International
Tables for
Crystallography
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. 594   | 1 | 2 |

Figure 23.3.2.9 

R. E. Dickersona*

aMolecular Biology Institute, University of California, Los Angeles, Los Angeles, CA 90095–1570, USA
Correspondence e-mail: red@mbi.ucla.edu

[Figure 23.3.2.9]
Figure 23.3.2.9

Watson–Crick pairing of a purine (A or G) with a pyrimidine to its right (T or C), and Hoogsteen pairing of the same purine with a pyrimidine above it. This combination of Watson–Crick and Hoogsteen pairing is found in triple helices or triplexes. Note that Hoogsteen pairing of G and C can only occur at a pH at which C is protonated, because the extra proton is essential for the second hydrogen bond.