Carbon monoxide and nitrogen monoxide ligand dynamics in synthetic heme and heme-copper complex systems

Article abstract of DOI:10.1021/ja906172c

(Chemical Equation Presented) Intermolecular nitrogen monoxide (·NO) and carbon monoxide (CO) transfer from iron to copper and back, a phenomenon not previously observed, has been accomplished by employing transient-absorbance laser flash photolysis metho

Full text of DOI:10.1021/ja906172c

Published on Web 09/08/2009
Carbon Monoxide and Nitrogen Monoxide Ligand Dynamics in Synthetic
Heme and Heme-Copper Complex Systems
Heather R. Lucas, Gerald J. Meyer,* and Kenneth D. Karlin*
Department of Chemistry, The Johns Hopkins UniVersity, Baltimore, Maryland 21218
Received July 23, 2009; E-mail: meyer@jhu.edu; karlin@jhu.edu
Scheme 1
Photoinitiated ligand dissociation with spectroscopic monitoring
(UV-vis, IR, and resonance Raman) has typically been used to
elucidate CO, O2, and NO binding dynamics in heme proteins,
including heme-copper oxidases (HCOs).1-3 Through time-
resolved spectroscopy, Woodruff and co-workers confirmed CuB
as the site of O2 entry into the HCO active site on the basis of
critical insights gained by monitoring CO transfer between hemea3
and CuB following photodissociation from hemea3-CO (Scheme
1).1b,c However, the detailed role of CuB in HCO/ ·NO(g) biochem-
istry is not well-understood; in fact, copper-NO interactions are
very difficult to study.4
Chart 1
Nitrogen monoxide is a known respiratory inhibitor (as is CO);5
it can also be a substrate undergoing (i) oxidation to nitrite6 or (ii)
reductive coupling, 2NO + 2e- + 2H+ f N2O + H2O.7 Thus, the
interaction of heme proteins, including HCOs, with ·NO(g) is a
subject of considerable importance and broad current interest. Large
differences in NO reduction capabilities among HCOs exist and
are thought to be related to variations in active-site structure/
chemistry. Despite this knowledge, little is known about the details
of CuB/NO interactions, creating a large gap in our understanding
of HCO/ ·NO(g) chemistry. Here it is shown how one can apply
heme-NO/CO photoinitiated chemistry to gain insights into NO
binding/dynamics via investigations employing synthetic heme/Cu
assemblies.3
Scheme 2
Previously, transient absorbance (TA) laser flash photolysis and
time-resolved IR spectroscopy were utilized to examine intramo-
lecular CO migration from an in situ-formed heme-CO species to
copper(I) and back to the heme by employing a heme/Cu complex
of the ligand 6L (Chart 1).3b Attempts to extend this work to
examine NO and/or O2 heme/copper dynamics were ineffective
because of complex instability.4 An advance here is the utilization
of synthetic heme/copper systems that consist of 1:1 components.
More experiments become possible because separate and stable
heme-CO and heme-NO complexes can be employed. Specifi-
cally, here we used the heme F8, the tetradentate chelate for copper
PyL (that found within 6L), as well as a tridentate ligand BzL (Chart
1).8a With these new systems, CO and NO transfer from iron(II)
to copper(I) have been measured (Scheme 2).
Single-wavelength excitation (λex ) 532 nm; 298 K) of
[(F8)FeII(CO)(DCIM)] (Chart 1)8a resulted in CO photodissociation,
formation of [(F8)FeII(Solv)(DCIM)], and subsequent CO rebinding.
In deoxygenated THF, CH3CN, and acetone solutions, k+FeCO values
were obtained in the presence and absence of exogenously added
copper(I) complex species PyLCuI or BzLCuI.8b For the CO-migration
experiments, samples were prepared in a FeII/CuI ratio range of
1:1 to 1:20 equiv (1 equiv ≈ 70 µM) under an inert atmosphere
within a glovebox.
in the presence of BzLCuI. The first-order rate constants result from
solvent coordination to the metal ion after CO photodissociation.2
The smaller k values measured in the presence of RLCuI are ascribed
to processes in which photoejected CO first reacts with RLCuI to
give RLCuI-CO (which is separately isolable) and returns to
[F8FeII(solv)(DCIM)] following CO deligation (Scheme 2). A
similar result was obtained with an HCO; when site-directed
mutagenesis led to the absence of active site CuB, a greater rate of
CO rebinding to hemea3 resulted.9 In CH3CN solvent, a change in
the rate for CO rebinding to [F8FeII(CO)(DCIM)] was not observed
in the presence of either RLCuI species; instead, k+FeCO ) 16 s-1
was consistently measured. The results suggest that CH3CN hinders
CO binding to RLCuI, since nitriles are strong Lewis basic ligands
for copper(I) ions.
In THF solvent and following CO photoejection from
[F8FeII(CO)(DCIM)], the heme-CO rebinding process followed
first-order kinetics and decreased from 65 s-1 (k+FeCO) to 10 s-1
Photoinitiated CO transfer from [F8FeII(CO)(DCIM)] to PyLCuI
also occurred in acetone, as evidenced by the observation of a
decrease in k+FeCO from 77 to 12 s-1 (k-CuCO/+FeCO). However,
addition of BzLCuI to [F8FeII(CO)(DCIM)] in acetone resulted in a
(k-CuCO/+FeCO) in the presence of PyLCuI and to 7 s-1 (k-CuCO/+FeCO
)
9
13924 J. AM. CHEM. SOC. 2009, 131, 13924–13925
10.1021/ja906172c CCC: $40.75  2009 American Chemical Society

C O M M U N I C A T I O N S
Scheme 3
results suggest that binding of ·NO to the reduced heme is faster
than that for CO (k+FeNO > k+FeCO), as is known from heme-protein
studies.2a,5
While the study of the binding of small gaseous ligands (·NO,
CO, O2) to hemes or heme-copper proteins is a mature field,
research activity in the area is still vigorous, as kinetic spectroscopic
interrogations continue to yield new insights into dynamics,
structure, and even mechanism of reaction. In this report, we have
shown the first example of reversible 1:1 intermolecular small-
molecule transfer, from a heme to copper(I), for both CO and ·NO;
kinetic parameters were obtained. The rates of reaction are less
than those observed in intramolecularly preorganized systems, such
thermal CO transfer reaction and a disproportionation reaction,
leading to BzLCuI-CO, [F8FeII(DCIM)2], and [F8FeII(Solv)2], as
evidenced by benchtop UV-vis absorption changes and IR analysis
of the product mixture (Scheme 3). These observations indicate
that the CO equilibrium binding constant for BzLCuI is higher than
that for [(F8)FeII(Solv)(DCIM)] (i.e., KCuCO > KFeCO).
Single-wavelength excitation (λex ) 532 nm; 298 K) of
[F8FeII(NO)(solv)]10 resulted in ·NO photodissociation, formation
of [F8FeII(solv)2], and subsequent ·NO rebinding. An absorption
difference spectrum, Abs{[F8FeII(thf)2] - [F8FeII(NO)(thf)]}, of this
·NO rebinding process in THF is shown in Figure 1. The calculated
∆A spectrum obtained through benchtop UV-vis spectroscopy
overlays perfectly, confirming the assigned process. Bimolecular
rate constants (kNO) could not be determined because of difficulties
in purifying NO during its passage through the gas mixer; this
situation will be addressed in future studies.
6
as in some HCOs or even the L heme/Cu framework, yet fast
enough to prevent or overcome other irreversible CuI/·NO reaction
chemistries.12 Since ·NO migration was observed, we can conclude
that like CO, ·NO kinetically favors binding to copper but
thermodynamically favors coordination to iron. Future experiments
will be directed toward obtaining complementary thermodynamic
data while employing 1:1 Fe/Cu component systems and elucidating
trends with systematic variation of Cu chelate (i.e., with different
denticity, donor-atom type, or CuII/I E1/2 value) and/or heme system.
Acknowledgment. We are grateful for support of this research
(K.D.K., NIH GM60353; G.J.M., NSF CHE0616500). We also
acknowledge Drs. A. A. N. Sarjeant and M. A. Siegler for the X-ray
structural determination of F8FeII(DCIM)2.
Supporting Information Available: Experimental details, Xray
crystallographic data for {F8FeII(DCIM)2} (CIF), ∆A spectra for CO
rebinding to F8FeII(DCIM)(Solv), and plots for 2nd order CO binding
rates.8b This material is available free of charge via the Internet at http://
pubs.acs.org.
In the presence of 1:1 and 1:20 (FeII/CuI) equiv of PyLCuI
(Scheme 2), biexponential · NO rebinding kinetics (k1, k2) were
observed upon photoejection of NO from [(F8)FeII(NO)(thf)] in
THF. The first, faster process (k1) involves direct rebinding of the
free ·NO molecule to the heme without transfer to PyLCuI; this
occurs with the same rate as measured independently (k1 ≈ k+FeNO
) 432 s-1). The second, slower process (k2) involves ·NO binding
to [(F8)FeII(thf)2] following initial coordination to PyLCuI; a
decreased rebinding rate of k2 ≈ k-CuNO/+FeNO ) 64 s-1 was
observed. Notably, unlike this present case of ·NO(g), direct
rebinding of CO to the heme in the presence of PyLCuI (k1) was
not observed (see above). This finding of “inefficient” NO iron-
to-copper migration (i.e., some ·NO rebinds to the heme) may
References
(1) (a) Lucas, H. R.; Karlin, K. D. Met. Ions Life Sci. 2009, 6, 295. (b) Bailey,
J. A.; Tomson, F. L.; Mecklenburg, S. L.; MacDonald, G. M.; Katsonouri,
A.; Puustinen, A.; Gennis, R. B.; Woodruff, W. H.; Dyer, R. B.
Biochemistry 2002, 41, 2675. (c) Dyer, R. B.; Peterson, K. A.; Stoutland,
P. O.; Woodruff, W. H. Biochemistry 1994, 33, 500. (d) Treuffet, J.;
Kubarych, K. J.; Lambry, J. C.; Pilet, E.; Masson, J. B.; Martin, J. L.;
Vos, M. H.; Joffre, M.; Alexandrou, A. Proc. Natl. Acad. Sci. U.S.A. 2007,
104, 15705. (e) Vos, M. H. Biochim. Biophys. Acta 2008, 1777, 15. (f)
Pinakoulaki, E.; Ohta, T.; Soulimane, T.; Kitagawa, T.; Varotsis, C. J. Am.
Chem. Soc. 2005, 127, 15161. (g) Larsen, R. W.; Miksovska, J. Coord.
Chem. ReV. 2007, 251, 1101. (h) Hayashi, T.; Lin, M. T.; Ganesan, K.;
Chen, Y.; Fee, J. A.; Gennis, R. B.; Moe¨nne-Loccoz, P. Biochemistry 2009,
48, 883.
suggest a lower affinity of PyLCuI for ·NO than CO, i.e. KCuCO
>
KCuNO. In fact, exactly such conclusions concerning CuB were drawn
by Vos et al.11 from NO dynamics experiments for cytochrome c
oxidase aa3. Further, since the NO and CO migration experiments
were conducted with the same component concentration range, the
(2) (a) Ford, P. C.; Lorkovic, I. M. Chem. ReV. 2002, 102, 993. (b) Ford, P. C.;
Laverman, L. E. Coord. Chem. ReV. 2005, 249, 391. (c) Wanat, A.; Wolak,
M.; Orzel, L.; Brindell, M.; van Eldik, R.; Stochel, G. Coord. Chem. ReV.
2002, 229, 37.
(3) (a) Fry, H. C.; Lucas, H. R.; Narducci Sarjeant, A. A.; Karlin, K. D.; Meyer,
G. J. Inorg. Chem. 2008, 47, 241. (b) Fry, H. C.; Cohen, A. D.; Toscano,
J. P.; Meyer, G. J.; Karlin, K. D. J. Am. Chem. Soc. 2005, 127, 6225.
(4) Primary interactions of copper(I) species with · NO and O2 typically lead
to species that are not stable or easy to handle at room temperature.
(5) Brunori, M.; Forte, E.; Arese, M.; Mastronicola, D.; Giuffre, A.; Sarti, P.
Biochim. Biophys. Acta 2006, 1757, 1144.
(6) Castello, P. R.; Woo, D. K.; Ball, K.; Wojcik, J.; Liu, L.; Poyton, R. O.
Proc. Natl. Acad. Sci. U.S.A. 2008, 105, 8203.
(7) Zumft, W. G. J. Inorg. Biochem. 2005, 99, 194.
(8) (a) See the Supporting Information. (b) Second-order rate constants for
CO binding to [(F8)FeII(Solv)(DCIM)] and RLCuI complexes were
obtained.8a
(9) Lemon, D. D.; Calhoun, M. W.; Gennis, R. B.; Woodruff, W. H.
Biochemistry 1993, 32, 11953.
(10) Wang, J.; Schopfer, M. P.; Sarjeant, A. A.; Karlin, K. D. J. Am. Chem.
Soc. 2009, 131, 450.
(11) Vos, M. H.; Lipowski, G.; Lambry, J. C.; Martin, J. L.; Liebl, U.
Biochemistry 2001, 40, 7806.
(12) Wasser, I. M.; de Vries, S.; Moe¨nne-Loccoz, P.; Schro¨der, I.; Karlin, K. D.
Chem. ReV. 2002, 102, 1201.
Figure 1. Absorption difference spectra (λex ) 532 nm; 298 K) representing
NO rebinding to [F8FeII(thf)2] following photoejection from
[F8FeII(NO)(thf)]; the inset is a kinetic trace with a first-order fit.
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