Communications
DOI: 10.1002/anie.200702120
Allosteric Receptors
Simple Heme Dimers with Strongly Cooperative Ligand Binding**
Daria Khvostichenko, Qing-Zheng Yang, and Roman Boulatov*
We report that the simple stable heme dimers (1-Fe)2 and (2-
Fe)2 display strongly cooperative ligand binding. For example,
(1-Fe)2 binds a second isocyanide molecule with an approx-
best known examples was reported by Collman et al. over
25 years ago.[6] However, our compounds stand out in their
simplicity: prior to our work, only hemes in crystals, in
polymers, or in the presence ofa large excess ofa competing
ligand displayed cooperativity.[7] Our receptors are coopera-
tive in solution with the affector as the only ligand present.
This simplicity necessarily limits their structural resemblance
with allosteric proteins, such as hemoglobin, but could open
up new opportunities for the applications discussed above.
The synthesis of( 1-Fe)2 and (2-Fe)2 is straightforward.[8]
UV/Vis spectra of( 1-Fe)2 in hexanes, toluene, or CHCl3 at
concentrations of0.1–50 mm manifest a strong sharp Soret
band at 434 nm (e = 2.3 105 mÀ1 cmÀ1) with a weaker com-
panion at 371 nm (e = 4.1 104 mÀ1 cmÀ1) and two well-defined
Q bands at 538 and 568 nm; the spectra of( 2-Fe)2 are
similar.[8] These spectra are highly characteristic ofa efrro-
heme coordinated axially to a single N-heterocyclic base,[9]
which in our case is only possible ifthe hemes dimerize. The
lower limit ofthe dimerization constant ( Ka > 5 108 mÀ1) is
determined by the absence ofthe spectroscopic signature of
the monomeric (four-coordinate) heme in 0.1 mm solutions.
Exposure ofparamagnetic ( 1-Fe)2 or (2-Fe)2 to CO,
Me2C6H3NC, or PhNO generates diamagnetic species with
NMR spectra that are highly indicative ofdimers with one
exogenous ligand per heme.[8] In these adducts, imidazole
hydrogen atoms resonate at frequencies 2–7 ppm upfield from
those in the free bases, at positions typical for imidazole
protons coordinated to a diamagnetic ferroheme.[8,9] Similarly,
the resonances ofthe b-pyrrolic H atoms and those ofthe
imidazole-bearing phenyl group shift upfield by as much as
3 ppm.[8] The pattern of shifts, which result from shielding by
the porphyrin ring current, unambiguously reveals the
dimeric heme core. The adduct formation is reversible:
evacuation ofthe samples at < 10 mbar regenerates the five-
coordinate hemes. (1-Fe)2 reacts rapidly with O2 to yield the
(por)Fe(mO)Fe(por) chromophore (por: porphyrin); oxygen-
ation of( 2-Fe)2 yields a product with a UV/Vis spectrum
imately 50-fold higher affinity than the first, or with an
allosteric interaction energy of DGa = 13 Æ 3 kJmolÀ1. The
degree ofcooperativity is comparable to that ofhemoglobin,
a prototypical cooperative receptor for small molecules,
whereas the mechanism appears to be distinct.
With these results, we aim to help address the unmet need
for simple, robust, and easily accessible synthetic receptors
that bind small molecules (O2, NO, CO) cooperatively. Such
receptors would be ofgreat value for a number ofapplications
(noncryogenic air separation,[1] chemical O2 delivery/stor-
age,[2] chemical sensors[3]) and fundamental studies[3–5] (for
example, kinetics ofmultisite catalysis with interacting sites,
conformational information transfer). The heme molecule is
unmatched as a receptor for small molecules and its
exploitation in synthetic cooperative receptors seems natural.
Herein we report a simple structural motifwithin which heme
ligation is strongly cooperative. We are not the first to achieve
cooperative heme ligation in a synthetic receptor: one ofthe
[9]
identical to that ofthe O 2 adduct ofpicket-fence porphyrin,
which persists at room temperature. It remains to be
confirmed that the resulting species is (2-FeO2)2. Under no
conditions did we observe the oligomers (1-Fe)n or (2-Fe)n
(n ꢀ 3), which are easily distinguishable from the dimers by
UV/Vis (by the presence ofsix-coordinate and of ur-coordi-
nate hemes) and NMR spectroscopy.
[*] Dr. D. Khvostichenko,[+] Dr. Q. Yang,[+] Prof. R. Boulatov
Department of Chemistry
University of Illinois
Urbana, IL 61801 (USA)
Fax: (+1)217-244-3186
E-mail: boulatov@uiuc.edu
In toluene or hexanes, the dimers bind N-Me-imidazole,
Me2C6H3NC, and nitrosoaryls cooperatively (Figure 1). The
isocyanide and nitrosoaryls were chosen as analogues ofCO
and O2, respectively.[10] Spectrophotometric titrations pro-
ceeded with well-defined isosbestic points,[8] a result indica-
tive ofinterconversion between only two chromophores: the
five-coordinate and six-coordinate hemes. The binding con-
stants and allosteric interaction energies are independent of
[+] These authors contributed equally to this work.
[**] This work was supported by the University of Illinois, the National
Center for Supercomputing Applications (grant nos. CHE050064N
and CHE060025N), and the Petroleum Research Fund of the
American Chemical Society (grant no. 43354-G3). We thank J. D.
Atkinson for the Evans measurements.
Supporting information for this article is available on the WWW
8368
ꢀ 2007 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
Angew. Chem. Int. Ed. 2007, 46, 8368 –8370