J. Phys. Chem. 1983,
2342
behavior. On the other hand, it has been shown that limited 170enrichment, i.e., sufficient to provide experimental convenience, but avoiding domination of proton relaxation by the lH-170 dipolar mechanism, provides important additional information to the case where proton
and deuteron relaxations are measured only, thus overcoming the above-describedambiguities be tween isotropic and anisotropic relaxation in the majority of situations. Registry No. Water, 7732-18-5.
Nuclear Magnetic Relaxation of Solvent Nuclei in Concentrated Aqueous Poly(methacrylic acid) Solutions C. W. R. Mulder, J. Schriever, W. J. Jesse, and J. C. Leyte' Gorlaeus Laboratories, Department Of Physical Chemistry, University of Leiden, 2300 RA Leiden, The Netherlands (Received: October 18, 1982)
The nuclear magnetic relaxation times of the water nuclei, i.e., 'H, 2H, and 170,and the polymer protons have been measured in concentrated, aqueous, fully neutralized solutions of three isotopically different forms of poly(methacry1icacid) (PMA, fully protonated, CD2deuterated, and fully deuterated). From these results it could be shown that cross relaxation between the polymer and water proton fractions is not important in the systems investigated. The relaxation rates of the three nuclei of the water molecules have been used to investigate the solvent microdynamics using isotropic and anisotropic models for the water motion in the concentrated polyelectrolyte solutions.
Introduction In this paper results are presented of NMR experiments on concentrated aqueous polymer solutions. We shall consider these results in relation to two aspects: spin diffusion or cross relaxation between polymer and solvent protons and the dynamic behavior of the solvent. The possibility of deciding on the isotropic or anisotropic nature of the solvent reorientation is examined in particular. In biological water systems many studies1-12suggest the possible importance of the "spin-diffusion" process. It is often difficult to make a distinction between cross relaxation, which is the result of direct magnetic interaction between two magnetically different nuclei (e.g., polymer and water protons) or chemical exchange by which process protons of magnetically different surroundings are exchanged physically by a chemical reaction (e.g., water protons are exchanged with the protons of the carboxylic acid or amino groups of a polymer). To distinguish clearly between cross relaxation and chemical exchange, all experiments were carried out on fully neutralized polymer solutions, so that chemical exchange between exchangeable polymer protons (Le., the carboxylic groups of PMA) and water protons would be negligible. To find evidence for cross relaxation an investigation (1) E. Hsi, G. J. Vogt, and R. G. Bryant, J. Colloid Interface Sci., 70, 338 (1979). (2) B. D. Sykes, W. E. Hull, and G. H. Snyder, Biophys. J., 21, 137
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(3) B. D. Hilton, E. Hsi, and R. G. Bryant, J. Am. Chem. SOC.,99,8483 (1977). (4) B. Benko, V. Buljan, and S.Vuk-Pavlovic, J. Phys. Chem., 84,913 (1980). ( 5 ) L. J. Lynch and D. S.Webster, J. Magn. Reson., 40, 259 (1980). (6) A. Kalk and H. J. C. Berendsen, J. Magn. Reson., 24,343 (1976). (7) S.H. Koenig, R. G. Bryant, K. Hallenga, and G. S.Jacob, Biochemistry, 17, 4348 (1978). (8) B. M. Fung and T.W. McGaughy, J . Magn. Reson., 39,413 (1980). (9) G. Valensin and N. Niccolai, Chem. Phys. Lett., 79, 47 (1981). (10) H. T. Edzes and E. T.Samulski, J. Magn. Reson., 31,207 (1978). (11) P. J. Andree, J. Magn. Reson., 29, 419 (1978). (12) J. D. Stoesz and A. G. Redfield, FEBS L e t t . , 91, 320 (1978).
0022-365418312087-2342$0 1.50lO
was carried out a t two field strengths. The proton relaxation rates were studied in concentrated aqueous solutions of poly(methacry1icacid) (PMA)and these were compared with the relaxation times in solutions of fully protonated, partially deuterated, and fully deuterated PMA. Indications of cross relaxations were found to be only just outside the experimental error. The important enhancement up to a factor of 3 of the solvent relaxation rates therefore indicates a reduction of the reorientation rate of the water molecules. An unequivocal conclusion as to the isotropy or anisotropy of the water molecules cannot be reached now. Using the theoretical results of the preceding paper, we show that eventual anisotropy in the solvent reorientational motion is insufficient to be detected by comparison of the 'H, 2H, and 170nuclear relaxation rates.
Experimental Section The three types of poly(methacry1ic acid), i.e., fully protonated (PMA), partially deuterated (CD2-PMA),and fully deuterated (per-D-PMA), were homemade atactic samples with an estimated purity of at least 98% on the basis of accumulated NMR resonance spectra. The solvent was a mixture of H20,D20,and H2170(30% lH, 70% D, and 5.5% 170).The H 2 0 was distilled and deionized. D20was obtained from Aldrich Chemical Co., Milwaukee, WI (minimum 99.96% D), and the H2170from Monsanto Research Corp., Miamisburg (a fraction containing 43.4 mol% I7O). All compositions were prepared on the basis of weight. The standard Wilmad 10-mm NMR tubes used in the experiments were steamed for at least 10 min and vacuum dried. Polymer solutions were made of about 0.5 mg equiv/g which were then concentrated over a period of several days by way of evaporation in a vacuum oven (at room temperature and a pressure of about 1 cmHg in the presence of P205) to concentrations of approximately 1 mg equiv/g of solution) and in the second instance (i.e., after the first set of NMR experiments) to concentrations of about 1.9 mg equiv/(g of solution). The solutions were fully neu@ 1983 American Chemical Society
Nuclear Magnetic Relaxation of Solvent Nuclei
tralized with NaOD obtained from Merck, Darmstadt, BRD (40% NaOD in 99% D20). To prevent C02 contamination during evaporation the concentrating was carried out in a N2 atmosphere in the presence of NaOH pellets. By this method it was possible to combine concentrating and deoxygenating without any risk to the glassware or the solutions. NMR experiments were performed on a home-modified Bruker BKR pulse spectrometer with a maximum field of 1.4 T, corresponding to resonance frequencies of 60,9.21, and 8.13 MHz for H, D, and 170,respectively. Temperature was maintained at 25 f 0.3 "C by fluid thermostating. Longitudinal relaxation rates were measured by the inversion recovery method and transverse rates by spin-echo or, if possible, CPGM sequences. Accuracies are estimated to be about 3-5% for the relaxation rates of the water nuclei and about 5-20% for the relaxation rates of the polymer protons. Synthesis of the Poly(methacry1ic Acids). The methyl ester of methacrylic acid was synthetized by reacting the dimethyl ester of malonic acid with methyl bromide, followed by partial saponification of the double-ester and a Mannich reaction with f0rmaldeh~de.l~By using deuterated forms of methyl bromide and/or formaldehyde in the methyl methacrylate synthesis, we obtained the desired isotopic forms of the methacrylic ester. The isotopic purity of the various methacrylic esters was determined by comparing in the NMR spectra the proton signals of the CH2 and CH, groups with the proton signal of the methyl ester group. Purities were always above 98%. After saponification of the methyl methacrylate the methacrylic monomer was polymerized under N2in an aqueous solution with H202initiation14 at a temperature of 85 OC for a period of 4 h. The polymerization product was cooled down, freeze-dried, and fracti~nated,'~ followed by dialysis and freeze-drying. As the CD2-PMA samples were slightly yellow, this polymer was subequently dialyzed against an aqueous dispersion of Norit, after which noncolored CD2-PMA samples could be prepared. Molecular weights were determined by viscosity measurement. For the fractions used, this yielded M,,,(CD2-PMA)= 103000,16M*(per-D-PMA) = 133000," and M~,,(PMA)= 400000 (not fractionated). Theoretical Section In the preceding paper expressions were derived describing the nuclear magnetic relaxations in water which was enriched both in deuterium and in oxygen-17. The total proton relaxation is a combination of three relaxational contributions: (1)the contribution due to the intramolecular interactions modulated by rotational reorientation, (2) the contribution due to intermolecular interactions modulated by both rotational reorientation and translation, and (3) the contribution due to the scalar coupling with 170modulated by chemical exchange. Expressions for the proton relaxation will be given for the intramolecular contribution only. Assuming all relaxation to be in the extreme narrowing limit, Le., w 7
