Three-coordinate Cu(II) complexes: Structural models of trigonal-planar type 1 copper protein active sites [5]

Full text of DOI:10.1021/ja991533e

7270
J. Am. Chem. Soc. 1999, 121, 7270-7271
sum analysis,12 and EPR spectroscopy (1 spin/Cu by integration;
Figure S2, Supporting Information).
Three-Coordinate Cu(II) Complexes: Structural
Models of Trigonal-Planar Type 1 Copper Protein
Active Sites
Patrick L. Holland and William B. Tolman*
Department of Chemistry and Center for
Metals in Biocatalysis, UniVersity of Minnesota
207 Pleasant Street SE, Minneapolis, Minnesota 55455
Reaction of 1 with sodium 4-methylthiophenolate resulted in
oxidation of the thiolate to a disulfide.8a However, treating 1 with
sodium triphenylmethylthiolate gave thermally stable, deep blue-
purple LCuSCPh3 (2).10 The X-ray crystal structure (Figure 1b)
shows that the bulky diimine L- continues to enforce three-
coordination in 2, despite the tendency of thiolate ligands to form
bridges.8a One isopropyl methine hydrogen atom approaches the
copper from each side of the plane at a distance of 2.8-2.9 Å,
but we cannot distinguish whether this geometry is a result of
weak bonding to the copper atom or from ligand constraints. The
short Cu-N and Cu-S bond lengths [1.922(3) and 2.124(1) Å,
respectively] are very similar to the distances in the crystal
structures of the trigonal-planar sites in the blue copper proteins
fungal laccase (1.9, 2.2 Å; Figure 1a)6a and ceruloplasmin (∼2.1
Å),6b as well as other type 1 copper sites.7 Interestingly, the Cu
geometry is distorted slightly toward pyramidal; the metal ion
lies 0.220(1) Å from the N2S plane, comparable to the deviation
in azurin (0.1 Å)4 but not as large as in the more tetrahedral
plastocyanin active site (0.4 Å).5 The pyramidalization in 2, which
lacks an axial ligand, should lead to reevaluation of claims that
weak (2.9 Å or greater) axial “bonding” in type 1 copper sites
leads to movement of the metal out of the N2S plane. This
distortion from planarity, as well as the Cu-S-C angle of
119.43(8)° that is more obtuse than that in the proteins (∼110°),
may result from steric crowding in 2.
ReceiVed May 10, 1999
Members of the type 1 class of copper protein active sites are
common redox reagents in biochemistry, and extensive studies
of them have been crucial to the development of modern
bioinorganic chemistry.1 A trigonally disposed His2Cys ligand
complement is found in all of these sites, with a short, highly
covalent Cu-S(Cys) interaction being the primary determinant
of their unique spectroscopic features.2 Despite their categorization
as a single type of copper center, the type 1 coppers exhibit
variable coordination geometries whose relationships to spectro-
scopic and redox properties are under intense study.2,3 These
geometries mainly differ in additional ligand(s) provided to the
copper atom by the protein: two donors at long distances (>3.0
Å), giving a 3 + 2 trigonal bipyramid (azurin);4 one donor at a
shorter distance (2.6-2.9 Å), giving a distorted tetrahedron (e.g.,
plastocyanin);5 or no donating residues, leaving a three-coordinate
trigonal-planar copper (fungal laccase,6a ceruloplasmin,6b,c and
certain azurin mutants6d).7 The trigonal-planar structure is an
arrangement for Cu(II) that has no precedent, to our knowledge,
in the coordination chemistry of this ion. Here, we report the first
examples of synthetic Cu(II) complexes with such a geometry,
including a thiolate complex that closely mimics the structures
of the trigonal fungal laccase and ceruloplasmin type 1 sites.8
The lithium salt of 2,4-bis(2,6-diisopropylphenylimido)pentane
(L-)9 was reacted with CuCl2 to give red-purple LCuCl (1).10 An
X-ray crystal structure (Figure S1, Supporting Information)11
revealed only three ligands coordinated to the metal ion, two
nitrogen atoms from L- at 1.870(3) Å and a chloride at 2.127(1)
Å, in an almost perfectly planar geometry (the metal lies only
0.005 Å from the N2Cl plane). The Cu(II) oxidation level was
confirmed from charge-balance considerations, a bond-valence
The axial signal in the X-band EPR spectrum of 2 (Figure 2a)
corroborates its Cu(II) formulation, but the g ) 2.17 and A )
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(8) For a review of Cu(II)-thiolate chemistry, see: (a) Mandal, S.; Das,
G.; Singh, R.; Shukla, R.; Bharadwaj, P. K. Coord. Chem. ReV. 1997, 160,
191-235. To date, the only Cu(II) complexes that accurately replicate the
N2S ligand set of the type 1 active site and its spectroscopic features are
TpCuSR (Tp ) 3,5-alkylated trispyrazolylborate; R ) t-Bu, s-Bu, CPh3, C6F5,
p-NO2C6H4), but the metal ions in these compounds are four-coordinate
because of the tridentate, facial coordination of the Tp ligand. (b) Thompson,
J. S.; Marks, T. J.; Ibers, J. A. J. Am. Chem. Soc. 1979, 101, 4180-4192. (c)
Kitajima, N.; Fujisawa, K.; Moro-oka, Y. J. Am. Chem. Soc. 1990, 112, 3210-
3212. (d) Kitajima, N.; Fujisawa, K.; Tanaka, M.; Moro-oka, Y. J. Am. Chem.
Soc. 1992, 114, 9232-9233. (e) Qiu, D.; Kilpatrick, L.; Kitajima, N.; Spiro,
T. G. J. Am. Chem. Soc. 1994, 116, 2585-2590.
(1) Baker, E. N. Copper Proteins with Type 1 Sites. In Encyclopedia of
Inorganic Chemistry; King, R. B., Ed.; Wiley: New York, 1994; pp 883-
905.
(2) Solomon, E. I.; Baldwin, M. J.; Lowery, M. D. Chem. ReV. 1992, 92,
521-542.
(3) For example, see: (a) Han, J.; Loehr, T. M.; Lu, Y.; Valentine, J. S.;
Averill, B. A.; Sanders-Loehr, J. J. Am. Chem. Soc. 1993, 115, 4256-4263.
(b) Andrew, C. R.; Yeom, H.; Valentine, J. S.; Karlsson, B. G.; Bonander,
N.; van Pouderoyen, G.; Canters, G. W.; Loehr, T. M.; Sanders-Loehr, J. J.
Am. Chem. Soc. 1994, 116, 11489-11498. (c) LaCroix, L. B.; Randall, D.
W.; Nersissian, A. M.; Hoitink, C. W. G.; Canters, G. W.; Valentine, J. S.;
Solomon, E. I. J. Am. Chem. Soc. 1998, 120, 9621-963. (d) Pierloot, K.; De
Kerpel, J. O. A.; Ryde, U.; Olsson, M. H. M.; Roos, B. O. J. Am. Chem. Soc.
1998, 120, 13156-13166.
(4) (a) Ainscough, E. W.; Bingham, A. G.; Brodie, A. M.; Ellis, W. R.;
Gray, H. B.; Loehr, T. M.; Plowman, J. E.; Norris, G. E.; Baker, E. N.
Biochemistry 1987, 26, 71-82. (b) Baker, E. N. J. Mol. Biol. 1988, 203, 1071-
1095.
(9) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese,
J. C.; Arthur, S. D. Organometallics 1997, 16, 1514-1516. A very similar
ligand has been used to synthesize a three-coordinate rhodium(I)-olefin
complex: Budzelaar, P. H. M.; de Gelder, R.; Gal, A. W. Organometallics
1998, 17, 4121-4123.
(10) Data for 1 (yield 51%): UV/vis (CH2Cl2) [λmax, nm (ꢀ, mM-1 cm-1)]
340 (∼25), 507 (4.1), 657 (1), 840 (1); EPR (9.61 GHz, CH2Cl2/toluene, 20
K) g ) 2.20, A ) 130 × 10-4 cm-1, g ) 2.05, A ) 8 × 10-4 cm-1. Anal.
Calc|d| for C29H|| CuClN2: C, 67.42; H, 8.00; N, 5.42. Found: C, 67.36; H,
41
8.05; N, 5.46. Data for 2 (yield 51%): UV/vis (heptane) [λmax, nm (ꢀ, mM-1
cm-1)] 340 (∼20), 427 (1.1), 487 (1.0), 561 (1.3), 749 (5.8); EPR (9.61 GHz,
toluene, 20 K) g ) 2.17, A ) 111 × 10-4 cm-1, g ) 2.04, A ) 13 × 10-4
||
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cm-1. Anal. Calcd for C48H56CuSN2: C, 76.20; H, 7.46; N, 3.70. Found: C,
76.21; H, 7.58; N, 3.68.
(5) Guss, J. M.; Bartunik, H. D.; Freeman, H. C. Acta Crystallogr. 1992,
B48, 790-811.
(11) X-ray data for 1: monoclinic, space group P21/n, a ) 12.5239(2) Å,
b ) 16.6649(1) Å, c ) 13.9652(1) Å, â ) 104.521(1)°, V ) 2821.59(5) Å3,
Z ) 4, Fcalcd ) 1.216 g/cm3. For 2: monoclinic, space group C2/c, a ) 38.799-
(8) Å, b ) 13.156(3) Å, c ) 17.577(4) Å, â ) 109.82(3)°, V ) 8441(3) Å3,
Z ) 8, Fcalcd ) 1.191 g/cm3. Nonhydrogen atoms were refined with anisotropic
thermal parameters, and hydrogen atoms were in idealized positions with riding
thermal parameters. Full-matrix least-squares refinement on F2 converged for
1 (R1 ) 0.0335, wR2 ) 0.0782, GOF ) 1.043 for 4036 independent reflections
with I > 2σ(I) and 308 parameters) and 2 (R1 ) 0.0418, wR2 ) 0.0867, GOF
) 1.019 for 5417 independent reflections with I > 2σ(I) and 479 parameters).
(12) Bond valence sum analyses on 1 and 2 yielded copper oxidation states
of 2.12 and 2.09, respectively. (a) Thorp, H. H. Inorg. Chem. 1992, 31, 1585-
1588. (b) Brown, I. D.; Altermatt, D. Acta Crystallogr. 1985, B41, 244-247.
(6) (a) Laccase: Ducros, V.; Brzozowski, A. M.; Wilson, K. S.; Brown,
S. H.; Østergaard, P.; Schneider, P.; Yaver, D. S.; Pedersen, A. H.; Davies,
G. J. Nature Struct. Biol. 1998, 5, 310-316. (b) Ceruloplasmin: Zaitseva, I.;
Zaitsev, V.; Card, G.; Moshkov, K.; Bax, B.; Ralph, A.; Lindley, P. J. Biol.
Inorg. Chem. 1996, 1, 15-23. It is likely that the trigonal-planar copper site
in this protein was in the Cu(I) state under the conditions of the crystal structure
in ref 6b. See: (c) Machonkin, T. E.; Zhang, H. H.; Hedman, B.; Hodgson,
K. O.; Solomon, E. I. Biochemistry 1998, 37, 9570-9578. (d) Azurin
mutants: Karlsson, B. G.; Nordling, M.; Pascher, T.; Tsai, L.-C.; Sjo¨lin, L.;
Lundberg, L. G. Protein Eng. 1991, 4, 343-349.
(7) Adman, E. T. AdV. Protein Chem. 1991, 42, 145-197.
10.1021/ja991533e CCC: $18.00 © 1999 American Chemical Society
Published on Web 07/27/1999

Communications to the Editor
J. Am. Chem. Soc., Vol. 121, No. 31, 1999 7271
(d-d bands may be buried under this feature2,3,14). The disparity
between the energies of this transition in the model and in the
proteins is under study.
The resonance Raman spectrum (λex ) 632.8 nm) of 2 in C6D6
exhibits multiple features (Figure 2b, inset), with a band at 430
cm-1 being most prominent. Similar spectra have been reported
for type 1 copper sites, and major bands between 340 and 435
cm-1 in these cases have been assigned as predominantly Cu-S
vibrations (νCu-S).3,16,1716,17 Higher νCu-S values imply a stronger
Cu-S bond, but interpretation is complicated by vibrational
coupling with S-C and C-C thiolate vibrations. Preliminary
assignment of the 430-cm-1 band in 2 to νCu-S is supported by
the similarity of its frequency to those reported in other model
complexes (e.g., 422 cm-1 for one with the same Ph3CS- ligand).8e
Also, its high energy is consistent with the strong Cu-S bonding
expected in this trigonal complex and compares well with the
major νCu-S bands between 428 and 435 cm-1 in fungal laccases
from several organisms.14,16
Figure 1. (a) Type 1 copper site in fungal laccase as determined by
X-ray crystallography (ref 6a). (b) Thermal ellipsoid (50%) diagram of
the X-ray structure of 2 (hydrogen atoms omitted).
In view of the function of the type 1 sites as electron-transfer
agents, it is generally believed that the unusual trigonal or
tetrahedral Cu(II) coordination geometry exists to minimize ligand
reorganization upon reduction to Cu(I).18 Thus a Cu(I)-like
coordination geometry is enforced, giving the copper ion a high
redox potential (+0.2-0.6 V vs NHE). At the extreme are the
three-coordinate fungal laccase and ceruloplasmin centers, which
have the highest redox potentials of the type 1 coppers (in some
cases, >+0.7 V).6c,14,19 The redox potential of 2 in THF is much
lower (E1/2 ) -0.18 V vs NHE, Figure S6, Supporting Informa-
tion),20 at least in part because of charge donation by the anionic
L-.
In summary, using a relatively simple synthetic approach, we
have characterized the first three-coordinate complexes of Cu(II),
including a close structural analogue of the trigonal type 1 copper
protein active sites. This work demonstrates that appropriate
protecting ligands can give this unusual Cu(II) geometry in the
absence of protein-enforced geometric constraints.21 There are
spectroscopic similarities between the synthetic model and the
protein centers, but important differences in absorption, Raman,
and electrochemical properties need to be explained through
further studies.
Acknowledgment. We thank Prof. Lawrence Que, Jr., for the use of
resonance Raman equipment, Prof. John Lipscomb for the use of the
EPR spectrometer, and Prof. Edward I. Solomon for providing access to
unpublished results from his laboratory. Financial support from the NIH
(GM47365 to W.B.T.; postdoctoral fellowship to P.L.H.) and the NSF
(NYI Award to W.B.T.) is gratefully acknowledged.
Figure 2. (a) X-band EPR spectrum of 2 in toluene (9.61 GHz, 20 K):
g ) 2.17, A ) 111 × 10-4 cm-1, g ) 2.04, A ) 13 × 10-4 cm-1. (b)
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UV-vis spectrum of 2 in heptane solution (298 K). Inset: Resonance
Raman spectrum of 2 in C6D6 (λex ) 632.8 nm, T ) 77 K).
Supporting Information Available: Experimental details and spec-
troscopic data for 1 and 2 (PDF), and two X-ray crystallographic files
(CIF). This material is available free of charge via the Internet at
http://pubs.acs.org.
111 × 10-4 cm-1 values differ significantly from those typical
for type 1 copper protein sites (g ) 2.2-2.3 and A < 70 ×
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10-4 cm-1). It is noteworthy, however, that EPR spectra of the
trigonal-planar sites in fungal laccase and ceruloplasmin are
exceptional for type 1 copper, displaying axial signals with lower
JA991533E
(16) Andrew, C. R.; Sanders-Loehr, J. Acc. Chem. Res. 1996, 29, 365-
372.
g values near 2.20 and higher A values (∼90 × 10-4 cm-1
)
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that have been related to a stronger Cu-S(cys) interaction.13-15
Thus, the EPR spectrum of 2, like its structure, is quite similar
to those of the trigonal-planar type 1 sites. The optical absorption
spectrum of 2 in heptane (Figure 2b) has multiple features, of
which the one with λmax ) 749 nm (ꢀ ) 5800 M-1 cm-1) is of
particular interest. It is reminiscent of, but at longer wavelength
(17) Blair, D. F.; Campbell, G. W.; Schoonover, J. R.; Chan, S. I.; Gray,
H. B.; Malmstro¨m, B. G.; Pecht, I.; Swanson, B. I.; Woodruff, W. H.; Cho,
W. K.; English, A. M.; Fry, H. A.; Lum, V.; Norton, K. A. J. Am. Chem. Soc.
1985, 107, 5755-5766.
(18) (a) Vallee, B. L.; Williams, R. J. P. Proc. Natl. Acad. Sci. U.S.A. 1968,
59, 498-505. (b) Gray, H. B.; Malmstro¨m, B. G. Comments Inorg. Chem.
1983, 2, 203-209.
(19) Xu, F.; Shin, W.; Brown, S. H.; Wahleithner, J. A.; Sundaram, U.
M.; Solomon, E. I. Biochim. Biophys. Acta 1996, 1292, 303-311.
(20) (a) Cyclic voltammetry used a platinum working electrode and a SCE
reference electrode. Potentials vs internal ferrocene (Fc) were corrected,20b
giving NHE reference. For 1 (0.4 M NBu4PF6 in CH2Cl2; in this electrolyte,
Fc/Fc+ E1/2 ) 0.70 V vs NHE),20b E1/2 ) -0.08 V; Epa - Epc ) 0.07 V, ia ≈
ic. For 2 (0.2 M NBu4PF6 in THF; in this electrolyte, Fc/Fc+ E1/2 ) 0.80 V vs
NHE),20b E1/2 ) -0.18 V; Epa - Epc ) 0.08 V, ia ≈ ic. (b) Connelly, N. G.;
Geiger, W. E. Chem. ReV. 1996, 96, 877-910.
than, the intense feature near 600 nm (ꢀ ≈ 5000 M-1 cm-1
)
responsible for the deep blue color of the type 1 sites, including
the trigonal-planar ones.2,3,14 By analogy to the protein data and
on the basis of its high extinction coefficient, we ascribe the 749-
nm band in 2 to a thiolate f Cu(II) charge-transfer transition
(13) Reinhammar, B.; Malmstro¨m, B. G. Blue Copper-Containing Oxidases.
In Copper Proteins; Spiro,T., Ed.; Wiley: New York, 1981.
(14) Palmer, A. E.; Randall, D. W.; Xu, F.; Solomon, E. I. J. Am. Chem.
Soc., in press.
(15) Werst, M. M.; Davoust, C. E.; Hoffman, B. M. J. Am. Chem. Soc.
1991, 113, 1533-1538.
(21) (a) Williams, R. J. P. Eur. J. Biochem. 1995, 234, 363-381. (b) Lumry,
R.; Eyring, H. J. Phys. Chem. 1954, 58, 110-120. It has been suggested
recently that the unusual geometry of type 1 copper follows instead from strong
thiolate-copper binding. (c) Guckert, J. A.; Lowery, M. D.; Solomon, E. I.
J. Am. Chem. Soc. 1995, 117, 2817-2844. (d) Ryde, U.; Olsson, M. H. M.;
Pierloot, K.; Roos, B. O. J. Mol. Biol. 1996, 261, 586-596.