http://www.okada-lab.es.uec.ac.jp/
Protain that Resembles Visual Pigments
Exists in the Earth’s Oldest Bacteria
Applications in Photo Sensors that Imitate
the Visual Function and Optical Memories
Yoshiko OKADA-SHUDO Laboratory
How does the eye see things?
What mechanisms are in play when you see things with your eyes? Reflected from
surrounding objects, light enters your eyes. Special photoreceptor cells in the eyes
convert this light to electrical signals, which are conveyed to the brain via nerves. The
retina and the brain process the received electrical signals, enabling you to see things.
Photoreceptor cells have the capability to convert light to electricity. They can be considered a type of photo sensor, like a photodiode or a solar cell. Unlike ordinary photo
sensors, photoreceptor cells are full-color sensors. Human eyes have two types of
photoreceptor cells with separate roles: rod cells are sensitive to light and cone cells
Yoshiko OKADA-SHUDO are sensitive to color (see Figure 1).
There are three types of color-sensitive cone cells, classified by the different colors
they are sensitive to. Cone cells can sense colors but they do not function well in
darkness. Cone cells require around 100 photons in order to sense light.
The brightness-sensitive rod cells, on the other hand, are very sensitive to light and
will generate an electric signal when just one photon enters a cell. Rod cells can be
compared to a photo sensor that can “read” the smallest value —the theoretical lowest limit — of light energy.
Within a rod cell is a visual pigment called rhodopsin, which is the material that is actually sensitive to light. Vitamin A and proteins are bonded in the rhodopsin, but they
dissociate in the presence of light. It is through this chemical change that energy is
transmitted as an electrical signal.
Meeting light-sensitive bacteria
The photoreceptor cells in mammals, birds, and other animals besides humans are
also divided into rods and cones, and their rod cells are sensitive to light through
rhodopsin.
Yoshiko Okada-Shudo, Associate Professor of Department of Engineering Science,
recalls that one discovery brought her a profound change in research life. Found in
bacteria was a protein which is similar to the protein in rhodopsin. The newly found
protein, a rhodopsin variant in bacteria, is called bacteriorhodopsin.
Keywords
Optoelectronics, Bioelectronics,
Quantum electronics, Nanophotonics,
Photo sensors, Image sensors, Nonlinear
optics, Vitamins, Vision, Visual pigments,
Extreme halophiles , Membrane proteins
SPIE, Japan Society of Applied
Physics, Physical Society of Japan,
Affiliations Laser Society of Japan, Association
of International Certified Optical
Engineers
Member
OPAL-RING
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Yoshiko Okada-Shudo, Associate
professor
Extreme halophiles are a type of archaea, the oldest living organisms
on Earth. In 1971, a joint research
team from the Max Planck Institute
in Germany and the University of
California identified that the violet color of saline lakes was the color of the
cellular membrane of extreme halophiles living in the lakes. The team also
discovered that the protein that forms
the violet cellular membrane functions
in nearly the same way as rhodopsin.
This protein is bacteriorhodopsin.
Ancient organisms use bacteriorhodopsin, a sight-function protein, in
photosynthesis like a solar cell without
having sight.
In 1989, a senior RIKEN researcher
working at the Max Planck Institute
brought samples of these extreme
halophiles back to Japan and looked
for a collaborator to investigate the application of extreme halophiles in opti-
視細胞・・・・光を受け止める細胞
└視物質・・光を感じる物質(ビタミンA+タンパク質)
Photoreceptor
cells: cells that react to light
・錐体(cone)：色を認識する細胞（赤、緑、青の３種類）
Visual
pigment: a light-sensitive material
• Cones:
color-sensitive cells (three types — red, green, and blue)
網膜に約600万個存在
Approx. six million cone cells in the retina
興奮に100光子以上必要
Stimulation threshold of about 100 photons
： cells
明暗を認識する細胞（ロドプシン→光受容タンパク質）
• Rods:・棹体(rod)
light-sensitive
(rhodopsin → photoreceptive proteins)
Approx.
120 million rod cells in the retina
網膜に約1億2000万個存在　Stimulated by one photon
1光子で興奮　Opsin
Chromophore
Cytoplasm
side
Disk side
(lipid bimolecular
membrane)
α-helix
Cytoplasm
side
Cis-retinal
シスレチナール
↓ Light
↓光
Trans-retinal
トランスレチナール
Figure 1: Operation of photoreceptor cells
Rod photoreceptor cells, which recognize lightness and
darkness, are sensitive to a single photon. This sensitivity is
derived from rhodopsin, a visual pigment contained in rod
cells, which undergoes a chemical change in the presence of
photons. Rhodopsin is a material in which retinal, a vitamin A
aldehyde, and opsin, a protein, are reversibly covalently bound.
When rhodopsin is exposed to light, cis-retinal changes to
trans-retinal, which has the same chemical composition but
a different structure. Trans-retinal cannot bind with opsin
and dissociates from opsin. The change in chemical energy
brought by this dissociation is transmitted along the optic
nerve as an electrical signal.
Nanotechnology and Materials
High sensitivity, long service life, high productivity,
strong heat resistance
Like rhodopsin, bacteriorhodopsin consists of retinal-protein
bonds. It is also very sensitive like rhodopsin, reacting to just
one photon. Where it differs is in the nature of the reaction.
Rhodopsin loses trans-retinal with light, whereas bacteriorhodopsin does not lose retinal. After releasing energy by photo
isomerization (trans-retinal into cis-retinal) , it returns to the original trans state owing to thermal energy. This photochemicalreaction cycle repeats more than a million times (see Figure 2).
Putting it another way, the rhodopsin in our eyes consumes
vitamin A when it senses light. This is why we must intake vitamin A through food. Bacteriorhodopsin, on the other hand,
quickly reverts even after reacting to light. This property is very
important when thinking about applications. This is because,
in principle, bacteriorhodopsin can be used repeatedly without
any deterioration.
The extreme halophiles that contain bacteriorhodopsin breed
and propagate in water environments with much higher salt
concentrations than seawater. Such saturated saline solutions
are very harsh environments for living organisms, so there is no
fear of contamination from unwanted bacteria. This boosts productivity because cultivating a pure culture is easy. This is another big advantage when considering applications. At OkadaShudo’s laboratory, researchers cultivate extreme halophiles
and extract bacteriorhodopsin.
Biologically derived materials are susceptible to heat (high temperatures), but bacteriorhodopsin thrives in high-temperature
environments. It functions as a photo sensor over a temperature range of -200 degrees Celsius to +140 degrees Celsius.
Highly functional photo sensors
Bacteriorhodopsin has several other properties, for which engineering applications are being considered (see Figure 3). For
example, Okada-Shudo’s group created a photo-responsive
cell using bacteriorhodopsin (see Figure 4). Photodiodes, solar
cells, and other photo sensors that use semiconductors only
produce electricity while exposed to light. Photo-responsive
cells using bacteriorhodopsin, on the other hand, generate a
current pulse the instant they are exposed to light and generate another current pulse in the opposite direction the instant
the incident light is interrupted. This process is essentially a
mathematical time differential of the input optical signal, and is
equivalent to the visual information processing of the retina.
What drove the evolution of the photo-responsive cell were sensors that detect the incident direction of light. The photo cell’s
structure consists of a thin film of bacteriorhodopsin coated
on transparent electrodes and sandwiched between opposite
poles. The cell has polarity; the current flows in one of two directions depending on whether light is incident on the front surface or incident on the rear surface. The Okada-Shudo laboratory made use of this property to fabricate a photo-responsive
cell called an artificial receptive field that places two types of
thin-film patterns face-to-face. It is hoped these cells can be
applied to sensors that detect the incident direction of light or
sensors that perceive the contours and borders in an image.
At the present time, photo-responsive cells are built on a substrate of ITO-coated glass, but Associate Professor OkadaShudo is thinking ahead to flexible sensors. Bendable sensors
could be realized by building the photo-responsive cells on a
substrate of flexible organic polymers instead of ITO-coated
glass. The current issue, then, is creating a substrate material
that is transparent, flexible, and conductive.
Okada-Shudo had a chance encounter with bacteriorhodopsin
and extreme halophiles that began a 20-year-plus relationship.
She admits that she occasionally feels an emotional attachment to the extreme halophiles, even though they are bacteria.
They are almost like children to her.
[Interview and article by Akira Fukuda (PR Center)]
hv
Q380
Millisecond
P490
hv
bR570
hv 0.5 picoseconds
All-trans-retinal
J 600
O640
All-trans-retinal
hv
Up to 200
nanoseconds
N560
13-cis-retinal
Millisecond
H
3 picoseconds
K590
+
H
Up to 1 microsecond
L550
13-cis-retinal
M412
13-cis-retinal
Up to 50 microseconds
H
+
+
Figure 2: Photochemical reaction cycle of bacteriorhodopsin
bR at the top of the circular cycle is in the ground state of bacteriorhodopsin. The
photochemical reaction progresses from this state to the J intermediate state. Colored
arrows represent light transitions, while the other arrows represent thermal transitions.
Letters after J indicate intermediate states, and the subscript numbers indicate the light
wavelength at which maximum absorption takes place. When the reaction progresses
to the M intermediate state (bottom), hydrogen ions (protons) are emitted. Protons are
absorbed while the bacteriorhodopsin returns from the M intermediate state to the bR
state — a return driven by thermal energy or the photo absorption of blue light.
Bacteriorhodopsin properties and engineering applications
• ‌Proton pump function (draws photons from inside the cellular membrane to
the outside)
→ Changes light to an electrical current
Characteristics:‌ultra-fast directional responses (< 5 picoseconds),
differential responses
Applications:‌artificial retinas, direction sensors, robot vision
• ‌Photochromism (changes color when exposed to light, reverts back to the
original color when light is extinguished)
→ Stores light intensity and complex amplitudes
Characteristics:‌variable storage retention times (humidity levels, pH, electric
fields, gene recombinations, etc.)
‌High-density storage with a size on the order of molecules (>
5000 lpm), high speed (low temperature), rewritable
Applications:‌real-time holography
• ‌Second-order and third-order nonlinearlity (response is proportional to the
second and third power of the input)
Characteristics:‌rhodopsin (retinal) is a colorant, so no fluorescent pigments
are needed (remains living)
Applications:‌nonlinear spectrum (Raman scattering), nonlinear
nanoimaging (Raman, SHG)
‌Phase conjugate wave generation → quantum optics (strong
correlation)
• ‌Photoinduced double refraction (phase velocity varies according to direction)
→ Stores polarized light
Applications:‌switching elements using polarized light information,
security holograms
Figure 3: Bacteriorhodopsin properties and their engineering applications
When bacteriorhodopsin is exposed to light, it undergoes several actions: a current
(protons) is generated, the color changes, light with twice the frequency is emitted, and
double refraction occurs.
1. 光波を用いた応用
★プロトンポンプ機能
Photo-responsive
cells using bacteriorhodopsin
応用例1：光電変換デバイス（網膜の神経節Y細胞）　40
30
Photocurrent [nA]
cal strage devices. It just so happened that Okada-Shudo, who
had just made her first step as a researcher at RIKEN’s optics
laboratory, joined the research collaboration. This was the start
of Okada-Shudo’s relationship with bacteriorhodopsin — a relationship that has lasted more than 20 years.
pH 7.2
pH 8.3
20
10
0
-10
-20
-30
-40
Light on
0
0.5
Light off
1
1.5
2
2.5
Time [s]
Bacteriorhodopsin
immobilized
バクテリオロドプシン固定化電極セル
electrode cells
Photoelectric response of
the bacteriorhodopsin cell
バクテリオロドプシンセルの光電流応答
Figure 4: Photo-responsive cells using bacteriorhodopsin
The left photo shows fabricated photo-responsive cells. The bacteriorhodopsin and an
electrolyte solution are sandwiched between two glass plates with electrodes attached.
The right diagram indicates the response characteristics. The blue and orange curves
indicate changes in electric currents. The light is turned on during the period indicated in
green. Current pulses are generated the instant the light is turned on and the instant the
light is turned off.
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