Section 36–3 The rod cells

(Plane structures / Chemical bond / Light absorption)

 

In this section, Feynman discusses plane structures, chemical bond, and light absorption of retinal in rod cells. Thus, the section could be titled as “the retinal of  rhodopsin (or photoreceptor cells).”

 

1. Plane structures:

“The rhodopsin, which is the pigment, is a big protein which contains a special group called retinene, which can be taken off the protein, and which is, undoubtedly, the main cause of the absorption of light (Feynman et al., 1963, p. 36–6).”

 

In his Nobel lecture, Wald (1968) mentions: “… Ball, Goodwin and Morton in Liverpool showed that retinene is vitamin A aldehyde. At Morton’s suggestion the names of all these molecules have recently been changed, in honor of the retina, still the only place where their function is understood. Vitamin A is now retinol, retinene is retinal; there is also retinoic acid.” That is, the term retinene is outdated and had been replaced by retinal, which is more commonly used in current literature. Retinal also refers to the aldehyde form of Vitamin A, which is an essential component of rhodopsin and plays a crucial role in visual phototransduction. Visual phototransduction is the photochemical reaction where light is converted to an electrical signal in the retina.

 

“There are layer after layer of plane structures, shown magnified at the right, which contain the substance rhodopsin (visual purple), the dye, or pigment, which produces the effects of vision in the rods (Feynman et al., 1963, p. 36–6).”

 

Feynman says there are layers of plane structures, which contain the rhodopsin molecules and adds that there is some reason for holding all the rhodopsin molecules parallel. Specifically, rod cells contain a light-sensitive pigment called rhodopsin, which consists of a protein called opsin and a light-absorbing molecule called retinal. However, the retinal molecule within rhodopsin is non-planar. In Wald’s (1968) words, “A cis-linkage always represents a bend in the chain; but because of this steric hindrance the 11-cis molecule is not only bent but twisted at the cis linkage. This departure from planarity, by interfering with resonance, was expected to make the molecule so unstable that one hardly expected to find it.” That is, the non-planarity is essential for the molecule’s light-absorption properties and the structural changes upon absorbing light.

 

2. Chemical bond:

“This kind of a structure, with layers, appears in other circumstances where light is important, for example in the chloroplast in plants, where the light causes photosynthesis. If we magnify those, we find the same thing with almost the same kind of layers, but there we have chlorophyll, of course, instead of retinene. The chemical form of retinene is shown in Fig. 36–6. It has a series of alternate double bonds along the side chain… (Feynman et al., 1963, p. 36–6).”

 

Feynman explains that retinene has a series of alternate double bonds in the side chain. The term “alternate double bonds” refers to a sequence of double bonds that is separated by single bonds (-C=C-C=C-). Double bond is a chemical bond between two atoms involving two pairs of electrons instead of a pair of electrons in a single bond. He also uses the term “conjugated double bonds,” which is extended to the analogous interaction involving a p-orbital containing an unshared electron pair (IUPAC, 2014). It means that the electrons can move freely across alternate double bonds allowing for delocalization of electrons in the retinal. While both terms describe the arrangement of double bonds in retinal, conjugated double bond emphasizes the electronic delocalization and unique optical properties of the molecule.

 

This substance is impossible for human beings to manufacture in their own cells—we have to eat it. So we eat it in the form of a special substance, which is exactly the same as retinene except that there is a hydrogen tied on the right end; it is called vitamin A, and if we do not eat enough of it, we do not get a supply of retinene, and the eye becomes what we call night blind, because there is then not enough pigment in the rhodopsin to see with the rods at night (Feynman et al., 1963, p. 36–6).”

 

Eating retinol (vitamin A) or beta-carotene (provitamin A) is crucial for preventing night blindness because these compounds are essential for the production of retinal, a key component of rhodopsin (Grune et al., 2010). Rhodopsin is responsible for vision in low-light conditions, such as those encountered during nighttime. Retinol and beta-carotene are precursors of retinal: Retinol can be directly converted into retinal in the body, while beta-carotene is converted into retinol and then retinal through metabolic processes. Thus, consuming foods rich in retinol (such as liver, fish and dairy) or beta-carotene (plant-based foods such as carrots, spinach, and lettuce) ensures that the body has a supply of retinal for the production of rhodopsin. However, it is incorrect for Feynman to say that human cannot manufacture retinene (or retinal) because they can convert beta-carotene into retinol and then retinal.

 

3. Light absorption:

“When light strikes this molecule, the electron of each double bond is shifted over by one step. All the electrons in the whole chain shift, like a string of dominoes falling over, and though each one moves only a little distance (we would expect that, in a single atom, we could move the electron only a little distance), the net effect is the same as though the one at the end was moved over to the other end! It is the same as though one electron went the whole distance back and forth, and so, in this manner, we get a much stronger absorption under the influence of the electric field, than if we could only move the electron a distance which is associated with one atom (Feynman et al., 1963, p. 36–6).”

 

Retinal is a conjugated chromophore (a molecule which absorbs light at a particular wavelength and emits color subsequently) that is responsible for our ability to see. When light strikes rhodopsin, retinal undergoes a transformation from its 11-cis form (rhodopsin) to its all-trans form (metarhodopsin), initiating a cascade of biochemical events that result in the generation of electrical signals in the rod cell, enabling vision in low-light conditions. The key to the absorption of light by retinal lies in the conjugated double bonds that are essential for the molecule’s light-absorbing properties. However, electrons do not move in a coordinated manner along a chain of atoms as dominoes do. The behavior of electrons in a conjugated system is more complex and involves delocalization, where electrons are spread out over the entire molecule.  

 

In its native form within visual pigments, retinal exists in the 11-cis configuration and all-trans configuration. The 11-cis prefix is due to the double bond at the 11^th carbon atom is connected to the two largest substituents whereby the largest chains are on the same side. The all-trans prefix (trans is a Latin prefix meaning across or on the other side) refers to the double bonds with the bulky substituents (e.g., hydrogens) are positioned on opposite sides. When light enters the eye and strikes the retina, it breaks the C=C double bond between C₁₁ and C₁₂ atom and causes a rotation to form the all-trans configuration (as shown below). After the change, the all-trans-retinal is later converted back into 11-cis-retinal through a series of enzymatic reactions and allowing the visual pigment for further light absorption.

Source: Das et al., 2024.

 

Review Questions:

1. Is the retinal molecule within rhodopsin planar or non-planar?

2. How would you explain the chemical bond of retinene (retinal)?

3. How would you explain the absorption of light by retinal?

 

The moral of the lesson: retinal molecule within rhodopsin has a non-planar structure and conjugated double bonds where electrons can move across the alternating single and double bonds, and thus, play a crucial role in light absorption.

 

References:

1. Das, U., Das, A., Das, R., & Das, A. K. (2024). Photochemistry of the retinal chromophore in the process of seeing (vision). ChemTexts, 10(2), 3.

2. Grune, T., Lietz, G., Palou, A., Ross, A. C., Stahl, W., Tang, G., ... & Biesalski, H. K. (2010). β-Carotene is an important vitamin A source for humans. The Journal of nutrition, 140(12), 2268S-2285S.

3. Feynman, R. P., Leighton, R. B., & Sands, M. (1963). The Feynman Lectures on Physics, Vol I: Mainly mechanics, radiation, and heat. Reading, MA: Addison-Wesley.

4. IUPAC (2014). Compendium of chemical terminology. Compiled by A. D. McNaught & A. Wilkinson. Oxford: Blackwell Scientific Publications. Retrieved October 8, 2014 from http://goldbook.iupac.org/.

5. Wald, G. (1968). Molecular basis of visual excitation. Science, 162(3850), 230-239.