B to Z transition in DNA fibre: the question of handedness of the duplex

The B-form of poly(dG-dC) is assumed to be a right handed duplex because its diffraction pattern in the same as that of the B form of DNA. Ploy(dG-dC), when in high salt, gives the Z pattern which has been interpreted as being due to a left handed zig-zag structure similar to that observed in single crystals of d(CpCpCpGpCpG) and d (CpGpCpG). We report here the observation that fibers of poly(dG-dC) drawn from 1:1 (v/v) water-ethanol also yield the Z pattern identical to the pattern obtained for the polymer in high salt. Surprisingly, the same fibre, which gives a B pattern initially, appears to undergo a transformation in the solid state such that its diffraction pattern changes to a final Z pattern after it has been equilibrated at room temperature (22°C) and humudity. This demonstrates that the handedness of the duplex in both forms of DNA should be the same since this transition from B pattern to Z pattern takes place in the solid state. The change of handedness of the duplex in the poly(dG-dC) fibre following prolonged annealing, as suggested by Arnott and co-workers, is incompatible with the observations reported here