Synthesis and x-ray absorption spectroscopy structural studies of Cu(I) complexes of HistidylHistidine peptides: The predominance of linear 2-coordinate geometry

Article abstract of DOI:10.1021/ja0708013

Modified His-His dipeptides have been reacted with copper(I) salts to model active-site Cu ions bound by contiguous His residues in certain oxygen-activating copper proteins, as well as amyloid β-peptide. Chelation of copper(I) by these ligands affords linear, two-coordinate complexes as studied structurally by X-ray absorption spectroscopy. The complexes are robust toward oxidation, showing limited to no reactivity with O₂, and they bind CO weakly. Reaction with a third ligand (N-methylimidazole) affords complexes with a markedly different structure (distorted T-shaped) and reactivity, binding CO and oxidizing rapidly upon exposure to dioxygen. Copyright

Full text of DOI:10.1021/ja0708013

Published on Web 04/06/2007
Synthesis and X-ray Absorption Spectroscopy Structural Studies of Cu(I)
Complexes of HistidylHistidine Peptides: The Predominance of Linear
2-Coordinate Geometry
Richard A. Himes,† Ga Young Park,† Amanda N. Barry,‡ Ninian J. Blackburn,‡ and
Kenneth D. Karlin*,†
Department of Chemistry, The Johns Hopkins UniVersity, 3400 N. Charles Street, Baltimore, Maryland 21218, and
Department of EnVironmental and Biomolecular Systems, OGI School of Science and Engineering at OHSU,
BeaVerton, Oregon 97006
Received February 4, 2007; E-mail: karlin@jhu.edu
The importance of protein-based ligand tuning of copper active
sites has been noted and recapitulated in model systems (e.g.,
sensitivity of Cu-O2 structure and reactivity to coordination
number, geometry, and bonding atoms).1,2 Several enzymessPHM,
DâH, and CcO3spossess active site Cu ions bound by contiguous
histidine residues, a binding motif unique to these redox/O2-
processing enzymes.4,5
We hypothesized that HisHis ligation of copper ion, in particular
CuI (the reduced form of the CuII/CuI redox pair) may enforce
structural/binding properties sin particular, a linear 2-coordinate
geometrysthat might also dictate particular redox properties and/or
reactivity patterns for the system, as has been observed in two-coor-
dinate CuI complexes of monodentate ligands (also see later).6-10
Thus, we set out to investigate these biologically relevant coordina-
tion aspects of HisHis. In this report, we describe the generation
of a series of such dipeptides and demonstrate via spectroscopic
Figure 1. EXAFS and XANES data and results carried out on copper(I)
complexes 1-4, and results of DFT calculations related to [LδCuI]+ (1).
interrogation and chemical behavior that indeed, strong preferences
for near-linear two-coordinate CuI geometries are observed.
Dipeptides with regioselectively substituted imidazole sidechains
(Nꢀ vs Nδ) have been synthesized by modifications of literature
procedures and standard solution-phase techniques.11 Tautomeric
preferences appear in Cu enzymes:12-14 Cu ion binds to NδNδ of
the HisHis fragment of the PHM CuH site,5,15,16 but NꢀNꢀ at the
cyt. c oxidase CuB site.4,12,17,18 The ligands synthesized for study
(diagram) were chosen in order to elucidate the implications of
these binding modes.
ligand scatterers with Cu-N ≈ 1.86-1.87 Å are indicative of linear
2-coordinate CuI. (Cu-N does not deviate from 1.86 to 1.88 Å for
known chemical examples.6-9,20) Three-coordinate complexes have
significantly longer Cu-N bond distances. Near-edge (XANES)
data (Figure 1) corroborate 2- and not 3-coordination, based on
strong precedent; the 2-coordinate [LδCuI]+ (1) near-edge absorption
is intense compared to 3-coordinate analogues (vide infra).21,22
DFT geometry optimization (B3LYP/6-311G**) supports the
experimental (i.e., EXAFS) structure analysis (Figure 1).11 A Cu-
HisNδHisNδ model minimizes to near-linear 2-coordinate geometry
with Cu-N bonds within 0.002-0.005 Å of the EXAFS values.23
In contrast, molecular mechanics and DFT calculations suggest that
an intramolecular 2-coordinate structure bound solely by the Nꢀ of
His imidazoles is thermodynamically disfavored, requiring severe
strain in the ligand. A dimeric Cu2L2 structure, that is, with
intermolecular Cu-HisNꢀ binding, is proposed to rationalize the
EXAFS data (also see below).
Solution (acetone) conductivity data for all three complexes were
also acquired. Onsager plots for [LδCuI]ClO4 and [LHCuI]ClO4 have
slopes in the range expected for 1:1 (monomeric) electrolytes, that
is, consistent with the mononuclear complex formulation.24 How-
ever, the slope for the Lꢀ complex indicates a 2:1 electrolyte behav-
ior; that is, a dimer, [(Lꢀ)2CuI2](ClO4)2, persists in acetone, as was
formulated for the solid (vide supra). The preference for Lꢀ to form
a dimeric structure further demonstrates the favorability of the
2-coordinate near-linear geometry for these CuI-ligand systems.
Additionally, [LΗCuI]+, with unblocked imidazole N-atoms, gives
a 2-coordinate structure and 1:1 solution conductivity, suggesting
it may preferentially bind to Nδ nitrogens of adjacent His residues.
CuI complexes of Lδ, Lꢀ, and LH were synthesized in either CH2-
Cl2 or acetone using [CuI(MeCN)4]Y salts (Y ) ClO4-, B(C6F5)4-).11
Solid complexes were isolated by precipitation and purified by
recrystallization; they give satisfactory C,H,N combustion analysis,
ESI-MS mass envelope isotope patterns are consistent with the
[LCuI]+ cation formulations and small shifts in the imidazolyl C-H
resonances are observed by 1H NMR spectroscopy of [LCuI]+
compared to that of the free ligand.11
EXAFS analysis of the solid complexes provides unequivocal
evidence for a near-linear 2-coordinate geometry, with His ligation19
in all cases. For [LδCuI]+, the Fourier transform and the results/
data for all complexes are given in Figure 1.11 Fits to two NHis
-
† The Johns Hopkins University.
‡ OGI School of Science and Engineering at OHSU.
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10.1021/ja0708013 CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Scheme 1
ments, such as occurs in CcO (vide supra) and also in the copper
binding portion of the amyloid beta (Aâ) peptide involved in
Alzheimer’s Disease.31
Acknowledgment. We gratefully acknowledge financial support
of this work (Grants NIH GM28962, K.D.K.; NIH Postdoctoral
Fellowship, R.A.H.; NIH NS27583, N.J.B.).
Supporting Information Available: Synthetic and computational
details, spectroscopic methods and data, and EXAFS fitting methods
are included. This material is available free of charge via the Internet
at http://pubs.acs.org.
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