Listeria-like motion of oil droplets

Article abstract of DOI:10.1246/cl.2006.708

Self-propelling movement of oil droplets associated with formation of giant vesicles on/in the surface of an oil droplet at the site opposite that of the direction of motion was observed. Fluorescence microscopic study using a synthesized fluorescent indi

Full text of DOI:10.1246/cl.2006.708

708
Chemistry Letters Vol.35, No.7 (2006)
Listeria-like Motion of Oil Droplets
Taro Toyota,1 Hirotatsu Tsuha,1 Koji Yamada,1 Katsuto Takakura,1;2 Takashi Ikegami,3 and Tadashi SugawaraÃ1
1Department of Basic Science, Graduate School of Arts and Sciences, The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
2Department of Chemistry and Biochemistry, Suzuka National College of Technology, Shiroko-cho, Suzuka 510-0294
3Department of General Systems Sciences, Graduate School of Arts and Sciences, The University of Tokyo,
3-8-1 Komaba, Meguro-ku, Tokyo 153-8902
(Received March 22, 2006; CL-060349; E-mail: suga@pentacle.c.u-tokyo.ac.jp)
Self-propelling movement of oil droplets associated with
formation of giant vesicles on/in the surface of an oil droplet
at the site opposite that of the direction of motion was observed.
Fluorescence microscopic study using a synthesized fluorescent
indicator confirmed that formation of the vesicular molecule
occurred through the coupling reaction between a reactive lipo-
phile and a micellar amphiphile in an aqueous dispersion.
Recently, autonomous motion of molecular aggregates in a
nano- and microscale has drawn much attention as a dynamic
model of protein complexes and even of living cells.1 Several
pathogenic bacteria, such as Listeria monocytogenes, propel
themselves by polymerizing actin, which is contained in the host
cell, at their tail edge.2 In this study, we describe the self-propel-
led motion of a reactive oil droplet that forms giant vesicles at
its tail edge. The movement of this oil droplet resembles the
self-propelled motion of such bacteria.
When an oily liquid (ca. 1 mL) of 3-(n-octyloxy)benzalde-
hyde (1, in Figure 1a) was added to ca. 75 mL of 20 mM aqueous
micellar solution3 of 10-(4-aminophenoxy)decyltrimethylam-
monium bromide (2, in Figure 1a), oil droplets were observed
in the dispersion under a phase-contrast microscope (IX70,
Olympus, Japan). While some droplets were adhered to the sur-
face of glass substrate, other droplets whose sizes were in range
of 50–120 mm, swam autonomously, with giant vesicles tailing
behind (Figures 2a and 2b). Initially, these droplets moved along
one direction (maximum speed ca. 11 mm/s); the rate of the self-
propelled swimming motion then decreased and the swimming
droplet made frequent turns (see Supporting Information). Note
that the inner aqueous particles inside the swimming droplets
demonstrated convectional movement. The rate of convection
was found to be related to that of the self-propelled swimming
droplet (Figures 2c and 2d). Namely, the stroboscopic micro-
graphs (Figure 2c, exposure time = 1 s) indicate that the direc-
tional swimming motion was associated with a pair of convec-
tion of aqueous particles and that the inner convection flow
temporarily stopped at the turning point of the swimming motion
(Figure 2d). The observed phenomenon is summarized in
Figure 2e.
Figure 1. (a) Schematic illustration of dehydrocondensation of
lipophile 1 and fluorescent indicator 4 with surfactant 2, produc-
ing amphiphilic azomethine derivatives 3 and 5. (b) Representa-
tive image of the autonomous swimming of oil emulsion droplet
of 1 in micellar solution of 2, propelled by generation of the
membrane of vesicular amphiphile 3.
dispersion of [1]/[2] = 20/20 (mM) was evaluated by 1H NMR
spectroscopy (see Supporting Information), and it turned out that
the conversion yield of 1 to product 3 was only ca. 1% at 2 h and
50% after 5 days (see Supporting Information). Therefore, it was
necessary to clarify that the trailing giant vesicles were com-
posed of vesicular amphiphile 3 (Figure 1b).4 We thus focused
on a fluorescent probe method that directly informs us the state
of this emulsion system at the microscopic level under a fluores-
cent microscope.
Among fluorophores, 4,4-difluoro-4-bora-3a,4a-diaza-s-in-
dacene (BODIPY) with its high fluorescent quantum yield is use-
ful because its fluorescent color can be varied by chemical mod-
ification.5 Moreover, some BODIPY derivatives, of which fluo-
rescence intensity or wavelength is switched by complexation
with a cation or by protonation, have also been reported.6 We
synthesized a BODIPY derivative (4, in Figure 1a) with a formyl
group linked directly to the phenyl ring as a reaction-sensitive
fluorescence indicator. This indicator can monitor the progress
The oil droplets adhering on the surface of glass substrates
also generated giant vesicles and slithered around on the glass
slides. This slithering continued about for 1 h. However, all of
the emulsion droplets in the dispersion eventually fixed on the
substrate and did not move any more after 2 h after the prepara-
tion. Accompanied with the generation of vesicles, all the oil
droplets turned out to be shrunken.
Conversion of 1 to vesicular amphiphile 3 in the aqueous
Copyright ꢀ 2006 The Chemical Society of Japan

Chemistry Letters Vol.35, No.7 (2006)
709
formation was monitored by phase-contrast and fluorescence
microscopes (Figure 3). When a band-pass filter (ꢀem: 515–
550 nm) was used, only the droplets glittered green. On the other
hand, the growing vesicles and emulsion droplets glittered red
through a cut-off filter (ꢀem: >580 nm). This observation clearly
indicates that the vesicular amphiphile 3 (and 5) was, indeed,
produced in/on the oil droplets by way of imine coupling
between 1 (and 4) and 2.
Investigations of triggering and sustaining mechanisms of
the self-propelled motion of the emulsion droplets tailing giant
vesicles in this system is now in progress with regard to follow-
ing factors: (i) the chemical Marangoni effect7 and (ii) the
symmetry-breaking jet.8 It is probable that the formed thin film
of 3 around the surface of the droplet is stripped off at an inlet
position of the convection, releasing vesicles at the opposite
side. This would be the reason why the oil droplet moves in
one direction associated with the flow pattern of convection in-
side of the running oil droplet (Figure 2e). Although the detailed
mechanism remains elusive, this is the first experimental system
made of simple organic molecules that exhibits self-propelled
motion that is coupled with chemical formation of the membrane
molecule.
Figure 2. (a) Phase-contrast microscopic images on self-pro-
pelled motion of an oil emulsion droplet (site P) with a trail of
giant vesicles (site Q). (b) Phase-contrast micrograph of a group
of giant vesicles in the trail [site Q in (a)]. Stroboscopic micro-
graphs (exposure time = 1 s) of the oil droplet including a
couple of convection of water particles during the directional
swimming motion (c); traces indicate that water particles
are moving as a pair of convection, and at a turning point (d);
stopped water particles are seen (see Supporting Information).9
Dynamics are schematically illustrated in (e).
This work was supported by Grant-in-Aid for Center of
Excellence (‘‘Research Center for Integrated Science’’) from
the Ministry of Education, Culture, Sports, Science and Technol-
ogy. T.T. is supported by Research Fellowships of the Japan
Society for the Promotion of Science for Young Scientists
(No. 08289).
References and Notes
1
2
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2002. b) M. Venturi, A. Credi, V. Balzani, Molecular
Devices and Machines, Wiley-VCH, Weiheim, 2003.
a) L. Blanchoin, K. J. Amann, H. N. Higgs, J. B. Marchand,
D. A. Kaiser, T. D. Pollard, Nature 2000, 404, 1007. b) M. D.
Welch, J. Rosenblatt, J. Skoble, D. A. Portnoy, T. J.
Mitchison, Science 1998, 281, 105. c) L. G. Tilney, D. J.
Rosier, M. S. Tilney, J. Cell Biol. 1992, 118, 71.
Dynamic light-scattering experiments (NIKKISO Microtrac
UPA150) at room temperature (23 Æ 1 ꢀC) revealed that, in
a micellar solution (20 mM) of 2, micelles with average
diameters of 5 nm (standard deviation = 1 nm) formed in
water.
Figure 3. Microscopic images of membrane formation at an oil
droplet of 1 containing 5 mol % of green-fluorescent indicator 4
one hour after addition of amphiphile 2. (a) Phase-contrast
microscopic image; (b and c) fluorescence microscopic image
with two filter units [(b) ꢀex, 460–490 nm; ꢀem, 515–550 nm;
(c) ꢀex, 520–550 nm; ꢀem, >580 nm]. The scale bar corresponds
to 10 mm. Since an exposure time (ca. 1 s) was needed for captur-
ing the fluorescence images, we recorded the fluorescence image
of immobilized or slowly moving droplets in the same butch as
we found the moving ones therein.
3
of the local imine-formation reaction, associated with the forma-
tion of giant vesicles because the formyl group is dehydrocon-
densed with an amino residue of the substrate 2 to afford azo-
methine derivative 5 (see Supporting Information). The absorp-
tion and fluorescence spectra of 4 showed its absorption and
emission maxima at 528 and 556 nm, respectively, whereas the
absorption and fluorescence maxima of several azomethine
BODIPY derivatives shifted to the range of 535–537 and 568–
570 nm, respectively (see Supporting Information).
The reaction-sensitive fluorescence indicator 4 was applied
to monitor the formation of a vesicular amphiphile through
imine coupling. To an emulsion of oil droplets of 1 containing
5 mol % of the indicator 4 was added an aqueous micellar solu-
tion (20 mM) of 2. As imine formation proceeded, membranes
began to grow from the oil droplets. The progress of membrane
4
5
6
In a 10-mM dispersion of 3, we also confirmed formation of
giant vesicles under a phase contrast microscope.
K. Yamada, T. Toyota, K. Takakura, M. Ishimaru, T.
Sugawara, New J. Chem. 2001, 25, 667.
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Int. Ed. 2001, 40, 385. b) M. Kollmannsberger, K. Rurack, U.
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E 2005, 71, 065301, and references cited therein.
R. F. Ismagilov, A. Schwartz, N. Bowden, G. M. Whitesides,
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7
8
9
Supporting Information is available electronically on the
CSJ-Journal Web site, http://www.csj.jp/journals/chem-lett/
index.html.
Published on the web (Advance View) May 30, 2006; doi:10.1246/cl.2006.708