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Highly Branched Liquid Crystalline Polymers with Chiral Terminal Groups.

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ing the volume expansion after nucleation does not progress
zonally as previously described for different reaction^,'^] but
rather concentrically, as long as a gas supply is available. O n
the inside of the wall, where the surface structure is broken
up. the amine 2a obviously has an easier route to reaction.
The effects of the crystal structure on the chemical reactivity
are clearly discernible. The product must rise up above the
original level, as is shown in Figure 3 b.
Compound 1c behaves differently again on reaction with
2a (morphologically dominant plane, small plates). Well-developed "volcanos"[41 of considerable height with a surrounding groove (Figs. 5a, b) form out of the very smooth
surface. A lack of crystal data means that a molecular interpretation is not possible here, however, it has been demonstrated once more just how important phase-reorganization
processes[41are in gas/solid reactions.
weight increase in each case corresponded to the uptake of 1 mol equivalent of
the gas 2.
3a [ 3 ] : M.p. 157'C (EtOH); "C NMR ([DJDMSO. in parentheses Specinfosimulation)- b =183.36 (173.1), 169.23 (170.8),47.29 (48.1). 25.66 (25.8): MS
(70eV):m/r(%): 148(7). 147(100)[M+], 130(14). 117(6), 116(44), 114(9).
88 (12), 73 (8). 60 (35).
3d: M.p. 144°C (EtOH); IJC N M R (CDCl,/[D,]DMSO. in parentheses
Specinfo-simulation): b = 178.35 (178.1). 166.15 (168.5). 237.57 (137.0). 127.17
(129.1, 2C). 122.97 (125.2, 2C). 121.65 (l25.1), 44.74(47.4). 34.28 (35.9). 33.78
192(32). 163(9). 136(25).
(35.91; MS(7OeV).m/z(%):237(31)[Mt],203(9),
135 (100). 131 (36), 102 (lo), 93 ( 5 2 ) , 77 (94). 72 (52).
5 a : IR(KBr):? = 3138 (NH), 2504.2198, 1663 c m - ' ; lJC NMR a s 4 with 2a.
Received: May 19, 1993 [Z 6091 IE]
German version: Angew. CI7e.m. 1993. 105. 1656
[I1 H. Fkdig, H . Mohr, Phwiol. Pluiir. 1992, 8 4 , 568-576, and references
121 R. S . Miller, D. Y Curtin. I. C. Paul. J. Am. Chem. Soc. 1974. 96. 63406349; G . Kaupp. J. Schmeyers, unpublished results.
[3] 3a: M . R. Salem, H. A. Abdel-Hamid, A. A. Shaker, Egjpr. J Chm7.
1983,26, 323-329; 3c: M. Pavlik. I. Kluh, F. Pavlikova. S. Vasickova. V.
Kostka. Collect. Czech. Chem. Commun. 1989, 54, 1940-1955.
141 G. Kaupp. Mol. Cryst. Liq. Cry,Yt. 1992. 211, 1 - 15.
[5] G . Kaupp, Angeu. Clzem. 1992. 104. 606-609. 609-612: Angew. Chcvn.
Int. Ed. EngI. 1992, 31, 592-595, 595-598.
[61 G . Kaupp, Proc. IUPAC Symp. Photochem. 14th 1992. 258-259; GIT
Fuchz. Luh. 1993.37.284-294; 581 -586.
[7] Under the experimental conditions used in Figure I. angles of inclination
of up to 5 0 could be measured, since the crystal plane was tilted by several
degrees t o the cantilever substrate. The quality of the lips way judged and
controlled under the microscope at 400 fold magnification.
[XI L. A. Walker, K. Folting, K. L. Merritt, Actu Cr,istullogr. Sect. B 1969.3.
19) I. A. Attia. T. Glowiak, I. 2. Siemion, Bull. Acud. Poi. Sci. Ser. S f .Chim.
1976, 24,781-790.
[lo] Schakal92; the next molecular layer on (103) is rotated through 90 in the
plane and so on; the phenyl groups on the side face (100) along the h-axis
[OlO], in the layer passing through the middle of the cell point outward as
d o the sulfur atoms on the layer passing through the cell surfwe. These
correspondingly hinder the attack of 2 o n the amide bond. Also the sterically shielded amide bonds on the (010) face of 1 b point inward
Fig. 5. Top: AFM surfaces of Ic. a) fresh, b) after a short reaction period with
gaseous 2a. Bottom: AFM surfaces of 4. c) fresh, d) after a second treatment
with gaseous 2a.
When studied by using AFM, the reaction of 4 with 2a
exhibits a dual mechanism. Initially craters up to 6 nm deep
form on the slightly puckered, faintly undulated surface
(Fig. 5c). After repeated treatment with 2 a these become
only marginally deeper (7 nni), but the surface develops in
addition to that deep, parallel trenches (inclination of the
flanks: 22 and 32"). Figure 5 d shows an already very deep
channel as well as further trench formation on both sides.
The ditches d o not progress along the original undulations,
but at an angle of approximately 55" t o them. This leads once
again to profound material rearrangements.
The reactions presented here indicate how free amines can
be absorbed out of the gas phase without the use of solvents,
through the formation of the compounds 3, which are interesting synthetic building blocks. AFM measurements of
chemical transformations have only just recently been pos51 They provide unparalleled resolution and create
the basis for the understanding of the gas/solid reactivity. A
seventh mechanism for the requisite solid-phase rebuilding
involving island formation has been identified and interpreted in molecular terms (for the first six mechanisms see [4,5]).
E,xperitnmtn( Procedure
Thiohydantoin (2 mmol) was treated at room temperature in an evacuated
500 mL llask through a vdcuum line with gaseous 2 (1 bar). After the mixture
had been left to stand for about 12 hours the excess gas was pumped off. The
Aiigiv. Chivn. 1171.Ed. Engl. 1993, 32. N o
Highly Branched Liquid Crystalline Polymers
with Chiral Terminal Groups
By Stephan Bauer. Hartrnut Fischer, and Helmut Ringsdoyj'*
In recent years an innovative field of polymer chemistry
has been the synthesis of dendrimers[*.21and highly
branched polymers.1334J These macromolecules are distinguished by their three-dimensional fractal structure. Their
synthesis requires monomers with an AB, functionalization.
Dendrimers can be constructed in successive synthetic steps,
resulting in the formation of defined, unimolecular compounds. In an alternative and easier approach hyperbranched polymers are synthesized from AB, molecules in a
Thermotropic liquid crystalline (LC) polymers constructed following this principle were first described by Percec
et al.141The approach described here is based on the consideration that branched, liquid crystalline polymers contain
numerous terminal groups, which can be functionalized in a
[*] Prof. H. Ringsdorf, Dip1.-Chem. S . Bauer
Institut fur Organische Chemie der Universitit
J. J.-Becher-Weg 18-20, D-55099 Mainz (FRG)
Telefax: Int. code + (6131)39-3145
Dr. H. Fischer
H. H. Wills Physics Laboratory
Tyndall Avenue, Bristol. BS8 ITL (UK)
(c VCH Verlag~~~selIsr/~ufi
m b H , 0-69451 Weinhrim, 1993
$ 10.00 +.?Sill
second reaction step (Scheme 1). We have selected a chiral
functional group in order to induce a cholesteric mesophase
in the highly branched polymer.
AB, monomer
, ,M
in the melt
of the terminal groups
1. KOH
2. Ac,O
Scheme 2. Synthesis of the key components 7 and 10
tionalized at its (n + 1 ) biphenyl acetate terminal groups by
a further condensation reaction with the chiral benzoyl chloride 10.
The hyperbranched polymer thus obtained is distinguished by its good solubility, for example in chloroform,
toluene, and tetrahydrofuran, and has a medium degree of
polymerization (P, = 6) as determined by gel permeation
chromatography (GPC). The 'H NMR spectrum (Fig. 1)
shows the almost complete disappearance of the resonance
signal of the acetate groups at 6 = 2.3. This proves that the
Scheme I . Synthetic approach to polymer 1.
The AB, monomer 7 chosen as the starting material consists of a mixed anhydride component and two mesogenic
biphenylacetate units. The synthesis of monomer 7 and the
chiral benzoyl chloride 10 is outlined in Scheme 2. Methyl
3,4-dihydroxybenzoate (2) was treated with excess 1,lO-dibromodecane under Claisen conditions to provide ether 3.
The remaining bromine terminal groups reacted with 4-hydroxy-4-benzyloxybiphenyl (4)15'under analogous conditions yielding 5. After cleavage of the benzyl protecting
groups, ester 6 was saponified and converted into
monomer7 with acetic anhydride. In the synthesis of the
chiral benzoyl chloride 10, compound 2 was treated with
2-(S)-methylbutanol following the Mitsunobu method[61to
give ether 8. Subsequent saponification and reaction with
thionyl chloride provided 10. Biphenyl benzoate mesogens
linked by a spacer were then synthesized by polycondensation of the AB, monomer 7 in the melt (see Scheme I). In the
second reaction step, the resulting highly branched polymer 11, with a degree of polymerization n, was then func1590
VCH ~ ~ r l u ~ s ~ e s i ~ l l nihH.
. ~ c A u0-69451
' I
t 6
Fig. I . ' H N M R spectrum of polymer 1 (CDCI,, 400 MHz). The signal indicated with OAc and an arrow is assigned to the acetate groups.
coupling of the chiral carboxylic acid chloride to polymer 11
was achieved with a yield of over 95 %. Polarimetric investigations verify the chirality of polymer 1, which has an optical
rotation of [4i0= 6.8 in dichloromethane.
O57O-0833/93/11l I - l S 9 O $ lO.OOi.2SjO
Angew Chcm. In!. Ed. Engl. 1993. 32. No. 11
The thermal behavior of polymer 1 was characterized by
polarization microscopy, differential scanning calorimetry
(DSC), and X-ray scattering analysis. The DSC thermograph shows a glass transition step at 60°C and two
phase transitions at approximately 95 "C and at 151 "C in the
second heating curve (Fig. 2). In the cooling curve, a phase
10 7
tions are being carried out to determine whether this is a
crystalline or a smectic phase.
The introduction of functional groups in the last reaction
step of the synthesis of highly branched liquid crystalline
polymers opens up a large palette of possible structural variations in LC systems. Examples are the introduction of polar
terminal groups for the syntheses micellar and lyotropic systems with treelike structures, and the application of these
highly branched LC polymers with variable surfaces for the
production of polymer blends.
Experimental Procedure
-20 0 20 4 0 60 80
Fig. 2. DSC curve of polymer 1 (heating rate 10 K min-'): a) heating curve,
b) cooling curve (g = giassy. Sx = crystalline or smectic, N* = cholesteric,
i = isotropic).
Synthesis of 7 . A mixture of 2 (10 mmol), 1,lO-dibromodecane, K,CO, (20 g).
and KI was refluxed in anhydrous methyl propyl ketone (4 8, molecular sieves)
for 24 h. After the excess dihromodecane was removed by distillation. the
residue was recrystallized from acetone (yield: 92%, m.p. 30°C). The conversion of 3 (8 mmol) into its ether derivative by treatment with 4 [5] (20 mmol)
under the above conditions gave, after recrystallization in chloroform, 5 (yield:
85%, m.p. 155'C). Compound 5 (6mmol) was hydrogenated using PdjC
(300 mg) in methanol as catalyst, and purified by flash chromatography eluting
with CHCI,/MeOH 10.1 (yield: 55%. m.p. 149°C). Ester 6 (3 mmol) was refluxed in 20 mL of EtOH/H,O and KOH (10 mmol) for 1 h, the free acid was
precipitated with 2 N HCI, dried, and refluxed in 20 mL of acetic anhydride for
20 min. The cooled reaction mixture was suction-filtered and the white precipitate recrystallized from acetone (yield: 87%. m.p. 157 "C).
7: ' H N M R (200 MHz, CDCI,): 6 =7.65 (d, 1 H, H,,6), 7.48 (m. 9 H , Hp,2,
transition is observed only at 148 "C. Investigations of this
mesophase with polarization microscopy show a typical uniform Grandjean texture,['] which is characteristic for
cholesteric phases (N*). The X-ray diffraction pattern of a
fiber extruded from the polymer melt reveals a scattering
diagram typical of nematic and cholesteric systems (Fig. 3).
8H, OCH,), 2.34 (s. 3H. CO-0-CO-CH,). 2.30 (s, 6H, -0-CO-CH,). 1.78 (m,
8H, 0-CHI-CH,), 1.5-1.2 (m. 24H, CH,); 13C NMR: (50 MHz, CDCI,):
6 =169.6. 162.1, 158.7, 154.6, 149.5, 148.7, 138.6, 132.6, 128.0, 127.6, 125.0,
121.7, 120.3, 114.7, 114.6, 111.7, 69.2, 69.0, 68.0, 29.5, 29.4, 29.3. 29.0, 28.9,
26.0. 25.9, 22.3, 21.1; FD-MS: mjz (%) 929.2 (100). [M'].
Synthesis of 10: Diethyl arodicarboxylate (25 mmol) was added dropwise to a
solution of 2 (10 mmol), triphenylphosphine (25 mmol), and 2-(S)-methylbutanol in 200 mL of anhydrous tetrahydrofuran at 0 "C and stirred at room
temperature for cd. 15 h. After filtration, the solvent was distilled off and the
residue purified by flash chromatography eluting with petroleum ether/acetic
acid 10.1 (yield 25%, colorless oily product). Compound 8 was saponified
under the previously described conditions (yield: 95 %, m.p. 108 "C). The resulting acid 9 (1.5 mmol) was stirred with thionyl chloride (3 mmol), catalytic
amounts of DMF, and 20 m L of benzene for ca. 15 h, freeze-dried, and used
immediately in the subsequent reaction.
9: 'H NMR (200 MHz, CDCI,): 6 =7.70(d, 1H, Hp,6), 7.56 (s, 1 H, Hph2),6.87
(d, 1 H, H,,5), 3.85 (m,4H, OCH,), 1.91 (m, 2H, 0-CHI-CH), 1.58, 1.25 (2m,
4H, CH,-CH,), 0.99 (d, 3H, CH,), 0.90 (t. 6H, CH,-CH,); FD-MS- mi; (X)
293.6 (loo), [ M ' I ; [a];' = + 14.1 ( c = i in CH,CI,).
Synthesis of 1: Monomer 7 (1 mmol) and p-toluenesulfonic acid (0.05 mmol)
were stirred under vacuum at 240°C for 8 h. The chiral benzoyl chloride 10
(1.2 mmol) and p-toluenesulfonic acid (0.05 mmol) were added to the cooled
reaction mixture, which was then heated to 200°C in an atmosphere of argon
for 1 h. The pressure was then slowly reduced to
mbar. The resulting
polymer was dissolved in chloroform and precipitated in methanol (yield:
1: ' H N M R (200MHz. CDCI,): 6 =7.80 (d, H,,6), 7.64 ( s , Hp,2). 7.55, 7.48
Fig. 3. X-ray diffraction pattern of polymer 1 in the cholesteric mesophase.
I n the wide-angle X-ray scattering patterns a diffuse halo of
high intensity is visible, which corresponds to an average
distance between the mesogens of 4.4 A. The low-intensity
small-angle reflections indicates a layer structure of the
cholesteric phase with a layer spacing of 28 A. Furthermore,
the small azimuthal elongation of the halo verifies that a very
good unidirectional orientation was achieved during the
drawing of the strand. The orientation factor S = 0.88 calculated from the X-ray patterns is comparable to that of main
chain side group polymers.[*' The structure of the low-temperature phase, which occurs in the heatingcurve between 60
and 95 "C,has not yet been elucidated. Ongoing investigaAVCW. Chein. Int. Ed. Engl. 1993, 32, No. 11
(m,OCH,), 2.0-1.7(m. 0-CH,-CH,,O-CHI-CH), 1.7-1.2(m, CH2),0.99(d,
CHI), 0.90 (t. CH,-CH,); ''C NMR (100 MHz, CDCI,): 6 ~ 1 6 5 . 1 158.8,
154.2, 153.9. 150.1, 149.2, 148.7, 138.5, 132.8, 128.1, 127.6, 124.4, 124.3. 122.0,
121.7. 121.6, 115.6, 114.8, 114.7, 112.0, 103.8, 74 1,73.8,69.4. 69.1, 68.1, 34.9,
34.8, 29.5, 29.4. 29.3, 29.2, 29.1, 26.2, 26.1, 26.0, 25.9, 16.6, 16.5, 11.3;
[XI;' = f 6 . 8 ( c =1 in CH,CI,); GPC (CHCI,, polystyrene standard): M,:
20000, M,:6500 gmol-I.
Received: July 7, 1993 [Z 6195 IEI
German version: Angew. Chem. 1993, 105, 1658
[ l ] Reviews. a) D. A. Tomalia, A.M. Naylor, W. A. Goddard 111, Angen.
Chcm. 1990,102.119; Angrw. Chem. In[. Ed. Engf. 1990,29,138; b) H.-B.
Mekelburger, W Jaworek, F. Vogtle, ibid. 1992, 104, 1609 and 1992, 31,
[2] Dendrimers: a) C. J. Hawker, J. M. J. Frechet, Macromolecules 1990. 23,
4726; b) 3. S. Moore, Z. Xu, hid. 1991, 24, 5893; c) T. M. Miller, E. W.
Kwock. T. X. Neenan, ibid. 1992,25,3143; d) L.-L. Zhou, J. Roovers, hid.
1993,26,963; e) K. L. Wooley. C. J. Hawker, J. M. Pochan, J. M. J. Frechet,
ibid. 1993, 26, 1514; f ) G. R. Newkome. C . N. Moorefield, G. R. Baker,
A. L. Johnson, R. K. Behera, Angew. Chem. 1991,103.1205; Angen. Chem.
Int. Ed. Engl. 1991,30,1176; g) G. R. Newkome, C. N. Moorefield. G. R.
VCH Verlag.~~rsells~hujr
mbH, 0-69451 Weinhelm, 1993
0570-0833/93/llll-tSYf $10.00+ ,2510
Baker. R. K . Behera, G. H. Escamillia, ihid. 1992, 104. 901 and 1992, 31.
917: h) F. Moulines, B. Gloaguen. D. Astruc, ;hid. 1992,104,452 and 1992,
31. 458; I ) S. Serroni, G. Denti. S. Campagna, A. Juris, M. Ciano. V. Balrani. ibid. 1992. 104. 1540 and 1992, 31. 1491; j ) C . J. Hawker, J. M. J.
Frechet, J. A m . Chem. Soc. 1992, 114. 8405; k) T. M. Miller. T. X. Neenan.
R. Zayas. H. E. Bair, ihrd. 1992, 114, 1018.
[3] Highly branched polymers: a) P J. Flory. J. Am. Chern. Soc. 1954, 74,2718;
b) Y. H. Kim. 0 W. Webster. ibrd. 1990, 112. 4592: c) L. J. Mathias. T. W.
Carothers, rhrd. 1991, 1/3. 4043; d) C. J. Hawker. I. M. J. Frechet. rhid
1991. 113. 4583; e ) K . E. Uhrich, C. J. Hawker. J. M . J. Frechet, Po@rn.
Murer. Sci. €rig. 1991. 65. 137: f ) K. E. Uhrich. C. J. Hawker, J. M. J.
Frechet. S. R. Turner. Mucroriiolecules 1992.25.4583. g) Y. H. Kim, 0 . W.
Webster. ;hid. 1992.28.5561. h) B. 1. Voit. S. R. Turner, Po/yi. P r e p . 1992,
33(1). 184: i ) F. Walter. S. R . Turner, B. I. Voit. rhrd. 1993, 34(1), 79.
[4] Hiphly branched LC polymers: a)V. Percec, M. Kawasumi. M W ~ W
r i i o l e ~ r r l ~1992,
~ . ~ 25. 1164; b) V. Percec, C. G. Cho, C. Pugh, D. Tomazos.
rhid. 1993. 26. 963: c) Y. H. Kim. J. Am Cham. Soc. 1992. 114. 4947.
[5] H. Kapitra. R. Zentel. Mukrofnol. Ch~fi7.
1991, 192. 1859.
[6] 0. Mitsunohu, S~.rir/7esi.s1988. 1
[7] D. Demus. L. Richter. P . y / u r c \ uf Liquid Cr~sruls,Verlag Chemie, Weinheim. 1978.
[XI B. W. Enders. M. Ehert. J. H. Wendorff. R. Reck, H . Ringsdorf. Liq. C'rrsr.
1990. 7. 217.
Refinement of a Model for the Nitrogenase Mo-Fe
Cluster Using Single-Crystal Mo and Fe EXAFS""
By Jie Chen, Jason Chrisliunsen, Mino Campohasso,
Jeffrey 7: Bolin, Roland C. Tittsworth, Brian J. Hales,
John J. Rrhr, and Stephen P. Cramer*
The enzyme nitrogenase catalyzes the reduction of dinitrogen to ammonia.['] Substrate conversion is thought to occur
at the M clusters, which are two MoFe,S, clusters embedded
in the x subunits of the x2P2 MoFe protein ( M , = 220000).r2'
Each M cluster presumably receives electrons from a companion P cluster, an Fe,S, structure located at the interface
between the x and B subunit. Accurate dimensions for these
clusters are important for synthetic modeling, for theoretical
studies, and for the observation of possible structural
changes during the catalytic cycle. The structures for MoFe
~~ A .
proteins C p l and Avl from C. p ~ s t e u r i a n u m [and
respectively, are being determined by X-ray diffraction with increasing accuracy, and models for the structures of the M cluster and P cluster have been presented.
However, given the limited resolution of the diffraction data
(2.2 A), it is likely that the root mean square error in the
Prof. S. P. Cramer
Department of Applied Science
University of California, Davis, CA 95616 (USA)
Telefax: Int. code + (916)752-2444
Energy and Environment Division. Lawrence Berkeley Laboratory
J. Chen. J. Christiansen
Department of Applied Science, University of California, Davis
N. Campohasso. J. T Bolin
Biology Department. Purdue University
R. C. Tittsworth, B. J. Hales
Department of Chemistry. Louisiana State University
J. J. Rehr
Department of Physics. University of Washington
This work was supported by the Department of Agriculture through
grants 91-37305-6514 (to S. P. C.) and 91-37305-6661 (to J. T. B.), the National Institutes of Health through GM-33965 (to B. J. H.), and the Department of Energy. Office of Health and Environmental Research. We
thank Prof. D. Coucouvanis (University of Michigan) for model compounds and M. Newville (University of Washington) for EXAFS analysis
software. Stanford Synchrotron Radiation Laboratory (SSRL) and National Synchrotron Light Source (NSLS) are supported by the Department
of Energy. Office of Basic Energy Science.-EXAFS = Extended X-ray
Absorption Fine Structure.
atomic positions of these models is on the order of 0.20.3 A, and errors in the interatomic distances are somewhat
larger. Since changes of metal-metal distances of even 0.1 8,
are chemically significant, more accurate cluster dimensions
are important. Now that the construction of the cluster
frameworks has been established by the X-ray diffraction
experiments, greatly improved metric information can be
obtained from solution and especially single-crystal EXAFS
We report here the first single-crystal Fe and Mo EXAFS
spectra of nitrogenase at low temperature. A new Mo-Fe
component at approximately 5 8, was observed along with
seven other distances between metal atoms and neighboring
atoms. These data were combined with that from previous
solution Fe EXAFSl5] and a new solution M o EXAFS to
refine the current model for the M cluster. The model is
compared with a current electron density map for the protein
from C. pasteurianum having a 2.3 A resolution.[3]Although
the spectroscopic results are qualitatively in complete agreement with the Kim and Rees model for the M cluster, some
average distances differ by roughly 0.2 8,. The EXAFS results also suggest less variation in individual Fe-Fe and
Mo-Fe distances than seen in the Kim and Rees model; in
other words, the Mo-Fe cluster is more symmetrical.
Fourier transforms of the M o and Fe K edge EXAFS for
a solution of Avl , a single crystal of C p l , and the cappedprismane model complex (Et,N)3[Fe6S,C16{Mo(CO)3}2]r61
are shown in Figure 1. The transform of the model complex
shows the expected first-shell Mo-C, Mo-S, and Mo-Fe
distances (2.0, 2.6, and 2.9 A, respectively) as well as a longrange Mo-Fe interaction at 4.28 A. Buried under the shorter Mo- Fe component is a multiple-scattering contribution
from a Mo-C-0 interaction. Fitting the EXAFS yields distances within 0.05 8, of the values obtained by X-ray crystallography[61(Table 1).
The Fourier transform of the Mo EXAFS spectrum of
Avl in solution exhibits two main peaks, which fit (Fig. 1) as
Mo-S and Mo-Fe interactions at 2.37 and 2.70 A, along
with an unresolved M o - 0 , N component at 2.208,
(Table 1). The Mo-S and Mo-Fe distances, like those from
previous EXAFS studies," -'I are clearly shorter than the
corresponding distances in both the capped-prismane model
and the respective average distances of2.46 and 2.92 8, in the
Kim and Rees model of the M cluster. As shown in Figure 1,
the M o EXAFS spectrum generated from the distances obtained by X-ray diffraction has different frequencies and
beats (because of different distances) and damps out more
rapidly (because of the greater spread in the distances between M o and neighboring atoms). In the 3-5 8, region of
the Fourier transform, a number of smaller peaks are observed, which may arise from interactions of C atoms of
homocitrate and histidine ligand with Mo, as well as Fourier
transform truncation ripple. We d o not interpret these features at this time. There is also a modest peak at approximately 5 A,where a second Mo-Fe interaction is expected.
However, the feature is quite weak, and numerous other
interactions might occur at such a distance.
We have used single-crystal EXAFS to enhance this signal
at 5 A and confirm the Mo-Fe assignment. The strength of
specific metal -neighbor EXAFS components varies by
cos2H, where H is the angle between the photon polarization
vector E and the metal-metal vector.['l Although there are
four M centers in the unit cell, their long axes tend to lie near
the hc* plane, as shown in Figure 1 . Furthermore, the longer
Mo-Fe vectors form an angle of only about 15" (cos' 15" =
0.93) with the approximately threefold symmetry axesf3]
Thus, by orienting a crystal with the E vector parallel to the
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