Apparent circular dichroism signature of stirring-oriented DNA and drug-DNA complexes.

It is shown that in water with no added salts calf-thymus DNA and its drug-loaded forms exhibit peculiar, completely reversible circular dichroism (CD) spectroscopic changes upon mechanical rotatory stirring. Due to the stirring-induced spatial alignment of the helices, the CD spectra are overwhelmed by the much more intense linear dichroism contribution. This apparent chiroptical response can be generated and detected without using any sophisticated attachment. It is a technically simple complement to existing methods suitable to obtain additional structural information which can not be derived from isotropic spectra. Drug-DNA interactions generating no or very weak CD spectroscopic changes under isotropic conditions become easily detectable upon stirring the sample solution. Stirring-induced changes of the CD profile also enable to clarify the DNA binding mode of various compounds (e.g., imatinib, thioflavin T) which would remain ambiguous considering isotropic spectral data only.