A SYSTEMATIC METHOD FOR STUDYING THE SPATIAL-DISTRIBUTION OF WATER-MOLECULES AROUND NUCLEIC-ACID BASES

A new method to analyze the distribution of water molecules around the bases in DNA is presented. This method relies on the notion of a ''hydrated building block,'' which represents the joint observed hydration around all bases of a particular type, in structures of a particular conformation type. The hydrated building blocks were constructed using atomic coordinates from 40 structures contained in the Nucleic Acid Database. Pseudoelectron densities were calculated for water molecules in each hydrated building block using standard crystallographic procedures. The electron densities were fitted to obtain ''average building blocks,'' which represent bases with waters only at average or probable positions. Both types of building blocks were used to construct models of hydrated DNA oligomers. The essential features of the solvent structure around d(CGCGAATTCGCG)2 in the B form and d(CGCGCG)2 in the Z form were reproduced.