The smell of rotten eggs wafting from chemistry labs is easily detected by our noses. Hydrogen sulphide is a noxious gas, and the WHO recommends limiting it to 15 parts per million in the air you breathe.
However, at even smaller quantities, hydrogen sulphide is a constituent of our lives. There are specialised enzymes that produce the gas in our brain, kidneys, liver, … The gas has important physiological roles to play. So detecting hydrogen sulphide in cells, tissues and biological fluids has important implications in the diagnosis of Alzheimer’s disease, Parkinson’s disease, Down’s syndrome, diabetes, liver cirrhosis, gastric mucosal injury, cancers…
Available techniques for detecting the gas at these levels are messy. Tedious sample processing, requirements for sophisticated instruments, often ending in not so reliable results.
Kiran R. Gore and Lavanya L. Mittapelli from the University of Mumbai collaborated with researchers from IIT Bombay and the Uppsala University, Sweden to devise an ingenious method to overcome the problem. The technique leverages on the chromophore of the green fluorescent protein which they call HBI for short, to escape saying 4-(p-hydroxybenzylidene)-5-imidazolidinone, whenever they wish to talk about the chromophore.
Fluorescence is quite a reliable way to detect gases even at low levels. What if we could attach this chromophore to something that reacts to the presence of hydrogen sulphide, they asked themselves.
So Lavanya set about synthesising acryloyl-protected analogues of HBI. Here is where it gets tricky. There is a benzylidene double bond in the HBI molecule. If it is free to rotate, there will be no fluorescence. But Lavanya could, through a series of chemical steps, synthesise such a material to meet this demand.
Now the question was: does it work?
The colourless material which exhibited maximum intensity at 360 nanometers fluoresced at 450 nanometers in the presence of the –HS group. The molecular sensor could detect hydrogen sulphide at concentrations as low as 15 parts per billion, at concentrations a thousand times less than the WHO recommended limits! Other physiologically relevant sulphur-containing molecules did not interfere with this phenomenon.
The low molecular weight molecule could easily go through the cell membrane and it was easy to visualise intracellular hydrogen sulphide.
It showed no toxicity to normal cells. Cancer cell lines that had higher amounts of hydrogen sulphide than found in normal cells, however, showed toxicity at higher concentrations of the sensor.
We now have a very sensitive and selective probe to explore the role of hydrogen sulphide in various physiological processes as well as in disease conditions, says Kiran Gore, University of Mumbai.
Sensors & Actuators: B. Chemical 298: 126875 (2019)
Udham Singh P K
Freelance science writer