Temperature effects on the structure of poly(dA).poly(dT): an X-ray fiber diffraction study

X-ray fiber diffraction of poly(dA).poly(dT) subjected to variation in the relative humidity, has allowed us to demonstrate the effects of temperature on the conformation of the polynucleotide. When the temperature of the poly(dA).poly(dT) is greater than 30 degrees C and the relative humidity near 80%, a new diffraction pattern is obtained. We observe a transition between the classical alpha B' form of poly(dA).poly(dT) and a double helical structure, B*, which remains stable at a temperature up to 70 degrees C. This new conformation of poly(dA).poly(dT) is a right-handed double helix with 11.4 nucleotide pairs per turn and a pitch of 36.7 A.