The magnetic orientation of native and synthetic nucleic acids has been observed for the first time by measuring the Cotton-Mouton effect in magnetic fields $B$ up to 14 T. Our data give evidence that a partial alignment of the nucleic acids takes place in the plane perpendicular to $B$ and that the diamagnetically anisotropic bases are responsible for this orientation. The method reported here represents a new way to determine the persistence length of flexible polymers.

• Received 30 October 1974

DOI:https://doi.org/10.1103/PhysRevLett.35.397