Retinal Conformations
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Transcript Retinal Conformations
SRJC May 12th 2008
Younes Ataiiyan
Physics 43
University of Rochester, Institute of Optics
Table 1.0 – Retinal forms
Biologically, retinal (a form of vitamin A) is stored in the
membranes of the rods and cones of optical organs
(e.g. the eye).
11-Cis-Retinal is the primary visual pigment for all animals,
while some animals have additional visual pigments
as listed in Table 1.0.
photon
Visual Pigment
Life Form
11-cis retinal
Land Animals
3,4-dehydroxyretinal
Aquatic Animals
3-hydroxyretinal
Insects
4-hydroxyretinal
Fireflies
2 µm in diameter
45 µm in length
Rod Cell
Source: Nakanishi 1991
Floating in the lipid membranes
discs of the rod and cone cells
are proteins known as opsin
(GMW ≈ 4000 g/mol).
Opsin holds the
11-cis-retinal molecule
between seven alpha helices.
Where does 11-cis retinal come from?
Beta-carotene
Molar Mass: 536.873 g/mol
Beta-carotene as an optical
material in non-linear
nano-circuits.
In the small intestine (mucosa), beta-carotene
(above) is cleaved in half by an enzyme called
beta-carotene dioxygenase to produce 11-Cis-Retinal.
11-Cis-Retinal
Molar Mass: 284.436g/mol
Beta-carotene is found in carrots,mangos,etc.
11-cis retinal is a non-polar molecule; like beta-carotene
it is in the molecular family known as the Cartenoids.
PHOTOISOMERIZATION:
When a photon of blue light (λ = 440-490 nm) strikes the retinal
molecule it breaks an alkene bond (C=C) by exciting an electron.
This allows the 11-cis retinal conformation to isomerize into
11-trans retinal (which is more stable, lower overall molecular energy).
Carbon-Carbon double bond (CIS: Hydrogen's on same side)
Source: Wikipedia
H
H
H
(A) 11-cis retinal
(B) 11-trans retinal
H
B
Carbon-Carbon double bond (TRANS: Hydrogen's on opposite side)
EXCITING AN ELECTRON:
When a photon excites an electron in a p-orbital,
It jumps from the ground state (zero potential energy)
at π into a higher potential energy state at π*.
V=
V=
As identified by the infinite potential well model.
Infinite Potential Well
V ( x)
0
En n
2
x 0, x L
0 x L
2 2
2
2mL
n ( x) 0
nx
L
n ( x) A sin
x=0
x=L
V=0
Breaks
AReformation
Rotation
photon
double
excites
about
bond
of Double
single
aπ
into
electron
bond
abond
single
Carbon
Carbon
n=2
n=1
11-trans retinal
11-cis retinal
Steric Hindrence
S = sharp
P = principal
MO Diagram
HOW DOES THIS CONFORMATION AFFECT VISION?
MONOCHROMATIC VISION:
Photoisomerization of 11-cis retinal into
11-trans retinal, induces a conformational
change in opsin that triggers a second
messenger cascade – ultimately responsible
for monochromatic vision in the dark.
COLOR VISION:
Closely related opsins which
differ only in a few amino acids
(and absorbed wavelengths)
are responsible for color vision.
Conclusion
Optical organs are comprised of rods and cones, which in turn house
rhodopsin (opsin + retinal). 11-Cis retinal (cofactor) undergoes
photoisomerization, to 11-trans retinal. More specifically; when an
electron in a p-orbital (of the π bond) is excited by a photon
(λ = 440-490 nm), the electron jumps (by infinite well model) into an
anti-bonding orbital π* (higher potential energy) - which breaks the π
bond. 11-cis retinal then rotates around the σ bond (C-C) due to steric
hinderence of the methyl substituents, and to a trans configuration.
The molecule is now in a lower molecular energy conformation. When
the electron returns to the ground state (zero potential energy) at π, the
double bond reforms (C=C) and the molecule is called 11-trans retinal.
11-trans retinal is longer in length then 11-cis retinal which expands the
circumference of the seven opsin alpha-helices, and triggers a second
massager cascade - which ultimately is responsible for the perception
of monochromatic light.
Works Cited
Websites:
http://www.diginfo.tv/archives/2006/02/09/national_institute_of_advanced_22.html#more
www.ks.uiuc.edu/Research/rhodopsin/
http://webexhibits.org/colorart/
www.circadian.org/biorhyt.html
http://www.accessexcellence.org/AE/AEC/CC/vision_background.php
http://www.optics.rochester.edu/workgroups/cml/opt307/spr06/joe/index.htm
http://en.wikipedia.org/wiki/Retinal
http://en.wikipedia.org/wiki/Rhodopsin
Images:
http://www.hortcouncil.ca/images/carrots.jpg
http://upload.wikimedia.org/wikipedia/commons/archive/d/da/20060520170505!Beta-carotene.png
http://education.vetmed.vt.edu/Curriculum/VM8054/EYE/ROD.HTM
http://www.dark-layouts.net/Backgrounds/purple/images/purple_nebula.jpg