People who receive photodynamic therapy to treat certain cancers often report an unusual side effect. Along with seeing strange shapes and silhouettes in the dark, many people also claim they gain a slight sense of “night vision”. 

Just recently, researchers in France have explained how this superhero-like effect occurs on an atomic level, the French National Center for Scientific Research (CNRS) reports. It has to do with the way rhodopsin, a light-sensitive protein in the retinas of our eyes, interacts with chlorin e6, a photosensitive molecule used in this type of cancer treatment.

First things first: photodynamic therapy is a treatment that involves light-sensitive medicine that is activated through a light source. Depending on the part of the body being treated, the photo-active agent (chlorine e6) is either put into the bloodstream through a vein or put on the skin, eventually making its way into the cancerous cells. Once a specialized light is beamed onto the affected area, the drug is “switched on” and it forms a chemical that kills the rogue cells.

It's also well known that this process increases the photosensitivity of the eyes. Some of the photoreceptors in your retina, known as rods, contain large quantities of rhodopsin, a photosensitive pigment that can absorb visible light thanks to an active compound called retinal. Retinal in the rhodopsin molecules react to visible light, a process that is eventually translated into visual information by our visual cortex.

Under low-light levels, most of the light exists at the infrared level, not visible light, which explains why we cannot see in low-light like some other species. However, it appears the introduction of chlorine e6 makes the rhodopsin react in the same way to infrared as it does when it receives visible light, explaining the "night vision" effect. 

"Thanks to experiments carried out by biologists in recent years, we now know that under infrared light the chemical structure of retinal is modified after the injection of chlorine (isomerization) in the same way as when it receives visible light. This explains the increase in night-time visual acuity,” chemist Antonio Monari, from the University of Lorraine in France, told CNRS.

“However, we did not know precisely how rhodopsin and its active retinal group interacted with chlorine. It is this mechanism that we have now succeeded in elucidating via molecular simulation.”

Reported in the Journal of Physical Chemistry Letters last year, the researchers used algorithms and molecular simulations to model how exactly this complex biochemical process occurs. As per their findings, chlorine e6 interacts with the oxygen present in the tissues of the eye after being “hit” with infrared radiation and transforms it into singlet oxygen, an electronically excited state of molecular oxygen. This singlet oxygen enters the rhodopsin molecule and accumulates alongside the retinal, sparking isomerization as if it was receiving “normal” visible light. 

“Our super-calculators ran for several months and completed millions of calculations before they were able to simulate the entire biochemical reaction triggered by infrared radiation. This reflects the extreme complexity of these phenomena, which occur within a few hundreds of nanoseconds,” Monari added.

[H/T: Science Alert]