Multiplexing sensory molecules map protons near micellar membranes

Article abstract of DOI:10.1002/anie.200801516

(Figure Presented) Hi-fi mapping: Multiplexing fluorescent sensors that simultaneously target proton concentration and polarity move to micellar nanospaces, self-regulate their positions, and report their pK_a values and wavelengths, which are c

Full text of DOI:10.1002/anie.200801516

Angewandte
Chemie
DOI: 10.1002/anie.200801516
Molecular Sensors
Multiplexing Sensory Molecules Map Protons Near Micellar
Membranes**
Seiichi Uchiyama,* Kaoru Iwai, and A. Prasanna de Silva*
Fluorescent sensors[1] have great potential to operate as
molecular-level devices[2] in nanospaces. Generally, a fluores-
cent sensor monitors a single parameter of its local environ-
ment, such as ion concentration. More functionalized systems
which operate according to similar principles are molecular
logic gates.[3] These gates respond to multiple parameters
simultaneously according to defined Boolean transforma-
tions. There are also a few examples of molecular sensors
which respond to multiple parameters, each by a different
Scheme 1. Fluorescent multiplexing sensors 1–18. The orders of 1!9
analytical technique.[4] Herein we demonstrate a new multi-
plexing fluorescent sensor which simultaneously monitors
multiple parameters (local proton concentration and polarity
in this instance) by multiple emission properties (intensity
and wavelength, respectively).[5] As the polarity of spherical
micelles in water is expected to change largely monotonically
along a radial coordinate,[6] polarity data translate into
positions. We can thus obtain local proton densities at various
positions by scattering a series of multiplexing sensors widely
over the aqueous micellar field. Therefore a nanoscaled
mapping of proton concentration emerges for this simple
membrane system. Proton concentration gradients are
responsible for the subject of bioenergetics.[7] Multiplexing
sensors also correspond to nanoscale versions of robotic
vehicles which go to humanly inaccessible spaces, map local
properties and send information back to us.
and 10!18 are determined by the logP (n-octanol/water partition
coefficient) value of a corresponding amine R1R2NH (see the Support-
ing Information).
is examined by a DpKa value (pKa in micellar solution–pKa in
water) of a conjugate acid of the receptor amine. This DpKa
value is affected by electrostatic potential and dielectric
constant at the sensor location but is independent of intrinsic
acidity/basicity of the sensor.[8] If local effective proton
concentration is higher than that of bulk water, a positive
DpKa value is obtained.[9] As our sensors possess a fluores-
cence “off–on” switching system by controlling photoinduced
electron transfer processes with a fluorophore–spacer–recep-
tor format,[1a] the DpKa values can be determined from
fluorescence intensity, with pH profiles arising from titrations.
2) The local polarity is estimated from the emission wave-
length of the polarity-sensitive fluorophore, 4-sulfamoyl-7-
aminobenzofurazan, as its emission wavelength is strongly
red-shifted with increasing environmental polarity and is
smoothly related to the dielectric constant e of the solvent.[10]
Thus, the relationship between the emission wavelength and
the e value is obtained beforehand for each sensor from the
fluorescence spectra in water, methanol, and so on (see the
Supporting Information). 3) The position of a sensor near
micellar membranes is altered by changing its substituents
R1–R3. The sensor bearing more hydrophilic substituents is
expected to stay at a more hydrophilic region in the nano-
space.[9a,11] Finally, by collecting the environmental data for 1–
18, proton concentration maps near micellar membranes can
be established in the form of DpKa–e diagrams. In the present
study, Triton X-100 (neutral, radius: < 4.8 nm[12]), octyl b-d-
glucopyranoside (OG; neutral, ~ 2.3nm [13]), sodium dodecyl-
sulfate (SDS; anionic, < 3.6 nm[14]), and cetyltrimethylammo-
nium chloride (CTAC; cationic, < 3.5 nm[14]) are used as
micelle media in which the nanoscaled proton gradients are
evaluated.
Scheme 1 shows the structures of the fluorescent multi-
plexing sensors 1–18 used in this study. These sensors consist
of a polarity-sensitive fluorophore (blue), a proton receptor
(orange), position tuners (red), and a spacer (green). The
sensors function as follows: 1) The local proton concentration
[*] Dr. S. Uchiyama, Prof. Dr. A. P. de Silva
School of Chemistry andChemical Engineering
Queen’s University, Belfast BT9 5AG (Northern Ireland)
Fax: (+44)28-9097-4687
E-mail: a.desilva@qub.ac.uk
Dr. S. Uchiyama
Graduate School of Pharmaceutical Sciences
The University of Tokyo
7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033 (Japan)
Fax: (+81)3-5841-4768
E-mail: seiichi@mol.f.u-tokyo.ac.jp
Prof. Dr. K. Iwai
Department of Chemistry, Faculty of Science
Nara Women’s University
Kitauoya-Nishimachi, Nara 630-8506 (Japan)
[**] We thank The Daiwa Anglo-Japanese Foundation, Invest NI (RTD
COE 40) (Japan) Society for the promotion of Science, W. T. Silva
andS. T. Herath for support andhelp.
The fluorescence properties of 9 in water and 18 in Triton
X-100 aqueous solution during titrations are shown in
Figure 1 as representatives of sensory functions. Regarding
proton concentration, the DpKa value for 18 in the Triton X-
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.200801516.
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Figure 1. Fluorescence behavior of a) 9 in water andb) 18 in Triton X-
100 solution with varying bulk pH. Left: spectra, right: intensities,
where I is the fluorescence intensity. Vertical axes for the insets:
log[(Imax-I)/(I-Imin)]. Excitation wavelength was 410 nm, andconcentra-
tions were 10 mm for 9 and 18 and0.52 m m for Triton X-100.
100 solution is obtained in a straightforward manner from the
[15]
two pKa values as ꢀ1.43(7.57–9.00).
In contrast, the
estimation of local dielectric constants from the emission
wavelengths required a longer procedure because of the
difference in polarity sensing properties between acidic and
basic conditions. The emission wavelengths of 1–18 are blue-
shifted by protonation of the amine receptor in homogeneous
medium. For example, the emission wavelength of 9 in acidic
water is 573nm, whereas that in basic water is 602 nm
(Figure 1a). This shift is due to an interaction between the
protonated receptor and the internal charge transfer excited
state of the fluorophore across the dimethylene spacer.[16]
Thus, two relationships (i.e., acidic and basic versions)
between the emission wavelength and the e value of solvent
are needed for the estimation of local polarity near sensors.
For the case of 18 in Triton X-100 aqueous solution
(Figure 1b), the local e values are estimated to be 34 and 2
in the acidic and basic conditions, respectively, from the
emission wavelengths (553and 525 nm; see the Supporting
Information).[17] An important phenomenon which emerged
from the collected e values is that the sensor changed its
position near the micellar membrane as the conditions
changed from acidic to basic. As the protonation of the
receptor increases its hydrophilicity, the sensor in acidic
condition was located at a more hydrophilic region of the
micelle.
Figure 2. DpKa–polarity (e¯) diagrams obtained for 1–18 (10 mm).
a) Triton X-100 (0.52 mm), b) OG (34 mm), c) SDS (0.20m), and
d) CTAC (5.0 mm). The numbers in italics represent the sensor
corresponding to each point. Dotted lines indicate trends in points.
hydrophobic series (10–18) stayed at more hydrophobic
regions (lower e values). In Figure 2a, the DpKa value
gradually decreases when the polarity decreases, i.e., as the
sensor goes towards the micellar interior.[6] In particular, 18
gave DpKa = ꢀ1.43, meaning that available protons near 18
are only 3.7% of that in bulk water. This negative DpKa value
can be attributed to the dielectric effect of the micelle,[8a]
which is unfavorable for protonated amine receptors. Our
sensors successfully plot the nanoscaled gradual decrease in
the effective proton concentration near the Triton X-100
micelle as moving from bulk water towards the micelle core.
Figure 2b shows the DpKa–e¯ diagram for another neutral
micelle, OG. Compared to that for Triton X-100, several
sensors (1–7 or 12–16) gathered around a narrow range of
polarity. Nevertheless, a similar DpKa–e¯ pattern is seen, which
Figure 2 shows the DpKa–e¯ diagrams for the four kinds of
micelles. As mentioned above, the positions of the sensors
varied between acidic and basic conditions. Taking this fact
into the consideration, the median e value (e¯) is adopted as a
parameter of polarity near a sensor.[18] Figure 2a is the DpKa–
e¯ diagram of Triton X-100. As expected, the sensors are
distributed at different positions, from bulk water (equivalent
to e¯ = 78.5) to the micellar interior (e¯ = 18). The more
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Chemie
suggests that the dielectric effect on the local proton
concentration is independent of the chemical structure of
non-ionic membranes (Figure 2).
[1] a) A. P. de Silva, H. Q. N. Gunaratne, T. Gunnlaugsson, A. J. M.
Huxley, C. P. McCoy, J. T. Rademacher, T. E. Rice, Chem. Rev.
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The DpKa–e¯ diagram for an anionic SDS micelle is
indicated in Figure 2c. Unexpectedly, the sensors show a
very narrow range of polarity (37 < e¯ < 44), with the exception
of 1 and 10. This means that the fluorophores are evidently
pinned at one place, regardless of the R1–R3 groups. This
positional pinning is probably due to the strong ion–dipole
interaction between the anionic head groups of the SDS
micelles and the fluorophore with the large dipole moment
(m = ꢁ 6.4 D).[19] In contrast, the receptor moieties of 1–18
changed their positions because the flexibility of the dimethy-
lene spacer allowed these positional variations. The inset in
Figure 2c shows the relationship between the DpKa value and
the hydrophobicity of the R1 and R2 groups. In the series of 1–
9 (red line), the positive DpKa value is found as the
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1.61 pKa units (= 4070% of that in bulk water). In the 10–18
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the head groups of the SDS micelles to give a positive DpKa
value. In both series, the most hydrophobic 9 and 18 sensed
the negative dielectric effect in addition to the positive
electric effect, which is why their DpKa values are close to
zero.
Figure 2d shows the map for cationic CTAC micelles,
where the ion–ion attraction exists between the cationic head
group of CTAC and 1, 2, 10, and 11 bearing one or two anionic
carboxylate group(s). In addition, the ion–dipole interaction
can be anticipated between the cationic head groups and the
fluorophore. However, some positional variation, especially
owing to the R3 group, is observed (39 < e¯ < 59). In the CTAC
micelles, both the dielectric and electric effects cooperatively
decrease the DpKa value. Therefore a large negative DpKa
value (ꢀ2.83) is seen for 18 and protons are much less
available at the location of 18 (0.15% of bulk water).
In conclusion, the proton concentration maps near the
Triton X-100, OG, SDS, and CTAC micelles are obtained by
the fluorescent multiplexing sensors 1–18. These sensors can
change their positions and report the multiple information
comprised of pKa values and emission wavelengths. As the
entire radial distances of the micelles investigated are less
than 5 nm, the space resolution opened up by the present
technique is remarkably high. This approach can be easily
applied to similar environmental mapping within other
nanospaces and of other chemical species. The charged head
groups of SDS and CTAC interfered with the positional
change of the sensors. Overcoming this interaction will be
among further developments of nanoscaled environmental
mapping with fluorescent multiplexing sensors.
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Received: March 31, 2008
Published online: May 21, 2008
Keywords: fluorescence spectroscopy · fluorescent probes ·
.
micelles · molecular devices · sensors
Angew. Chem. Int. Ed. 2008, 47, 4667 –4669
ꢀ 2008 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
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