J. Phys. Chem. 1981, 85,3367-3369
3367
Solutlon Structures of Cadmlum-Glycine Complexes Probed by Cadmium-113 NMR of Supercooled Aqueous Solutions Hans J. Jakobsen"' Department of Chemistry, University of Aarhus, 8000 Aarhus
C,Denmark
and Paul D. Ellis" Department of Chemistry, Unlverslfy of South Carolina, Columbia, South Carolina 29208 (Received: August 18, 198 1)
l13Cd-16N spin-spin coupling constants have been observed for the individual complexes of the aqueous cadmium-glycine system, Le., CdLGly+, Cd"-Glyz, and Cdn-Glyc. These coupling constants were determined at -40 "C from supercooled aqueous emulsions of Cd(C10& and lSN-1abeled glycine by employing natural abundance '13Cd NMR. The results have thrown new light on the solution structures of these complexes and have allowed unambiguous assignment of the l13Cd resonances observed for this system upon supercooling. Cadmium-113 NMR spectroscopy, which has become a valuable tool in inorganic and bioinorganic chemistry, owes its applicability in biological studies to the ability of cadmium to replace calcium and zinc in those systems. This is reflected by a number of '13Cd NMR studies of metalloproteins.2 Recently we demonstrated that detailed information on the solution structures of cadmium complexes may be obtained from l13Cd spin-lattice relaxation parameters, chemical shifts, and spin coupling ons st ants.^ Furthermore, solid-state l13Cd NMR, with and without magic-angle spinning, proves to be of equal importance as the liquid-state NMR techniques. This is evidenced by recent studies on solid-state structures of inorganic cadmium sa1ts,4i5metalloproteins,6 and in combination with anisotropic motion studies in the liquid state.7 However, application of metal nuclide NMR to aqueous based inorganic solution chemistry is often hampered by chemical exchange among labile metal complexes. When the chemical exchange is fast or intermediate on the NMR time scale at ambient temperature, a detailed characterization of the NMR parameters for the individual metal sites is prevented. This is caused by the observation of only a single time-averaged (sharp or exchange-broadened) resonance and because low-temperature studies are limited to around 0 "C. Employing supercooling of aqueous solutions with temperatures down to ca. -50 "C, the Ackerman's6 recently achieved the slow-exchange limit of the NMR time scale for various labile inorganic complexes and (1) Visiting professor at South Carolina Magnetic Resonance Laboratory, University of South Carolina, 1981. (2)For a collection of references prior to 1980 see ref 3. D. B. Bailey, P. D. Ellis, and J. A. Fee, Biochemistry, 19,591 (1980);A. R. Palmer, D. B. Bailey, W. D. Benhke, A. D. Cardin, P. P. Yang, and P. D. Ellis, ibid., 19,5063(1980);B. R. Bobsein and R. J. Myers, J. Am. Chem. Soc., 102, 2453 (1980);J. D. Otvos and I. M. Armitage, Biochemistry, 19,4031 (1980);A. J. M. S. Uiterkamp, I. M. Armitage, and J. E. Coleman, J. Biol. Chem., 255,3911 (1980);B. R. Bobsein and R. J. Myers, ibid., 256,5313 (1981);J. D. Otvos and I. M. Armitage, R o c . Nutl. Acud. Sci. U.S.A., 77, 7094 (1980);N. B. H.Jonsson, L. A. E. Tibell, J. Erelhoch, S.J. Bell, and J. L. Sudmeier, ibid., 77,3269 (1980);J. L. Sudmeier, S. J. Bell, M. C. Strom, and M. F. Dunn, Science, 212,560 (1981). (3)C. F. Jensen, S. Deshmukh, H. J. Jakobsen, R. R. Inners, and P. D. Ellis, J. Am. Chem. Soc., 103,3659 (1981). (4)J. J. H. Ackerman, T. V. Orr, V. J. Bartuska, and G. E. Maciel, ibid., submitted for publication. (5)T.T.P. Cheung, L. E. Worthington, L. E. Murphy, P. DuPois Murphy, and B. C. Gerstein, J. Magn. Reson., 41,158(1980);P.DuPois Murphy and B. C. Gerstein, J. Am. Chem. SOC.,103,3282 (1981). (6)R. R. Inners, P. D. Ellis, and J. D. Potter J. Am. Chem. SOC., submitted for publication. (7)P. D. Ellis, R. R. Inners, and H. J. Jakobsen, J. Am. Chem. Soc., submitted for publication. (8) M. J. B. Ackerman and J. J. H. Ackerman, J.Phys. Chem., 84,3151 (1980). 0022-3654/81/2065-3367$01.25/0
TABLE I: l13Cd Chemical Shiftsa and '13Cd-lSN Coupling Constantsb for Cd11-'5N-Gly,(*n)+ Complexes Supercooled to - 40 " C [ I5N[Cd(II)I,C G l y l , Cd I1 "free" Cd -Gly+ CdII-Gly, Cd" -Gl y -
h'
M 0.10 0.10 0.10 0.10
M
PH 0.25 7.0 0.25 7.0 0.25 7.0 1.00 8.0
6 ('13Cd), 'J('"Cd-
ppm
15N),Hz
-35.9 53.6 153.9 262.8
170 165 >140d
'13Cd chemical shifts (+1 p p m ) are referenced to 0.1 In Hz with an estimated e r r o r o f f 5 Hz. Total c o n c e n t r a t i o n o f Cd(I1) using Cd(ClO,),. Minimum value o f t h e coupling c o n s t a n t extracted f r o m t h e two inner lines of the exchange-broadened q u a r t e t observed f o r solution 111.
M Cd(ClO,), at 25 "C.
thereby observed separate NMR resonances for the various complexes in solution. The technique, demonstrated for the cadmium-glycine system, is based on the method introduced by Rausmussen and McKenzieg using aqueous emulsions for supercooling aqueous solutions. However, details on the solution-state structures for the various Cd'I-glycine complexes are still lacking despite several investigations of this system both in solution and the solid state (IR,l0 Raman,l' NMR,12 and X-ray13). The X-ray crystal structure study13of Cd(NH2CH2C00)2.H20reveals octahedral metal coordination with the two glycine ligands chelating the metal through the nitrogen and oxygen atoms in a trans planar configuration, while the two other coordination sites are occupied by oxygens of neighboring glycine ligands. For the solution-state structures, coordination through the nitrogen atom is generally assumed in accord with a recent l13Cd NMR study of Cd"-EDTA ~omplexes.~ On the other hand, evidence for monodentate coordination through the carboxylic oxygen atoms of the glycine zwitterion has also been presented.l' This lack of details on the solution-state structures and stoichiometries (9)D. H.Rasmussen and A. P. McKenzie in "Water Structure at the Water-Polymer Interface", H. H. G. Jellinek, Ed., Plenum Press, New York, 1971,pp 126-144. (10)A.J. Saraceno, I. Nakagawa, S. Mizushima, C. Curran, and J. V. Quagliano, J. Am. Chem. Soc., 80,5018(1958);T.J. Lane, J. A. Durkin, and R. J. Hooper, Spectrochim. Acta, 20, 1013 (1964). (11)K. Krishnan and R. A. Plane, Inorg. Chem. 6,55 (1967). (12)R. J. Kostelnik and A. A. Bothner-By, J . Magn. Reson., 14,141 (1974);I. M.Armitage, R. T. Pajer, A. J. M. S. Uiterkamp, J. F. Chlebowski, and J. E. Coleman, J. Am. Chem. SOC.,98,5710 (1976);B. Birgersson, R. E. Carter,and T. Drakenberg, J. Magn. Reson., 28,299(1977). (13)B. W.Low, F. L. Hirshfeld, and F. M. Richards, J. Am. Chem. SOC.,81,4412 (1959).
0 198 1 American Chemical Society
3368
The Journal of Physical Chemistry, Vol. 85, No. 23, 1081
I
200
loo
"
'
ppm
I
"
'
0
Figure 1. Natural abundance l13Cd NMR spectra of -40 OC supercooled aqueous (D20/H20, 1:l) solutions of 0.1 M Cd(C10,)2 and 0.25 M glycine in 5 M NaN03 at pH 7.0. (a) Obtained by using 95% 16Nenriched glycine and 2300 transients; expansions of the doublet and triplet for resonances C and D are inserted. (b) Obtained by using isotoplcally normal glycine and 1000 transients. Gated 'H decoupllng and a line broadening of 20 Hz has been applied for both spectra.
severely hampered the assignment of the various l13Cd resonances observed by the Ackerman's upon supercooling solutions of Cd"-glycine complexes. We wish to report that, in addition to unravelling the chemical shift, important spin-spin coupling information (e.g., l13Cd-15N) may also be derived employing the approach of supercooled aqueous emulsions. This is demonstrated here for the Cd'Lglycine system by using 15N-labeledglycine and has provided new unequivocal information on the solution state structures and configurations of these complexes together with an unambiguous assignment for the '13Cd resonances observed at low temperatures. Emulsions of the aqueous Cd"-glycine solutions were prepared according to Rasmussen and McKenziegfollowing the modifications described re~ent1y.l~Three different aqueous CdE[l5N]glycine solutions, all 0.1 M in Cd(C104)z and 5 M in NaN03, were employed. The [15N]glycine concentrations and pH of these solutions were as follows: (I) 0.25 M [15N]glycine,pH 7.0; (11)1.0 M [15N]glycine,pH 9.0; (111) 1.0 M [15N]glycine,pH 8.0. The l13Cd NMR spectrum (88.75 MHz)15 of the emulsion for solution I obtained at a temperature of -40 "C is depicted in Figure la. Also shown (Figure lb) is the l13Cd NMR spectrum obtained by using isotopicallynormal glycine and otherwise (14)J. S.Thompson, H. Gehring, and B. L. Vallee, Proc. Natl. Acad. Sci. U.S.A., 1977,132(1980);C. Balny and P. Douzou, Biochinie, 61,445 (1979). (15)W d NMR spectra were obtained at 88.75 MHz on a Bruker WH-400superconducting spectrometer with 10-mm 0.d. tubes. Gated 'H decoupling with suppression of the "3Cd-('H} nuclear Overhauser effect was applied throughout. Internal deuterium (D,O)field/frequency lock was employed. Typical acquisition parameters were as follows: spectral width, 38 kHz; acquisition time, 0.43 s; repetition time 1 s; pulse width, 15 ps (75O flip angle); number of transients, 500-2500.
Letters
identical conditions. The l13Cd chemical shifts for the three resonances observed (Table I) are similar to those reported for the peaks A, C, and D in the work by the Ackerman's.8 Furthermore, the resonances C and D in Figure l a show distinct multiplet structure attributable to l13Cd-15N spin-spin coupling. From the 1:l doublet, J(l13Cd-15N)= 170 f 5 Hz, and 1:2:1 triplet, J(113Cd-16N) = 165 f 5 Hz, observed for the resonances C and D, respectively, we conclude that (i) resonances C and D originate from the species Cd"-Gly+ and CdII-Gly,, respectively, and A from "free" Cd2+;(ii) cadmium coordination for C and D involves (at least) the formation of a Cd-N bond for these species in solution. The latter conclusion is inferred from a comparison of the observed l13Cd-16N coupling constants with the magnitudes obtained to date for one-bond l13Cd-15N couplings in two other Cd(I1 complexes. For cadmium [15Nz]ethylenediaminetetraacetic acid (Cd"-EDTA) IJ(113Cd-15N)I = 81 f 1 Hz has been reported,16 while for the pyridine adduct of cadmium [15N4]meso-tetraphenylphorphyrin both the magnitude and sign have been determined for 'J(l13Cd-16N) = + 150.1 Hz.17 Interestingly, the magnitudes of the l13Cd-16N couplings for the Cd"-Gly+ and Cd"-Gly, complexes are approximately a factor of two larger than the value for CdILEDTA. The Cdn-Gly2 complex in the solid state is planar, with bridging carboxylate groups, and with the two glycine ligands chelating the metal in a trans planar config~rati0n.l~ However, on the basis of Raman studies it has been suggested that CdII-Gly,, like ZnII-Gly,, is tetrahedral in solution.ll Whether the difference in the one-bond l13Cd-15N couplings for Cd'I-EDTA and Cd"Gly2reflects (i) a change in the nitrogen configuration from cis to trans planar in going from CdILEDTA to Cd"-Gly, (i-e.,the solid-state octahedral coordination configuration is preserved in solution), (ii) a conversion from the solidstate octahedral coordination for CdWly, to a tetrahedral coordination sphere in solution, or (iii) other factors influencing lJ('13Cd-15N), are unresolved questions that will have to await the results of future studies. In this respect a comparison with solid-state '13Cd NMR data for the Cd"-glycine complexes should be useful. The l13Cd NMR spectrum of the emulsion for solution I1 obtained at -40 "C gives raise to a single rather poorly resolved multiplet with a chemical shift (6 = 262.8 ppm) identical with that observed for resonance by the Ackerman's.8 On the basis of our results for solution I we assign this resonance to the Cdrr-Gly3- complex; i.e., the " T d resonance is expected to show multiplet structure corresponding to l13Cd-15N spin-spin coupling with three 16N isotopes. However, the total width (-195 Hz) and the observed splittings (-50-60 Hz) for this resonance are not in accord with the l13Cd-15N couplings obtained for C and D from solution I. This indicates that the [15N]glycine ligands for CdILGly3- in solution I1 are most likely undergoing intermediate intermolecular chemical exchange on the NMR time scale with excess free ligand. This hypothesis was confirmed by examining the l13Cd spectrum of an identical solution at a slightly lower pH (pH 8.0, Le., solution 111. In the l13Cd spectrum of solution I11 at -40 "C both resonances D and E show up. However, at this pH resonance E appear as an exchange-broadened 1:3:3:1 quartet (or two overlapping exchange-broadened 1:2:1 triplets) the total width of the multiplet being 400-450 Hz and the separation between the two exchange-broadened (16)R. Hagen, J. P. Warren, D. H. Hunter, and J. D. Roberts, J. Am. Chem. SOC.,95,5712 (1973). (17)P.D.Ellis, R. R. Inners, H. J. Jakobsen, and C. F. Jensen, to be submitted for publication.
J. Phys. Chem. 1981, 85, 3369-3371
inner lines being 140 f 10 Hz. In addition, resonance D for the CdLGly, species at pH 8.0 no longer appear as the well-resolved triplet in Figure l a but rather as a broadened featureless resonance of width 350-400 Hz. The multiplet structure for resonance E provides direct proof for its assignment to the CdILGly3- complex. Furthermore, at the higher pH values which is required for the observation of resonance E, the exchange rate of glycine ligand “in and our” of the Cd’I-GlyF and CdILGly2 complexes at -40 “C becomes comparable to J(’13Cd-15N). At lower pH (e.g., pH 7) resonance E cannot be observed even with a 10-fold excess of glycine. Obviously, the intermolecular exchange rate of ligands for solution I11 is slow enough on the l13Cd chemical shift scale (in our case -9700 Hz) at -40 “C to allow the detection of separate resonances D and E. Finally, our observation of the increasing ligand exchange rate for the Cd”-Gly2 and Cd”-Gly,- complexes with increasing pH may account for the rather broad D and E resonances observed at 22.09 MHz and -50 “C by the Ackerman’s (Figure 2, VI).8 In conclusion, l13Cd-16N spin-spin coupling constants appear promising for probing the aqueous solution-state structures of metal-amino acid complexes. In cases of rapid or intermediate exchange among labile complexes
3389
these parameters may be obtained from supercooled aqueous emulsions by 15N-labeledamino acids. This approach also proves useful in probing the environment and configuration of the metal binding site($ in studies of metal-peptide interactions and metalloproteins using specifically 15N-labeledpeptides/proteins. Furthermore, combining the results of such solution studies (e.g., for the Cd’I-glycine system) with solid-state l13Cd NMR investigations and X-ray crystal structure data should allow determination of any differences between the solution and solid-state structures of individual complexes. Finally, the structural role of water in the solution-state structures of metal complexes is often unknown. Such information may be derived from l13Cd NMR relaxation studies as has been recently demonstrated for Cd’I-EDTA.,
Acknowledgment. The authors gratefully acknowledge support of this research by the NATO Science Affairs Division, Brussels (Research Grant No. 1831),the Danish Natural Science Research Council (J. No. 11-2147), and the National Institute of Health (GM26295). The use of the facilities at the University of South Carolina Regional NMR Center, funded by the National Science Foundation (CHE78-18723), is acknowledged.
Ion-Pair Formation in Multiphoton Fragmentation N. Ohmlchi, J. Sllbersteln, and R. D. Levlne” The Fritz Haber Research Center for Molecular Dynamics, The Hebrew University, Jerusalem 9 1904, Israel (Recelved: August 11, 198 1)
Computational examples and physical considerations suggest that, in the statistical limit, ion-pair formation should be an important mechanism for the appearance of ions in the post-threshold region. This is particularly the case when the threshold energy for ion-pair formation is comparable or even lower than the first ionization potential. As the mean energy absorbed by the molecule increases, the ionization mechanism rapidly increases in importance. Deviations from the statistical limit are expected to favor the ionization pathway.
The formation of ionic fragments following multiphoton excitation (using lasers in the visible or UV spectral range) is currently thought to proceed via the parent molecular ion as an intermediate.14 The “two-color” e~perimentsl-~ and the dependence of the extent of fragmentation on the absorption cross section of the parent ion5 have lent considerable support to this mechanism. For many molecules the ionization potential is comparable to or even higher than the threshold energy for ion-pair formation. It is therefore reasonable to enquire whether ion pairs can be formed to any significant extent, as compared to ionization. We report computational results (and their interpretation) which show that in the statistical limit ion-pair formation is quite facile in the post-threshold region. Examples where essentially all ions in the threshold region are pro(1)U. Boesl, H.J. Neusser, and E. w. Schlag, J. Chem. Phys., 72,4327 (1980). (2) K.R.Newton, D. A. Lichtin, and R. B. Bernstein, J. Phys. Chem., 85,15 (1981). (3)D.M. Lubman, R. Naaman, and R. N. Zare, J. Chern. Phys., 72, 3034 (1980). (4)K.L. Kompa in “Lasers and Applications”, W. 0. N. Guimaraes, C. T.Lin, and A. Mooradian, Ed., Springer, Berlin, 1981,p 182. (5) D. H. Parker, R. B. Bernstein, and D. A. Lichtin, J. Chem. Phys., in press.
duced as pairs will be provided below. The importance of the ion-pair mechanism does, however, decline quite rapidly as the energy absorbed per molecule increases. Eventually, all ions are produced via the ionization pathway. I t is reasonable to expect a statistical approach to overestimate the branching ratio in favor of ion-pair formation. To see this consider the reasons why ion-pair formation may be disfavored. The first argument which is usually brought forward is that the ion-pair correlates with a particular electronic state of the parent. Hence, only excitation of that state will lead to ion pairs. This argument is, however, somewhat questionable even for single-photon excitation of diatomics, due to the extensive curve crossings between the ionic and covalent curves! The recent “alternative doorway state’’ test’ suggesta that, for multiphoton excitation of polyatomics, energy is indeed remarkably scrambled in the manner suggested by the statistical t h e ~ r y . ~That , ~ test, however, necessarily probes (6) M. B. Faist and R. D. Levine, J. Chem. Phys., 64,2953 (1976);J. J. Ewing, R. Milstein, and R. S. Berry, J. Chem. Phys., 54,1752(1971). (7)D.A. Lichtin, R. B. Bernstein, and K. R. Newton, J. Chem. Phys.,
in press. (8)J. Silberstein and R. D. Levine, Chern. Phys. Lett., 74,6 (1980).
0022-3654/81/2085-3369$01.2510 0 1981 American Chemical Society