Niederfrequente Dynamik von überstreckter DNS

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Micromechanical experiments on single DNA molecules have shown that the force required to stretch DNA beyond its geometrical contour length lo does not steeply increase, but reaches a plateau which extends to elongations up to 1.7lo. This plateau has been interpreted as a coexistence between normal B-DNA and a new structure of DNA called S-DNA where the base pairs are thought to be tilted with respect to the helical axis. The reduced van-der-Waals interaction between tilted base pairs should thus entail a significant change in the acoustical properties of the DNA which are directly probed with inelastic light scattering. In addition, the conformational fluctuations at the B-S transition are expected to give rise to a rich quasi-elastic spectrum. The goal of the present project is a detailed study of the dynamics and conformational fluctuations of overstretched DNA using inelastic and quasi-elastic light scattering from carpets of oriented DNA molecules attached to the plates of a surface force apparatus or, alternatively, to super-paramagnetic colloidal beads which are used as force transducers using a magnetic field gradient. These experiments are expected to reveal novel insight into the microscopic nature of the force plateau, into conformational fluctuations in DNA and their importance for the binding of proteins such as the transcription protein RecA.