Molecular umbrellas

Full text of DOI:10.1021/ja953261v

J. Am. Chem. Soc. 1996, 118, 1573-1574
1573
Scheme 1
Molecular Umbrellas
Vaclav Janout, Marion Lanier, and Steven L. Regen*
Department of Chemistry and
Zettlemoyer Center for Surface Studies
Lehigh UniVersity, Bethlehem, PennsylVania 18015
ReceiVed September 25, 1995
In this paper we introduce a new concept in surfactant
chemistry that is based on molecules that mimic the structure
and function of umbrellas, i.e., molecules that can cover an
attached agent and shield it from an incompatible environment.
We term such structures “molecular umbrellas” and report herein
the first representative example. Potential applications of this
new class of compounds in the areas of drug design and drug
delivery are briefly discussed.
Activation of cholic acid by conversion to its N-hydroxysuc-
cinimide ester, followed by condensation with the primary amino
groups of spermidine and subsequent coupling with dansyl-
glycine afforded the double-walled umbrella, I (Scheme 1). An
analogous single-walled structure (II) was also prepared using
monodansylcadaverine as starting material. Compound III,
which was chosen as a control, was synthesized by acetylation
of spermidine with acetic acid N-hydroxysuccinimide ester (A-
NHS), followed by condensation with dansylglycine.5
The fluoresence emission spectra of 2 µM solutions of I in
varying dimethoxyethane/water mixtures are shown in Figure
1. Incremental replacement of DME with water resulted in a
continuous decrease in fluorescence intensity and a shift in λmax
to longer wavelengths, until a composition of 10/90 (DME/
water, v/v) was reached. Further increases in water content
resulted in a reVersal in the spectral changes; i.e., the fluores-
cence intensity increased and λmax shifted toward shorter
wavelengths.6 In pure DME, water, and hexane, the fluores-
cence intensities that were observed were very similar; the
corresponding λmax values were 501, 496, and 495 nm,
respectively. In sharp contrast, incremental increases in water
content for DME/water solutions of II produced a continuous
shift in λmax to longer wavelenghs and a continuous decrease
in fluorescence intensity oVer the entire range of solVent
mixtures used (in 100% water, the observed fluorescence was
barely detectable); exactly analogous behavior was observed
for III.
Our construction of molecular umbrellas hinges on the use
of amphiphilic molecules that maintain a hydrophobic as well
as a hydrophilic face.1-3 In essence, two or more such
amphiphiles (umbrella walls) are coupled to a suitable scaffold
either before or after a desired agent is attached to a central
location. Under appropriate environmental conditions, the
amphiphilicity of each wall combines with the hydrophobicity
or hydrophilicity of the agent to produce a “shielded” conforma-
tion. For those agents that are hydrophobic, “immersion” in
water favors a shielded conformation such that intramolecular
hydrophobic interactions are maximized and the external face
of each wall is hydrated. When immersed in a hydrocarbon
solvent, the umbrella favors a fully exposed conformation where
solvation and intramolecular dipole-dipole and hydrogen-
bonding interactions can be optimized. For those umbrellas
that bear a hydrophilic agent, these same forces are expected
to produce shielded and fully exposed conformations in
hydrocarbon and aqueous environments, respectively, i.e., the
opposite conformational preferences.
In this study, we have prepared an umbrella molecule using
cholic acid as “wall material”, spermidine as the scaffold, and
an environmentally-sensitive fluorescent probe, 5-dimethyl-
amino-1-naphthalenesulfonyl (dansyl), as the agent. Due to the
relative hydrophobicity of the dansyl moiety, it was anticipated
that such umbrellas would favor exposed or fully exposed
conformations in solvents of low polarity and a shielded
conformation in water. Cholic acid was specifically chosen
because it possesses the requisite amphilicity and because it
can be readily conjugated through its carboxylic acid group.2,4
In addition, umbrella frameworks derived from cholic acid and
spermidine were expected to be potentially biocompatible, since
both compounds occur naturally in mammalian cells.
The observed changes in the fluorescence emission spectrum
of I with increasing water content, on going from pure DME
(5) Dansylgycine, monodansylcadaverine, and A-NHS were commercially
available (Sigma) and used as obtained. Compounds I, II, and III gave
1
satisfactory H NMR (360 MHz) and HRMS analyses.
(6) The cmc of I, which was determined in pure water by UV methods
(Shoji, N.; Ueno, M.; Meguro, K. J. Am. Oil Chem. 1978, 55, 297) was 2.5
µM. In addition, the fluorescence intensity of I in pure water was directly
proportional to its concentration within the range of 0.5 and 5.0 µM; a
discontinuity at the 5.0 µM concentration, however, was noted. This
discontinuity is presumed to reflect the onset of aggregation. The fact that
the “turning point” for I (i.e., the DME/water ratio at which a minimum
fluorescence intensity is observed) occurs at exactly the same DME/water
ratio (10/90 v/v) when 1.0 and 0.5 µM solutions of umbrella are employed
also argues against the possibility that this turning point reflects a critical
micelle concentration.
(7) MacGregor, R. B.; Weber, G. Nature 1986, 319, 70.
(1) (a) Stein, T. M.; Gellman, S. H. J. Am. Chem. Soc. 1992, 114, 3943.
(b) McQuade, D. T.; Barrett, D. G.; Desper, J. M.; Hayashi, R. K.; Gellman,
S. H. J. Am. Chem. Soc. 1995, 117, 4862.
(2) (a) Cheng, Y.; Ho, D. M.; Gottlieb, C. R.; Kahne, D.; Bruck, M. A.
J. Am. Chem. Soc. 1992, 114, 7319. (b) Venkatesan, P.; Cheng, Y.; Kahne,
D. J. Am. Chem. Soc. 1994, 116, 6955.
(3) Burrows, C. J.; Saute, R. A. J. Inclusion Phenom. 1987, 5, 117.
(4) Hjelmeland, L. M.; Nebert, D. W.; Osborne, J. C. Anal. Biochem.
1983, 130, 72.
(8) In preliminary studies, we have found that a molecular umbrella can
enhance the partitioning of a polar compound from water into an organic
phase. Thus, spermidine, bearing a cholic acid group at each end,
significantly enhances the partitioning of picric acid from water into
chloroform. Although the structure of this umbrella/picric acid complex
remains to be established, a likely possibility is one in which the umbrella
adopts a shielded conformation that covers over picric acid and holds it in
place through acid/base interaction with the secondary amine group of
spermidine: Vigmond, S.; Janout, V.; Regen, S. L., unpublished results.
0002-7863/96/1518-1573$12.00/0 © 1996 American Chemical Society

1574 J. Am. Chem. Soc., Vol. 118, No. 6, 1996
Communications to the Editor
highly polar media. In this regard, the λmax for I in water, which
lies between those observed in DME and in hexane, reflects a
microenvironment that is very hydrophobic. The fact that a
single-walled analog (II) cannot effectively shield the dansyl
moiety indicates that a minimum of two walls is necessary in
order to have a functional umbrella. This finding also provides
compelling evidence that the shielded conformation of I has
both walls covering over the dansyl group; with II, one side of
the fluorophore must remain completely exposed to the solvent
at all times.
Molecular umbrellas of the type reported herein offer intrigu-
ing possibilities for creating new classes of drugs and drug
carriers. One can envision, for example, the sequestration of
cytotoxic, nonpolar cross-linking agents within an umbrella
(agents that would otherwise decompose in water or react with
water-soluble nucleophiles) and their release into target mem-
branes by “flipping” from a shielded to a fully exposed state.
Alternatively, molecular umbrellas may serve as novel vehicles
for transporting polar drugs across biological membranes.
Specifically, a logical sequence of events would be (i) diffusion
of a drug/umbrella conjugate to a biomembrane surface in a
fully exposed state, (ii) insertion into the outer monolayer leaflet
by flipping into a shielded state, (iii) diffusion to the inner
monolayer leaflet, and (iv) entry into the cytoplasm via the
sequential reversal of steps ii and i.
Figure 1. Representative fluorescence emission spectra of 2 µM
solutions of I in DME/water mixtures measured at 23 °C. Highest to
lowest intensity spectra were observed using the following ratios of
DME/water (v/v): 0/100 (pure water), 100/0 (pure DME), 50/50, 20/
80, and 10/90. The excitation wavelength and spectral bandwidth used
in all cases were 330 and 5 nm, respectively. Inset: Plot of relative
emission intensity at λmax for I as a function of water content (v/v) for
a series of DME/water mixtures measured at 23 °C. Similar data were
obtained using 0.5 and 1.0 µM concentrations of the molecular umbrella.
Efforts aimed at exploiting molecular umbrellas with a view
toward drug design and drug delivery are now under intensive
investigation in our laboratories.8
Acknowledgment. We are grateful the National Institutes of Health
(PHS Grant GM51814) for support of this research.
Supporting Information Available: One figure containing the data
used to determine the cmc of I (1 page). This material is contained in
many libraries on microfiche, immediately follows this article in the
microfilm version of the journal, can be ordered from the ACS, and
can be downloaded from the Internet; see any current masthead page
for ordering information and Internet access instructions.
to 10/90 DME/water (v/v), clearly reflect an increase in polarity
of the microenvironment surrounding the dansyl moiety.7 Such
changes indicate the presence of exposed and/or fully exposed
conformations. The dramatic reversal in these spectral changes
that accompanies further increases in water content shows that
I functions like an umbrella by shielding the dansyl moiety in
JA953261V