The possibility of using DNA in nanometer scale molecular electronic devices has spurred renewed interest in exploring its underlying physical properties in the various fields of physics and engineering. Much effort has been made to demonstrate the fundamental nature of electron transport in these molecules, though the experimental evidences continue to be controversial spanning the whole spectrum of conduction mechanisms from insulator, semi-conductor, metal to proximity induced superconductor. Such disagreement illustrates the intricate atomic level interaction between the DNA molecules and their environment, i.e., buffer solutions, electrical contacts, etc.
The magnetization measurement, unlike the electrical conductivity measurement, does not require any electrode attachment to the sample, thus the problem of contact-induced modification of DNA properties can be alleviated. A limited amount of work exists, however, on the intrinsic magnetic properties of DNA molecules. It is widely known that DNA is diamagnetic at room temperature with a small anisotropy that is believed to stem from the presence of aromatic rings of the base pairs. But how does the magnetic state of DNA depend on physical parameters such as temperature? Can it also be influenced by the external factors (salt concentrations, hydration levels) as well as the internal factors (molecular structure, base-pair sequence) in a way reminiscent to the electrical transport? To answer these pressing questions, we have studied the low temperature susceptibility and magnetization of randomly oriented l-DNA molecules and its relation to their molecular structure. We have witnessed the gradual apparition of intrinsic paramagnetic state in DNA molecules at low temperatures that is related to the hydration and the counter ion induced structural transition from A- to B-DNA . These new findings shed light upon the unconventional low temperature electronic property reported in the l-DNA molecules.

Ecole Supérieure de Physique et Chimie Industrielles, Paris