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

The absence of proportionality between fiber length and interbase axial dis tance 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 incr ease 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 th e r.h. An unusual minimum of fiber length is observed around 65% r.h. and e xplained as been due to an amorphous part of the fiber, essentially compose d 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 wit h an important decrease of fiber length in spite of constancy of the helica l rise per base pair. It is shown that this behavior is also due to the amo rphous part of the fiber and not to a conformational transition. (C) 1998 A cademic Press.