Aspetti strutturali e dinamiche di fibre di DNA Oriented

We have studied structural and dynamical behavior of fiber DNA under osmotic pressure and without by means of neutron scattering and differential scanning calorimetry (DSC). A classical three-axis spectrometer, IN3 at ILL, and Micro DSC III at CNRS, Grenoble, were employed in our experiments. We focused on B-form Na-DNA fibers due to the technical issues of sample preparation, less complicated diffraction patterns comparing to the A-form, and most frequent presence of this DNA conformation in nature. By means of wet spinning apparatus (WSA) we prepared two oriented fiber DNA samples, and one of them was subjected to osmotic pressure, i.e. placed in polyethylene glycol (PEG) solution, while another one was left under special relative humidity to gain the B-form. With DSC we ran preliminary experiments to obtain melting profiles for both samples. This method was used, firstly, to verify that the necessary conformation was gained by both samples during the preparation process and that B-form was preserved in the sample under osmotic pressure, and, secondly, to get information about the denaturing process in both samples. Out of curiosity we also ran several DSC experiments on Li-DNA B-form fibers to see the influence of counter-ion used in the DNA preparation on the dynamics of the melting process. A comparative structural studies for both samples were carried out with the use of neutron diffraction at IN3 spectrometer. The experiments were made for both samples in longitudinal and transverse Bragg peak configurations. The scans in longitudinal configuration showed typical Bragg peak for B-from DNA, which indicates a periodic arrangement of the bases along the helical axis, and the scans in transverse configuration revealed the orientation of molecules along a fiber and correlation of fibers themselves with their neighbors.