J. Am. Chem. Soc. 2000, 122, 6331-6332
A Structural Model of the Type 1 Copper Protein
6331
Active Site: N2S(thiolate)S(thioether) Ligation in a
Cu(II) Complex
Patrick L. Holland and William B. Tolman*
Department of Chemistry and Center for
Metals in Biocatalysis, UniVersity of Minnesota
207 Pleasant St. SE, Minneapolis, Minnesota 55455
ReceiVed April 17, 2000
The ubiquitous type 1 copper biological electron-transfer sites
have a single Cu ion bonded to two histidine imidazoles and a
cysteine thiolate (Cu-S ∼2.15 Å) in an approximately trigonal
geometry, usually with a fourth donor (typically a methionine
thioether) present at a long yet bonding distance (2.6-2.9 Å).1
Recent work has shown that this classic motif is in fact variable,
and there is great interest in understanding how the subtle changes
in the Cu coordination sphere in the various type 1 sites relate to
differences in their spectral and redox properties.1,2 Studies of
synthetic copper complexes are helpful for understanding such
structure/property relationships.3 Despite attempts over several
decades to prepare inorganic complexes that reproduce the type
1 coordination geometry, however, no Cu(II) compound with sole
N2S(thiolate)S(thioether) ligation has been characterized defini-
tively.4,5 We describe herein the successful attainment of this
longstanding goal.
We recently reported6 the synthesis of the first three-coordinate
Cu(II) complexes, including thiolate 1 that models the trigonal
type 1 site in fungal laccase,7 by using a highly hindered
â-diketiminate as a supporting ligand (L).8 In an extension of
this approach, we treated solutions of LCuCl in THF with NaSC-
(Ph)2CH2ECH3 (E ) O or S).9 For the case of E ) O, product 2
was isolated as a stable blue-purple crystalline solid. The pendant
ether is not coordinated to the Cu(II) ion in the complex and it
adopts a solution geometry similar to that of 1, as shown by
Figure 1. (a) X-band EPR spectra of toluene solutions of 2 (dashed
line) and 3 (solid line), T ) 20 K. (b) UV-visible spectra of solutions
in heptane (1), pentane (2), or toluene (3) at ambient temperature.
as well as axial EPR signal parameters.10 The X-ray crystal
structure of 2 (Figure 2a) also is similar to that of 1. The Cu
geometry is trigonal planar [the Cu atom lies 0.125(2) Å from
the N2S plane], and the metrical parameters mimic the coordina-
tion sphere of the three-coordinate type 1 Cu(II) site in fungal
laccase in much the same way as described earlier for 1.6
In contrast, using a ligand with a thioether functionality (E )
S) in the place of the ether resulted in the formation of a product
(3) with significantly perturbed spectroscopic and structural
features. Importantly, the X-ray crystal structure of 3 (Figure 2b)
reveals coordination of the thioether group to the metal ion [Cu-
S(2) ) 2.403(1) Å] to yield the first example of a structurally
defined Cu(II) complex with the same donor set as the classical
type 1 biological site. As a result of strong thioether binding, the
remainder of the Cu coordination sphere in 3 differs significantly
from those of 1 and 2. The metal-ligand distances in 3 are
lengthened, consistent with its higher metal coordination number.
In addition, in 3 the â-diketiminate ligand is “folded” along the
spectral data (Figures 1 and S1). Thus, the UV-vis and EPR
spectral features of 1 and 2 are nearly identical, key features being
the analogous S f Cu(II) LMCT band energies and intensities
(1) (a) Adman, E. T. AdV. Protein Chem. 1991, 42, 145. (b) Solomon, E.
I.; Baldwin, M. J.; Lowery, M. D. Chem. ReV. 1992, 92, 521. (c) Randall, D.
W.; Gamelin, D. R.; LaCroix, L. B.; Solomon, E. I. J. Biol. Inorg. Chem.
2000, 5, 16.
(2) Representative literature: (a) Palmer, A. E.; Randall, D. W.; Xu, F.;
Solomon, E. I. J. Am. Chem. Soc. 1999, 121, 7138. (b) Buning, C.; Canters,
G. W.; Comba, P.; Dennison, C.; Jeuken, L.; Melter, M.; Sanders-Loehr, J. J.
Am. Chem. Soc. 2000, 122, 204. (c) Pierloot, K.; De Kerpel, J. O. A.; Ryde,
U.; Olsson, M. H. M.; Roos, B. O. J. Am. Chem. Soc. 1998, 120, 13156. (d)
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.
(3) Karlin, K. D. Science 1993, 261, 701.
(4) (a) Mandal, S.; Das, G.; Singh, R.; Shukla, R.; Bharadwaj, P. K. Coord.
Chem. ReV. 1997, 160, 191. (b) Kitajima, N. AdV. Inorg. Chem. 1992, 39, 1.
(c) Bouwman, E.; Driessen, W. L.; Reedijk, J. Coord. Chem. ReV. 1990, 104,
143. (d) EPR and UV-vis spectra were obtained for a complex proposed to
contain a N2S(thiolate)S(thioether) donor set, but they do not resemble spectra
of type 1 or 1.5 sites: Mandal, S.; Bharadwaj, P. K. Indian J. Chem. 1991,
30A, 948.
(8) Feldman, J.; McLain, S. J.; Parthasarathy, A.; Marshall, W. J.; Calabrese,
J. C.; Arthur, S. D. Organometallics 1997, 16, 1514.
(9) For synthetic procedures and complete characterization data, see the
Supporting Information.
(10) Data for 2 (yield 54%): UV/vis (pentane) [λex, nm (ꢀ, mM-1 cm-1)]
343 (∼35), 423 (1.4), 480 (1.6), 566 (1.4), 738 (5.6); EPR (9.61 GHz, toluene,
Ν
20 K): g| ) 2.17, A|Cu ) 111 × 10-4 cm-1, g ) 2.04, A ) 13 × 10-4
cm-1; cyclic voltammogram (0.2 M NBu4PF6/THF electrolyte, referenced to
internal ferrocene): E1/2 ) -0.12 V vs NHE, Epa - Epc ) 0.12 V, ia ≈ ic.
Anal. Calcd for C44H56CuN2OS: C, 72.94; H, 7.79; N, 3.87. Found: C, 72.78;
H, 7.89; N, 3.84. Data for 3 (yield 54%): UV/vis (toluene) [λex, nm (ꢀ, mM-1
cm-1)] 354 (∼15), 430 (1.9), 538 (2.1), 691 (2.3); EPR (9.61 GHz, toluene,
20 K): g| ) 2.15, A|Cu ) 98 × 10-4 cm-1, gx ) 2.01, gy ) 2.06, AxN ) AyN
) 10 × 10-4 cm-1; cyclic voltammogram (0.2 M NBu4PF6/THF electrolyte,
referenced to internal ferrocene): -0.20 V vs NHE, Epa - Epc ) 0.11 V, ia ≈
ic. Anal. Calcd for C44H56CuN2S2: C, 71.36; H, 7.62; N, 3.78. Found: C,
71.92; H, 8.23; N, 3.48. Selected data for 1 (ref 6): λmax ) 749 nm, ꢀ ) 5.8
(5) Synthetic type 1 copper protein design efforts: Hellinga, H. W. J. Am.
Chem. Soc. 1998, 120, 10055 and references therein.
(6) Holland, P. L.; Tolman, W. B. J. Am. Chem. Soc. 1999, 121, 7270.
(7) Ducros, V.; Brzozowski, A. M.; Wilson, K. S.; Brown, H.; Østergaard,
P.; Schneider, P.; Yaver, D.; Pedersen, A. H.; Davies, G. J. Nat. Stuct. Biol
1998, 5, 310.
Ν
mM-1 cm-1; : g| ) 2.17, A|Cu ) 111 × 10-4 cm-1, g ) 2.04, A ) 13 ×
10-4 cm-1; E1/2 ) -0.18 V vs NHE.
10.1021/ja001328v CCC: $19.00 © 2000 American Chemical Society
Published on Web 06/16/2000