A need for re-examination of the fibre diffraction data of A-DNA

A-DNA pattern, obtained using a flat plat camera, was indexed by Fuller $\text{et}\text{al}{}^6$ on the basis of a c-face centred monoclinic cell with a = 22.24 Å, b = 40.62 Å, c = 28.15 Å and β = 97.0°. A precession photograph of A-DNA which gives an undistorted picture of the lattice, showed that the unit cell parameters as given by Fuller $\text{et}\text{al}$ were not quite correct. The precession photograph showed a strong meridional reflection (R = 0.00 Å^?1) on the 11th layer line. But the occurrence of the meridional reflection on the 11th layer line could not be explained on the basis of the cell parameters given by Fuller $\text{et}\text{al}{}^6$; using those cell parameters the reflection which comes closest to the meridian on 11th layer line is at R = 0.025 Å^?1. However, a simple interchange of a and b values accounted for the meridional reflection on 11th layer line. The corrected cell parameter refined against 28 strong spots are a = 40.75 Å, b = 22.07 Å, c = 28.16 Å and β = 97.5°. In the new unit cell of A-DNA, the packing arrangement of the two molecules is different from that in the old one. Nonetheless, our earlier contention is again reaffirmed that both right and left-handed A-DNA are stereochemically allowed and consistent with the observed fibre pattern.