Chemistry Lecture Notes Light and Vision

1. Chemistry Lecture Notes Light and Vision

2. Absorption of light in conjugated systems molecular orbitals: ground state excited state conjugated double bonds smaller DE  lower frequency, longer wavelength

3. b-carotene and retinol lmax = 450 nm (absorbs in blue, appears orange) found in carrots and someyellow and green vegetables found in fish-liver oils and dairy products

4. Geometric isomers  single bonds are free to rotate  no free rotation in double bonds cis-2-butene trans-2-butene geometric isomers (a class of stereoisomers)

5. The primary vision process

6. The overall vision process

7. 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.