German researchers have used standard circular dichroism spectroscopy to probe interactions between drug molecules and DNA within living cells. This avoids the need for fluorescent or radionuclide labelling, allowing easy insight into potential mechanisms of drug action.

Birgit Krewer, Lutz Tietze and colleagues from the Georg-August-University of Göttingen were trying to understand the anti-cancer action of a series of molecules based on a natural antibiotic called CC-1065, which are thought to bind to DNA and prevent its replication. 'CC-1065 is very toxic,' explains Krewer, 'so our approach was to synthesise some pro-drugs with reduced toxicity that could be activated inside the cancer tissue, and this study was to try and see how they work.'

Krewer first studied the interaction of the molecules with isolated DNA in solution using circular dichroism (CD) spectroscopy, which is a common technique for studying the way polarised light interacts with chiral molecules or complexes. When the chiral molecules bound to the DNA, there was a distinctive signal in the CD spectrum. 

The team then decided to try repeating the experiment in living cells to see if the same kind of binding happened. 'CD spectroscopy of cells has been done before,' says Krewer, 'but using a very sophisticated setup to amplify the signals. We just used a normal machine.' The signal showed up without any special amplification, and remained even after the cells were killed and the DNA extracted - showing that the drugs had covalently bonded to the DNA structure.

Bengt Nordén from Chalmers University of Technology in Gothenburg, Sweden, says that the work is interesting since there has been some debate as to whether these drugs act by binding to DNA. 'The results are qualitative, but still indicative of DNA binding,' he says. However, he adds that making the technique quantitative would require some difficult amendments to the analysis.

The technique could be quite general, explains Krewer: 'you need a signal that's strong enough and separated from the [unbound] DNA signal, but if you have that it's very easy to do.' She adds that the method works best with chiral molecules, but it is still possible with achiral molecules since binding to DNA or other biomolecules that are themselves chiral can induce a CD signal.

Phillip Broadwith

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