A chirped mirror compressor, used in the laboratory of Associate Professor Zhi-Heng Loh to generate ultra-short laser pulses. Photo credit: M. Fadly.
Nuclear radiation is dangerous because of its insidious ability to damage the molecules in our bodies. The ionizing radiation emitted by radioactive substances, such as the nuclear waste released in the Chernobyl and Fukushima disasters, can fundamentally alter DNA and other biological molecules by disintegrating the chemical bonds holding the molecules together. Although the dangers of radiation have been recognized since the 1930s, when Marie Curie died from anemia caused by long-term exposure to radioactivity, scientists still lack many details about how exactly ionizing radiation alters molecules, especially those found in biological tissue.
Now, researchers from Nanyang Technological University, Singapore (NTU Singapore) have performed the first experimental study showing how ionizing radiation alters organic molecules dissolved in water, over time-frames of just one quadrillionth of a second – a femtosecond.
The team, led by Associate Professor Zhi-Heng Loh from NTU's School of Physical and Mathematical Sciences, used an ultra-fast laser apparatus to identify the complex vibrations of molecules struck by ionizing radiation. They showed that these vibrations occur only when the molecules are dissolved in water, a finding that provides new insight into how biological tissue is damaged by radiation. The study was published in the journal Nature Communications in July 2019.
When a molecule collides with a particle of ionizing radiation, such as an X-ray photon, it undergoes violent stretching, bending, and twisting motions that can eventually cause it to break apart or deform into a different shape. These motions are hard to observe since they occur over several femtoseconds, faster than most scientific instruments can detect.
“Our study is the first time anyone has observed ionisation-induced molecular dynamics in aqueous solutions over femtosecond time scales,” explains Associate Professor Loh. “In previous studies, scientists only observed the products of ionisation, after the molecule had already been broken apart.”
Lasers provide an ultra-fast probe for radiation damage
The new study used methods from femtochemistry, an area of chemistry dedicated to understanding ultra-fast phenomena in atoms and molecules, like the formation or breaking of chemical bonds. In order to observe ultra-fast chemical processes from start to end, femtochemistry researchers use lasers that emit extremely brief pulses of light, creating snapshots that can be stitched together like the frames of a video.
Associate Professor Loh and his team set out to use such methods to probe how ionising radiation affects biological molecules. As a starting point, they focused on the phenoxide ion, a relatively simple organic molecule containing many of the same chemical bonds found in the proteins making up living tissue. To simulate the features of a biological environment, they dissolved the phenoxide in water.
A technique called high-resolution spectroscopy had previously been used to study phenoxide in its gaseous form, and researchers had observed relatively simple behaviour: when struck by ionising radiation, each phenoxide molecule vibrated at a single frequency, like a bell ringing in a single clear tone. However, this method could not be used to study organic molecules dissolved in water, which is closer to the state in which molecules are found in biological tissue.
“Our research group specialises in femtochemistry, and once we got interested in the topic, it turned out to be relatively simple to adapt our methods to study the vibrations of ionised molecules dissolved in water. To our surprise, no one had ever tackled this problem before,” says Associate Professor Loh.
Using a pulsed-laser apparatus, the team observed radiation damaging the water-dissolved phenoxide molecules with unprecedented precision and clarity. They discovered that when radiation causes the molecules to eject an electron, the molecule vibrates in a highly complex pattern, more akin to the sound of a cymbal or gong than a ringing bell. They identified multiple vibrational frequencies, distinct from the single frequency observed in gaseous phenoxide.
“In the future, we will build on this to investigate how radiation affects larger and more complicated molecules, such as proteins and nucleic acids, which are the building blocks of life,” say Associate Professor Loh.
Tushar Debnath, Muhammad Shafiq Bin Mohd Yusof, Pei Jiang Low, and Zhi-Heng Loh, Ultrafast structural rearrangement dynamics induced by the photodetachment of phenoxide in aqueous solution, Nature Communications 10, 2944 (2019)