FIGURE 6.

Ion coordination and packing interactions. (A) Coordination of cobalt hexamine ions (cyan and blue for Co3+ and N, respectively) and a chloride anion (large sphere in magenta) in the major groove of the A-ANA duplex. Selected residues are labeled, water molecules are red spheres and hydrogen bonds are dashed lines. (B) Example of a packing interaction in the A-ANA and A-FANA structures, involving terminal base pairs of a duplex (green bonds) and the sugar-phosphate backbone of a neighboring one (blue bonds). In the example shown here, the sugar moieties of residues T4 and 2′OMeA5 from an A-FANA1 decamer form a pseudo-base pair that stacks against the terminal C:G base pair from an A-FANA2 duplex.