Chemistry Lecture Notes

Light and Vision

C C

HH

H H

largerE

smallerE

light lightLUMOHOMO

hchE smaller DE lower frequency,

longer wavelength

Absorption of light in conjugated systems

conjugated double bonds

molecular orbitals:

ground state excited state

OH

-carotene

retinol (vitamin A)

found in carrots and someyellow and green vegetables

found in fish-liver oilsand dairy products

max = 450 nm(absorbs in blue,appears orange)

b-carotene and retinol

Geometric isomers

single bonds are free to rotateC C

H

H

H H

H

H

C C

HH

H3C CH3

C C

H CH3

H3C H

no free rotationin double bonds

cis-2-butene trans-2-butene

geometric isomers(a class of stereoisomers)

OHretinol

O

H

all-trans-retinal

enzyme,ATP

OH

11-cis-retinal

H N

H2Nopsin

opsin

opsin H2Nformation of Schiff base

absorption of photon of lightcauses isomerization of cis to transand dissociation from rhodopsin

h

rhodopsin

enzymatic oxidation

The primary vision process

The overall vision process

Vision Event Occurs With Blinding Speed

The time scale for the primary event in vision is faster than expected, according to researchers in Berkeley, CA. Their measurements show that the first step in vision, the 11-cis 11-trans isomerization of the rhodopsin chromophore, is essentially complete in only 200 femtoseconds, making it one of the fastest photochemical reactions ever studied. Rhodopsin consists of the 11-cis retinal prosthetic group bound within the protein opsin. It was determined in 1963 that the absorption of light results in isomerization of 11-cis retinal to form the trans photoproduct, bathorhodopsin. Although previous studies have suggested that the isomerization occurs in less than a picosecond, the time scale for the formation of the primary photoproduct had never been experimentally determined. An advance earlier this year by Schoenlein and Shank in the generation of femtosecond laser pulses in the blue-green spectral region made it possible to study the ultrafast process.