1 - Article ; The influence of monovalent cations on DNA conformation and readout is an open question. This NMR study of DNA with either Na+ or K+ at physiological concentrations shows that the nature of the cation affects the 31P chemical shifts ([delta]P) and the sequential distances H2'i-H6/8i+1, H2[modifier letter double prime]i-H6/8i+1, and H6/8i-H6/8i+1. The [delta]P and distance variations ascertain that the nature of the cation affects the DNA overall structure, i.e. both the conformational equilibria between the backbone BI ([epsilon]-[zeta] 0°) states and the helical parameters, via their strong mechanical coupling. These results reveal that Na+ and K+ interactions with DNA are different and sequence-dependent. These ions modulate the overall intrinsic properties of DNA, and possibly its packaging and readout.