ARCHIVED - NRC Scientists Uncover a Clue to Survival of Early Life on Earth
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September 06, 2006— Ottawa, Ontario
Scientists at the NRC Steacie Institute for Molecular Sciences (NRC-SIMS) have gained new insight into how DNA deals with harmful ultraviolet light (UV) radiation. Their findings shed new light on how molecules use ultrafast mechanisms to protect themselves against UV damage. These studies have implications for how early life was able to sustain itself on Earth without the protective ozone layer.
The team, led by Dr. Albert Stolow of NRC-SIMS, published its findings in the Proceedings of the National Academy of Sciences (PNAS) of the United States. The team reported a comprehensive model explaining how the constituents of DNA (deoxyribonucleic acid) can quickly convert the UV light into heat. The team studied the adenine molecule, one of the four building blocks of the genetic code. Surprisingly, the protection mechanisms operate on picosecond (a millionth of a millionth of a second) timescales, far faster than any biological function.
Learning how nature protects these molecules has implications for future developments in both science and technology. For example, in the emerging field of 'molecular photonics', molecules themselves are being proposed as light-activated 'nanoscale' switches or modulators. By understanding how to protect molecules from light-induced damage, the efficiency and stability of these new photonic devices can be greatly improved.
DNA, the giant molecule that carries genetic information in living things, is made up of just a few chemical building blocks that bond together in a very particular fashion. These building blocks of life are also the primary absorbers within DNA of UV radiation. Since DNA was around even before plant life had time to create the ozone layer, it was assumed that DNA must inherently have some kind of UV protection mechanism or "photostability". For several decades now, scientists have been engaged in theoretical and experimental efforts to understand this mechanism.
The NRC team studied adenine using time-resolved photoelectron spectroscopy, an ultrafast measurement technique developed at NRC. In their experiment, laser flashes, that last only femtoseconds (a femtosecond is a thousand times less than a picosecond), were used to "film" the response of the adenine molecule to UV light. Together with computer simulations, the researchers were able to elucidate the details of adenine's stability.
The absorption of UV light makes the molecule so excited that it can break. In order to drain off this excess energy, the molecule must quickly convert it to heat. This is analogous to rapidly pouring water into a sink. If the drain is too slow, the sink will overflow and damage will occur. However, if the drain is very fast then the damage will be avoided. It is the competition between two fast processes, the filling of the sink (absorption of UV light) and the draining of the sink (converting the energy into heat), which determines whether or not damage will occur.
Dr. Stolow explains, "One might think that processes which occur on such fast time scales cannot have any consequences for biology. This is not correct. There are unavoidable processes, UV damage being an example, which are very fast and lead to the destruction of biomolecules. In order to compete against these, nature had only one choice, namely to design protection mechanisms which are even faster."
Future experiments by the NRC team will include the study of other biological molecules to further investigate these protection mechanisms. Discovering the rules of ultrafast dynamics in molecules provides an important new perspective for understanding biological processes such as vision and photosynthesis and for the design of new nanoscale molecular devices.
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