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Dr. Roland Netz
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Conference abstract - invited talk:
COMPLEXES FORMED BY DNA
Charged polymers such as DNA exhibit a number of different complexation modes when mixed with other charged objects such as spheres or cylinders.A simple model for the complexation of semiflexible polyelectrolytes with oppositely charged spheres is considered, which can exhibit tightly wrapped polymer structures. Using the appropriate parameters for DNA-histone complexes, we find complete wrapping for intermediate salt concentrations only, in agreement with experiments. We calculate the effective charge of the complex as a function of the DNA length, which shows charge reversal of the sphere only for intermediate lengths. We also discuss the forces needed to pull the DNA off from the histone, leading to plateau forces of the order of 10-40 pN (depending on salt concentration) and thus amenable to detection by atomic force microscopy (AFM).
In the case of complexation of a semiflexible polyelectrolyte with an oppositely charged cylinder, we find helical as well as straight wrapping structures. Again, salt can be used to switch from one morphology to the other. These system are realized by DNA complexing with polylysine (where the more flexible polypeptide wraps around the DNA) and with dendrimeric polycations (where the DNA wraps around the bulky dendrimeric polymer).
Finally, the response of a single DNA molecule to pulling (for example by AFM) is discussed. At high forces the DNA double helix unwinds and shows a transition from the normal B-DNA to the S-DNA state. The precise critical force strength depends on salt concentration, which can be explained by a coupling between i) bending fluctuation modes, ii) electrostatic repulsion between polymer segments, and iii) polymer stretching.