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Logo of nihpaAbout Author manuscriptsSubmit a manuscriptHHS Public Access; Author Manuscript; Accepted for publication in peer reviewed journal;
 
Inorg Chem. Author manuscript; available in PMC 2010 August 3.
Published in final edited form as:
PMCID: PMC2744091
NIHMSID: NIHMS128533

Multifrequency EPR Studies of [Cu1.5Cu1.5]1+ for Cu2(μ-NR2)2 and Cu2(μ-PR2)2 Diamond Cores

Neal P. Mankadζ
Department of Chemistry, Massachusetts Institute of Technology, Cambridge MA 02139
Seth B. HarkinsI
Department of Chemistry, California Institute of Technology, Pasadena CA 91125
William. E. Antholineξ
Department of Biophysics, Medical College of Wisconsin, Milwaukee, Wisconsin 53226

Abstract

Multifrequency EPR spectroscopy is used to explore the electronic structures of a series of dicopper complexes of the type the {(LXL)Cu}2+. These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two [LXL]- ligands featuring bridging amido or phosphido ligands and associated thioether or phosphine chelate donors. Specific chelating [LXL]- ligands examined in this study include bis(2-tert-butylsulfanylphenyl)amide (SNS), bis(2-di-iso-butylphosphinophenyl)amide (PNP), and bis(2-di-iso-propylphosphinophenyl)phosphide (PPP). To better map the electronic coupling to copper, nitrogen, and phosphorus in these complexes, X-, S-, and Q-band EPR spectra have been obtained for each complex. The resulting EPR parameters implied by computer simulation are unusual for typical dicopper complexes and are largely consistent with previously published XAS and DFT data, where a highly covalent {Cu2(μ-XR2)2}+ diamond core has been assigned in which removal of an electron from the neutral {Cu2(μ-XR2)2} can be viewed as ligand centered to a substantial degree. To our knowledge, this is the first family of dicopper diamond core model complexes for which the compendium of X-, S-, and Q-band EPR spectra have been collected for comparison to CuA.

Keywords: EPR, CuA, mixed-valence, class III, redox-active ligands, dicopper, diamond core

Introduction

A long-standing subject of interest in the field of bioinorganic chemistry concerns the mode by which metalloenzymes mediate electron transfer (ET).i,ii This is especially true of enzymes that feature redox-active copper centers that serve as ET relays, such as the type 1 sites in blue copper proteins iii,iv,v and the CuA sites of cytochrome c oxidases and nitrous oxide reductases. To mediate rapid ET, the local geometry of the copper center(s) must undergo rather little structural change during a redox event so as to minimize the reorganizational term λ,vi which when large serves to attenuate rates of ET. Therefore it is of particular interest to understand how copper centers achieve such low structural reorganization in proteins, and by analogy in small molecule model complexes.vii,viii,ix,x One hypothesis that has been widely advanced is that of the `entatic' or strained state, whereby it is presumed that in order to minimize λ a protein matrix confers a geometry at the redox active site that is similar to the transition state for the ET reaction, a geometry (and function) that would not be prevalent in the absence of the protein matrix. This hypothesis was first advanced by Vallee and Williams,xi and various researchers, most notably Rorabacher and coworkers,xii have sought small molecule model systems to explore the viability of this approach.

An alternative explanation as to how copper-containing enzymes mediate rapid ET concerns their intimate electronic structures, and the role that metal-ligand covalency plays in providing stabilized electronic states, as opposed to a geometrically strained state. Solomon and Ryde have been amongst those advancing this idea,xiii both in type 1 copper enzymes and also in CuA in which short and highly covalent Cu-S bonds are prevalent.

Efforts in our laboratories have focused on the preparation of small molecule model complexes that feature the Cu2(μ̵XR2)2 diamond core motif.xiv xv,xvi,xvii These complexes contain two four-coordinate copper centers of highly distorted tetrahedral geometries linked by two bridging XR2- ligands (where XR2- = NR2- or PR2-), where the R groups provide neutral donor groups (e.g., thioether or phosphine) to complete the copper coordination spheres. To date, the three systems shown in Figure 1 have been reported.xiv,xvi,xv,xvii A common feature of each of these systems is a fully reversible one-electron redox couple between the reduced dicopper(I,I) state and a one-electron oxidized fully delocalized mixed-valence dicopper(1.5,1.5) state. In this regard, each system is a good functional model of CuA. Indeed, the rate of electron self-exchange can be very rapid (> 107 M-1 s-1), as has been estimated for the {(SNS)Cu}2 system by NMR line-broadening analysis ((SNS) = bis(2-tert-butylsulfanylphenyl)amide; see Figure 1).xiv X-ray diffraction studies have established that the overall structural reorganization between each set of redox pairs is small. This is especially true of Cu2(μ-NR2)2 systems supported by the (SNS) and (PNP) ligands, where (PNP) represents the bis(2-di-iso-butlyphosphinophenyl)amide ligand.xiv,xvii In these cases, the most noteworthy structural change pertains to a Cu…Cu compression of ~ 0.15 Å upon one-electron oxidation. This compression is far more pronounced for the (PPP) system (> 0.5 Å),xv where (PPP) = bis(2-di-iso-propylphosphinophenyl)phosphide.

Figure 1
Cu2(μ-XR2)2 diamond core systems featured in the present study.

A recent comparative X-ray crystallographic, XAS, and DFT study of the series of compounds {(tBu-PNP)Cu}2n+ and {(PPP)Cu}2n+ (where n = 0, 1, 2; tBu-PNP = bis(2-di-iso-butlyphosphino-5-tert-butylphenyl)amide) sought to establish the role played by the bridging X and Cu atoms during successive oxidations. Based upon that study,xvii it was inferred that a substantial degree of ligand oxidation was prevalent. Indeed, a majority of oxidation occurs at the bridging diarylamido or diarylphosphido units compared with the copper centers, though there is appreciable copper d-character in the redox active molecular orbitals (RAMOs). What is very clear is that the Cu2(μ-XR2)2 diamond core is a highly covalent unit, and one cannot decouple the oxidation of the metal or bridging atoms from one another. This scenario is also true of the CuA site, where it has been estimated that the RAMO in the Cu2(μ-SR)2 mixed-valence state is ~46% sulfur and ~44% copper.xiii

To more fully develop the electronic structure description of these redox active diamond core dicopper systems we undertook EPR studies of the Cu1.5Cu1.5 complexes. Whereas initial studies reported the X-band EPR spectra,xiv,xv,xvi a detailed analysis of the convoluted spectra had not been performed. To aid such an analysis, we have now collected comparative Q-, X-, and S-band EPR spectra for each system shown in Figure 1. This set of data is unique for dicopper model systems, and allows for the possibility of quantifying hyperfine interactions between various centers and the unpaired spin, critical in describing their electronic structures and for comparison to published XAS and DFT studies, and related data available for CuA. Indeed, deconvolution of the EPR parameters by computer simulation reveals values unusual for typical dicopper systems and consistent with a large degree of Cu-XR2 spin delocalization.

Experimental Section

Dicopper complexes were prepared according to previously published proceduresxiv,xv,xvi,xvii as their B[3,5-(F3C)2C6H3]4 salts and EPR spectra were recorded on frozen glasses in 2-methyltetrahydrofuran (5 mM). EPR spectra were obtained at the National Biomedical EPR Center in Milwaukee using Varian E-9 and E109 spectrometers operating at 9 GHz (X-band), 3.3 GHz (S-band) and 35 GHz (Q-band). The low frequency 3.3 GHz (S-band) spectrometer is based on the loop-gap resonator designed by Froncisz and Hyde.xviii The Q-band bridge was modified with the addition of a GaAs field-effect transistor signal amplifier and low-noise Gunn diode oscillator.xix Microwave frequencies were measured with an EIP model 331 counter. Simulations were carried out using the program Xsophe (Bruker). Xsophe simulations were calculated using matrix diagonalization. Unless otherwise indicated, simulations assumed hyperfine interactions of the unpaired electron with 2 equivalent Cu atoms, 2 equivalent bridging (N or P) atoms, and 2 equivalent non-bridging P atoms where available (vide infra).

Spectrometer conditions were as follows. Q-band: microwave power 36 dB, temperature 16.7K, mod. amplitude 5G set [actual about 3 G], time constant 0.1 sec, 100 KHz mod. frequency, scan time 4 min., microwave frequencies 35.011 GHz (PPP), 35.0008 GHz (SNS), 35.028 GHz (PNP). S-band: microwave power 22 dB, temperature -140 °C, mod. amplitude 5 set [actual about 3 G], time constant 0.064 sec, scan time 2 min., microwave frequencies 3.3661 GHz (SNS), 3.3642 GHz (PNP); PPP 3.3416 GHz (PPP). X-band: microwave power 16 dB, temperature 120 K, mod. amplitude 5 G, time constant 0.128 sec, scan time 4 min., microwave frequencies 9.434 GHz (PPP), 9.431 GHz (PNP), 9.457 GHz (SNS).

Results

X- and Q-band EPR spectra of {(PPP)Cu}2+, {(PNP)Cu}2+, and {(SNS)Cu}2+

X-band EPR spectra for the mixed valence compounds {(PPP)Cu}2+,xv, {(PNP)Cu}2+,xvi and {(SNS)Cu}2+,xiv are similar to spectra recorded previously (Figure 2, Table 1).xx The g-values are more apparent from the Q-band (35 GHz) spectra (Figure 3, Table 1). The gmin values determined from Q-band can then be used for X-band simulations. The hyperfinefine structure in the Q-band spectrum for {(PPP)Cu}2+ is still resolved indicating that there is little g-strain. Neither gmax nor gmin is separated from the center g-value for {(PPP)Cu}2+, consistent with an isotropic g-value at this resolution. A high field line for gmin in the Q-band spectra for {(PNP)Cu}2+ and {(SNS)Cu}2+ is separated (Figure 3). This high field g-value is apparent in the X-band spectrum for {(SNS)Cu}2+, but not as apparent for {(PNP)Cu}2+ or {(PPP)Cu} +2 (Figure 2). The hyperfine structure is not resolved in the Q-band spectra for {(PNP)Cu}2+ and {(SNS)Cu}2+ where g-strain contributes to the broadening of the lines.

Figure 2
X-band EPR spectra of {(PPP)Cu}2+, {(PNP)Cu}2+, and {(SNS)Cu}2+
Figure 3
Q-band EPR spectra of {(PPP)Cu}2+, {(PNP)Cu}2+, and {(SNS)Cu}2+
Table 1
EPR parameters for {(LXL)Cu}2+ from simulations, and comparative literature values for CuA and the mixed-valence complex Cu2L#.

S-band spectra of {(PPP)Cu}2+, {(PNP)Cu}2+, and {(SNS)Cu}2+

The hyperfine structure in the spectra for {(PPP)Cu}+2, {(PNP)Cu}2+, and {(SNS)Cu}2+ is better resolved in the low frequency S-band spectra (Figure 4). Additional lines in the S-band spectrum for {(PNP)Cu}2+ are resolved on the high field side where there are only inflections in the X-band spectrum (Figure 2). The g-value close to the crossover point in Figure 4 is giso for {(PPP)Cu}2+ and is close to g for {(PNP)Cu}2+ and {(SNS)Cu}2+ as taken from the Q-band data. The g-anisotropy is not as evident at S-band because the g-values are getting closer as determined by field position. Using the g-value determined for {(SNS)Cu}2+ and {(PNP)Cu}2+ at Q-band, gmid is determined in the center of the spectrum. Equally spaced lines around gmid are the hyperfine lines. There should be a 1:2:3:4:3:2:1 seven line pattern centered at gmid if the hyperfine lines are only from two coppers. The observed eleven lines are consistent with a 1:4:10:20:26:28:26:20:10:4:1 pattern for {Cu2(μ-NR2)2}+ where the hyperfine couplings are about equal for copper and nitrogen (Figs 2 and and44).

Figure 4
S-band EPR spectra of {(PPP)Cu}2+, {(PNP)Cu}2+, and {(SNS)Cu}2+

Second derivative multifrequency EPR spectra and simulations for {(PPP)Cu}2+

The second derivative of an EPR spectrum emphasizes sharp lines and deemphasizes broad lines. Of the three complexes described in this paper, the hyperfine lines for {(PPP)Cu}2+ are most apparent and easiest to compare at X-, Q-, and S-band frequencies (Figure 5). The spectra are centered at the apparent, to first order, isotropic g-value, 2.003. The large number of resolved lines in the X- and S-band spectra of each species, most apparent in the second derivative spectrum for {(PPP)Cu}2+ in Fig. 5, suggests that the hyperfine lines are not from only copper. Two almost equivalent coppers in {(PPP)Cu}2+ are expected to give seven lines for each g-value. If the lines are from more than one g-value, they should not line up at three different microwave frequencies. About twenty lines are almost aligned at the three frequencies. This indicates that the hyperfine lines are due to not only Cu (I = 3/2), but also P (I = 1/2) assuming that the copper hyperfine lines are from equivalent coppers and not inequivalent coppers with different hyperfine values. The lines are better resolved as the frequency is lowered. Moreover, almost all of the hyperfine lines line up at all three frequencies. At the higher frequencies, the relative intensities are not evident due to line broadening and partial overlap of the lines. At S-band, the lines are so well resolved that the relative intensity of the lines becomes more evident. For example, on the low field side of the spectrum, some of the lines have relative intensities of 1:2:1 consistent with coupling to two almost equivalent nuclei with I = 1/2, i.e. a pattern due to two almost equivalent P atoms (Figure 5, circled area). Assigning the 1:2:1 pattern to two almost equivalent P atoms gives AP(terminal) = 12.5 G. The next three lines also form a 1:2:1 pattern. The difference in field for the centers of both 1:2:1 patterns gives a second hyperfine value assigned to copper and phosphorous, i.e. ACu ~ AP(bridge) = 45 G. Note that a 1:4:6:4:1 pattern is expected for four equivalent P atoms, and it is difficult to distinguish between a 1:2:1 and a 4:6:4 pattern where the lines with intensity 1 are superimposed onto more intense lines. It is also possible that geometric factors dictate inequivalent coupling of the unpaired spin to the terminal phosphines, much like coupling that is observed with the CH2 protons in the tyrosine radical in ribonucleotide reductase.xxi Simulation of the spectra with ACu = 45 G, AP(bridge) = 45 G and AP(terminal) = 12.5 G and a line width of 5 G gives a multi-line pattern that is better resolved than the experimental S-band spectrum for {(PPP)Cu}2+ because the line width is less for the simulation than for the experimental spectrum (Supplemental Figure S1). The number of lines and the splitting of the lines in the simulation are similar to the lines in the experimental spectrum. A limitation of this simulation is that the variables are underdetermined. The g- and A-axes are taken as coincident with all Euler angles at zero. MI-dependent line width parameters and quadruple terms were not used. The simulation is thus consistent with, but not proof of, the values for the experimental spectrum.

Figure 5
Second derivative of Q-, X-, and S-band spectra for {(PPP)Cu}2+

Second derivative EPR spectra and simulations for {(PNP)Cu}2+ and {(SNS)Cu}2+

The X-band spectrum for {(PNP)Cu}2+ (Figure 2) has ten clearly resolved lines in the center of the spectrum, which is attributed to the gmid region. The resolved lines for {(PNP)Cu}2+ are broader than the lines for {(SNS)Cu}2+ presumably due to the superhyperfine lines from the terminal P-atoms. Initial simulation of a single isotropic line with a line width of 6 G and subsequent addition of hyperfine lines for two phosphorous atoms with AP(terminal) = 5 G doubles the peak-to-peak width from 7 G to 15 G (data not shown). It is estimated that AP(terminal) ~ 5 G from this simulation of the single line. If four phosphorous atoms are almost equivalent, AP(terminal) should be slightly less than 5 G. The second derivative of this spectrum emphasizes the lines that are resolved (Figure 6). Since only seven lines for the gmid region are expected for the copper hyperfine from a class III mixed-valence complex, hyperfine lines from nitrogen were again considered to increase the number of lines. Simulation of the Q-band data, which is the most sensitive to g-values, provided the rhombic tensors gmax, gmid, and gmin of 2.085, 2.060, and 2.000, respectively (Supplemental Figure S2). A good simulation of the X-band spectrum for {(PNP)Cu}2+ can be then obtained with gmax, gmid, and gmin values of 2.085, 2.057, and 2.000; ACu values of 40, 27, and 15 G; and AN values of 12, 24, and 15 G (Figure 6). The S-band spectrum may also be fit using these parameters (Supplemental Figure S3), lending weight to their assignments.

Figure 6
Second derivative X-band spectrum for {(PNP)Cu}2+ and simulations. EPR parameters for simulations: gmax, gmid, gmin, 2.085, 2.057, 2.000; ACumax, ACumid, ACumin, 40, 27, 15 G; ANmax, ANmid, ANmin, 12, 24, 15 G; line width 15, 11, 10 G (top simulation), ...

As already noted, the X-band EPR spectrum of {(SNS)Cu}2+ has been published and a crude simulation is consistent with the class III mixed-valence species.xx As for {(PPP)Cu}2+, second derivative spectra for {(SNS)Cu}2+ emphasize the sharp features in the spectra (Figure 7). The lines and line shapes in the high field and low field regions are consistent with rhombic g-values. There are about 10 lines in the center of the spectrum, which comprise the gmid region. Assuming more than seven lines in the gmid region, a seven line pattern for Cu(1.5)Cu(1.5) does not fit the experimental spectrum. Thus superhyperfine lines are observed involving splittings from, presumably, the bridging nitrogens. Using EPR parameters from Q-, X-, and S-band spectra, a simulation with gmax, gmid, and gmin equal to 2.069, 2.066, and 2.00; ACu values of 44 G, 17 G, and 5 G; AN values of 12 G, 17 G, and 5 G; fits the experimental spectrum extremely well (Figure 7). While simulations are consistent with, not proof of, the EPR parameters, the EPR experimental and simulated parameters do appear to be very close. One criterion for the goodness of fit is to simulate at another frequency without changing the EPR parameters. The simulation of the S-band spectrum is equally good, thus increasing the confidence in the EPR parameters (Supplemental Figure 4).

Figure 7
Second derivative X-band spectrum for {(SNS)Cu} +2 and simulation. EPR parameters for simulation: gmax, gmid, gmin, 2.069, 2.066, 2.00; ACumax, ACumid, ACumin, 44, 17, 5 G; ANmax, ANmid, ANmin, 12, 17, 5 G; line width 7, 7, 4 G; microwave frequency, 9.377 ...

Discussion

Asymmetry in the coordination environment of the copper centers for {(tBu2-PNP)Cu}2+ and {(tBu2-PNP)Cu}2 2+ has been observed via solid-state XRD analysis and is presumably a phenomenon specific to the solid-state.xvii This phenomenon was not observed for the (SNS)- and (PPP)-supported systems. For completeness, and direct comparison to the EPR data presented here, the solid-state X-ray structure of {(PNP)Cu}2+ has also been determined and placed in the Supporting Information. It likewise shows substantial asymmetry about the diamond core motif. In solution or upon freezing to a glass, however, {(PNP)Cu}2+ is a fully-delocalized, class-III mixed valence species based upon the EPR data available.

CuA has g-values of gmin = 2.007, gmid = 2.024, and gmax = 2.180 (Table 1).xxiv For CuA, gmax > gmid ~ gmin and the ground state for the coppers is primarily dx2-y2. A g-value close to 2.00 for CuA suggests admixture of a low-lying excited state. This unusual behavior is also evident in our model compounds. For {(SNS)Cu}2+, gmin = 2.00, gmid = 2.066, and gmax = 2.069. The ground state for the coppers involves the dz2 orbitals more than the dx2-y2 orbitals as reflected by gl > gll where gmid and gmax are gl. For {(PNP)Cu}2+, gmin = 2.00, gmid = 2.055, and gmax = ~2.08. Here the g-values are rhombic. In contrast, for {(PPP)Cu}2+, only one g-value from the crossover point, 2.003, is assigned as the signal is nearly isotropic at the resolutions we have achieved.

The line widths of the spectra in Figure 2 are consistent with sharp lines for {(SNS)Cu}2+, which have only copper and nitrogen hyperfine lines. For {(PNP)Cu}2+, the hyperfine lines are broader, consistent with an additional superhyperfine contribution from non-bridging phosphorus. A key feature for the spectrum of {(PPP)Cu}2+ is the increase in the number of hyperfine lines, which are assigned to copper and bridging phosphorous atoms, where an increase in line width accounts for terminal phosphorous atoms.

The interpretation of the hyperfine and superhyperfine structure is simplest for {(SNS)Cu}2+ because there are no superhyperfine couplings from the terminal ligands. The lowest field lines in the spectrum (Figure 7) are separated from the other lines, equivalent to lines expected in a single crystal, and well simulated with gmax = 2.069, ACumax = 44 G, and ANmax = 12 G. The g- and A-tensors in the simulation are coincident, but it is not known how sensitive the spectra are to the Euler angles. The number of lines in the gmid region is similar in both the experimental gmid region and the simulated spectrum (Figure 7). It is concluded that there are more than seven lines in the gmid region, which implies hyperfine lines from both copper and nitrogen. A crude simulation assuming a seven-line pattern for equivalent coppers has been publishedError! Bookmark not defined.21 but does not fit nearly as well as the simulation shown in Figure 7. The shape of the lines for gmin in the high field region are not as close to the experimental spectrum as for the lines in the gmax and gmid region, but, other than the line width being narrow, little information about the parameters was gained from the experimental spectrum (Figure 7). Simulation parameters for the gmin region are gmin = 2.00, ACumin = 5 G, and ANmin = 5 G. ACumin and ANmin are much less than the parameters for Amax and Amid, and hence the tensors are anisotropic. AN for a terminal nitrogen is expected to be more isotropic. The anisotropic values for AN for {(SNS)Cu}2+ and {(PNP)Cu}2+ thus appear to be characteristic for the bridging N-atoms. The isotropic values for AP(bridging) for {(PPP)Cu}2+ are approximate values because the g-values are quenched, making assignments for the x, y, and z directions more difficult and because the time to simulate with anisotropic values was prohibitive. Therefore the degree of anisotropy for AP(bridging) for {(PPP)Cu}2+ was not determined. Curiously, despite greater calculated Cu spin density in CuA relative to the {(LXL)Cu}2+ model compounds (Table 2), the values for ACu are more or less similar to the values for ACuA, with ACu for {(PPP)Cu}2+ somewhat greater than for ACuA and with ACu for {(PNP)Cu}2+ and {(SNS)Cu}2+ about the same or less than for ACuA. It may be that the value for A(Cu) for {(PPP)Cu}2+ is not isotropic, but anisotropic and the anisotropy in the EPR spectrum is not readily evident. Under this case the spin density for Cu would be overestimated.

Table 2
Estimated spin density distribution from DFT calculations.

Recently XAS and electronic structure calculations were obtained for {(PNP)Cu}2 n+ (n = 0,1,2) and {(PPP)Cu}2 n+.xvii Rhee and Head-Gordon have independently undertaken a theoretical study of {(PPP)Cu}2 n+.xxv It has been generally concluded that the redox chemistry for {(PPP)Cu}2 n+ and {(tBu2-PNP)Cu}2 n+ is substantially delocalized throughout the Cu2(μ-XR2)2 cores, with a majority component of the redox chemistry occurring at the bridging N or P units (Table 2). The values obtained for AN and AP(bridging) are consistent with this hypothesis. Values from Table 1 for AP(terminal) for {(PPP)Cu}2+ of 12.5 G and for {(PNP)Cu}2+ of about 5 G are in accord with modest terminal phosphorous contributions. AN(terminal) is 5.6 G for CuA (Table 1), but the ratio of the nuclear moments for P to N is 2.26320 to 0.4037607. If the electron density for the terminal N and phosphorous were similar, a value of about 30 G would be expected for AP(terminal). The electron density on the bridging phosphorous or nitrogen is substantially greater (Table 1) than for the terminal phosphorous, which is in accordance with a large bridging atom A values. Moreover, quenching of the g-values for {(PPP)Cu}2+ to an almost isotropic value, together with delocalization of spin density on the bridging phosphorous, suggest the presence of P-centered radical character. AN values for {(PNP)Cu}2+ and {(SNS)Cu}2+ (Table 1) are comparable to those observed for N-localized radicals such as Me2N (AN = 14.7 G)xxvi and nitroxide radicals (AN approx 32 G).xxvii P-centered radicals are less common, and AP values range from 42.5 G in highly delocalized systemsxxviii to 96.3 G in P[CH(SiMe3)2]2.xxix The A value in {(PPP)Cu}2 (45 G) is in the range expected for free P-centered radicals. Such direct observation of AX(bridging) in the Cu2(μ-X)2 core of CuA is not possible because the bridging S atoms are spin inactive (I = 0).

Supplementary Material

Supplementary Data

Acknowledgement

JCP acknowledges support from the NSF (GOALI). NPM is grateful for an NSF graduate fellowship. WEA acknowledges the National Biomedical EPR Center Grant EB001980 from NIH. The authors are grateful to a reviewer for a helpful suggestion.

Footnotes

Supporting Information Available: Crystallographic data (cif) and supporting EPR spectra are available free of charge via the Internet at http://pubs.acs.org.

References

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