Determinants of DNA quadruplex structural type: sequence and potassium binding.

There are DNA sequences which adopt the same quadruplex structural type in the presence of sodium as in the presence of sodium and potassium. There are also sequences that appear to have a requirement for the presence of potassium for the adoption of a particular quadruplex structural type. Information about the basis for these potassium effects has been obtained by examining the structures of a set of DNAs with differing numbers of loop residues and different lengths of runs of dG residues in the presence of sodium alone and in the presence of potassium and sodium. On the basis of the results, obtained primarily via solution-state NMR, it appears that very small loops favor parallel stranded quartet structures which do not require the presence of potassium. DNAs with loops of two to four residues and runs of two dG residues can form quadruplex structures of the "edge" or "chair" type in the presence of potassium but not in the presence of sodium alone. When all of the loops contain four residues, a "crossover" or "basket" type structure can be formed in the presence of sodium as well as in the presence of sodium and potassium. Structures with runs of three or four dG residues and with loops from two to four residues can form basket or crossover type structures in the absence of potassium. The presence of a purine in a loop can block both potassium binding and formation of chair type structures. Modeling of the interactions of cations with these quadruplex structures indicates that the potassium ions required for chair type structures interact with a terminal quartet and residues in the adjacent loop.