Retinal: An Aldehyde Critical to Vision

Retinal is a terpene aldehyde generated from Vitamin A (carotene). Because of the extended series of π bonds, the molecule is a chromophore, absorbing visible light (λmax appr. 400 nm) giving it an orange color.

Retinal structure: a terpenoid structure with extended conjugation of 5 pi bonds, one of which is cis. The aldehyde functional group reacts with free amines in the side chains of lysine residues in the rhodopsin protein in your macular cells. This is the same condensation reaction we talked about in Chapter 17. When this derivatized protein captures a photon, it undergoes E/Z isomerization. That causes a major conformational change in the protein that initiates a sequence resulting in an electrical signal in your brain: and you see the light!
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Display isomerizing C=C bonds