3 Vision

3.1 Human Vision on the Molecular Scale

All sensors, biological or technological, have several elements: the sensing element that

senses the signal, the transducer that transfers the signal, and an amplification and/or

analysis/reporting element that increases the signal and/or analyzes it. In human vi-

sion, the sensing element is the eye (Figure 3.1a). On the molecular scale, the sensing

elements are specifically the molecules rhodopsin and iodopsin in the rod and the cone,

respectively, which are activated by light photons (Figure 3.1b). Rhodopsin and iodopsin

contain 11-cis retinal derivatives bound to different proteins called opsins. Light activa-

tion turns 11-cis retinal into 11-trans retinal, changing the molecule from a bent, bulky

molecule into a long, thin one. The opsin, though, cannot bind the long, thin molecule

well anymore, and thus releases it and changes its own conformation in the process.

This new opsin conformation fits and binds well to a specific G-protein-coupled receptor

called transducin [1, 2] (Figure 3.1b). As the name suggest, this is the initial stage of trans-

ducing the signal. In this case, the signal is amplified by a signal transduction pathway

that eventually closes an ion channel, which hyperpolarizes the outer cell membrane

of the rod or cone. The amount of change in membrane potential is dependent on the

amount of light activation and is transferred not via action potentials but as a current in

the cytoplasm [3]. This current induces the rod or cone to release less of the inhibitory

neurotransmitter glutamate, thus activating the following nerve cell. In the brain, these

activated and firing neurons lead to the analysis of the original signal; e. g. with quick

scanning and temporal resolution we now understand that we saw a red rose. This anal-

ysis might even connect to other neurons in the brain that tell you that the young man

giving you the red rose wants to say that he is in love with you.

Let us summarize what happened here: a photoreceptor (11-cis retinal bound to

a protein) was activated by photons, which resulted in a change of protein conforma-

tion, which started a signal cascade that amplified and transferred the signal to an ion

channel, which changed the potential of the cell membrane. This potential change was

further transferred to the brain, where the signal was analyzed and recognized as a red

rose. Is it possible to use the molecules and methods of the human vision system to make

an artificial, molecular-sized photosensor with similar functions?

3.2 Photosensors Using Biological Molecules and Methods

It is not easy to keep native protein structures in an artificial system; in most cases pro-

teins denature, i. e., they lose their specific structure and become random, losing their

function in the process. In the specific case of rhodopsin, not only does the native protein

structure need to be preserved but the structure must also cycle through two specific

conformations repeatedly, which is difficult to achieve.

https://doi.org/10.1515/9783110779196-003