Structural parameters of the monohydrated DNA bases

During the last two years, monohydrated forms (including the geometries of the water assisted transition states for intramolecular proton transfer) of the 7H-and 9H- adenine (the B3LYP/6-31 l(d,p) level), cytosine (MP2/6-31 G(d) level) and mono and dihydrated forms of 7H- and 9H- guanine (MP2/6-31 G(d) level) have been studied extensively [119 -124]. Furthermore, the post Hartree - Fock investigations of the struetural parameters of the isolated DNA bases are the subject of a large number of publications (for the reviews see [61,125]). 

The analyses of the structural parameters of the investigated monohydrated complexes reveal that the hydrogen bonds between the base and water molecule should be classified as rather weak. The geometrical parameters of the monohydrated amidic fragment of guanine and cytosine are very similar. 

The second source of the nonplanarity is the interaction of the amino group hydrogen atoms with the closest atoms belonging to the remaining part of the DNA base. To analyze the nonplanarity of this type, it is also convenient to introduce the parameter A, the meaning of which is described in Tables 5-8. 

Let us start the analysis of the influence of the hydration on the nonplanarity phenomena from the nonplanarity of the first type. There are two important observations here. First of all one may see that the nonplanarity of the first type (sp- hybradization of the amino group) does not depend significantly either on chemical structure of the tautomers or on the interaction with water molecules in the case of guanine and cytosine. 

Selected values of torsional angles MP2/6-31G(d) level. of 7-guanine tautomers calculated at the 

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