Among all different types of cancer treatment, photodynamic therapy, in which light is used to destroy malignant cells, could have one of strangest side effects: Patients are often able to see better in the dark.
Last year, researchers finally figured out why this is happening: Rhodopsin, a light-sensitive protein in the retina of our eyes, interacts with a photosensitive compound called chlorin e6, a crucial part of this type of treatment for cancer.
The work was based on what scientists already knew about the organic compound retinal, which is found in eye & is generally not sensitive to infrared light.
Visible light triggers the separation of the retina from rhodopsin – it is converted into an electrical signal that our brains interpret to see. While we don’t get much visible light at night, it turns out that this mechanism can also be activated with another combination of light & chemistry.
Under infrared light & with an injection of chlorine, the retina changes in the same way as does under visible light.
“This explains the increase in night visual acuity”, Chemist Antonio Monari, from University of Lorraine in France, told Laure Cailloce at the CNRS in January 2020.
“However, we did not know precisely how rhodopsin & its active retinal group interacted with chlorin. It is this mechanism that we have now been able to elucidate through molecular simulation.
In addition to some high-level chemical calculations, the team used molecular simulation to model the movements of individual atoms (in terms of respective attraction or repulsion), as well as to break or creating chemical bonds.
The simulation was performed for several months & chewed via millions of calculations – before being able to precisely model the chemical reaction caused by infrared radiation. In real life, the reaction would take place in nanoseconds.
“For our simulation, we placed a virtual rhodopsin protein insert into its lipid membrane in contact with different molecules of chlorin e6 & water, or several tens of thousands of atoms,” Monari told CNRS.
Since chlorine e6 absorbs infrared radiation, it interacts with oxygen in eye tissue, turning it into highly reactive singlet oxygen – in addition to destroying cancer cells, singlet oxygen can also react with the retinal & allow an increase in night vision, simulation shows.
Now that scientists know the chemistry behind this strange side effect, they may be able to limit the possibility of it occurring in patients undergoing photodynamic therapy, who have reported seeing silhouettes & outlines in dark.
Further, this chemical reaction could also be exploited to help treat certain types of blindness or over sensitivity to light, although it is certainly not recommended to attempt to use chlorin e6 to give you-self super human night vision.
This is another example of the information we can get even from molecular simulations & how the most powerful computers on planet are able to give us a deeper understanding of science than we would otherwise have.
“Molecular simulation is already being used to shed light on fundamental mechanisms – for example, why some DNA damage is better repaired than others – and to allow the selection of potential therapeutic molecules by mimicking their interaction with a target chosen, ”Monari told CNRS.