Authors: Rachel Casiday and Regina Frey
Key Concepts: Electronic absorption spectroscopy; molecular orbitals; molecular structure and function; particle-in-a-box.
The vision process depends on the interaction of many different factors, such as the optics of the eye, the isomerization of retinal, nerve impulses, and the brain's ability to reconstruct the image. But the fundamental processes underlying vision are the absorption of a photon by the retinal molecule, and the subsequent isomerization from 11-cis-retinal to all-trans-retinal. The main chemical concept used in the vision process is the change in retinal's properties due to the change in the molecular orbitals (and therefore, the electron density) of retinal when it absorbs energy from photons that are reflected off of an object. When visible light hits retinal, a p electron is promoted to a higher-energy orbital, which results, approximately half the time, in the isomerization of retinal when the p electron returns to a lower molecular orbital. This isomerization of retinal forces a conformational change in the protein opsin and initiates a cascade of biochemical reactions resulting in a large potential difference across the plasma membrane of the photoreceptor cell. This potential difference is passed along a nerve cell, as an electrical impulse, to the brain, which interprets the visual information.
This tutorial describes the vision process, the isomerization of retinal in terms of electronic absorption spectroscopy, the protein conformational changes, and the potential-difference buildup that generates the nerve impulse. Both monochrome vision (rod cells) and color vision (cone cells) are discussed. The tutorial also emphasizes the relationship between protein structure and function and describes how retinal-containing proteins with different absorption spectra contribute to color vision.
At Washington University in St. Louis, students complete this tutorial in conjunction with a first-semester experiment in which they study the optical-absorption properties of conjugated dye molecules and analyze the results using the particle-in-a-box model. The students receive three (3) of four (4) cyanine dyes. They must obtain the lmax for each of their unknown dyes using visible-light absorption spectroscopy, and determine which 3 of the 4 dyes in the series they have using the particle-in-a-box model. Given the general form of the series, the students then must draw the structures of their 3 unknown dyes.
A common difficulty for students first encountering quantum theory is visualizing the changes in electron configuration as physical changes occurring in real molecules. This tutorial helps students to see how a real molecule, retinal, changes in response to absorption of an electron, and that this change has important consequences, i.e., vision.
