Hydrogen bonding to guanine

Fig. 3.1-7 The crystal structure of EF-Tu bound to a nonhydrolyzable CTP analog shows Aspl38 hydrogen bonding to guanine. (PDB 1EXM).

Double-stranded helix structure of DNA. Adenine is hydrogen-bonded to thymine (A T), and cytosine is hydrogen-bonded to guanine... 

Molecules of DNA consist of two complementary polynucleotide strands held together by hydrogen bonds between heterocyclic bases on the different strands and coiled into a double helix. Adenine and thymine form hydrogen bonds to each other, as do cytosine and guanine. 

The ability of DNA to replicate lies in its double-helical structure. There is a precise correspondence between the bases in the two strands. Adenine in one strand always forms two hydrogen bonds to thymine in the other, and guanine always forms three hydrogen bonds to cytosine so, across the helix, the base pairs are always AT and GC (Fig. 19.29). Any other combination would not be held together as well. During replication of the DNA, the hydrogen bonds, which are... 

In DNA, base pairing occurs between guanine and cytosine in its most stable form [87], as in [92]. However, cytosine in its imino form [91] is able to form a hydrogen bond to adenine as in [93], and such a base pairing could lead to the formation of mutations if it continued in DNA replication. The kinetics of the tautomerisation of purine and pyrimidine bases has been... 

The second example in Figure 11.2 shows guanine bound to a phospho-ribosyl transferase. Only one of the two possible tautomers is able to form the correct hydrogen bond to an active site aspartate. 

The DNA molecule is a double strand of nucleic acids, each consisting of a sequence of nucleosides, which are paired ribose sugar and phosphoric acid, held together in sequences of [—R(X)—P—], where X is one of four bases of thymine, cytosine, adenine, or guanine. Thymine can bond to adenine by two hydrogen bonds, and cytosine can bond to guanine by three hydrogen bonds. The two strands of DNA molecules run... 

An interesting feature was discovered by Sharma and co-workers494,495 in the crystal structure of isocytosine. Two tautomers of isocytosine (42 and 43) exist in an exact 1 1 ratio in the crystal. The tautomers are hydrogen-bonded to each other in a manner analogous to that proposed by Watson and Crick496,497 for the guanine-cytosine pair in DNA. It is worth underlining that the base pair of isocytosine was not obtained by expedient cocrystallization of the two components. It seems therefore probable that both forms 42 and 43 of isocytosine are of approximately equal stability and are present in comparable amounts in solution. 

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