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[ I S ] The density of 2 in the absence of solvent is just 0.87 Mgm--' compared to
1.856 Mgm
in the corresponding dihydrate 1. The density was not deter-
mined cxprrimcntally because of the tendency of the crystals to lose solvent
rapidly
[16] As noted by 'I reviewer and revealed by Figure 1 a, there is considerable distortion of the thermal ellipsoids associated with carbon atoms 2,3,5.6,2',3',5' and
6' o l the 4.4'-bipyridine ligands. This observation is consistent with slight twisting of the the pyridine moieties and either static or dynamic disorder between
two orient11tions.
[I71 C-H" F and C . . F contacts, 2.342 and 3.429(9) A, respectively. are well
within ranges expected for significant C-H ' . . F interactions. 1.Shimoni, H. 1.
Carrell. J. P Glusker, M. M. Coombs, J. Am. Chrm. Soc. 1994, ff6. 8162.
[IS] Compounds with comparable or larger pores have recently been reported but
cither cations (T. J. McCarthy, T. A. Tanzer, M. G. Kanatzidis, J. Am. Chem.
So( 1995. 117. 1294) or anions are necessarily present in the pores (B. F.
Ahrdhams. U. F. Hoskins, D. M. Michall, R. Robson, A'urure, 1994,369, 727).
Rational Design of Orthogonal
Receptor - Ligand Combinations**
Peter J. Belshaw, J o s e p h G. Schoepfer,
Karen-Qianye Liu, Kim L. Morrison, and
S t u a r t L. Schreiber*
Signal transduction in cells occurs primarily by two mechanisms, one involving allostery and the other involving proximity.['] The role of ligand-induced allosteric change in proteins has
been appreciated for many years, and has led to the synthesis of
numerous molecules that either promote or inhibit the allosteric
change required to transduce information in cells. More recently, cell biological studies have illuminated the role of regulated
protein dimerization or oligomerization as a means of information transfer. Such an event can promote a proximal relationship between an enzyme and its substrate or a receptor and its
ligand, thereby facilitating a molecular interaction leading to a
signaling evcnt. Examples include the dimerization of growth
factor receptors,[*]oligomerization of antigen
and
dimerization of transcription factors.[41These insights have created new opportunities for chemists to synthesize molecules
with two protein-binding surfaces and thereby to induce protein
association. Such chemical inducers of dimerization (CIDs)
have the potential to activate many cellular processes, including
ones of biological and medical significance. We recently described a method to inducibly control the association of proteins
in cells.[', 61 This was accomplished through the expression of
chimeric proteins in mammalian cells, consisting of a dimerization domain fused to a protein or protein domain of interest. By
treating these cells with a synthetic, cell-permeable C I D that
binds to thc dimerization domain, self-association of the protein
occurs and a signal is transmitted (Fig. 1).
Although in theory many receptor-ligand systems can be
used, the immunophilins FKBP12 and cyclophilin A (CyP) and
their ligands FK506 and cyclosporin A (CsA) were selected for
this purpose. The ligands are cell permeable and can be modified
[*I
[*"I
Prof S L Schreiber, P. J. Belshaw. Dr. J. G. Schoepfer, K-Q. Liu,
K.L Morrison
Howard 11 ughes Medical Institute, Department of Chemistry
Harvard Uniwrsity, Cambridge, MA 02138 (USA)
TCkfaX'
+ (617) 495-0751
e-mail . bclshawfn slsiris.harvard edu
This research was supported by a grant from the National Institute of General
Medical Sciences (GM-52067). We thank theNSERCfor a postgraduatescholarahip awarded to P. J. B., and the Swiss National Science Foundation, the
Roche Kcsearch Foundation, and Ciba-Geigy Jubilaums Stiftung for fellowship) to J G S
An,qw Cliem. Inr Ed. Engl. 1995. 34, No. 19
Fig. 1. Regulated intracellular dimerization with cell-permeable synthetic ligands
named CIDs.
synthetically in a rational way to remove their intrinsic immunosuppressive and toxic properties. This requires modifying the
calcineurin-binding (but not the immunophilin-binding) domain of these naturally occurring CIDs. FKBP12 and CyP bind
their ligands with high affinity (K,, = 0 . 5 n ~and 5 n ~ respec,
tively), are monomeric, have no discernible effect when expressed in cells, and their small size (12 kDa and 18 kDa, respectively) facilitates the incorporation of their cDNA into
expression vectors. A shortcoming is that the natural immunophilins are expressed at high levels in many cells and can
therefore diminish the potency of C I D ligands toward immunophilin fusion proteins by forming nonproductive receptor-ligand complexes (Fig. 2a). The resulting loss in specificity
is expected to be especially troublesome in whole organisms,
where cells expressing the fusion proteins may be relatively few
in number, thus magnifying the buffering effect of natural immunophilins.
Fig. 2. Improved receptor-ligand pairs for the regulated inlracellular protein association with synthetic ligands a) Possible protein associations occurring in cells
expressing wild-type immunophilins as dimerization domains. b) Protein associations predicted to occur in cells by using modified ClDs and iinniunophilin~with
compensatory mutations.
To solve this problem we envisioned the creation of new receptor-ligand pairs with interacting surfaces that ensure a high
degree of specificity. Immunophilin ligands were designed to
contain substituents that clash sterically with amino acid
sidechains in the immunophilin receptor, thereby abolishing
their interaction with endogenous immunophilins. Compensatory mutations in the receptor were sought that would remove
the offending interaction, thereby creating a unique receptor
that would be used as a dimerization domain (Fig. 2b). Herein,
we report the implementation of this strategy using CyP-CsA as
a test system. In addition to providing an effective solution to
the specific research problem outlined above, we propose that
the creation of new receptor-ligand pairs in this manner will
result in new experimental systems for testing our understanding of molecular recognition involving protein receptors.
A, VCH Ver-lu~sge.s~~llschrrfi
mhH, D-694if Weinheim, 1995
O571~-0833/95;34fY-ZIZY$ / O flOT .Z5X
2129
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The high-resolution X-ray structure of the CyP-CsA comp l e ~ [ ' . *served
~
as the starting point for the design of modified
ligands. The primary hydrophobic interaction between CyP and
CsA involves the sidechain of MeVal11 of CsA and a deep
pocket on the receptor that is lined with the hydrophobic
residues Phe60, Met61, AlalOl, Phell3, and Leu122 (Fig. 3A).
Several CsAs with modifications at residue 11 have been synthesized previously.[91Binding assays showed that an increase in
the size of the MeValll sidechain abrogated binding to CyP
(MeIlelI, Me-allo-Ilel I), and conversely that compounds with
smaller sidechains (MeAlal 1) still bind with substantial affinity.
Our approach would lead to an increase in buried hydrophobic
surface, and as hydrophobic interactions can provide a significant contribution to the free energy of binding,["] this pocket
was selected for modification.
For simplicity, the first target selected was MeIlelICsA. A
hypothetical model of MeIlellCsA bound to CyP (Fig. 3B) suggests there should be a steric clash between the strategic methyl
~
~
~
~~~
group of the modified ligand and the phenyl ring of P h e l l 3 of
the receptor. Removal of this ring (Phell3 to Ala) should relieve
the steric clash (Fig. 3C), yet it should also create a considerable
volume of unoccupied space. To minimize these voids and maximize binding energy, two secondary mutations were designed
wherein nearby sidechains were increased in size. A Ser99 to Thr
mutation (Fig. 3D) adds an extra methylene group and a
CysllS to Met mutation (Fig. 3E) adds two methylenes. Computer models were generated with MacroModel4.0 by performing Monte Carlo substructure minimizations with all torsions
specified for the sidechains of residues MeIlel1, Phe60, Met61,
Met1 15, and Thr99. All other atoms within 6 A of these atoms
were considered but were assigned a 100 k J A - ' movement
penalty. These calculations indicated that the conformer required for efficient packing was the one least frequently observed for threonine residues in protein X-ray structures.["]
These modeling studies also predicted that the two secondary
mutants could simultaneously assume their preferred conformations, thus the triple mutant was also investigated
(Fig. 3F). These mutant proteins were prepared by PCR
mutagenesis using the megaprimer method['21 and verified by sequencing.
Our synthetic route begins
with the readily available
CsA, and produces a common
intermediate for the synthesis
of compounds with modifications at residues 1 and 11
(Scheme 1). This plan is particularly attractive since position 1 is the site we have used
previously to prepare dimeric
versions of CsA, named
( C S A ) ~ . [ ' ~Thus,
I
by using
modified amino acids at positions 1 and 11, we should be
able to simplify the synthesis
of CIDs with greater specificity than (CsA)2. CsA was ringopened and degraded by first
converting it to i s ~ - C s A [ ' ~ ~
and then subjecting the resulting amine to a modified Edman degradation sequence." ']
This procedure excises the
MeBmt amino acid, producing a linear peptide that was
protected at its amino- and
carboxy-termini. The selection of the M E M ester was essential for the subsequent reduction,[161as all other combinations of common esters
and reducing agents failed.
Following reduction, another
acid-catalvzed N to 0 shift
and an N-acylation produced
Fig. 3. Graphical representation of the binding interfaces between Cyp and CsA and vanants. The solid blue surface and white
the target intermediate from
mesh represent the solvent-accessible surfaces of CsA and Cyp. respectively (nonpolar hydrogens omitted). The contact
sidechains of Cyp are represented as tubular bonds and viewed from the insidc of the protein; the lowest proti-usion ofsolid blue
which CsAs with modifica.
corresponds to the sidechain of residue 1I of CsA. A) Cyp-CsA crystal structure. B) Hypothetical model ofCyp-MeIlelICsA
tions at residues 1 and/or 11
complex. C) Cyp(F113A)-MeIlel lCsA complex. D) Cyp(S99T, FI I.iA)-MellellCsA complex. E) Cyp(F113A. C115M)are accessible. T W O cycles of
MeIlel lCsA complex. F) Cyp(S99T, FI 13A. CI 15M)-Mellel lCsA complex. Graphical representations were created by using
GRASP [23]
deprotection and coupling
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yielded a linear, protected undecapeptide that was deprotected
and cyclized to give MeIlel1 CsA. This unprecedented cyclization between residues 10 and 11 proceeds with good yield and no
detectable racemization. The choice of coupling agent and base
proved to be critical for efficient cyclization.[' '1
CyP experiences an approximate twofold enhancement in fluorescence upon binding CsA. This effect provides a convenient
assay for the determination of Kd values.['8-201Using this fluorescence assay. the binding constants for MeIlellCsA and CsA
with the wild-type and mutant CyP proteins were determined
and are shown in Table 1. The data show that the extra methyl
group of MeIlel1 CsA dramatically decreases binding to CyP.
lable I . Binding constants K , [ n ~of] CsA-based ligands for wild-type and mutant
('yPr.
Protein
MeIlel lCsA
CsA
hCyP(wild-type)
hCyP(F113A)
hCyP(F113A. ( ' l l 5 M )
hCyP(S99T. Fl l.3A)
hCyP(SYY1. F l l i A . C115M)
> 3000 [a]
53+9
75+13
2k0.5
4+1
5*1
55+15
19+7
5+1
7fl
[a] This \,aIuc represents the lowest possible value, as precise determination is hindered by Ihc solubility 01' the hgand.
Mutation of Phel13 to Ala has an even more dramatic effect,
restoring the binding affinity to within an order of magnitude of
the wild-type receptor. The double mutant CyP(S99T, F113A)
binds MeIlel lCsA with an affinity of 2 n ~ Thus,
.
by adding a
substituent to CsA and by creating a binding pocket for it on its
receptor using site-directed mutagenesis, a new receptor-ligand
pair results that binds with two- to threefold greater affinity
than the wild-type system. The other double mutant,
CyP(F113A. C115M), has less affinity for MeIlellCsA than the
F113A single mutant, indicating that this mutation is slightly
destabilizing. This appears to be due to an interfering contact
between the terminal methyl of MI15 and the terminal methyl
of the MeIlel1 sidechain. Models show that this interaction is
slightly closer than any other contact in the binding pocket,
although we cannot rule out a disturbance of the
local structure due to the Met. Interestingly, the
triple mutant CyP(S99T, F113A, C115M) binds
MeIlel 1CsA more poorly than the double mutant
CyP(S99T, F113A), and the free energy difference
between these two mutants is approximately the
same as the difference between CyP(F113A) and
CyP(F113A, C115M). This indicates that the destabilizing effect of the C115M mutation is independent of the S99T mutation. All of the mutants show
good binding to CsA with affinity generally increasing as the predicted void volume decreases.
We have successfully generated new high affinity
receptor-ligand pairs through structure-guided rational design. By selecting other residues with large,
hydrophobic, conformationally constrained sidechains as replacements for MeIlel1 and by judicious
introduction of point mutations, we may be able to
extract even greater free energy gains from the hydrophobic effect. In addition to this goal, we intend
to synthesize dimeric versions of these new ligands
in order to induce the dimerization of'target proteins
in vivo, thereby assessing the effect of endogenous
immunophilins in these types of experiments. The
generation of "bumps" and compensatory "holes" provides a
controlled system for studying molecular recognition involving
protein receptors. Through the iterative interplay of synthesis,
molecular and structural biology, and calorimetry and/or other
physical methods, the contribution of specific molecular interactions to the free energy of binding can be assessed
Experimental Procedure
Fluorescence measurements were made on a Hitachi F2000 fluorescence spectrometer following a 10 min incubation with Iigand at 20 :C. Excitation was at 280 nm,
10 nm bandwidth and 331100 filter. Emissions were scanned at 300-400 nm, 10 nm
bandwidth. Proteins were diluted to 3 m L in Tris-buffered saline pH 7.4 at about
100 nM (for&> IOnM) o r 2 0 0 - 5 0 0 n ~(for K , <lOnM). Small aliquots ofligand in
50% ethanol were added between successive measurements such that the total
volume added was < 1.5%.
An equation relating the observed fluorescence change (A&,,.), the total fluoresthe protein concentration (E,,),the ligand concencence change at saturation
tration (fJ. and the Kd can be derived from consideration of tight binding kinetics
and the assumption that Fob,= ( H ) 8 J E o [Eq. (a)].
(Ace,),
AFmr = [ ( E , + 1, +
KJ
~
{ ( E D+ lo
+ &Iz
-
4 ( L ) ( ~ J ) i ' 2(AF,,d/2)
1
(a )
The fluorescence change at 338 nm (after adjustment for fluorcscence of the hgand
solution alone) was plotted vs the concentration of the hgand. The data for each
determination of K , (t 14 measurements) were fitted to Equation (a) with
Kaleidagraph 3. 0. All fits have r >0.99.
Physical data for all compounds are in accord with the expected structures and are
available upon request from the authors.
1: To a solution of IsoCsA [14] (2.886 g. 2.401 mmol) in pyridine. (144 mL) triethylamine (7.2 mL) and methyl isothiocyanate (7.2 mL) were added. The solution was
heated under nitrogen a t 50 'C for 30 min. More triethylamine (2.9 mL) and methyl
isothiocyanate (2.9 mL) were added and the reaction mixture was heated for an
additional 30 min. Following evaporation at room temperature (RT), the residue
was dissolved in chorobutdne/trifluoroacetic acid (TFA) (9111. stirred at 10°C for
1 h, evaporated, and the residue was precipitated three times from EtOAclhexane to
give the crude precursor of 1. To this crude product (2.4g) in dichloromethane
(DCM) (100 mL) was added fert-butyoxycarbonyl (Boci anhydride (654 mg,
3.0 mmol) and diisopropylethylamine (DIPEA) (1.4 mL. 8.00 mmol). After 4h at
RT, MEMCI (570 pL, 5 mmol) was added and the mixture was stirred overnight.
The solution was diluted with DCM (200 mL), washed with sodium bisulfate (1 M).
water, sodium bicarbonate (sat.). water, and brine (50 mL each). The organic layer
was dried (Na,SO,), filtered. and evaporated. The residue a a s chromatographed
(silica gel, DCM/MeOH (9812, 95/5) to afford 1 (1.933 g, 66 74 from IsoCsA).
2 : To a solution of 1 (975 mg, 0.795 mmol) in DCM (10 m L ) . 1 mL of 2~ lithium
horohydride in T H F was added dropwise at O T . The reaction mixture was stirred
overnight at room temperature and an additional 0.5 mL of lithium borohydride
solution was added. After 2 4 h the reaction mixture wah cooled to O ' C and
COMMUNICATIONS
quenched with a solution of citric acid. The mixture was diluted with DCM
(100 mL) and washed with sodium blcarbonate (sat.). water. and brine (25 mL
each). The organic layer was dried (Na,SO,), evaporated. and the residue was
dissolved in DCM/MeOH (98/2) and filtered through a plug of silica gel. Evdporation gave the crude precursor of 2 (773 mg). To this crude product (746 mg) in dry
T H F (40 mL) was added methanesulfonic acid (300 pL). After stirring the mixture
at RT for 72 h under nitrogen, pyridine (400 pL), DIPEA (1.5 mL), and acetic
anhydride were added at O'C. After 2 h the solution was diluted with DCM
(lOOmL), washed with sodium bisulfate (1 M), water. sodium bicarbonate (sat.),
water, and brine (25 mL each). The organic layer was dried (Na,SO,), filtered,
evaporated. and chromatographed (silica gel, DCM/MeOH (98/2)) to afford 2
(360 mg, 40 % based on 1).
3: To 2 (60 mg, 0.051 mmol) in DCM (1 mL), TFA (0.5 mL) was added dropwise
and stirred for 1 h a t 0 C. The reaction mixture was diluted with DCM (50 mL) and
washed three times with sodium bicarbonate (sat.) then water and brine (10 mL
each). The organic layer was dried (Na,SO,), evaporated, and redissolved in DCM
(0.5 mL). Boc-(2S,3R,4R,6E)-3-hydoxy-4-methyl-2-(methy~dmino)-6-octenoicacid
(Boc-MeBmt) [21] (18.7 mg, 0.062 mmol), bromotripyrrolidinophosphonium hexafluorophosphate (PyBroP)[22] (27 mg, 0.060 mmol) and DIPEA (27 pL) were
added at 0 ° C under nitrogen. After 2 h a t 0°C the reaction was allowed to warm to
RT, diluted with DCM (50 mL), washed with sodium bisulfate (1 M ) , water, sodium
bicarbonate (sat.), water, and brine (20 mL each). The organic layer was dried
(Na,SO,), filtered, evaporated and chromatographed (flash silica gel, EtOAciacetone (9/l)) to give 3 (56 mg, 81 "A).
4: To 3 (27 mg, 0.022 mmol) in DCM (1 mL), TFA (0.5 mL) was added dropwise
and stirred for 1 h at 0 'C. diluted with DCM (50 mL) and washed three times with
with sodium bicarbonate (sat.). water and brine (10 mL each). The organic layer
was dried (Na,SO,), filtered and evaporated. To the residue in DCM (30 pL),
Fmoc-N-Melle (12 mg, 0.032 mmol). PyBroP (14 mg, 0.032 mmol) and DIPEA
(15 pL) were added and the mixture was stirred for 6 h at O'C and 30 min at RT.
Fmoc-N-MeIle (5 mg, 0.013 mmol), PyBroP (5 mg, 0.011 mmol) were added again
at 0 "C and allowed to warm to R T overnight. The mixture was diluted with DCM
(50 mL), washed with sodium bisulfate (1 M). water, sodium bicarbonate (sat.),
water, brine (20 mL each). The organic layer was dried (Na,SO,). filtered. evaporated, and chromatographed (silica gel, EtOAchcetone 9: I) to afford 4 (19 mg.
60 Yo).
5: To 4 (4.6mg, 0.0028 mmol) in THF/water (10:l. 200 pL), DBU ( 3 pL) and
lithium bromide (2 mg) were added. After stirring overnight at RT. DBU (4 pL) and
lithium bromide (3 mg) were added again After 5 h. the reaction was quenched with
acetic acid (20 pL) and purified by reverse-phase HPLC (Beckman ODS ultrasphere
5 p 10 mm x 25 cm, 0.1 % TFAiMeCN 70/30 +10/90 in 30 min, 70 C, 3 runs) to
yield the peptide precursor of 5 (2.1 mg, 60%). A solution ofthis peptide precursor
(1.2 mg, 970 nmol), AOP[17] (4 mg, 0.009 mmol) and 2,6-lutidine (4 pL) in DCM
(1.2 mL) was stirred for 48 h at RT. The reaction mixture was quenched with acetic
acid (30 pL), the DCM evaporated, and the residue dissolved in acetonitrile and
purified by reverse-phase HPLC (Beckman ODS ultrasphere 5 p 10 mm x 25 cm.
0.1 % TFAiMeCN 70/30 + 10/90 in 30 min. 70 -C) to afford the pure cyclic peptide
5 (0.65 mg, 55 YO).
[16] R. E. Ireland, W. J. Thompson, Tefruhedron Left. 1979, 4705.
1171 L. A. Carpino, E A. El. J. Org. Chem. 1994, 59, 695.
[18] R. E. Handschumacher, M. W. Harding, J. Rice, R. J. Drugge, D. W. Speicher.
Science 1984. 226. 544.
[19] H. Husi. M. Zurini, A n d . Biochem. 1994, 222, 251.
[20] L. D. Zydowsky. F. A. Etzkorn, H. Y Chang, S. B. Ferguson, L. A. Stolz, S. 1.
Ho. C. T. Walsh, Protein Sci. 1992. I , 1092.
[21] W. D. Lubell, T. F. Jamison, H. Rapoport, 1 Org. Chem. 1990, 55. 3511.
[22] J. Coste, E. Frerot, P. Jouin, J. Org. Chem. 1994. SY, 2437.
[23] A. Nicholls. K. A. Sharp, B. Honig, Pror. Sfruct. Funcr. Genet. 1991, if. 283.
Strong Binding of Paraquat and Polymeric
Paraquat Derivatives by Basket-Shaped Hosts**
Albertus P. H. J. Schenning, Bas de Bruin,
Alan E. Rowan,* Huub Kooijman, Anthony L. Spek,
and Roeland J. M. Nolte
Clip-shaped host molecules of type 1 can bind uncharged
aromatic guest molecules, for example resorcinol, by T I - % stacking and hydrogen bonding interactions."] Basket-shaped
derivates of 1 containing crown ether moieties (compounds of
type 2) are, in addition, able to bind alkali metal ions and protonated amines.12] We report here on the binding affinities of
these host molecules towards charged aromatic compounds,
such as paraquat 3 and the polymeric paraquat derivatives 4 and
Meo2p"
.<;I=;>.
Me0e
O
M
e
2a, R = C6H5
b , R = CH,C,H,
Received: June 9, 1995 [ZX0801E]
German version: Angew. Chem. 1995. 107, 2313-2317
-
Keywords: cyclophilin * cyclosporin * immunophilins protein
dimerization
-
D. J. Austin, G. R. Crabtree, S. L. Schreiber, Chem. d Bid. 1994, I . 131
M. A. Lemmon, J. Schlessinger. Trendv Binchem. Sci. 1994, 19. 459.
A. C. Chan, D. M. Desai, A. Weiss, Annu. Rer. Immunof. 1994, 12. 555.
W. H. Landschulz, P. F. Johnson, S. L. McKnight, Science 1988, 240, 1759.
D. M. Spencer, T. J. Wandless, S. L. Schreiber, G . R. Crabtree, Science 1993,
262. 1019.
[6] M. N. Pruschy, D. M. Spencer. T. M. Kapoor, H. Miyaki, G. R. Crabtree, S.
S. L. Schreiber. Chem. & B i d . 1994, I . 163.
[7] H. Ke, D. Mayrose, P. J. Belshaw. D . G. Alberg, S. 1.Schreiber. Z. Y. Chang.
F. A. Etzkorn, S. Ho. C. T. Walsh, Structure (London) 1994. 2, 33.
[XI G . Pflugl. J. Kallen, T. Schirmer. J. N. Jansonius, M . G. Zurini, M . D. Walkinshaw, Nature 1993, 361, 91.
[9] V. F. Quesniaux. M. H Schreier, R. M. Wenger, P. C. Hiestand, M. W. Harding, M. H. V. Van Regenmortel, Eur. 1 Immunol. 1987, 17, 1359.
[lo] T. Clackson, J. A. Wells, Science 1995, 267, 383.
[ l l ] J. S. Richardson, D. C. Richardson in Predicfioir of prolein strucmre and !he
principles of protein conformution, Vol. X I I I ; (Ed. G . D. Fasman), Plenum.
New York, 1989.
[12] G. Sarkar, S. S. Sommer, BioTechniquPs 1990, 8,404.
[13] P. J. Bekhdw, S. L. Schreiber, Chem. d Biol.. submitted.
[14] R. Oliyai, V. .I. Stella, Phurm. Rex 1992. Y, 617.
[15] A. Ruegger, M. Kuhn, H. Lichti, H. R. Looali, R. Huguenin, C . Quiquerez,
W. A. Von, Hell'. Chim. Acto. 1976, 59. 1075.
[l]
[2]
[3]
[4]
[5]
21 32
(CI VCH Verlugsgesdlschuft mhH. 0 4 9 4 5 1 WcJinheim.1995
5 . There is currently a great deal of interest in paraquat-binding,
which has resulted in the design and construction of new molecular structures, as exemplified by the elegant work by Stoddart
et al. on catenands and r o t a x a n e ~ . [We
~ ] describe here that com[*I
[**I
Dr. A. E. Rowan, DipLChem. A. P. H. J. Schenning. DipLChem. B. de Bruin,
Prof. Dr. R. J. M. Nolte
Department of Organic Chemistry, NSR Center
University of Nijmegen
Toernooiveld. NL-6525 ED Nijmegen (The Netherlands)
Telefax: Int. code +(go) 553450
Dr. H. Kooijman, Dr. A. L. Spek
Bijvoet Center for Biomolecular Research, Crystal and Structural Chemistry
Utrecht University (The Netherlands)
This work was supported by the Netherlands Foundation for Chemical Research (SON) with financial aid from the Netherlands Organization for Scientific Research (NWO)
0570-0N33/95/3419-2132$ 10.00f ,2510
A n g w . Chern. h t . Ed. Engl. 1995. 34, N o . i Y