Base reverse Watson-Crick

Fig. 8. Non-Watson-Crick base pairs occurring in double-stranded RNA where — represents the site of attachment to the sugar (a) A—U reverse-Watson-Crick (b) G—C reverse-Watson-Crick (c) A—U Hoogsteen (d) A—U reverse-Hoogsteen (e) G—U wobble and (f) G—U reverse-wobble.

Combination AU62 is the Watson-Crick base pair, AUS2 is called reversed Watson-Crick (Fig. 16.10, left). AU32 is the base pair discovered by Hoogsteen, and AU22 is the reversed Hoogsteen base pair (Fig. 16.10, right). 

Fig. 16.10. Schematic description of A-U base pairs found in crystals and in double helical nucleic acids. AU62 Watson-Crick A(JS2 reversed Watson-Crick AU32 Hoogsteen AU22 reversed Hoogsteen...

Fig. 16.11. Watson-Crick GC23 and reversed Watson-Crick GC42 base pairs formed by guanine and cytosine...

Zhanpeisov, N. U. Sponer, J. Leszczynski, J. Reverse Watson-Crick isocytosine-cytosine and guanine-cytosine base pairs stabilized by the formation of the minor tautomers of bases. An ab initio study in the gas phase and in a water cluster, J. Phys. Ghent. A 1998,102,10374-10379. 

Tautomerism in 5,8-diaza-7,9-dicarbaguanine (alloguanine) has been studied An X-ray structure analysis of the title compound revealed that this purinoid exists in the crystal as the two tautomers which interact with each other in the mode of a reverse-Watson-Crick base pair <05HCA1960>. 

Shows reverse Watson-Crick base pairing behaviour. 

Finally we would like to mention the investigation of the so called reverse Watson-Crick isocytosine - cytosine base pair which mimic the parallel strand of the DNA [145]. Among the structures investigated at the MP2/6-31 G(d)//HF/6-3 lG(d) level, the complex presented in Figure 13, which includes the interaction of two rare tautomers, is only 9 Kcal mof less stable than canonical cytosine-isocytosine base pair. Inclusion of the hydration decreases this difference to 6 Kcal-mof. Thus the calculations indicate that possible involvement of minor tautomeric pairs should be considered when considering the structure of a parallel strand of DNA. 

H-bonds. The interaction (complexation) energies of all four AT pairs (Watson-Crick, Hoogsteen, Reverse Watson-Crick, Reverse Hoogsteen) are between -12.3 and -13.3 (—11.7 to —12.6) kcal mol, while the complexation energies of the weakest pairs are —9 to -10 kcal mol" only. The reorganization energy of bases for neutral pairs ranges from -f2.5 kcal mol (GGl pair) to about -fO.5 kcal mol for the weak pairs. 

FIGURE 1.7 Base pairing schemes of Watson-Crick and non-Watson-Crick types. Among the non-Watson-Crick base pairs, wobble pairs are formed by H-bonds between tautomeric forms of hydroxyl (OH) and imino (NH) groups. Non-Watson-Crick base pairs also comprise Hoogsteen base pairs and reverse Watson-Crick base pairs which form the basal structural units in triplex DNA and In parallel-stranded DNA (psDNA), respectively. 

The non-Watson-Crick type of base pairing also comprises the base pairs formed by inosine (I) with C, T, or A, purine hase pairs A G(or I), A - A, and G C, and the reverse Watson—Crick base pair A T. In the reverse Watson—Crick A T pairing, the T ring is rotated 180° around the N-3—C-6 axis from the normal Watson-Crick pair. One peculiarity of the reverse Watson—Crick hase pairing is that the two pairing strands can form an as yet unnatural para//e/-stranded, right-handed double helix (2, 3). 

The hydrogen bonds are employed by the nucleobases to form base pairs interactions. It is known that there are 28 possible base-pairing motifs that involve at least two hydrogen bonds, which can be formed between the four common nucleobases. These include reverse Watson-Crick, (Fig. 3a), Hoogs-teen and Wobble base pairs (Fig. 3b), and base-triplets (Fig. 3c). In the case of adenine and guanine larger aggregates are also possible. 

A logical extension of the work on phosphoryl cleavage reactions is the study of the reverse, ligation reaction. The principle of microscopic reversibility offers the comforting thought that the requirements for catalysis are basically the same, and Watson-Crick base-pairing provides a simple... 

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