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Synthetic Models for IronЦOxygen Aggregation and Biomineralization.

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plies new, surprising, and unpredictable insights on the interaction of these compounds with unmodified oligonucleotides. The data presented also show that computer modeling
is a helpful tool in the design of new DNA analogues.
German version: Angew. Chem. 1992, 104, 1039
[I] H. E. Moser, P. B. Dervan, Science 1987, 238, 645-650; P. A. Beal, P. B.
Dervan, ibid. 1991, 251, 1360-1363.
[2] M. H. Caruthers, Science 1985, 230, 281-285; J. Engels, E. Uhlmann,
Angew. Chem. 1989, 101, 733-753; Angew. Chem. I n t . Ed. Engl. 1989, 28,
[3] J. Engels, Nurhr. Chem. Tech. 1991, 39, 1250, and literature cited therein.
[4] One should keep in mind the successful regulation of gene expression in
tomatoes: P. Eckes, Angew’. Chem. 1992, 104, 182; Angew. Chem. I n ( . Ed.
Engl. 1992, 31, 175, and literature cited therein.
[5] E. Uhlmann, A. Peyman. Chem. Rev. 1990.90, 544-584.
[6] a) M. Egholm, 0. Burchardt, P. E. Nielsen, R. Berg, Science 1991, 2.54,
1497-1500: b) J. Am. Chem. Sue. 1992, 114, 1895-1897: cj Other authors
have designed peptides as analogues of the backbone of DNA/RNA; however, none of the derivatives examined showed hybridization properties: H.
De Koning, U. K. Pandit. Rer. Trav. Chim. 1971, 91, 1069; J. D. Buttrey,
A. S. Jones, R. T. Walker, Tetrahedron 1975, 3 i , 13-75.
[7] R. B. Merrifield, Angeu. Chem. 1985, 97,801 ; Angew. Chem. Inr. Ed. Engl.
1985, 24, 799.
[S] W. Stec, A. Grajkowski, M. Koziolkiewicz,B. Uznanski, Nucleic Acids Res.
1991,19, 58883-58888.
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1991.56.6000-6006; S.-B. Huang, J. S. Ne1son.D. D. Weller, ibid.1991.56,
6007- 6018.
Synthetic Models for Iron-Oxygen Aggregation and Biomineralization
By Karl S . Hagen*
Iron, the most abundant transition element in the earth’s
crust, plays a major role in biological systems primarily because of its rich redox chemistry, and to a lesser extent the
structural and magnetic properties of its various mineral
forms. A streamlined biological apparatus for handling free
iron is imperative, since free Fe(r1) ions will react with dioxygen to form highly reactive and destructive radicals, whereas
uncomplexed Fe(rir) ions form insoluble iron hydroxide under physiological conditions. The iron-storage protein ferritin, made up of 24 subunits forming a shell approximately
70 8, in diameter which can contain up to 4500 iron atoms,
is primarily responsible for solubilization and storage of excess Fe(In).[’l The structure of the hydroxo(oxo)iron core of
ferritin is not well-defined and is variable in composition.
The best information is obtained under low phosphate concentrations for which considerable long-range order, resembling that of the mineral ferrihydrite, is detected by X-ray
diffraction and electron microscopy. The variable phosphate
component in the core is thought not to play a dominant
structural role, although phosphate may play a significant
role in binding to isolated iron ions, small aggregates, or the
surface of large crystallites.
Iron oxides have a rich mineral chemistry, and more than
twenty years ago stable, soluble nanoparticles averaging
70 A in diameter were prepared by hydrolytic polymerization of iron(iiI).[2]Yet the chemistry of molecular iron-oxygen clusters has been dominated by the fundamental oxobridged diiron(n1) unitc3’ and the trinuclear basic iron
carboxylates. The crystal structure determination in 1981 of
a binuclear iron-oxygen species in hemerythrin and the discovery of related 0x0-bridged bimetallic centers in other
proteins has revived the study of iron-oxygen chemistry. An
extensive chemistry of binuclear model complexes has been
developed, and larger Fe,, Fe,, Fe,,, and Fe,, aggregate^,'^]
Prof. K. S. Hagen
Department of Chemistry, Emory University
1515 Pierce Drive, Atlanta, GA 30322 (USA)
0 VCH Veriugsgeselkhuft m b f f , W-6940 Weinherm. 1992
predominantly ligated by bridging carboxylato ligands, have
been characterized.
However, the precipitation process of iron hydroxide oxides remains ill-defined, and unlike the rich polyoxoanion
chemistry of early transition metal ions, no molecular
polyiron(m) species containing only aqua, hydroxo, or 0x0
ligands has been crystallized. The inability of high-spin Fe”’
ions to strongly polarize and stabilize terminal 0x0 ligands
favors the uncontrolled growth of polyiron complexes. Therefore the formation of molecular aggregates necessarily relies
on multidentate or bulky organic capping ligands. A few
model studies of the basic aggregation process are now available to aid our understanding of the molecular basis of polymer growth. This report highlights the advances in this field
culminating in a recent report in this journal’”’ on the
largest polyiron aggregates isolated and crystallographically
characterized to date.
The electronic spectroscopy of ferritin indicates octahedral coordination about the high-spin iron(irr) ions. Therefore aggregates are best conceptualized as a construction of
FeL, octahedra which share vertices, edges, or faces
(L = H,O, OH-, 0’-, phosphate, or donor atoms from
amino acid side chains). The Fe,(pO) unit in lL3]
has the
simplest structure in which two octahedra share a vertex at
oxygen.[*I Dibridged cores Fe,(p-OR), as in 213b1
In the structures of 1-11 depicted here, only the central skeletons of the
polyiron compounds are shown for reasons of clarity. Atom representations
are: Fe dotted, 0 cross-hatched, (carhoxylate 0 atoms in 9 are white), N
partially shaded circles. The same perspectives are presented for ball-andstick and polyhedral drawings except for those of 5 where orthogonal views
are depicted.
0570-0833~92j0808-i010$ 3 . 5 0 + .2SSj0
Angew. Chem. Ini. Ed. Engl. 1992, 31, Nu. 8
Subsequent growth in iron-oxygen aggregates can be
viewed either from variation in the role of bridging 0x0 ligands or increasing nuclearity. An example of the former is
the introduction of the p4-0 unit as in the [Fe8(p4-0)(p3O)4]14fcore in 6.[7b]
This results in a compact core with four
R = H or alkyl or aryl groups) are also known. The only
structurally characterized trinuclear iron complex without
carboxylate bridges contains the planar Fe,O,(OH) core in
3[’] in which tetradentate ligands block all but two coordination sites at the octahedral iron center. Use of tridentate
central octahedra sharing a common vertex and each sharing
two edges with neighbors. This central core is a fragment of
the alkoxido(oxo)hexaferrate [Fe6(p6-O)(p-OR),,(OR)6]’in 7,[7bIa structural motif common to early transition-metal
polyoxoanions.[’] The next step in polyoxoanion growth is a
facially coordinating ligands affords the “three-dimensional” core Fe,O,(OR), in compounds of type 4 in which a
fourth octahedron caps the three octahedra arranged like
fusion of two hexametalates
those in 3.[61
This core may be prepared by the hydrolysis of
[Fe(tacn)CI,] (tam = 1,4,7-triazacyclononane). Further
hydrolysis affords the octanuclear Fe-0 aggregate
to the decametalates,
[MloO,,l”-, a fragment of which is seen in the hexairon 0x0
alkoxide, [Fe6(p4-0)2(p-OR),(OR),(tren),]Z+ 8[’“’ (tren =
tris(2-aminoethy1)amine). The coordination of terminal
amine ligands has presumably prevented the full decametalate structure from forming.
[Fe,0,(OH),,]8+ in 5””’ which, for the first time, contains
two discrete layers of close-packed ligand atoms (0 and N)
with Fe(Iir) ions in octahedral holes. This is also the first
compound containing the Fe,O unit which does not have
bridging carboxylate groups.
Angew. Chem. I n / . Ed. Engl. 1992, 31, No. 8
An example of Fe-0 aggregates with higher nuclearity is
the carboxylate [Fe, ,06(OH),(0,CPh),,] shown in 9.[,=]
The bidentate carboxylate ligands dictate the overall geome-
Veriagsgeseiischaft mbH, W-6940 Wemheim, 1992
try and prevent layered structures from forming. Such aggregates may represent the structures of amorphous iron oxides
which are present in ferritin and a prerequisite for subsequent nucleation and crystal
The largest ironoxygen aggregates crystallized to date are 10 and 11
(H,heidi = N(CH,COOH),(CH,CH,OH)).rl
They are excellent models for the incipient stages of the biomineralization of layered iron oxides. The Fe,O,,, cores are clearly
Verlagsgesellschaft mbH, W-6940 Weinheim, 1992
fragments of the layered AX, structural type and are surrounded by less highly structured shells made up of eight and
ten iron atoms capped and bridged by the tetradentate ligand
These synthetic models shed light on two basic aggregation processes : the formation of soluble aggregates with high
nuclearity related to well-defined polyoxometalates of the
early transition metals, and the formation of insoluble layered minerals such as goethite. They lay the groundwork for
the synthesis of models of technologically important iron(r1)
and the mixed-valence mineral magnetite, which
has recently been synthesized in ferritin.['2b1
German version: Angew,. Chem. 1992, 104, 1036
[l] Biomineralization: Chemical and Biochemical Perspectives (Eds.: E. S.
Mann, J. Webb, R. J. P. Williams), VCH, Weinheim, 1989, Chap. 9and 10.
121 a) T. G. Spiro, S. E. Allerton, J. Renner, A.Terzis, R. Bils, P. Saltmann, J.
A m Chem. SOC.1966,88,2721-2726; b) S. E. Allerton, J. Renner, S. Colt,
P. Saltman, ibid. 1966, 88, 3147-3148.
[3] a) K. S. Murray, Coord. Chem. Rev. 1974 i 2 , l ; b) D. M. Kurtz, Jr., Chem.
Rev. 1990, 90, 585-606.
[41 a) S. J. Lippard, Angew. Chem. 1988,100,353-371; Angew. Chem. Inl. Ed.
Engl. 1988,27,344-361; b) J. B. Vincent, G. L. Olivier-Lilley, B. A. Averill, Chem. Rev. 1990, 90, 1447-1467.
[5] [Fe,O,(OH)(tren),l'+: K. S. Hagen, V. S. Nair, unpublished.
[6] Examples: [Fe,L,O,(OH),],
L = 2-hydroxy-S-methyl-l,3-~yIylenediaminetetraacetate: a) B. P. Murch, P. D. Boyle, L. Que, Jr., J. Am. Chem.
Suc. 1985, 107, 6728; b) B. P. Murch, F. C. Bradley, P. D. Boyle, V. Papaefthymiou, L. Que, Jr. ibid. 1987, 109, 7993-8003; c) [Fe,O,(OH),(tacn),14' : from [Fe(tacn)CI,]: S.Drueke, K. Wieghardt, B. Nuber, J.
Weiss, E. L. Bominaar, A. Sawaryn, H. Winkler, A. X. Trautwein, Znorg.
Chem. 1989, 28, 4477-4483; from [Fe(tacn)J'+: K. S. Hagen, unpublished .
[7] a) [Fe,(ta~n),(~~-0),(1(,-0H),,1~'
: K. Wieghardt, K. Pohl, 1. Jibril, G.
Huttner, Angew. Chem. 1984, 96, 66-67; Angew. Chem. Int. Ed. Engl.
1984, 23, 77-78. b) [Fe,0,(0,CMe),(tren),16t and [Fe,O,(OMe),,(tren),]": K . S. Hagen, V. S . Nair, Inorg. Chem., submitted.
[8] a) [Fe,0((OCH,),CCH,),]2- : K. Hegetschweiler, H. Schmalle, H. M.
Streit, W. Schneider, Inorg. Chem. 1990, 29, 3625-3627; b) [Fe,O(OCH,),,]'- : K. Hegetschweiler, H. Schmalle, H. M. Streit, V. Gramlich,
H.-U. Hund, I. Erni, ibid. 1992, 31, 1299-1302.
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New York, 1983; b) V. W Day, W G. Klemperer, Science 1985,228, 533541; c) M. T. Pope, A. Miiller, Angewj. Chenz. 1991, 103, 56-70; Angew.
Chem. In!. Ed. Engl. 1991, 30, 34-48.
[lo] a) Biomineralization: Chemical and Biochemical Perspectives (Eds.: S.
Mann, J. Webb, R. J. P. Williams), VCH, Weinheim, 1989., b) L. Addadi,
S. Weiner, Angew. Chem. 1992,104,159-176; Angew. Chem. Int. Ed. Engl.
1992,31, 153-169.
[11] S. L. Heath, A. K. Powell, Angew. Chem. 1992, 104, 191-192; Angew,.
Chem. Inl. Ed. Engl. 1992, 31,191-193.
1121 a) R. F. Ziolo, E. P. Giannelis, B. A. Weinstein, M. P. O'Horo, B. N.
Ganguly, V. Mehrotra, M. W Russell, D. R. Huffman, Science 1992, 257,
219-223; b) E C. Meldrum, B. R. Heywood, S. Mann, ibid. 1992, 257,
0570-0833/92/0808-10f2S3.50-t ,2510
Angew. Chem. Int. Ed. Engl. 1992, 31, N o . 8
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synthetic, model, ironцoxygen, biomineralization, aggregation
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