Scientists have taken high-resolution images of shape changes in retinal – a molecule important for vision – by using a carbon nanotube as a sample holder.
|cis and trans retinal chromophores attached to the C60 fullerenes inside nanotubes. (Credit: K Suenaga)|
Using a transmission electron microscope (TEM), Kazu Suenaga and colleagues at the National Institute of Advanced Industrial Science and Technology (AIST) imaged a single retinal molecule by attaching it to a carbon-60 molecule trapped inside a carbon nanotube.
“The dynamic behaviour of retinal is responsible for vision,” explained Suenanga. “Our experiment therefore allows us to observe how we see with our eyes at the molecular level. People say that human eyes are one of the most efficient detectors. If we can correctly modify the retinal molecules at the single-molecule level, we may be able to fabricate a device that mimics eyes.”
When retinal molecules are stimulated by light, they change shape – going from being bent (cis) to straight (trans). It is this shape change that triggers a cascade of biochemical reactions that ultimately results in an electrical impulse being sent along the optic nerve.
The carbon nanotube protected the retinal molecules against any denaturation that may have been caused by the electron beam in the TEM. This protective effect meant the researchers achieved a resolution of 0.14nm, which is the same as the distance between two carbon molecules. This meant they were able to see the C–C bonds in the retinal molecule changing shape from cis to trans.
Since the samples are not damaged in the microscope, the team says that its technique could be used to visualise other molecules, not just retinal. “For example, to investigate molecular recognition events such as protein folding or steric hindrance,” said Suenaga.
The researchers reported their work in Nature Nanotechnology.