, that stringy stuff in our
DNA cells , contains four important chemicals : adenine ( A ), thymine ( T ), guanine ( G ) and cytosine ( C ). These are the nucleobases , the molecules that encode the information for life . They are also willing absorbers of ultraviolet ( UV ) light , and because of this sunlight can be a major nuisance for our skin .
Absorption is the process where energy from light ( a photon ) is taken in by a molecule , causing an electron to jump into an ‘ excited state ’. Excited states are energetic and unstable , and when they occur within a DNA nucleobase , they can initiate the damaging chemical reactions that result in sunburn and skin cancer .
It has been known for many years that UV light attacks DNA , but until just over a decade ago the excited states of DNA were elusive , because they were too short-lived for any lab instrument to record . However , with advances in femtosecond lasers ( that produce flashes of light lasting about 10 -15 seconds ), we can now study these excited states in great detail .
The ability to study very fast processes in DNA is a welcome development , as the behaviour of the excited state in those first few picoseconds ( 10 -12 seconds ) after absorption decides much of what eventually happens to our skin cells . Although this can sometimes result in the adverse health effects mentioned above , our DNA is actually quite resilent to UV light and can even be regarded as its own sunscreen .
This self-defence mechanism is most vividly expressed in the behaviour of the individual nucleobases , and represents the first achievement of femtosecond laser studies of DNA . Despite having different chemical structures , the nucleobases all react similarly to the absorption of UV light ; they deactivate , fast . Within one picosecond ( 10 -12 seconds ) of absorption , the nucleobase excited states have lost most of their ability to cause damage . Computer
Fixing DNA a flash
Páraic Keane writes that the first few trillionths of a second makes a big difference in how sunlight affects our DNA
simulations show that this is achieved by a process of molecular gymnastics . The nucleobases , which are usually flat , distort their shape and release the excess energy through vibrations of the chemical bonds , which persist for about ten picoseconds after the excited state has been swiched off . In effect , the high energy UV light is rapidly dispersed through a series of lower energy processes , which can be described simply as ‘ heat ’, but is more technically known as ‘ ultrafast internal conversion ’. No bonds are broken , and the very rapid quenching of the excited state minimises the chance of any harmful reactions happening .
However , if the chemical structure of a nucleobase is altered slightly , the deactivation can slow considerably , and the potential for chemical reactivity increases . It appears , therefore , that the nucleobases are optimised to survive UV radiation . This survival feature may partly explain why A , T , G and C became building blocks of life . Nowadays , we can be thankful that atmospheric oxygen and the ozone layer removes most of the dangerous UV rays ( UVC and most UVB ) before they reach us , but this protection may not have been present billions of years ago when the chemistry of primordial life was evolving . As a result , a high tolerance for UV light was likely to have been a prerequisite for biological success .
The ultrafast deactivation of the excited nucleobases is a neat example of natural selection on the molecular scale , but cannot completely explain the interactions of UV with DNA . In double-stranded DNA the nucleobases are all joined up by a sugar / phosphate backbone , they are base-paired with their complement in another strand through hydrogen bonds ( A with T , G with C ), and they stack on top of other
nucleobases within their own strand . All these features are significant , as the behaviour of excited states is very sensitive to chemical structure .
In fact , having other nucleobases so close together is where much of the trouble starts . When two thymine nucleobases appear next to each other in a sequence , UV light can cause them to fuse together to form a dimer , which inhibits further replication of the strand . Although relatively common , these TT dimers are well repaired by enzymes in the cell that recognise and remove the damaged sequences . By contrast , cytosine dimers ( CC ) are rarer , but are much more dangerous because they are poorly repaired . One reason is that they rapidly transform to uracil ( the nucleobase that replaces thymine in RNA ), which is complementary to A rather than G . If the damaged strande is copied , CC becomes TT , and a potentially cancerous mutation occurs .
In general , UV damage to DNA is concentrated at the ‘ pyrimidines ’ ( T and C ) rather than the larger ‘ purines ’ ( A and G ). This is mainly due to the structural features of pyrimidines versus purines , but ultrafast laser experiments have thrown up some other intriguing coincidences . When the sugar / phosphate molecules are attached to T or C ( but not A or G ), about 15 per cent of the UV energy diverts through another excited state , which retains the energy up to 100 times longer than the nucleobase alone . These are known in spectroscopy parlance as ‘ dark ’ states , a rather apt name when neither the reasons nor consequences of their formation are understood . It is possible that they are involved in the formation of dimers , although some experiments suggest that these dimers are formed too rapidly for dark states to matter . Alternatively , they could be involved in the other photo-
SCIENCE SPIN Issue 58 Page 10