Polymorphism and disorder of poly(dA).poly(dT) in fibers.

The absence of proportionality between fiber length and interbase axial distance reveals a high degree of disorder in fibers of poly(dA).poly(dT) when the relative humidity (r.h.) varies. According to X-ray patterns, the increase of fiber length does not correspond to any conformational change. The B' structure is present alone and in a better-organized form at higher r.h. We noted a dependence of length values on the direction of variation of the r.h. An unusual minimum of fiber length is observed around 65% r.h. and explained as been due to an amorphous part of the fiber, essentially composed of single strands of poly(dA) and poly(dT). At a temperature higher than 30 degrees C, the transition B'(alpha)-B* takes place but is associated with an important decrease of fiber length in spite of constancy of the helical rise per base pair. It is shown that this behavior is also due to the amorphous part of the fiber and not to a conformational transition.