Guest recognition with micelle-bound cavitands

Article abstract of DOI:10.1021/ja0723378

We report that a benzimidazole cavitand is incorporated in aqueous phosphocholine (PC) micelles, folds into the vase conformation, and functions as small-molecule host. As a micelle-bound host it has the ability to sequester selective hydrophobic guest "a

Full text of DOI:10.1021/ja0723378

Published on Web 07/18/2007
Guest Recognition with Micelle-Bound Cavitands
Michael P. Schramm, Richard J. Hooley, and Julius Rebek Jr.*
Contribution from The Skaggs Institute for Chemical Biology, The Scripps Research Institute,
10550 North Torrey Pines Road, Mail MB 26, La Jolla, California 92037
Received April 3, 2007; E-mail: jrebek@scripps.edu
Abstract: We report that a benzimidazole cavitand is incorporated in aqueous phosphocholine (PC) micelles,
folds into the vase conformation, and functions as small-molecule host. As a micelle-bound host it has the
ability to sequester selective hydrophobic guest “anchors” into its interior. These anchors include
cycloalkanes, adamantanes, and nitrogen heterocycles that compete favorably with the large excess of
PC alkyl side-chains that make up the micelle interior. The adamantyl anchor was further functionalized
with a fluorophore, and in another instance a dipeptide and both guests retain their recognition properties
with the micelle-bound cavitand. Additionally, we report that variations in the cavitand periphery and rim
are well-tolerated under our experimental conditions. We find that enhanced binding toward certain guests
in both micelles as well as in solution occurs in response to titration with base; this previously unknown
property of benzimidazole cavitands is reported in detail.
molecules to adopt helical conformations within its interior.21
This role reversal of induced-fit recognition, where the guest
Introduction
Resorcinarenes have proven to be a useful platform on which
to develop a variety of cavitands.1-4 The deeper cavitands served
as small-molecule hosts that demonstrated guest selectivity,2
slow guest exchange,5 and acceleration6,7 or even catalysis of
reactions.8,9 Some deep cavitands have even exhibited these
functions in water, despite their seemingly overwhelming
amount of exposed π surface.10-13 The deepened hydrophobic
interiors facilitate sequestration of both neutral14 and charged15
organic molecules from bulk solution, most commonly Via the
hydrophobic effect.16-20 In addition to recognition, a water
soluble cavitand was demonstrated to entice long surfactant
adapts in response to a rigid host, has been recently reviewed.22
The concentration of surfactant was found to determine its ability
to form kinetically stable complexes in the cavitands. Above
the critical micelle concentration (cmc) the water-soluble
cavitand was shown to migrate into the micelle and became
the guest rather than the host.23 In an effort to develop a general
cavitand for guest recognition in micelles we prepared hydro-
phobic cavitand 1 (Figure 1) and report here our experiences.
Results and Discussion
We synthesized benzimidazole cavitand 1 in three linear steps
following well-established procedures.24,25 These prior reports
indicated the necessity of four interleaved HOR (R ) H, Me,
Et, C ) OR′) molecules to effect folding and stabilization of a
vase conformation in CDCl3 solution. Water was directly
observed in a similar calixarene imidazole cavitand Via NOE,26
and we will continue to include these helper molecules in our
(1) Moran, J. R.; Karbach, S.; Cram, D. J. J. Am. Chem. Soc. 1982, 104, 5826-
5828.
(2) Rudkevich, D. M.; Hilmersson, G.; Rebek, J. J. Am. Chem. Soc. 1998,
120, 12216-12225.
(3) Rudkevich, D. M.; Hilmersson, G.; Rebek, J. J. Am. Chem. Soc. 1997,
119, 9911-9912.
(4) Ho¨gberg, A. G. S. J. Am. Chem. Soc. 1980, 102, 6046-6050.
(5) Hooley, R. J.; Van Anda, H. J.; Rebek, J. J. Am. Chem. Soc. 2006, 128,
3894-3895.
1
depictions. The H NMR spectra in wet DMSO-d6 or THF-d8:
(6) Purse, B. W.; Gissot, A.; Rebek, J. J. Am. Chem. Soc. 2005, 127, 11222-
D2O and high-resolution MS provided unambiguous character-
ization of 1 and demonstrated that under these conditions 1 exists
in a (time-averaged) C4V vase conformation. Although 1 features
no striking water solubilizing features, it was slightly soluble
in pure D2O and gave sharp NMR signals (Figure 2A)
characteristic of an unfolded “kite” conformation, exhibiting
either C2V or D2d symmetry (for a monomeric or dimeric
11223.
(7) Hooley, R. J.; Rebek, J. J. Am. Chem. Soc. 2005, 127, 11904-11905.
(8) Zelder, F. H.; Rebek, J. Chem. Commun. 2006, 753-754.
(9) Hooley, R. J.; Biros, S. M.; Rebek, J. Angew. Chem., Int. Ed. 2006, 45,
3517-3519.
(10) Haino, T.; Rudkevich, D. M.; Rebek, J. J. Am. Chem. Soc. 1999, 121,
11253-11254.
(11) Haino, T.; Rudkevich, D. M.; Shivanyuk, A.; Rissanen, K.; Rebek, J.
Chem.-Eur. J. 2000, 6, 3793.
(12) Hof, F.; Trembleau, L.; Ullrich, E. C.; Rebek, J. Angew. Chem., Int. Ed.
2003, 42, 3150-3153.
(13) Biros, S. M.; Rebek, J. Chem. Soc. ReV. 2007, 36, 93-104.
(14) Hooley, R. J.; Biros, S. M.; Rebek, J. Chem. Commun. 2006, 509.
(15) Menozzi, E.; Onagi, H.; Rheingold, A. L.; Rebek, J. Eur. J. Org. Chem.
2005, 3633-3636.
(21) Trembleau, L.; Rebek, J. Science 2003, 301, 1219-1220.
(22) Schramm, M. P.; Rebek, J. Chem.-Eur. J. 2006, 12, 5924-5933.
(23) Trembleau, L.; Rebek, J. Chem. Commun. 2004, 58-59.
(24) Far, A. R.; Shivanyuk, A.; Rebek, J. J. Am. Chem. Soc. 2002, 124, 2854-
2855.
(25) Choi, H. J.; Park, Y. S.; Song, J.; Youn, S. J.; Kim, H. S.; Kim, S. H.;
Koh, K.; Paek, K. J. Org. Chem. 2005, 70, 5974-5981.
(26) Botana, E.; Da Silva, E.; Benet-Buchholz, J.; Ballester, P.; de Mendoza, J.
Angew. Chem., Int. Ed. 2007, 46, 198-201.
(16) Gibb, C. L. D.; Gibb, B. C. J. Am. Chem. Soc. 2006, 128, 16498-16499.
(17) Gibb, C. L. D.; Gibb, B. C. J. Am. Chem. Soc. 2004, 126, 11408-11409.
(18) Sebo, L.; Diederich, F.; Gramlich, V. HeIV. Chim. Acta 2000, 83, 93-
113.
(19) Diederich, F.; Dick, K. J. Am. Chem. Soc. 1984, 106, 8024-8036.
(20) Tanford, C. Science 1978, 200, 1012-1018.
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cyclohexane (forcing the host into the folded C4V vase confor-
mation), no dissolution in water was detected.
1. Behavior of 1 in Lipid Micelles. The geometry of cavitand
1 in water changed drastically with the addition of dodecylphos-
phocholine (DPC) above the cmc. Here the 1H NMR spectrum
reveals that cavitand 1 adopts the vase conformation (Figure
2B) even in the absence of added guest, and that this previously
insoluble conformation had been coaxed into water by the
micelle. The methine proton (HE) now appears at 5.7 ppm. This
initial finding indicated that the hydrophobic interior of DPC
micelles effected folding of 1. No peaks for bound phospho-
choline were observed,23 most probably due to weak binding
with rapid in/out exchange. Surprisingly, cavitand 1 was not
soluble in sodium dodecyl sulfate (SDS) above the cmc despite
reports that the water-soluble tetracarboxylate analogue was
soluble.23
Figure 1. The ethyl-footed benzimidazole cavitand 1 and minimized
representation of 1 with four interleaved water molecules (PM3, Spartan
‘04).29
1.1. Guest-Host-Micelle Interactions. Suitable guests
adamantylamine and cyclohexane showed strong binding in
micelle-bound cavitand 1, as was obvious from the upfield
region of the 1H NMR spectra (Figure 2 C,E). The guests were
able to compete with a sea of long chain alkyl groups of the
surfactant in which 1 is submerged. The 1H NMR spectra allow
the observation of guest binding, as the guest shows large upfield
shifts due to the preorganized aromatic rings of the cavitand
creating a magnetically shielded environment, a phenomenon
that micelles do not exhibit due to their construction from
magnetically docile saturated hydrocarbon chains. 1H NMR does
not exclusively prove that the host-guest complex is located
inside the micelle (although the solubility of the complex
provides compelling evidence). Diffusion ordered spectroscopy
(DOSY)30 was able to show the presence of one multicomponent
system. The signals for host (7.6 × 10-11 m2/s), guest (8.4 ×
10-11 m2/s) and micelle (8.4 × 10-11 m2/s) displayed diffusion
coefficients indicating the presence of one unified molecular
assembly (see Supporting Information for raw data; HOD was
measured to be 1.8 × 10-9 m2/s). These results are in agreement
with literature reports.31
The addition of inorganic base to the guest-host-micelle
assemblies had a marked effect on guest binding. With no added
guest (Figure 2B) or with bound cyclohexane (E) the downfield
aromatic residues of cavitand 1 were essentially unchanged.
Proton HA of the cavitand (Figure 1) resonates at 9.2 ppm in
both cases. Addition of NaOH to the system with bound
cyclohexane (Figure 2F) resulted in the upfield shift of cavitand
peaks (e.g., HA ∆∂ ) -1.10 ppm) as well as those of guest
(cyclohexane ∆∂ ) -0.29 ppm). In the process the binding
affinity of cyclohexane for 1 decreased. Adamantylamine, on
the other hand, did not display the same drastic changes upon
addition of NaOH. In this case, the guest is itself basic, and so
the downfield cavitand peaks seen in Figure 2C are more akin
to those in F (+NaOH) than those in neutral media (B, E). On
the basis of integrations, adamantylamine shows only 50%
binding. Addition of external NaOH (D) did not cause a large
change in host or guest chemical shifts, but sharpened the signals
significantly and increases binding affinity; integration reveals
a 1:1 host-guest complex. The broadened, split peaks in B and
E are due to multiple or flexible cavitand conformations; these
peaks sharpen dramatically as seen in D and F where it appears
Figure 2. 600 MHz 1H NMR (D2O, 600 µL) of (A) 3 mM cavitand 1, (B)
cavitand 1 + 40 mM DPC, (C) B + 30 mM adamantylamine, (D) C + 10
µL 3% NaOH, (E) B + 30 mM cyclohexane, (F) E + 10 µL 3% NaOH.
Traces of DMF marked with “X.”
complex respectively).27,28 Most characteristic is the methine
resonance (HE), that occurs at 3.9 ppm in this kite conformation.
Electrospray ionization MS gave a series of peaks with the
correct isotopic distribution for a singly positive charged dimer
(dimer + H+ found ) 2115), confirming that cavitand 1 forms
a dimeric velcrand in water.11 Presumably the kite dimer presents
a smaller surface area to the water solvent; indeed, if the
cavitand was prepared as a stoichiometric complex with
(27) Cram, D. J.; Choi, H. J.; Bryant, J. A.; Knobler, C. B. J. Am. Chem. Soc.
1992, 114, 7748-7765.
(28) Moran, J. R.; Ericson, J. L.; Dalcanale, E.; Bryant, J. A.; Knobler, C. B.;
Cram, D. J. J. Am. Chem. Soc. 1991, 113, 5707-5714.
(29) Spartan’04, Wavefunction, Inc., Irvine, CA.
(30) Price, W. S. Concepts Magn. Reson. 1997, 9, 299-336.
(31) Gao, X.; Wong, T. C. Biophys. J. 1998, 74, 1871-1888.
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Figure 3. 1H NMR competition of encapsulated hydrocarbons (3 mM 1, 41 mM DPC, 45 mM of each guest in D2O). Column A: cyclooctane, cyclooctylamine,
and cyclooctane/cyclooctylamine. Column B: cyclohexane, cyclohexylamine, and cyclohexane/cyclohexylamine. Column C: methylcyclohexane,
4-methylpiperidine, and methylcyclohexane/4-methylpiperidine.
that NaOH has slowed the kinetic motion of the cavitand on
the NMR time scale.
furthest from the point of amine attachment deep in the cavity.
Competitive binding experiments between the alkanes and their
amine-substituted analogues revealed that nitrogen had a
remarkable effect. In all instances the nitrogen-substituted
alkanes were the only bound species observable with greater
than 99:1 selectivity. There appeared to be little or no selectivity
between cyclohexylamine and cyclooctylamine. When adaman-
tylamine was compared with either cyclooctylamine or cyclo-
hexylamine there also was no clear preference (not shown).
1.2. Guest Selectivity. Cavitand 1 formed stable complexes
with several other small molecules in DPC micelles including
trans-decalin, trans-1,4-dimethylcyclohexane, cyclohexane, me-
thylcyclohexane, cyclooctane, cyclohexylamine, cycloocty-
lamine, cyclooctanol, and 4-methylpiperidine. A few ostensibly
compatible species exhibited either weak affinity for the cavitand
(e.g., 4-tert-butylaniline, adamantylmethylamine, 1,1,4-trimeth-
ylcyclohexane), or none at all (e.g., adamantylacetic acid,
adamantanecarbonitrile). On the basis of these initial results,
we conducted a series of competition experiments to probe the
role of both the shaped anchor (hydrocarbon ring) as well as
heteroatom substitution. The hydrocarbons cyclooctane, cyclo-
hexane, and methylcyclohexane served as good guests (Figure
3). In the case of cyclooctane and cyclohexane, the presence of
one major peak is indicative of rapid guest tumbling on the
NMR time scale. Methylcyclohexane also appears to tumble
rapidly because of the close grouping of peaks around -2.5
ppm.14 The addition of a heteroatom stops the tumbling of these
guests on the NMR time scale, presumably through interaction
with the rim or perhaps repulsion from the electron-rich aromatic
walls of the cavity. This is clearly illustrated in the second row
1.3. Binding Functionalized Anchors. The experiments thus
far revealed that a combination of an appropriately sized and
shaped hydrocarbon anchor in conjunction with a heteroatom
substituent was most beneficial for cavitand binding and good
selectivity could be obtained between qualitatively similar guests
for 1 in micelles. This selectivity is not dependent on the
substituent residing outside the cavity (in previous cavitand
studies finite guest lengths were uncovered, above which binding
did not readily occur).14 We prepared several guests with a small
variety of anchors to which we attached additional functionality
that would reside outside the binding pocket of the cavitand.
Dansyl-appended adamantylamine 2 (Figure 4) was shown to
bind under the standard conditions, affording a fluorescently
labeled guest. In addition, adamantyl-Gly-Gly-OH 3 and ada-
mantyl maleimide bind strongly, demonstrating that the complex
between 1 and micelles can act as a host for a variety of species
functionalized with suitable binding anchors. The lack of
selectivity of cavitand 1 for unfunctionalized adamantylamine
vs cyclooctylamine encouraged us to explore the cyclooctyl
analogue of 3, cyclooctyldansylsulfonylamide; this compound
showed no uptake in micelle bound 1, revealing one example
of a limitation on this strategy. The successful functionalized
1
of Figure 3; selected H signals for the aliphatic amines are
assigned and their spread of chemical shifts gives support for
slowed rotation as well as reveals the orientation of the guest.
The magnetic anisotropy of the cavitand has the effect of
providing the most shielding for species located near the
resorcinarene: 4-methylpiperidine clearly places its methyl
group deep into this cavity (δ -4.2 ppm, MeC). Cyclooctylamine
and cyclohexylamine behave similarly, burying the methylene
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a variety of substitution patterns of resorcinarene-based cavitands
are well-tolerated for micelle localization and guest recognition.
Variation at both the feet and the rim is possible, as well as in
the composition of the walls, which are built up from a common
resorcinarene scaffold. The breadth of tolerated substitution
patterns in conjunction with the relative ease of cavitand
preparation bode well for future exploration toward more
complex membrane mimics.
2. Solution Studies of 1. While the cavitands could be
extracted into solution by the micelles, and in several cases
exhibit guest recognition, the broadest activity as a host was
observed in the presence of added base, either NaOH or organic
amine. In order to determine the reason for this, the host-guest
properties of 1 were studied in the absence of micelles. In either
DMSO-d6 (Figure 6A) or 2:1 THF-d8:D2O (Figure 7A) cavitand
1 was soluble and existed in the folded C4V conformation. In
2:1 THF-d8:D2O the spectrum was much sharper, attributed to
THF’s well-established ability to serve as a guest for systems
like this.32 Efforts to find suitable guests for 1 in THF:D2O were
hindered by an inability to displace THF from the cavitand
interior. DMSO, on the other hand, only poorly occupies the
cavity and led to a less kinetically stable receptor system, but
one that proved more amenable to the binding of added guests.
Figure 4. Adamantane “anchor” with attached functionality.
adamantyl anchors employed here by no means define the limits
of this approach; rather they set the stage for further exploration.
1.4. Cavitand Variation. The short ethyl “feet” and unfunc-
tionalized benzimidazole wall afforded a cavitand that was
soluble, folded, and well-behaved as a small molecule host while
immersed in DPC micelles. Were these properties unique to
cavitand 1 or could others be found? In an effort to answer this
question we screened a variety of cavitands through varying
both cavitand foot (R′) and rim (R) substitution (Figure 5). We
decided to test several variables to get a better understanding
of the properties of our prototypical cavitand 1. Having a variety
of functionality to choose from at both locations, we limited
the functionalization of the feet to either the short ethyl group
or the long C11H23 chain. At the rim of the cavitand we decided
to explore a wider range of groups including an alkyl chain,
alkyl esters, substituted aromatics, and an octaamide cavitand
that differs in composition from the benzimidazole scaffold on
which the others are based. We found that a number of these
features were amenable to extraction into the micelle. The NMR
spectra of the complexes of cavitands 4-9 in micelles are
located in the Supporting Information.
2.1. Guest Binding in DMSO. The addition of adamanty-
lamine to a DMSO-d6:D2O solution of 1 failed to yield evidence
of guest binding or host solubility, contrary to what was
observed in aqueous micelles. However, treatment of this
solution with NaOH resulted in concomitant host solubilization
and guest uptake, so an increase in binding affinity for the host:
guest complex occurred. Adamantylacetic acid proved to be a
very poor guest at neutral pH, and in this case the addition of
NaOH resulted in a disappearance of all host and guest signals
1
in the H NMR spectrum. We also explored using tetramethy-
The presence of ethyl esters at the rim with either ethyl (4)
or C11 feet (5) gave results similar to 1 under the experimental
conditions. The binding of adamantylamine was obvious in the
upfield region of the NMR, and upon addition of base enhanced
binding was observed. The incorporation of ester groups at the
rim and the C11 chains at the feet (5) gave NMR spectra with
broader signals. Longer alkyl chain surfactants could allow for
better exploration of cavitand functionality, and we employed
hexadecylphosphocholine (HPC). Cavitands 1, 4, and 5 were
well-behaved in HPC when compared to DPC, and a slight
increase in peak sharpness for the longer 5 was noted. Perhaps
HPC allows a greater range of cavitands to be bound, purely
due to its increased dimensions. With aromatic functionalized
rims we found that 7 behaved as a weak host for adamanty-
lamine, even in the presence of base. It is unclear at this time
whether this is purely an issue of cavitand solubility in HPC
micelles or if other factors are at work. Cavitand 8 with
p-nitrophenyl groups served as a good host. Cavitand 6, which
was functionalized with n-octyl chains at the rim, resisted
micelle uptake, presumably because of poor solubility. The
protrusion of the octyl rim into the bulk solution may be the
cause.
lammonium bromide as a host for cavitand 1 in solution. The
assignment of the (time-averaged) C4V conformation of 1 in
DMSO is shown Figure 6A. Upon addition of excess teatram-
ethylammonium bromide a very small signal in the upfield
region is observed, indicative of weak guest binding. Treatment
of the sample with NaOH results in enhanced binding as a 1:1
host guest complex is now observed (Figure 6B vs C). The
binding properties of tetraalkylammonium salts and other
organics in related cavitands is well-known, so we decided to
examine the role of base more closely.
2.2. The Role of Base. Why does the conformational stability
and binding activity of the cavitand increase in the presence of
base? The cavitand is held in the vase conformation by four
water molecules that provide eight hydrogen bonds to the
benzimidazole groups at the cavitand rim. Only four of the eight
hydrogens from the water molecules are involved in hydrogen
bonding; the remaining four are mere spectators. If NaOH (or,
to a lesser extent, an organic amine base) is added, some of the
hydrogens at the rim (either from the N-H or from H2O) can
be removed, increasing the electron density along the hydrogen-
bonding network. Presumably, this could strengthen the weak
forces holding the cavitand together. While a solvent mixture
of THF:D2O was incompatible with host-guest studies, the
sharpness of the aromatic cavitand peaks in the 1H NMR spectra
The benzimidazole functionality is not an essential feature
for cavitand uptake and function; octamide 9 was also a
successful host for adamantylamine in HPC micelles. In this
case added NaOH has no effect on binding. This result is not
surprising, as the walls of 9 lack the acidic benzimidazole
functionality of the other cavitands. These results illustrate that
(32) Biros, S. M.; Ullrich, E. C.; Hof, F.; Trembleau, L.; Rebek, J. J. Am. Chem.
Soc. 2004, 126, 2870-2876.
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Figure 5. Benzimidazole (1, 4-8) and octamide (9) cavitands studied as hosts in DPC and HPC micelles.
1
Figure 6. 600 MHz H NMR of 19 mM cavitand 1 in 600 µL DMSO-d6 (A), after addition of 65 mM tetramethylammonium bromide (B); subsequent
addition of 20 µL of 3% NaOH results in 1:1 stoichiometric encapsulation (C). Trace solvents DMF and DCM indicated by X.
presented us with the opportunity to explore the role of pH more
carefully. The downfield region of cavitand 1 shows sharp peaks
for all unique protons (Figure 7A), the farthest downfield being
the highest up on the cavitand wall, namely the C-H of the
imidazole at 8.5 ppm (labeled HA in Figure 1). The peak at 7.4
ppm corresponds to the aromatic proton HD at the base of the
cavitand. When HCl was titrated, cavitand peaks migrated
downfield (Supporting Information), confirming an earlier
account of the analogous C11 benzimidazole cavitand upon
treatment of TFA in CDCl3.25
When a solution of NaOH was titrated into a 1.4 mM solution
of 1, large upfield shifts occurred for the protons on the cavitand
wall (Figure 7). Upon addition of 4 equiv of NaOH, HA had
shifted 0.6 ppm upfield. The remaining peaks of the cavitand
wall also experienced significant changes, while the methine at
the base of the resorcinarene ring remained relatively unchanged
(see data for HE in Figure 7B), indicating that the vase
conformation remained intact. Protons HA and HB show the
largest shift upon deprotonation in the H-bonding network,
because they are sited in rings that are in direct conjugation
with the benzimidazoles, whereas HC and HD reside in the
resorcinarene and do not experience the electronic changes as
strongly.
It is not completely clear from where the protons are removed
in the hydrogen-bonded network; either deprotonation of one
or more of the imidazole NHs or the removal of the spectator
hydrogen on a bridging water molecule is plausible. In any case,
the rapid exchange of these hydrogens renders the point moot.
Modified Job’s plot analysis was conducted33 in THF:D2O by
titration of 1 with NaOH to discern the number of acidic protons
(33) Hirose, K. J. Inclusion Phenom. Macrocyclic Chem. 2001, 39, 193-209.
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1
Figure 7. 600 MHz downfield H NMR of 1.4 mM 1 (initial concentration) in 2:1 THF-d8:D2O with the addition of 0, 1, 2, 4, 6, and 11 equiv of NaOH
(A) and graphical response in ∆∂ (∂final - ∂initial) (B).
Figure 8. Modified Job’s plot analysis of 1 with NaOH. R ) 2 mM
(constant) ) [cavitand]0 + [base]0, ø ) [cavitand]0/[cavitand]0 + [base]0;
all measurements performed at 300 K using 600 MHz 1H NMR, 2:1 THF-
d8:D2O.
Figure 9. Hydrogen-bonding model of 1 after removal of two protons from
the H-bonding network.
removed by added base (Figure 8). The intersection of lines
revealed a maximum at [cavitand 1] × ∆δ ) 0.48. This
tonation the hydrogen-bonding network could be strengthened,
maximum occurs when ø ) 0.3 (where ø ) [cavitand]0/
[cavitand]0 + [base]0). To a first approximation this gives a
2.33:1 base:cavitand relationship. Both solubility issues at high
base:cavitand ratios and acid-base equilibration are believed
to result in this noninteger relationship. Nevertheless, this result
in conjunction with dilution studies that show no change in ∆δ
as a function of concentration supports the hypothesis that
deprotonation is responsible for the observed changes in
chemical shift. We illustrate these effects with a model of the
hydrogen-bonding seam that is supported by the experimental
evidence (Figure 9).26 The addition of base has a remarkable
effect on the electronic environment of the capsule, resulting
in buildup of negative charge, that in turn affects the binding
ability of the host toward guests. This can be explained in one
of two ways. With 1, it is clear that a variety of conformations
are adopted in response to environmental conditions (e.g.,
solvent, presence of guest, presence of micelles). Upon depro-
resulting in a more kinetically stable conformation where the
walls are more tightly held together and less likely to flex or
“breathe.” Deprotonation contracts the cavitand’s time-averaged
structure and increases its shape complementarity to suitable
guests. Alternatively, the increase in electron density allows
greater London dispersion forces to occur between the electron
rich cavitand and the thin layer of positive charge displayed by
the hydrocarbon guests. This latter explanation seems less likely
to play as significant a role as an increase in the kinetic stability
of the cavitand host would, yet at this time it cannot be
discounted. It may even complement the effects of enhanced
kinetic stability of guest binding as a result of changing the
electronic nature of the hydrogen-bonding seam.
Conclusion and Outlook
In conclusion we report that deep, self-folding benzimidazole
cavitands such as 1 are incorporated in aqueous phosphocholine
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reports for the synthesis of the C11-footed cavitand.24,25 Ethyl-footed
octanitro cavitand38 (300 mg, 0.24 mmol) was dissolved in DMF (12
mL) under N2. SnCl2·2H2O (1.5 g, 6.6 mmol) was added followed by
concentrated hydrochloric acid (6 mL). The reaction was heated to
105 °C for 16 h, cooled, and poured into iced water (75 mL). The
slurry was centrifuged, and the supernatant was decanted. The solid
was taken up in a minimal amount of methanol:CH2Cl2 and then
concentrated to remove excess water and DMF. The solid was treated
with 50 mL of dichloromethane, sonicated, filtered, and dried under
(PC) micelles and fold into the C4V vase conformation, serving
as small molecule hosts while residing in lipid micelles. As a
micelle-bound host, 1 has the ability to sequester hydrophobic
guests into its interior even though both host and guest are
submersed in a formidable sea of competing alkyl side chains.
The cavitands exhibit guest selectivity, which allowed us through
the use of a hydrophobic anchor to localize both a fluorophore
(2) and a dipeptide (3) within the micelle-bound receptor.
Subsequent exploration of other cavitand substitution patterns
reveals some flexibility and tolerance in both substitution at
the feet and rim toward micelle localization. These variables
should afford future opportunities in both the types of hosts
and their respective guests that are amenable to migration into
a variety of both synthetic and natural membranes. Additionally,
the ability to incorporate aromatic groups at the cavitand rim
should allow us to fluorescently label the host and perhaps
further expand upon the types of suitable handles we can
employ.
The transport of sensitive small molecules across biological
membranes has been a long-standing goal of medicinal chem-
istry. Conceptually, several researchers have envisioned encap-
sulating drugs in larger chemical frameworks or capsules to
effect targeted transport to a specific location.34,35 These small
molecule cavitand hosts are themselves guests within the
hydrophobic interior of the micelle and are simple biomimetic
receptors. The next steps of this research program will be to
transport fluorophores and drug-like molecules into more
complicated vesicular and lipid-bilayer systems.
1
vacuum to give 1 as a pale yellow powder (175 mg, 69%). H NMR
(600 MHz, DMSO-d6): δ 0.93 (t, J ) 7.2 Hz, 12H); 2.39 (qn, J ) 7.2
Hz, 8 H); 5.44 (t, J ) 7.8 Hz, 4H); 7.69 (s, 4H); 8.02 (s, 4H); 8.25 (s,
8H); 8.51 (br s, 4H), ESIHRMS m/z: calcd for C64H49N8O8 (M + H)
1057.3668; found 1057.3658.
Representative Procedure for the Synthesis of Cavitands 5 and
8. C11 Ester Cavitand 5. To an oven-dried, 25 mL round-bottomed
flask equipped with a magnetic stirrer and water-cooled condenser were
added C11H23-footed octaamine cavitand, HCl salt24 (100 mg, 0.060
mmol), ethyl 3-ethoxy-3-iminopropanoate hydrochloride39 (0.36 mmol,
70 mg), and anhydrous ethanol (10 mL). The mixture was placed under
argon and heated to reflux for 14 h. The solvent was removed by rotary
evaporation, and the resulting solid was suspended in dry MeOH (25
mL). The suspension was filtered and washed with dry MeOH (3 ×
20 mL) and then dried under high vacuum to yield cavitand 1a (60
mg, 57%) as an off-white solid. 1H NMR (600 MHz, CDCl3/D2O): δ
0.90 (t, J ) 6.6 Hz, 12H); 1.2 - 1.4 (m, 16H); 1.31 (t, J ) 6.6 Hz,
12H); 1.43 (qn, J ) 6.6 Hz, 8H); 2.25 (q, J ) 7.8 Hz, 8H); 4.26 (q, J
) 7.2 Hz, 8H); 4.30 (s, 8H); 5.73 (t, J ) 7.8 Hz, 4H); 7.25 (s, 4H);
7.44 (s, 4H); 8.01 (s, 8H); ESIHRMS m/z: calcd for C116H145N8O16
(M + H+) 1906.0773; found 1906.0739.
C11 p-Nitrobenzylcavitand 8. Cavitand 8 was synthesized on a 0.06
mmol scale according to the procedure used for 5, employing ethyl
2-(4-nitrophenyl)acetimidate hydrochloride S1 (Supporting Informa-
tion), giving an off-white solid (109 mg, 86%). H NMR (600 MHz,
Experimental Section
1. General Information. 1H and DOSY30 NMR spectra were
recorded on a Bruker DRX-600 spectrometer with a 5 mm QNP probe.
Proton chemical shifts are reported in parts per million (δ) with respect
to tetramethylsilane (TMS, δ ) 0) and referenced internally with respect
to the protio solvent impurity. The DOSY spectra were acquired using
an LED pulse sequence with bipolar gradient pulses and two spoil
gradients, as supplied with the Bruker software.36 Sine-shaped pulsed
gradients were incremented from 2.7 to 51.4 G cm-1 with an Acustar
gradient system in 32 steps, with each step consisting of 256 scans.
The raw data was processed using the MestreC program (Mestrelab
Research, Santiago de Compostela). Deuterated NMR solvents were
obtained from Cambridge Isotope Laboratories, Inc., Andover, MA,
and used without further purification. Anhydrous solvents and reagents
were obtained from Aldrich Chemical Co., St. Louis, MO, and were
used as received. Cavitands 4,32 6,37 7,5 and 96 were synthesized
according to reported procedures. All micelle experiments were
conducted by sonication of the indicated amounts of host, guest, and
PC in D2O for 10 min prior to NMR acquisition.
1
THF-d8/D2O 4:1): δ 0.84 (t, J ) 7.2 Hz, 12H); 1.2 - 1.4 (m, 16H);
1.41 (qn, J ) 7.2 Hz, 8H); 2.23 (q, J ) 7.2 Hz, 8H); 4.11 (s, 8H); 5.73
(t, J ) 7.2 Hz, 4H); 7.29 (s, 4H); 7.48 (d, J ) 7.8 Hz, 8H); 7.71 (s,
4H); 7.87 (s, 8H); 8.05 (d, J ) 7.8 Hz, 8H); ESIHRMS m/z: calcd for
C
128H141N12O16 (M + H+) 2102.0582; found 2102.0565.
Acknowledgment. We are grateful to the Skaggs Institute
and the National Institutes of Health (GM 50174) for financial
support. M.P.S. and R.J.H. are Skaggs Postdoctoral fellows. We
wish to thank Dr. Enrique Mann for compound 3.
Supporting Information Available: 1H NMR spectra for all
new cavitands. All spectra for encapsulation results that were
discussed but not illustrated in the text. DOSY results for 1
with adamantylamine in DPC micelles showing one assembly
along with DCl titration data of 1 in THF:D2O. This material
is available free of charge via the Internet at http://pubs.acs.org.
2. Synthesis of New Compounds. Procedure for the Synthesis of
Cavitand 1. Ethyl-footed cavitand 1 was prepared following related
JA0723378
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