraEian J. Sci. Research, A8, 679 (1949).
7.21
V
0.06 .06
VI
VI1
0.02 .24
0.0182 ,916
* 54 ... 3.04f0.01 3.18i0.01 7.7 1.19 14.2 2.29
...
3.09 0.29
1.0-
0.8-
o(,
0.60.4 -
0.2-
I.Or
0.41- / 0.2 -
-
I 5
I
3
4
I
6
PH.
Fig. 1.-Cu(I1) chelation in PMAL and valine solution: amino acid 0.025 MI Cu(II), 0.0012 M ; 0,valine; 0 , PMAL.
The data obtained with solutions V and VI at an ionic strength of 0.72, lead to log K A = 8.03 k 0.01 and A ~ Avalues of 16 and 31 at 600 and 700 mp, respectively. (3) When valine is in very large excess over copper at a p H = 3, practically all of the valine is converted to (CuVal)+. In this case, even large errors in the estimate of KI and K A affect only the calculation of the valine fraction which escapes complex formation and have little effect on the estimate of A ~ A . Using this approach with solution VII, in which 95% of the valine forms (CuVal)+, it is found that a t 600 mp AEA = 15. No measurements could be carried out a t 700 mp because of excessive absorption by free Cu(I1) ion. The estimate of A e 2 obtained ~ by method 1 is believed to be reliable since (YZA = 0.99 in solution VI. However, method 2 and 3 led to A ~ A values up to 25% lower than those obtained by method 1. In view of the good agreement between the results obtained by method 2 and 3, A ~ A= 15.5 a t 600 mp and AEA= 31 a t 700 mp were used in subsequent calculations. Cu(I1) Chelation by PMAL.-Table I1 summarizes spectral data obtained with Cu(I1) solutions in the presence of PMAL. Using the AEA and AEZA values obtained from the valine
Oct., 1957
CP x 10’
CHELATE EQUILIBRIA OF Cu(I1) AND POLYMER WITH CPAMINOSIDE-CHAINS
TABLE I1 SPECTRAL ABSORPTION AND CHELATE DISTRIBUTION IN SOLUTIONS CONTAININQ PMAL AND Cu(11) CC” x
25.6 14.5 7.5 13.5 24.3 16. 16. 4b 16. 4b 24.3 24.3 24.3 24.3 24.3 24. 3b 24. 3b a For positive lized MAL.
10’
a5
0.37 0 .37 0 0 .37 .67 0 0 1.20 0 0.50 0 .86 0 1.50 + O . 283 1.20 .205 1.20 .093 1.20 .023 1.20 0.000 1.20 .048 1.20 -091 1.20 values, a = (HCIOr)/Cp, for
+ + +
-
PH
Aesoo
Aeroo
58.1 39.1 3.83 54.3 37.2 3.64 34.2 3.48 48.6 45.2 33.4 3.33 43.2 33.8 3.23 46.4 35.3 3.65 41.3 32.4 3.37 33.4 26.4 3.23 2.59 12.2 13.6 17.7 18.5 2.66 29.7 27.0 2.88 40.1 32.3 3.10 43.2 33.2 3.21 38.3 3.67 51.4 59.4 38.3 4.43 negative values, a = -(NaOH)/Cp.
studies, the fraction of copper CYAchelated with one amino acid and the fraction L Y ~ chelated A with two amino acid groups were calculated from LYA = 0.023 A6600 - 0.0123 Ae7w and CYzA = 0.0436 ALEBOO 0.0225 Ae7oo. The results show that the ratio of the monochelate and the dichelate concentration is quite insensitive to the concentration of PMAL or the copper/polymer ratio. The observed chelate distribution with PMAL is compared in Fig. 1 with values calculated for equivalent concentrations of valine showing the striking displacement of the dichelate stability range to lower p H when the amino acid groups are attached to the polymer. Discussion For several of the PMAL solutions studied, the sum of the calculated concentrations of monochelate and dichelate exceeded the stoichiometric copper concentration, pointing to the presence of very strongly absorbing complexes which were not taken into account. There are two possible explanations of this discrepancy: (1) Penta-coordinated Cu(I1) complexes with very high extinction coefficients are formed in concentrated ligand solutions”-14 and it is not unlikely that the local concentration of amino acid groups in the polymer coil leads to the formation of higher chelates than those observed in solutions of simple amino acids. The phenomenon would be analogous to the formation, with poly-(acrylic acid), of copper chelates higher than those which are formed in solutions of monocarboxylic acids.3~4 (2) Copper forms very strongly absorbing chelates with diglycine and triglycine which involve ionized peptide groups. Again, the high local -CONH- concentration in PMAL may lead to this type of complex, although it was not observed in solutions of BL. The effect would be (11) J. Bjerrum, Chsm. Revs., 46, 381 (1950). (12) J. Bjerrum and E. J. Nielsen, Acta Chem. Scand., 2, 297 (1948). (13) J. Bjerrum, C. J. Ballhausen and C. K. Jplrgensen, ibid., 8, 1275 ClQ64). (14) H. B. Jonasaen, R. E. Reeves and L. Segal, J . Am. Chem. Soc., 77, 2748 (1955). (15) H. Dobbie, W. 0. Kermack and H. Lees, Biochsm. J . , 59, 246, 257 (1955).
PA
QzA
1359
udal
0.39 0.90 2.3 2.1 .39 .84 1.9 .39 .74 1.5 .43 .67 1.2 .49 .61 1.4 * 49 .67 1.2 .48 .59 .39 * 47 1.2 0.4 .32 .13 .40 .20 0.5 .50 .38 0.8 -50 .56 1.1 1.3 .47 .62 .50 .75 1.5 .32 -94 2.9 Polymer obtained from uncrystal-
expected to increase with rising p H and therefore only data obtained below p H 5 are reported. The relatively high stability of the dichelate at low p H and the insensitivity of the C Y ~ A / ~ ratio A to variations in PMAL concentration reflect the high ligand concentration within the swollen polymer coil. If the amino acid groups of two neighboring monomer units are involved in the formation of a dichelate, a 20-membered ring is produced. The probability of forming such a ring can differ only slightly from that of, e.g., the 22-membered ring formed by cooperation of next but nearest monomer units and, in fact, a wide range of ring sizes must make a comparable contribution to the spectroscopically observed dichelate. Under these circumstances the “effective local concentration” C ~ Rof ~amino ~ , acid ~ ~groups is the factor controlling the equilibrium between the monochelate and the dichelate concentrations. When the net charge of the polymer is small, we may neglect its effect on the ionization and chelate formation equilibria. Since most amino acid groups are zwitterionic, the fraction present in the basic form is K z / ( H + ) and where K ~ is A the second chelate formation constant of Cu(I1) with an analog amino acid. This leads to Ceffvalues of around 1 mole/liter. The mean concentration of interacting groups within the volume of a sphere which is hydrodynamically equivalent to the polymer coil (V,) has previously been suggested16 as an approximate measure of Gee. Using Ve = 0.14R3 where R is the root mean square displacement of the chain ends18 the light scattering data lead to a mean concentration of 0.056 N monomer units within the polymer coils. This is more than an order of magnitude below the Ceffcalculated from spectral data, indicating that the equilibrium between monochelate and dichelate is determined by the interaction of amino acid groups spaced relatively close to each other in (16) 8. Y.Chmg and H. Moraweti, THISJOURNAL, 60, 782 (1956). (17) H. Moraweti, J . PolVmcr Soi., 2S, 427 (1947). (18) P. J. Flory, “Principles of Polymer Chemistry,” Cornell University Press, Ithaca, N. Y..1953, pp. 605-611.
FRANK E. HARRISAND STUART A. RICE
1360
the chain molecule. When random flight statistics of chain configuration are applicable, Kuhn has shown1g that the probability of ring closure is inversely proportional to the 2/2 power of the chain length. This leads to the estimate that half of the dichelates formed in PMAL involve amino acid groups separated by less than 30 monomer units. The insensitivity of the chelate distribution t o variations in the copper concentration when PMAL is in large excess, indicates that there is little interference between regions of the polymer chain participating in the complexing of different Cu(I1) ions. At a lower excess of polymer such interference would undoubtedly work against the formation of the dichelate, but such conditions could not be studied since they led to polymer precipitation. The experiments in which the degree of neutralization of PMAL was varied, corresponded to a net charge per monomer unit from 2 = $0.01 to 2 = +0.22.20 Since the charged groups are placed at the end of long side chains attached to the polymer
-
(19) W. Kuhn, KollOid-Z., 68, 2 (1934). (20) 2 a [2Cw (H+)]/Cp where Cub ia the concentration
+
-
of bound copper ion.
Vol. 61
backbone the electrical field due to the polymer is relatively weak. It has, therefore, a correspondingly small effect on the equilibria involving the polyion and its counterions, with the apparent first ionization constant of the amino acid groups being increased only by a factor of 2 at the highest positive charge of PMAL which was investigated. Nevertheless, the electrostatic free energyz1 associated with the displacement by a Cu(I1) ion of the proton of a zwitterionic amino acid group attached to a polymer bearing a net positive charge should hinder the formation of the monochelate. It is hard to see why PMAL seems to form this complex more readily than valine (Fig. 1). On the other hand, in the coordination of a second amino acid group to a monochelate carried by the polymer, protons are lost by the PMAL and a net positive charge of the polymer should favor, on electrostatic grounds, the dichelate formation. A slight tendency for C,B to increase with 2 was observed, but the uncertainties in the interpretation of the spectral data preclude a quantitative evaluation of thiti effect. (21) A. Katchalsky and J. Gillis, Reo. ~ W U . chim., 88, 879 (1949).
A MODEL FOR ION BINDING AND EXCHANGE I N POLYELECTROLYTE SOLUTIONS AND GELS BYFRANK E. HARRIS AND STUART A. RICE^ Department of Chemistry, University of California, Berkeley, California, and Department of Chemistry, Harvard University, Cambridge, Massachusetts Received February $6, 1967
A molecular model for linear and cross-linked polyelectrolytes is described. The model emphasizes the effect of interactions between neighboring charged groups upon both configurational and thermodynamic properties of the polymeric systems. Ion binding is introduced in a ,phenomenological manner, and it is shown that the model predicts far larger amounts of binding to polymers than to small molecules containing similar functional groups. It is found that ion binding is necessary to explain the configurational properties and titration curves of linear polyelectrolytes. Moreover, equilibria among ion pairs and unbound ions are shown to provide a means for understanding of the variation of ion-exchange resin selectivity with cross-linking, exchange capacity and the composition of the solution in contact with the resin.
Recent years have seen much progress toward an understanding of the configurational and titration properties of linear polyelectrolytes in solution. This progress has evolved from the synthesis of several concepts, each of which was originally developed for use elsewhere. A current picture of a polyelectrolyte solution includes consideration of the statistics of coiled chains, of electrostatic interactions between ions in solution, and, of particular interest a t this time, of binding of counter ions t o the polymer. Concurrently, many investigators were studying ion exchange in polyelectrolyte gels. The more comprehensive efforts toward elucidation of the exchange phenomena were not from a microscopic structural point of view, but instead treated the gel as a macroscopic phase of suitable thermodynamic properties. The authors have recently shown, however, how the theory of linear polyelectrolytes can be extended to cross-linked polyelectrolyte gels. They found that a consideration of the binding phenomena in the gels leads naturally to a description of their ion-exchange (1) Society of Fellows, Harvard University.
properties. It is the purpose of this paper to survey the methods by which a simple molecular model has been applied to both linear and cross-linked systems, and t o indicate, without mathematical detail, how the assumed model leads to a relatively satisfactory reproduction of many experimental observations. I. Linear Polyelectrolytes.-A basic step in the study of linear polyelectrolytes was taken in 1948 by Kuhn, Kunzle and Katchalsky,a who described the configuration of a polyion as that of a chain which was randomly coiled with the restriction that its mean end-to-end distance be such as to minimize the sum of the configurational and electrostatic free energies. This simple model was able t o predict qualitatively the size changes accompanying changes in charge on a polymer. How(2) Summaries of experimental data and comparison with this model appear in ref. 8 and 11. Much general information is given in: P. Doty and 0. Ehrlich. Ann. Rev. Phus. C h e n . , 8, 81 (1952), and P. J. Flory, “Prinoiples of Polymer Chemistry,” Cornell University Press, Ithaca, N. Y.,1953. (3) W. Kuhn. 0. Kunzle and A. Katchalsky, Hslv. Chim. Acta, 81, 9419 (1948).
