by Roland Piquepaille
You can see above the optical components of the femtosecond fluorescence up-conversion spectrometer used at CAU. (Credit: Friedrich Temps, CAU) Here is a link to a larger version of this photo.
You can see above Nina Schwalb adjusting the femtosecond laser spectroscope. (Credit: J. Haacks, CAU) Here is a link to a larger version of this photo.
So what exactly did the research group find? “It has been known for many years that the individual bases that code the genetic information contained in DNA show a high degree of photostability, as the energy that they take up from UV radiation is immediately released again. Surprisingly, however, it is found that in DNA, which consists of many bases, those mechanisms are ineffective or only partially effective. It seems that the deactivation of UV-excited DNA molecules must instead occur by some completely different mechanisms specific to DNA, which are not yet understood. Through measurements by a variety of methods on DNA molecules with different base sequences, the research group led by Professor Friedrich Temps at the Institute of Physical Chemistry of Kiel University has now been able to confirm and clarify that assumption.”
Here is a quote from Professor Friedrich Temps about this research project. “DNA achieves its high degree of photostability through its complex double-helix structure. The interactions between bases that are stacked one above another within a DNA strand, and the hydrogen bonds between the base pairs of the two complementary single strands in the double-helix play key roles. Through the different interactions that we have observed the DNA acts to some extent as its own sun-protection.”
And here are more details about Nina Schwalb’s work. “Nina Schwalb investigated many different base combinations in synthetically-produced DNA molecules. Using a femtosecond pulsed laser spectroscope, she measured the characteristic energy release for each combination. She was able to measure the time for which the molecules continued to fluoresce, and thus how long they stored the light energy. She found that for some base combinations these fluorescence ‘lifetimes’ were only about 100 femtoseconds, whereas for others they were up to a thousand times longer. A femtosecond is one millionth of a billionth of a second.”
This research work has been published in Science under the title “Base Sequence and Higher-Order Structure Induce the Complex Excited-State Dynamics in DNA” (Volume 322, Issue 5899, Pages 243-245, October 10, 2008). If you know this research field, you might understand the abstract. “The high photostability of DNA is commonly attributed to efficient radiationless electronic relaxation processes. We used femtosecond time-resolved fluorescence spectroscopy to reveal that the ensuing dynamics are strongly dependent on base sequence and are also affected by higher-order structure. Excited electronic state lifetimes in dG-doped d(A)20 single-stranded DNA and dG·dC-doped d(A)20·d(T)20 double-stranded DNA decrease sharply with the substitution of only a few bases. In duplexes containing d(AGA)·d(TCT) or d(AG)·d(TC) repeats, deactivationof the fluorescing states occurs on the subpicosecond time scale, but the excited-state lifetimes increase again in extended d(G)runs. The results point at more complex and molecule-specific photodynamics in native DNA than may be evident in simpler model systems.”
Personally, I don’t fully understand this abstract. But I’m happy to report that Nina Schwalb has probably a bright future as a researcher. Please visit this page and scroll to “Ph.D. student Nina Schwalb wins poster prizes at international conferences.”
Christian-Albrechts-Universität zu Kiel news release, October 10, 2008; and various websites
You’ll find related stories by following the links below:Roland Piquepaille lives in Paris, France, and he spent most of his career in software, mainly for high performance computing and visualization companies. For disclosures on Roland's industry affiliations, click here.