2388
Inorg. Chem. 1983,22, 2388-2392
linear relationship. The only significant deviations are observed for Me3N-BF3. Further work is under way to investigate the significance of the apparent steric effects. In conclusion, therefore, we can add the measurement of boron-nitrogen couplings to the list of effective measures of the relative bond strength of adducts of the boron trihalides, but the nitrogen-15 chemical shift data do not reflect a measure of a simple electron density picture of the B-N bond. Thus, especially in view of the experimental difficulties of the I5N N M R experiment, it would appear to be of greater use and interest to measure J(lLB-I5N) from the l l B N M R spectra, which are much simpler to obtain. Acknowledgment. We thank the Natural Sciences and
Engineering Research Council of Canada for support of this work and the Southwestern Ontario N M R Centre at the University of Guelph for WH-4000 time. We also thank T. R. B. Jones for assistance with the WP-60 spectra and R. Geanangel for helpful discussions and for the XL-100 llB data. Registry No. Me3N.BF3,420-20-2;Me3N.BF2C1,25889-87-6; Me3N.BF2Br,25889-93-4;Me3N.BF21,25889-95-6; Me3N.BFC12, 25889-88-7;Me3N.BFBr2, 25889-94-5;Me3N.BCl3, 15 16-55-8; Me3N.BF12,25889-96-7;Me3N.BC12Br,25889-90-1; Me3N.BC121, 25889-97-8;Me3N.BC1Br2,25889-91-2; Me3N.BC1BrI, 39708-29-7; Me3N.BBr3, 1516-54-7;Me3N.BC112, 25889-98-9;Me3N.BBr21, 39708-24-2; Me3N.BBr12,39708-25-3;Me3N.B13,5041-59-8;lSN, 14390-96-6;I'B, 14798-13-1.
Contribution from the Departments of Chemistry, State University of New York at Stony Brook, Stony Brook, New York 11794,and University of Guelph, Guelph, Ontario NlG 2W1, Canada
Aqueous Shift Reagents for High-Resolution Cationic Nuclear Magnetic Resonance. 2. 25Mg,39K, and 23NaResonances Shifted by Chelidamate Complexes of Dysprosium(111) and Thulium(II1) MARTIN M. PIKE,+DAVID M. YARMUSH,t JAMES A. BALSCHI,t ROBERT E. LENKINSKI,$ and CHARLES S. SPRINGER, J R . * ~ Received October 26, 1982
The tris complexes of anions of chelidamic acid (H3CA, 4-hydroxypyridine-2,6-dicarboxylicacid) with dysprosium(111) and thulium(II1) have been tested as aqueous shift reagents for the NMR peaks of metal cations. The Dy(CA),& complex was observed to produce significant upfield isotropic hyperfine shifts of the 2sMg2+,3gK+,and 23Na+resonances (as well as that of *'Rb+ and the 14Nresonance of NH4+). The T I ~ ( C A ) ~complex + produces a downfield shift of the 23Na+peak. The shifts are produced by an interaction between cation and shift reagent anion, which is labile on the chemical shift NMR time scale. The shifts are strongly pH dependent due to protonation of the coordinated ligands. We have recently reported the development of aqueous shift reagents (SR) for metal cationic NMRLv2and have demonstrated their usefulness in bioinorganic chemistry, particularly in the study of transport of alkali-metal ions (Na+ and Li+) across model3 and real4 biological membranes. Transmembrane transport of the other physiological metal cations, K+, Mg2+,and Ca2+,is of course also important, and we wish to show that N M R can be used to study these processes as well. Here, we report improved S R and their effectiveness with natural-abundance 25Mgand 39Kas well as 23Na NMR. In a separate paper, we report their use in the study of transmembrane transport of Mg2+ and K+.5 The usefulness of various paramagnetic lanthanide ions for shifting and/or relaxating nuclear magnetic resonances in aqueous solutions has been known for more than a decade.6 Most subsequent studies have employed various lanthanide coordination complexes as aqueous hyperfine shift7-12or rel a x a t i ~ n ' J ~reagents -~~ and as aqueous susceptibility shiftL9or relaxation20 reagents. Of the resonances of the physiological alkali-metal or alkaline-earth-metal ions, only that of 23Nahas been the subject of specific hyperfine relaxation21.22or shiftingL*2,L2*23,24 and susceptibility shiftingL9experiments and these have employed paramagnetic lanthanide complexes. Our resultsLt and those of others23," indicated that increased charge on the anionic shift reagent was a major determinant of increased effectiveness: our best early SR were trianions,1*2 Elgavish and Elgavish have employed a p e n t a a n i ~ n ,and ~~ Gupta and Gupta, a h e ~ t a a n i o n .Thus, ~ ~ we have considered
State University of New York at Stony Brook. 'University of Guelph.
ways of increasing the charge of the complex. Chelidamic acid (H3CA, 4-hydroxypyridine-2,6-dicarboxylic acid) has a tridentate coordinating ability almost identical with that of diPike, M. M.; Springer, C. S. J . Magn. Reson. 1982, 46, 348. Balschi, J. A.; Cirillo, V. P.; le Noble, W. J.; Pike, M. M.; Schreiber, E. C.; Simon, S. R.; Springer, C. S. Rare Earths Mod. Sci. Technol. 1982, 3, 15. Pike, M. M.; Simon, S. R.; Balschi, J. A.; Springer, C. S . Proc. Natl. Acad. Sci. U.S.A. 1982, 79, 810. Balschi, J. A.; Cirillo, V. P.; Springer, C. S. Biophys. J. 1982, 38, 323. Yarmush, D. M.; Pike, M. M.; Balschi, J. A.; Lenkinski, R. E.; Springer, C. S., manuscript in preparation (for abstract, see: Biophys. J . 1982, 37, 337a). Morallee, K. G.; Nieboer, E.; Rossotti, F. J. C.; Williams, R. J. P.; Xavier, A. V. J . Chem. SOC.D 1970, 1 1 32. Dobson, C. M.; Williams, R. J. P.; Xavier, A. V. J. Chem. SOC., Dalton Trans 1974, 1762. Elgavish, G. A.; Reuben, J. J . Am. Chem. SOC.1976, 98, 4755. Elgavish, G. A.; Reuben, J. J . Am. Chem. SOC.1977, 99, 1762. Horrocks, W. D.; Hore, E. G. J . Am. Chem. SOC.1978, 100, 4386. Geraldes, C. F. G. C. J . Magn. Reson. 1979, 36, 89. Bryden, C. C.; Reilley, C. N.; Desreux, J. F. Anal. Chem. 1981, 53, 1418. Lettvin, J.; Sherry, A. D. J . Magn. Reson. 1977, 28, 459. Gansow, 0.A.; Kavsar, A. R.; Triplett, K. M.; Weaver, J. J.; Yee, E. L. J . Am. Chem. SOC.1977, 99, 7807. Gansow, 0.A.; Triplett, K. M.; Peterson, T. T.; Bolto, R. E.; Roberts, J. D. Org. Magn. Reson. 1980, 13, 71. Dechter, J. J.; Levy, G. C., J . Magn. Reson. 1980, 30, 207. Bearden, W. H.; Cargin, V. M.; Robersen, W. R. Org. Magn. Reson. 1981, 15, 131. Wenzel, T. J.; Ashley, M. E.; Severs, R. E. Anal. Chem. 1982.54, 615. Malloy, C. R.; Smith, T. W.; Delayre, J.; Fossel, E. T., personal communication. Brindle, K. M.; Brown, F. F.; Campbell, I . D.; Gratwohl, C.; Kuchel, P. W. Biochem. J . 1979, 180, 37. Degani, H.; Elgavish, G. A. FEBS Lert. 1978, 90, 357. Elgavish, G. A. Rare Earths Mod. Sci. Technol. 1982, 3, 193. Gupta, R. K.; Gupta, P. J . Magn. Reson. 1982, 47, 344. Elgavish, A,; Elgavish, G. A., personal communication.
0020-1669/83/1322-2388$01.50/00 1983 American Chemical Society
Inorganic Chemistry, Vol.’22,No. 17, 1983 2389
Shift Reagents for Metal Cationic NMR picolinic acid (H2DPA, pyridine-2,6-dicarboxylicacid),25but it has three ionizable protons as compared with two for H2DPA.26 The (DPA)3 complexes of the lanthanides, Ln(DPA)33-, are well characterized in the solid state2’ and in s ~ l u t i o n , ~and * - ~we ~ found the DY(DPA)~’anion to be a good SR for the ’Li and 23Na aquo cations.’ Thus, the possible DY(CA)~+anion would seem to be a good candidate as a SR for metal cationic resonances. Also, since three-compartment transport studies such as in some epithelial tissue preparationsa require both down- and upfield SR, we sought to prepare the Tm(CA)36- complex. According to dipolar hyperfine shift theory, analogous Dy(II1) and Tm(II1) complexes will always produce opposite shifts of the same substrate
I
.
3
* ’
I
acid equivalents added to Dy(CA):2
Experimental Section
3
Chelidamic acid (Aldrich) was recrystallized according to the method of Bag et aLZ6 The tris complex of Dy3+ (or Tm”) was prepared in situ by adding up to 9 equiv of LiOH slowly (buret), with stirring, to a stoichiometric aqueous dispersion of DyC1, (or TmCl,) (Alfa) and the insoluble H,CA. The synthesis is described by the reaction DyC1,
+ 3H3CA + 9LiOH
-
4
Li6Dy(CA), + 3LicI
The pH must be monitored continuously and never allowed to rise above 6 or 7 (even transiently) during the early stages lest Dy3+ hydrolysis and precipitation become a problem. After complex formation is complete (clear solution, obtained at pH ca. 8 or by warming at slightly lower pH values), the pH can be raised further (see below) provided the concentration of divalent metal cations is not too high. An in situ synthesis from DyzO3 that avoids the extra countercations, as we reported earlier for the bis(nitri1otriacetate) (NTA3-) complex of Dy3+,’has also been accomplished. Since this is a doubly heterogeneous synthesis (the oxide is also insoluble), it requires stirring and warming, even before base is added, for hours if not days. Compositions of solutions for NMR spectroscopy and for pH measurement are given in the text and figure captions. The NMR spectra were obtained on Varian XL-100 (2.35 T), Nicolet NT-300 (7.05 T), and Bruker WH-400 (9.40 T) spectrometers. These were always field-frequency locked on the 2H resonance of ZHzOpresent in the solvent, which thus served as an internal reference. The isotropic hyperfine shift, A, was measured as the difference of the observed resonance position from the resonance position of the cation in the absence of shift reagent. Upfield shifts are reported as positive. Field-frequency locking on the ZHresonance eliminates contributions to the observed values from bulk magnetic susceptibility. This correction would be imperfect if the 2H resonance of ZHzOitself suffered any hyperfine shift. However, this is expected to be very small since the molar ratio of SR:H20 never rises above 3 X lo-, in this work
(25) Thich, J. A.; Ou,C.-C.; Powers, D.; Vasiliou, B.; Mastropaolo, D.; Potenza, J. A,; Schugar, H. J. J . Am. Chem. SOC.1976, 98, 1425. (26) Bag, S. P.; Fernando, Q.; Freiser, H.Inorg. Chem. 1962, 1 , 887. (27) Albertsson, J. Acra Chem. Scand. 1970,34, 1213; 1972,26,985, 1005, 1023. (28) Grenthe, I. J. Am. Chem. SOC.1961,83, 360. (29) Grenthe, I.; Tobiasson, I. Acta Chem. Scand. 1963, 17, 2101. (30) Donato, H.;Martin, R. B. J. Am. Chem. SOC.1972, 94, 4129. (31) Desreux, J. F.; Reilley, C. N. J. Am. Chem. SOC.1976, 98, 2105. (32) Horrocks, W. D.; Sudnick, D. R. Science (Washington,D.C.) 1979,206, 1194. (33) ForSen, S. Eur. Con/. NMR Macromolecules 1978, 243. (34) Lloyd, D. A.; Wishnia, A.; Springer, C. S., submitted for publication. (35) Civan, M. M.; Shporer, M. In “Biological Magnetic Resonance”; Berliner, L. J., Reuben, J., Eds.; Plenum Press: New York, 1978; p 1. (36) Chrzeszczyk, A.; Wishnia, A.; Springer, C. S. Biochim. Biophys. Acta 1981, 648, 28. (37) Chu, S.; Pike, M. M.; Fossel, E. T.; Smith, T. W.; Balschi, J. A.; Springer, C. S., submitted for publication. (38) Becker, E. D. “High Resolution NMR”, 2nd ed.;Academic Press: New York, 1980. (39) Liao, M.-J.; Prestegard, J. H.Biochim. Biophys. Acra 1978, 550, 157. (40) Pike, M. M.; Springer, C. S., unpublished results. (41) Inagaki, F.; Miyazawa, T. Prog. Nucl. Magn. Reson. Spectrosc. 1981, 14, 67.
4:
,! d 5
2
4
8
6
IO
12
14
PH
Figure 1. pH dependenceof the isotropic hyperfine shift of 23Na(26.5 MHz, 2.35 T), AN,, by Dy(CA),& (0)and Dy(DPA),> ( 0 )(left-hand ordinate). The concentrationsof the shift reagents, L&Dy(CA)3.3LiC1 and Li3Dy(DPA),-3LiC1,were held constant at 20 mM. Sodium was present as the chloride, and the pH was adjusted by the addition of HC1 (except at very high pH values, where NaOH was added). The solutions were such that the sodium concentration was held constant at 100 mM below pH 8.8 in the Dy(CA),& titration and below pH 10.2 in the Dy(DPA),,- titration. (It rose smoothly to 130 mM at pH 12.1 in the DY(CA),~-titration and to 109 mM at pH 11.5 in the Dy(DPA),,- titration.) The chloride concentration varied from 230 mM at pH 2.5 to 160 mM at pH 8.0 and above in the Dy(CA),& titration and from 243 mM at pH 1.7 to 209 mM at pH 10.2 and above in the Dy(DPA),,- titration. The solvent was 40% DzO.The dashed curves are intended merely to guide the eye. Slight precipitation was noted only at the very basic end of the Dy(DPA),’- experiment and at the very acidic end of the DY(CA),~-experiment. The temperature was ca. 301 K. A simple pH titration of Dy(CA),& is also shown (dotted points, right-hand ordinate). The complex (Le., L&Dy(CA),-3LiCl)concentration is diluted from 82 to 22 mM during the titration with HC1. The temperature was ca. 296 K.
and since these SR have few, if any, inner-spherecoordination positions available for Hz0.32For similar reasons, the pH variation of Figure 1 does not itself alter the resonant frequency of the 2Hz02H peak. Other spectroscopic details are given in the figure captions.
Results
We sought first to test the efficacy of Dy(CA),“ as a shift reagent with 23Na,the most sensitive of the physiological metal cation magnetic nuclei.33 Figure 1 depicts the upfield hyperfine shift of the 23Na+resonance (left-hand ordinate), induced by the tris(che1idamate) complex of Dy3+, as a function of pH. The stoichiometric complex concentration is 20 mM over the entire p H range. The stoichiometric Na+ concentration is 100 m M over most of the p H range (
