Models for Amide Ligation in Nonheme Iron Enzymes

Full text of DOI:10.1021/ic970541y

5424
Inorg. Chem. 1997, 36, 5424-5425
Models for Amide Ligation in Nonheme Iron Enzymes
Sanjay K. Mandal and Lawrence Que, Jr.*
Department of Chemistry and Center for Metals in Biocatalysis, University of Minnesota, 207 Pleasant Street, S.E.,
Minneapolis, Minnesota 55455
ReceiVed May 8, 1997
Lipoxygenase and isopenicillin N synthase belong to an
emerging class of mononuclear nonheme iron(II) enzymes
involved in dioxygen activation.1 Though these two enzymes
catalyze very different reactions, they possess common structural
features. Crystal structures2,3 of these enzymes (Figure 1) show
the divalent metal centers to be coordinated to a 2-His-1-
carboxylate facial triad1c as well as an amide ligand. These
enzymes represent the first examples of amide ligation to iron
in biology, a feature not anticipated by prior spectroscopic
studies. The amide ligand appears to play a role in catalysis,
as indicated by site-directed mutagenesis studies of the two
enzymes.4,5 To determine the effects of an amide ligand on
the properties of an iron center, we have synthesized iron
complexes of the tetradentate ligand bis(2-pyridylmethyl)-
glycinamide (BPGm). [FeII(BPGm)(O2CCH3)(CH3OH)](BPh4)
(1) represents the first example of a complex with a ligand
combination that corresponds closely to the coordination
environments found for the metal centers in lipoxygenase and
isopenicillin N synthase. For comparison, we also report the
structures of [FeII(TPA)(O2CC(CH3)3)(CH3OH)](BPh4) (2, TPA
) tris(2-pyridylmethyl)amine) and [FeIII2(µ-O)(µ-O2CCH3)-
(BPGm)2](ClO4)3 (3) and their properties.
Anaerobic reaction of equimolar amounts of Fe(ClO4)2,
BPGm,6 and NaO2CCH3 in methanol at ambient temperature
followed by the addition of NaBPh4 afforded the yellow
crystalline solid 1.7 The crystal structure8 of the cation in 1
(Figure 2a) resembles that of the related 29,10 (Figure 2b); both
complexes have a mononuclear, distorted octahedral iron center
Figure 1. Active sites of lipoxygenase and isopenicillin N synthase.
bound to a tetradentate tripodal ligand, a monodentate carboxy-
late, and methanol. The metal centers are in the high-spin
Fe(II) state as indicated by average Fe-N distances of 2.22 Å
and their isotropically shifted 1H NMR signals (see Supporting
Information). The amide function in 1 binds to the iron through
its carbonyl oxygen atom with an Fe-Oamide distance of
2.170(5) Å),11 which is slightly longer than those observed in
Fe(II)-DMF complexes (average 2.12 Å).12 However it is
significantly longer than those found for the terminal carboxy-
lates in 1 (2.024(5) Å) and 2 (1.988(4) Å), which probably
reflects the difference in charge between the amide and
carboxylate groups. From this perspective, the amide ligand
behaves more like a pyridine, as indicated by the comparable
length of the corresponding Fe-Npyridine bond in 2 (2.212(6)
Å) when the differing covalent radii of oxygen and nitrogen
are taken into account.
The near-IR spectra of 1 and 2 in CH3CN can be compared
to assess the ligand field strength of the amide function.
Complex 1 exhibits a broad feature with λmax at 1000 nm (ꢀ )
(1) (a) Que, L., Jr.; Ho, R. Y. N. Chem. ReV. 1996, 96, 2607-2624. (b)
Feig, A. L.; Lippard, S. J. Chem. ReV. 1994, 94, 759-805. (c) Hegg,
E. L.; Que, L., Jr. Eur. J. Biochem., in press.
(2) Minor, W.; Steczko, J.; Stec, B.; Otwinowski, Z.; Bolin, J. T.; Walter,
R.; Axelrod, B. Biochemistry 1996, 35, 10687-10701.
(3) (a) Roach, P. L.; Clifton, I. J.; Fu¨lo¨p, V.; Harlos, K.; Barton, G. J.;
Hajdu, J.; Andersson, I.; Schofield, C. J.; Baldwin, J. E. Nature 1995,
375, 700-704. (b) Roach, P. L.; Clifton, I. J.; Hensgens, C. M. H.;
Shibata, N.; Schofield, C. J.; Hajdu, J.; Baldwin, J. E. Nature 1997,
387 827-830.
(4) Kramer, J. A.; Johnson, K. R.; Dunham, K. R.; Sands, R. H.; Funk,
M. O., Jr. Biochemistry 1994, 33, 15017-15022.
5
8 M-1 cm-1), which can be attributed to the T2g
f
5Eg
transition of an octahedral d6 ion. The corresponding band for
2 has a λmax at 956 nm (ꢀ ) 7 M-1 cm-1), the blue shift
indicating that the amide in BPGm exerts a weaker ligand field
than the corresponding pyridine on TPA. This result is
consistent with a comparative study of soybean and mammalian
lipoxygenases,13 whose active sites differ by the substitution of
the amide ligand in the former with a histidine in the latter.
A further assessment of the effects of an amide group can be
obtained from the corresponding (µ-oxo)diiron(III) complex.
Exposure of 1 to O2 results in the formation of 3,14 whose crystal
structure15 (Figure 2c) shows a (µ-oxo)diiron(III) unit supported
by an acetate bridge. The dimensions of this dibridged core
resemble those of the closely related complexes [Fe2(µ-O)(µ-
(5) (a) Landman, O.; Borovok, I.; Aharanowitz, Y.; Cohen, G. FEBS Lett.
1997, 405, 172-174. (b) Sami, M.; Brown, T. J. N.; Roach, P. L.;
Schofield, C. J.; Baldwin, J. E. FEBS Lett. 1997, 405, 191-194.
(6) The BPGm ligand was prepared by refluxing equimolar amounts of
bis(2-pyridylmethyl)amine and bromoacetamide and excess Na2CO3
in CH3CN for 2 h and isolated as the HBr salt. After neutralalization
and extraction with CH2Cl2, the ligand was obtained in 85% yield.
(7) Anal. Calcd for [FeII(BPGm)(O2CCH3)(MeOH)](BPh4)‚CH3OH‚
3H2O: C, 62.40; H, 6.56; N, 6.93. Found: C, 62.50; H, 5.84; N, 7.05.
(8) Crystal data for 1‚2CH3OH: yellow needles, space group P21/c (No.
14), with a ) 17.4278(13) Å, b ) 9.0809(7) Å, c ) 27.361(2) Å, â
) 105.936(2)°, V ) 4163.7(5) Å3, and Z ) 4. The structure was refined
by using 2385 reflections (I > 3σ(I)) and 374 parameters to final
discrepancy indices R1 ) 0.0678 and wR2 ) 0.1243, based on F2
derivatives.
(11) The oxygen and nitrogen atoms of the amide ligand were assigned in
the structure shown by a number of criteria: the comparable sizes of
the thermal ellipsoids, the C-X bond lengths, and the presence of a
hydrogen-bonding network for the NH2 group.
(12) (a) Constant, G.; Daran, J. C.; Jeannin, Y. J. Inorg. Nucl. Chem. 1971,
33, 4209-4217. (b) Mu¨ller, A.; Schladerbeck, N. H.; Krickemeyer,
E.; Bo¨gge, H.; Schmitz, K. Z. Anorg. Allg. Chem. 1989, 570, 7-36.
(13) Pavlosky, M. A.; Zhang, Y.; Westre, T. E.; Gan, Q.-F.; Pavel, E. G.;
Campochiaro, C.; Hedman, B.; Hodgson, K. O.; Solomon, E. I. J.
Am. Chem. Soc. 1995, 117, 4316-4327.
(14) Alternatively, 3 can also be obtained from Fe(ClO4)3, BPGm, and
NaO2CCH3 in a 2:2:1 ratio in methanol in 90% yield. Anal. Calcd for
[FeIII2(µ-O)(µ-O2CCH3)(BPGm)2](ClO4)3: C, 36.11; H, 3.51; N, 11.23.
Found: C, 35.86; H, 3.56; N, 10.94. Caution! Metal complexes with
organic ligands and perchlorate anions are potentially explosive.
(9) Anal. Calcd for [FeII(TPA)(O2CC(CH3)3)](BPh4): C, 73.62; H, 6.13;
N, 7.31. Found: C, 73.90; H, 6.20; N, 7.05.
(10) Crystal data for 2‚CH3OH: space group P212121 (No. 19) with a )
9.4635(2) Å, b ) 23.8909(4) Å, c ) 39.7436(4) Å, V ) 8985.7(3)
Å3, and Z ) 8. The structure was solved by using 8740 reflections (I
> 2σ(I)) and 1068 parameters to final discrepancy indices R1 ) 0.0634
and wR2 ) 0.1278, based on F2 derivatives. There were two
independent molecules of 2 in the crystallographic asymmetric unit.
S0020-1669(97)00541-7 CCC: $14.00 © 1997 American Chemical Society

Communications
Inorganic Chemistry, Vol. 36, No. 24, 1997 5425
Table 1. Comparison of the Properties of [FeII(L)(O2CR)(CH3OH)] and [FeIII2(O)(L)2(O2CR)] Complexes
La
r(FeII-X) (Å)
λ
max (nm)
r(FeIII-X) (Å)
λ
max (nm)
E1/2red (mV vs NHE)
TPA (X ) py)
2.212(6)
2.170(5)
956
1000
2.116(6)b
2.043(5)
1.988(5)e
700, 1050b
645, 950
+150 (rev)c
-200 (irrev)
-350 (irrev)
BPGm (X ) C(O)NH2)
BPG (X ) COO-)
[2.024(5)]d
636, 920e
a X is the variable pendant arm on the tripodal ligand. b From ref 16. c From ref 20. d We have not been able to synthesize the corresponding
mononuclear BPG complex; we approximate this value based on the Fe-O2CR distance observed in 1. e From ref 17.
Figure 2. Crystal structures of the cation in [Fe(CH3CO2)(BPGm)(MeOH)](BPh4) (1), [Fe(TPA)(O2CC(CH3)3)(CH3OH)](BPh4) (2), and [Fe2(µ-
O)(µ-O2CCH3)(BPGm)2](ClO4)3 (3). Selected interatomic distances and an angle are as follows. For 1: Fe(1)-O(1) 2.185(6) Å, Fe(1)-O(2) 2.035-
(5) Å, Fe(1)-O(4) 2.132(6) Å, Fe(1)-N(1) 2.288(7) Å, Fe(1)-N(3) 2.208(7) Å, Fe(1)-N(4) 2.184(7) Å. For 2: Fe(1A)-O(1A) 1.988(4) Å,
Fe(1A)-O(3A) 2.154(4) Å, Fe(1A)-N(1A) 2.264(5) Å, Fe(1A)-N(2A) 2.207(5) Å, Fe(1A)-N(3A) 2.212(6) Å, Fe(1A)-N(4A) 2.207(5) Å. For
3: Fe(1)-O(1) 1.800(4) Å, Fe(1)-O(2) 2.063(4) Å, Fe(1)-O(4) 2.045(5) Å, Fe(1)-N(1) 2.252(5) Å, Fe(1)-N(2) 2.126(6) Å, Fe(1)-N(3) 2.109-
(5) Å, Fe(2)-O(1) 1.783(4) Å, Fe(2)-O(3) 2.053(4) Å, Fe(2)-O(5) 2.042(5) Å, Fe(2)-N(5) 2.240(5) Å, Fe(2)-N(6) 2.125(5) Å, Fe(2)-N(7)
2.139(6) Å, Fe(1)‚‚‚Fe(2) 3.202(1) Å, Fe(1)-O(1)-Fe(2) 126.6(2)°.
O2CCH3)(TPA)2](ClO4)3 (4)16 and [Fe2(µ-O)(µ-O2CC6H5)-
(BPG)2](ClO4) (5)17 (BPG ) bis(2-pyridylmethyl)glycinate),
with Fe-Fe distances of 3.22 ( 0.02 Å and Fe-O-Fe angles
of 126-130°. The remaining sites of the iron ions in 3 are
occupied by the BPGm ligand, with the amine nitrogens
positioned trans to the oxo group as found in 5. The amide in
3 binds trans to one of the pyridines with an Fe-Oamido bond
of 2.04 Å, comparable in length to that found for an FeIII-
DMF complex18 (2.03(1) Å) but much shorter than that of the
other recently reported BPGm complex, [Fe2(µ-O)Cl2(BPGm)2]-
(ClO4)2 (2.165(6) Å),19 where the amido oxygens are trans to
the oxo bridge. Unlike in the Fe(II) complexes, the Fe-Oamide
bond in 3 is only 0.05 Å longer than the terminal Fe-Ocarboxylate
bond in 5 and is comparable in length to the Fe-O bonds of
the bridging carboxylate. This comparison suggests that the
interaction of the amide with the Fe(III) center makes it behave
more like a carboxylate ligand.
strength for complexes with similar Fe-O-Fe angles. For high-
spin d5 ions, increasing ligand field strength results in transitions
of lower energy. From Table 1 it can be seen that the ligand
field bands of 3 approach those of 5 and differ significantly in
energy from those of 4. Similarly, the redox properties of 3
resemble those of 5. Both 3 and 5 exhibit irreversible one-
electron reductions, at -200 and -350 mV vs NHE, respec-
tively, while 4 exhibits a quasireversible reduction at +150
mV.20 Taken together, the data suggest that the amide group
acts like a carboxylate group with a partial negative charge;
this may come about by greater population of the imino form
of the amide as a result of its interaction with the Lewis acidic
FeIII center.
In conclusion, we have prepared and characterized FeII and
FeIII complexes of BPGm, a tripodal ligand with a pendant
amide group to model the coordination environments of the iron
centers in lipoxygenase and isopenicillin N synthase. By
comparison with structurally related complexes having pendant
carboxylate and pyridine ligands, the amide ligand appears to
behave like a neutral pyridine ligand in the FeII complex but
becomes more like a carboxylate with a partial negative charge
when the iron center becomes FeIII. This ability of the amide
ligand to modulate its electronic effects may be useful for tuning
the properties of an iron center as it goes through a redox cycle.
The optical and electrochemical properties of 3 support the
notion that the amide ligand in 3 is more carboxylate-like. The
optical spectrum of 3 in CH3CN contains features with λmax (ꢀ,
M-1 cm-1) ) 411 (2100), 435 (1700), 479 (800), 489 (1000),
521 (320), 645 (180), and 950 (8) nm, which are very similar
to those observed for 4 and 5. The bands in the 550-800 and
6
800-1100 nm spectral regions can be assigned to the A1 f
4T2 and 6A1
f
4T1 ligand field transitions of a d5 ion,
Acknowledgment. Funding for this work was provided by
the Unilever Research Laboratory, Vlaardingen, The Nether-
lands, and the National Institutes of Health (GM-33162). We
thank Dr. V. G. Young, Jr., of the University of Minnesota X-ray
Crystallographic Laboratory for the crystal structures.
respectively,21 and spectral shifts reflect changes in ligand field
(15) Crystal data for 3‚2CH3CN: space group P21/n (No. 14) with a )
13.6381(2) Å, b ) 23.5590(3) Å, c ) 15.7021(2) Å, â ) 108.594-
(1)°, V ) 4781.7(8) Å3, and Z ) 4. The structure was solved by using
5609 reflections (I > 2σ(I)) and 650 parameters to final discrepancy
indices R1 ) 0.0822 and wR2 ) 0.2132, based on F2 derivatives.
(16) Norman, R. E.; Yan, S.; Que, L., Jr.; Backes, G.; Ling, J.; Sanders-
Loehr, J.; Zhang, J. H.; O’Connor, C. J. J. Am. Chem. Soc. 1990,
112, 1554-1562.
Supporting Information Available: 1H NMR spectra of 1 and 2
(2 pages). X-ray crystallographic files, in CIF format, for complexes
1-3 are available on the Internet only. Ordering and access information
is given in any current masthead page.
(17) Me´nage, S.; Que, L., Jr. New J. Chem. 1991, 15, 431-438.
(18) Reiff, W. M.; Witten, E. H.; Mottle, K.; Brennan, T. F.; Garafalo, A.
R. Inorg. Chim. Acta 1983, 77, L27-L30.
IC970541Y
(19) Ito, S.; Okuno, T.; Matsushima, H.; Tokii, T.; Nishida, Y. J. Chem.
Soc., Dalton Trans. 1996, 4479-4484.
(20) Holz, R. C.; Elgren, T. E.; Pearce, L. L.; Zhang, J. H.; O’Connor, C.
J.; Que, L., Jr. Inorg. Chem. 1993, 32, 5844-5850.
(21) Reem, R. C.; McCormick, J. M.; Richardson, D. E.; Devlin, F. J.;
Stephens, P. J.; Musselman, R. L.; Solomon, E. I. J. Am. Chem. Soc.
1989, 111, 4688-4704.