Tables for
Volume F
Crystallography of biological macromolecules
Edited by M. G. Rossmann and E. Arnold

International Tables for Crystallography (2006). Vol. F. ch. 23.3, p. 599   | 1 | 2 |


R. E. Dickersona*

aMolecular Biology Institute, University of California, Los Angeles, Los Angeles, CA 90095–1570, USA
Correspondence e-mail:

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Comparison of structures of A, B and Z helices

Handedness Right Right Left
Helix axis relative to base pairs Major groove side Through centre of base pair Minor groove side
Major groove Very deep and narrow Wide, same depth as minor Very shallow and broad
Minor groove Shallow and broad Variable, same depth as major Very deep and narrow
Glycosydic bonds anti anti C: anti
      G: syn
Minor groove backbone chain sense Clockwise Clockwise Counterclockwise
Sugar conformation C3′-endo (narrow range) C1′-exo/C2′-endo (broad range) C: C2′-endo
      G: C3′-endo
Base pairs per helix repeat 1 1 2
Base sequence limitations None None Alternating [\hbox{(C-G)}_{n}] or close variants
Rise per base pair (average) 2.9 Å 3.4 Å C-G: 4.1 Å
      G-C: 3.5 Å
Base pair inclination 10–20° ca ca
Mean twist angle 30–33° 34–36° C-G: −8°
      G-C: −52°
Helix repeats per turn 11–12 10–10.5 6 (2 base pairs)
Propeller twist Often substantial, 0–25° Often substantial, 0–25° Usually small
Common biological occurrence RNA DNA None?
Relative 5′-to-3′ directions of the two backbone chains, when viewed into the minor groove.