Photochemical regulation of the electrical properties of the novel planar bilayer lipid membrane incorporating spiropyran derivatives

Article abstract of DOI:10.1021/jp953340t

We incorporated the alkyl chain substituted spiropyran derivatives, 1-methyl-3,3-dimethyl-6′-nitrospiro[indoline-2,2′-[2H][1]benzopyran] (SP1), 1-octadecyl-3,3-dimethyl-6′-nitrospiro[indoline-2,2′-[2H][1] benzopyran] (SP18), and 1-octadecyl-3,3-dimethyl-6′-nitro-8-[docosanoyloxymethyl]spiro[indoline-2, 2′-[2H][1]benzopyran] (SP1822) into the planar bilayer lipid membranes (BLM) of soybean lecithin for the first time, and the photoresponse of the BLM was monitored. When a dc voltage was applied across the BLM, the transmembrane current changed under alternate irradiation with ultraviolet light and visible light. Such a photoelectric response of the BLM seems to be caused by the change in the membrane conductivity due to photoisomerization of spiropyran.

Full text of DOI:10.1021/jp953340t

5160
J. Phys. Chem. 1996, 100, 5160-5162
Photochemical Regulation of the Electrical Properties of the Novel Planar Bilayer Lipid
Membrane Incorporating Spiropyran Derivatives
Motomu Tanaka* and Yoshiro Yonezawa
DiVision of Molecular Engineering, Graduate School of Engineering, Kyoto UniVersity, Yoshida, Sakyo-ku,
Kyoto 606-01, Japan
ReceiVed: NoVember 13, 1995; In Final Form: February 14, 1996X
We incorporated the alkyl chain substituted spiropyran derivatives, 1-methyl-3,3-dimethyl-6′-nitrospiro[indoline-
2,2′-[2H][1]benzopyran] (SP1), 1-octadecyl-3,3-dimethyl-6′-nitrospiro[indoline-2,2′-[2H][1]benzopyran] (SP18),
and 1-octadecyl-3,3-dimethyl-6′-nitro-8-[docosanoyloxymethyl]spiro[indoline-2,2′-[2H][1]benzopyran] (SP1822)
into the planar bilayer lipid membranes (BLM) of soybean lecithin for the first time, and the photoresponse
of the BLM was monitored. When a dc voltage was applied across the BLM, the transmembrane current
changed under alternate irradiation with ultraviolet light and visible light. Such a photoelectric response of
the BLM seems to be caused by the change in the membrane conductivity due to photoisomerization of
spiropyran.
Introduction
Experimental Details
Soybean lecithin was purchased from Wako Chemicals Co.
SP1 and SP18 were from Japanese Research Institute for
Photosensitizing Dyes Co. SP1822 was obtained as a gift from
Matsushita Electric Industrial Co. Ltd.3 The structure formulas
of spiropyrans are included in the scheme. The formation of
the BLM was obeyed to a brush technique11 under irradiation
with visible light (wavelength λ ) 550 nm). The BLM was
fabricated in a 0.5-0.6 mm hole in a Teflon thin wall (thickness,
less than 0.5 mm) of the inner chamber. The membrane-forming
solution consisted of 100 mg of soybean lecithin plus 4.6 mg
of SP1 (7.8 mg of SP18, or 13.5 mg of SP1822) in 1.30 mL of
n-decane: CHCl3 solution (10:3 by volume). The molar
concentration ratio of spiropyrans to soybean lecithin was kept
at 0.10. The inner cell was placed in an outer chamber made
of Teflon, which had a quartz window for irradiation. The cell
compartments were filled with a 1 M aqueous solution of KCl.
Two Ag/AgCl microelectrodes prepared with the electroplating
method were connected with cell compartments by salt bridges.
Applied dc voltage to the cell, Vapp, was 0-100 mV and the
transmembrane current was measured by a CEZ-2300 patch-
clamp amplifier (Nihon Kohden Co.). A 500-W high-pressure
mercury lamp (USH-500HD, Ushio Denki Co.) was used as
the light source. Ultraviolet (UV) light (λ ) 365 nm) and visible
light (λ ) 550 nm) were selected by the combinations of a 10
wt % CuSO4 aqueous solution filter and glass filters, UV-35
and UV-D35 for UV light, and L-39 and KL-55 for visible light
(Toshiba Co.), respectively. Absorption spectra of dye solutions
and extinction spectra of liposome dispersions were recorded
on a spectrophotometer UV-260 (Shimadzu Co.). All the
measurements were carried out at room temperature in air.
Artificial bilayer lipid membranes containing photochromic
compounds are interesting as a model system for studying
several physicochemical processes because the membrane
properties can be regulated by the photochemical reactions of
the dopant molecules.1 Reversible photochromic reactions are
suited to demonstrate the role of the molecular interaction
between the dopants and the lipid molecules. The photochromic
reaction causes a change in the shape of the chromopholic
moiety of the molecules that leads to a reversible change in the
membrane properties. Among the model membranes, the planar
bilayer lipid membrane (BLM) is particularly suitable to monitor
the transport of charged species across the membrane directly
by electrical measurements. There have been several studies
on the photochemistry of spiropyran derivatives, by incorporat-
ing them into artificial membranes as Langmuir-Blodgett (LB)
films,2,3 lipid bilayers,4 and polymeric films.5 Spiropyran
derivatives take the colorless form corresponding to the “closed”
spiropyran structure under irradiation with visible light. Irradi-
ated with UV light, they take the colored form corresponding
to the “open” planar merocyanine structure.6-10 In this study,
we have fabricated the novel BLM-containing the spiropyran
derivatives 1-methyl-3,3-dimethyl-6′-nitrospiro[indoline-2,2′-
[2H][1]benzopyran] (SP1), 1-octadecyl-3,3-dimethyl-6′-nitro-
spiro[indoline-2,2′-[2H][1]benzopyran] (SP18), and 1-octadecyl-
3,3-dimethyl-6′-nitro-8-[docosanoyloxymethyl]spiro[indoline-
2,2′-[2H][1]benzopyran] (SP1822) and attempted to regulate the
electrical properties of the BLM photochemically. The differ-
ences between the photoresponse of the BLM doped with SP1,
SP18, and SP1822 are briefly discussed.
Results and Discussion
The variations of the absorption spectrum of a 0.1 mM
chloroform solution of SP18 under irradiation with UV and
visible light are shown in Figure 1. SP18 molecules took
spiropyran structure under irradiation with visible light (λ )
550 nm). Irradiated with UV light (λ ) 365 nm), they were
converted to the merocyanine structure as evident from the
increase of the strong absorption peak at λ ) 570 nm.2 SP18
molecules undergo reversible photoisomerization under alternate
irradiation with UV light and visible light. Almost all spiro-
pyran molecules changed to the merocyanine structure after 100
X Abstract published in AdVance ACS Abstracts, March 15, 1996.
0022-3654/96/20100-5160$12.00/0 © 1996 American Chemical Society

Letters
J. Phys. Chem., Vol. 100, No. 13, 1996 5161
Figure 1. Absorption spectrum of a 0.1 mM chloroform solution of
SP18. Light path length ) 1 cm. (a) Spiropyran structure, after
irradiation with visible light (λ ) 550 nm) for 100 s. (b) Merocyanine
structure, after irradiation with UV light (λ ) 365 nm) for 100 s. (c)
Spiropyran structure, after irradiation with visible light (λ ) 550 nm)
for 100 s.
Figure 3. Current-voltage characteristics of the BLM containing SP18
and SP1. (A) Current (I(SP)) - voltage (Vapp) curves in spiropyran
structure (SP18, O; SP1; 0). (B) The difference current (δI) - voltage
(Vapp) curves between merocyanine (MC) structure and spiropyran (SP)
structure (SP18, b; SP1, 9). δI ) I(MC) - I(SP). Vapp ) 0-100 mV.
[SP]/[lecithin] ) 0.10.
Figure 2. Changes in the dc current across the BLM containing SP18
and SP1 under continuous irradiation with UV and visible light. The
arrows indicate the change from UV to visible light or vice versa. (a)
[SP18]/[lecithin] ) 0.10. Vapp ) 50 mV. (b) [SP1]/[lecithin] ) 0.10.
Vapp ) 100 mV.
structure of SP1. On the other hand, under irradiation with UV
light, an increase of the dc current (13.1 pA) was observed,
which is due to merocyanine structure of SP1. The photore-
sponse is reversed by alternation of wavelength of the incident
light. Figure 3A shows the current-voltage curves of the BLM
containing SP18 and SP1 in spiropyran structure. The difference
current between merocyanine structure and spiropyran structure
of the BLM is plotted versus Vapp in Figure 3B. In both cases,
the steady dc current is proportional to Vapp under irradiation
with UV light and visible light, except for the residual current
at Vapp ) 0 mV. The membrane conductivity of the BLM with
SP18, G, amounted to G(SP) ) 7.96 × 10-10 S (spiropyran
structure) and G(MC) ) 9.75 × 10-10 S (merocyanine structure).
The ratio of the membrane conductivity, G(MC)/G(SP), was
1.22. On the other hand, the membrane conductivity of the
BLM with SP1, G, amounted to G(SP) ) 1.26 × 10-10 S and
G(MC) ) 1.39 × 10-10 S. The ratio of the membrane
conductivity, G(MC)/G(SP), was 1.05. The photoresponse of
the BLM is responsible to the change in the electric conductivity.
We have recently observed the photoresponse of the BLM
of soybean lecithin doped with an amphiphilic azobenzene
derivative, which is interpreted as the change in the electric
conductivity of the BLM due to photoisomerization of azoben-
zene.11 It is plausible that the photoinduced electric conductivity
s of UV irradiation. The dc current in the BLM cell before
and after formation of the BLM was monitored under irradiation
with visible light. The cell resistivity was (0.5-1.0) × 104 Ω
before the membrane formation. After the BLM formation, it
was increased to (1.6-7.7) × 109 Ω. The photochemical
regulation of the steady dc current across the BLM containing
SP18 is demonstrated in Figure 2. Under irradiation with UV
light (λ ) 365 nm), SP18 took merocyanine structure, and the
steady dc current across the membrane was 50.0 pA (Vapp ) 50
mV). Irradiated with visible light (λ ) 550 nm), SP18 changed
to spiropyran structure, and a remarkable decrease in the dc
current (37.5 pA) was observed. Such a photoresponse is
reversible, indicating that the changes in the dc current across
the BLM are caused by photoisomerization of SP18. We
incorporated SP1 instead of SP18 into the BLM and examined
the photoresponse (Figure 2). The photoresponse of the BLM
doped with SP1 was similar to that of the BLM doped with
SP18. Irradiated with visible light, the dc current was 11.6 pA
(Vapp ) 100 mV), which is responsible for the spiropyran

5162 J. Phys. Chem., Vol. 100, No. 13, 1996
Letters
suggests that the J-aggregation of SP1822 in merocyanine
structure blocks the ion permeation across the local packing
mismatch due to the stacking of dye molecules. These results
would demonstrate that merocyanine structure plays an impor-
tant role in the ion permeation in the BLM.
The dc current across the BLM doped with SP18 is larger
than that observed in the BLM with SP1, in both the spiropyran
structure and the merocyanine structure. The ratio of the electric
conductivity of the BLM with SP18, G(MC)/G(SP), is larger
than that of the BLM with SP1. Considering the limited
information derived from electrical measurements as well as
the possibility of several stereoisomers of the merocyanine,8
an interpretation of the origin of the difference between two
dyes is not so easy at present. However, it is plausible that the
different behaviors are related to, at least in part, the packing
mismatch of lipids around dye molecules. If we employ a
concept of hydrophobicity, the penetration depth of spiropyran
molecules depends on the hydrophobic/hydrophilic interaction
between the dopant and soybean lecithin when the spiropyran
molecules are incorporated into the BLM. As the methyl group
in SP1 is less hydrophobic than the octadecyl group in SP18,
the penetration of SP1 into the BLM would not be so deep,
resulting in a rather small membrane current and the small ratio
of the electric conductivity. It is likely that the deeper
penetration of SP18 causes larger membrane current and the
enhanced ratio of the electric conductivity as a result of more
serious packing mismatch of lipid molecules induced by
photoisomerization. However, further study would be necessary
to confirm such assumptions. It is expected that spiropyrans
in merocyanine structure have certain affinity with specific ions,
because they take zwitterion form. The comparative study on
the ion-selective permeation of the BLM doped with various
photochromic dyes, such as spiropyran and azobenzene, would
be helpful to obtain further insight into the photochemically
regulated ion permeation mechanism through the small packing
mismatch.
Figure 4. Changes in the dc current across the BLM containing SP1822
under continuous irradiation with UV and visible light. [SP1822]/
[lecithin] ) 0.10. Vapp ) 20 mV.
change is caused by the expansion of the local packing mismatch
of lipid molecules due to trans-to-cis isomerization of azoben-
zene molecules.12 According to Polymeropoulos and Mo¨bius,
photoisomerization of SP18 in the monolayer at the air/water
interface leads to the increase in the area per molecule.2
Therefore, it is not so unreasonable to assume that the
photoresponse in the BLM doped with SP1 and SP18 is caused
by a similar mechanism: the change in the ion permeability
through the local packing mismatch of the lipids induced by
photoisomerization from spiropyran structure to merocyanine
structure. Ando et al. synthesized the spiropyran having two
long chains, SP1822. As a result of UV irradiation, SP1822
molecules in LB films change from spiropyran structure to
merocyanine structure, which finally form the stable J-ag-
gregate.3 Seki et al. reported that the merocyanine structure of
SP1822 molecules forms the very stable J-aggregate in am-
monium-type bilayer membranes.4 We have examined the
photoresponse of the BLM doped with SP1822. As shown in
Figure 4, the dc current across the BLM was first increased
under irradiation with UV light in just the same manner as SP18
and SP1. However, the dc current was gradually decreased after
prolonged irradiation, resulting in a value almost the same as
the former. At this stage, the current did not change under
irradiation with visible light. After that, photoresponse of the
BLM disappeared. As a reference, we fabricated the liposome
dispersion of soybean lecithin incorporating SP1822 ([SP1822]/
[lecithin] was ca. 0.10). SP1822 molecules in the liposomes
underwent photoisomerization from colorless spiropyran struc-
ture to merocyanine structure (λmax ) 570 nm) under UV
irradiation. SP1822 molecules in merocyanine structure were
finally converted to the J-aggregate (λmax ) 615 nm). The back
reaction to spiropyran structure due to irradiation with visible
light was almost completely suppressed as a result of dye
aggregation. These observations are the supporting evidence
that SP1822 molecules could form the J-aggregate via mero-
cyanine structure in the BLM. The photoresponse of the BLM
References and Notes
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(3) Ando, E.; Miyazaki, J.; Morimoto, K.; Nakahara, H.; Fukuda, K.
Thin Solid Films 1985, 133, 21.
(4) Seki, T.; Ichimura, K.; Ando, E. Langmuir 1988, 4, 1068.
(5) Shimidzu, T.; Yoshikawa, M. Polym. J. 1983, 15, 631.
(6) Hirshberg, Y.; Fisher, E. J. Chem. Soc. 1954, 297; 1954, 3129.
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(10) Kolc, Y.; Becker, R. S. J. Phys. Chem. 1967, 71, 4045.
(11) Tanaka, M. ; Yonezawa, Y.; Sato, T. Proc. 7th International
Conference on Organized Molecular Films; Ancona: Italy, 1995; pp 116.
(12) Tanaka, M.; Sato, T.; Yonezawa, Y. Langmuir 1995, 11, 2834.
JP953340T