Condensed Matter and Materials Physics(DCMMP)
Physique de la matière condensée et des matériaux (DPMCM)
David BENSIMON
École Normale Supérieure, France
The elastic behaviour of a real polymer: the case of ssDNA
The possibility to manipulate single molecules has opened a new vista on the study of their physical properties. It has thus been shown that the elastic behaviour of double stranded DNA (dsDNA) is very well described by the Worm Like Chain model of an ideal (non-interacting) polymer. That excellent agreement is the fortuitous result of the unusually large rigidity (persistence length) of dsDNA. For more usual polymer chains (single stranded DNA (ssDNA), proteins and artificial polymers such as polyethylene or polystyrene) whose persistence length is of the same order of magnitude as the monomer size, the WLC model cannot provide a good description of their behaviour under stress. We have used ssDNA as a model to investigate the elastic behaviour of these polymers, both experimentally and theoretically. We will show that the response to stress of ssDNA can be well described by a more refined model of a polymer (an elastic Freely Jointed Chain) that takes into account electrostatic repulsion and basepairing interactions between complementary nucleotides.
This model allows a parameter free description of the elastic behaviour of ssDNA over a wide range of ionic conditions and over 4 orders of magnitude of force. Similar results have been observed on the stretching of proteins where the WLC model is usually (but wrongly) used to fit the data.