DNA Structure Puzzle #8 - The Crystallographers' Paradox
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Sat Nov 20 03:19:54 EST 1999
DNA STRUCTURE PUZZLE NUMBER 8 The Crystallographers' Paradox
Several teams of nucleic acid crystallographers have reported on the interconversion of the B to the Z form under very mild conditions inside fibres of drawn DNA (22,23), as well as the B to form D conversion under similar, very mild conditions (24).
The very mild conditions are interpreted by some (23,24) to mean that the B and Z forms, and the B and D forms, respectively, must have the same helical handedness. (Another team (22) seems to offer no interpretation of its observation of the B to Z transition inside a fibre.) The reversible conversion inside fibres of the A to, and from, the B form has been known for many years, and therefore the A, B, D and Z forms would all seem to have the same helical handedness.
However, there are many studies of diffraction from true oligonucleotide crystals which purport to show that the A and B forms are right-handed, for example (25,26), and purport to show that the Z form is left-handed, for example (27).
How can all the crystallographers be right ?
22 Polymorphism of DNA Double Helices; A.G.W. Leslie, S. Arnott, R. Chandrasekaran & R.L. Ratliff; J Mol Biol Vol 143 (1980) 49 - 72
23 B to Z Transition in a DNA Fibre: The Question of Handedness of the Duplex; V. Sasisekharan & S.K. Brahmachari; Current Science Vol 50 (1981) 10 - 13
24 Time-resolved X-ray Diffraction Studies of the B to D Structural Transition in the DNA Double Helix; A. Mahendrasingham, V.T. Forsyth, R. Hussain, R.J. Greenall, W.J. Pigram & W. Fuller; SCIENCE Vol 233 (1986) 195 - 197
25 Conformation of d(GGGATCCC) x 2 in crystals and in solution studied by X-ray diffraction, Raman spectroscopy and molecular modelling; H. Fabian, W. Holzer, U. Heinemann, H. Sklenar & H. Welfle; Nucleic Acids Research Vol 21 (1993) 569 - 576
26 Crystal Structure Analysis of a Complete Turn of B-DNA; R. Wing, H. Drew, T. Takano, C. Broka, S. Tanaka, K. Itakura & R.E. Dickerson; Nature Vol 287 (1980) 755 - 758
27 Molecular Structure of a Left-handed Double Helical DNA Fragment at Atomic Resolution; A.H.-J. Wang, G.J. Quigley, F.J. Kolpak, J.L. Crawford, J.H. van Boom, G. van der Marel & A. Rich; Nature Vol 282 (1979) 680 - 686
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