Figure 3.

Statistical analysis of ions in nucleic acid crystal structures. Distribution of the fraction of nucleic acid charges that are neutralized by ions resolved in the crystal structures of 6,238 nucleic acids obtained from the nucleic acid data base (http://ndbserver.rutgers.edu/). Nucleic acid charges were estimated by counting atoms of type P (phosphorus); ion charges were estimated by counting all elemental ions of the first and second groups as well as Mn²⁺, Fe²⁺, Co²⁺, Ni²⁺, Cu²⁺, and Zn²⁺. The “fraction neutralized” is defined as the total positive charge due to the ions divided by the total negative charge of the nucleic acid. More than half of all crystal structures of nucleic acids do not resolve any ions, and in the structures that do report ions, the total charge of the crystallographically resolved ions, on average, accounts for only ~20% of the total nucleic acid charge.