General Hydrogen-Bonding Patterns in Nucleoside and Nucleotide Crystal Structures

The structure contains a chelated three-center bond from the -N(H)H to the phosphate group. 

3 General Hydrogen-Bonding Patterns in Nucleoside and Nucleotide Crystal Structures 

If the crystal structures and hydrogen-bonding patterns of uridine and thymidine and their simpler derivatives are compared with those of cytidine, it becomes obvious that the -NH2 functional group has a characteristic influence. The nucleosides without the -NH2 groups have less than average hydrated crystals, although the one nucleotide represented, uridine-3 -monophosphate, crystallizes as a monohydrate. 

In the uridines and thymidines, there is a tendency to form long finite sequences of hydrogen bonds, and there are a few examples of infinite chains. In the cytidines, the NH2 group does not function as a hydrogen-bond acceptor, and is most frequently the starting point of two shorter finite chains. 

The purine nucleoside crystal structures adenosine, inosine, and guanosine provide less reliable data from the hydrogen-bonding point of view. This is particularly true of the guanosines, where only four structures can be reported. 

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