Of all the different cancer treatments, photodynamic therapy, where light is used to kill cancer cells, can have one of the strangest side effects: Patients often see better in the dark.
Now researchers have figured out why this is happening: rhodopsin, a light-sensitive protein in the retina in our eyes, interacts with a compound called chlorin e6, a critical component of this type of cancer treatment.
The work is based on what scientists already know about the organic compound of the retina, which is found in the eye and is usually insensitive to infrared light.
Visible light entering the eye causes the retina to separate from the rhodopsin – this is converted into an electrical signal that the brain can interpret. While we don't have a lot of visible light at night, it turns out this mechanism could also be caused by a different combination of light and chemistry.
With infrared light and with chlorine injection, the retina changes in the same way as with visible light.
“This explains the increased visual acuity at night,” said chemist Antonio Monari of the University of Lorraine in France, Laure Caylos of CNRS.
“However, we did not know exactly how rhodopsin and its active retinal group interacted with chlorin. It is this mechanism that we have been able to find out using molecular modeling. '
Along with some high-level chemical calculations, the team used molecular modeling to move individual atoms (in terms of their respective attraction or repulsion) and break or create chemical bonds.
The simulation ran for months – and ran millions of calculations – before it could accurately simulate the chemical reaction caused by infrared radiation. In real life, the reaction will occur in just nanoseconds.
“For our simulation, we put a virtual rhodopsin protein inserted into its lipid membrane in contact with a few chlorin e6 molecules and water or tens of thousands of atoms,” Monari told CNRS.
Because chlorin e6 absorbs infrared radiation, it interacts with oxygen in the tissue of the eye, converting it into highly reactive singlet oxygen – as well as destroying cancer cells, singlet oxygen can also react with the retina and stimulate increased night vision.
Scientists now know the chemistry behind this strange side effect, and they may be able to limit the likelihood of this happening in patients undergoing photodynamic therapy who report seeing silhouettes and outlines in the dark.
In the future, this chemical reaction can even be used to treat certain types of blindness or hypersensitivity to light, although it is absolutely not recommended to try to use chlorin e6 to provide superhuman night vision.
The study was published in the journal Physical Chemistry Letters.
Sources: Photo: Antonio Monari
