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Normal and less polarised water models are used as the solvent to investigate Hofmeister effects and alkali metal ionic effects on dodecamer d(CGCGAATTCGCG) B-DNA with atomic dynamics simulations. As normal water solvent is replaced by less polarised water, the Hofmeister series of alkali metal ions is changed from Li+> Na+≃ K+≃ Cs+≃ Rb+to Li+> Na+> K+> Rb+> Cs+. In less polarised water, DNA experiences the B→A conformational transition for the lighter alkali metal counterions (Li+, Na+and K+). However, it keeps B form for the heavier ions (Rb+and Cs+). We find that the underlying cause of the conformation transition for these alkali metal ions except K+is the competition between water molecules and counterions coupling to the free oxygen atoms of the phosphate groups. For K+ions, the ‘economics’ of phosphate hydration and ‘spine of hydration’ are both concerned with the DNA helixes changing. [ABSTRACT FROM PUBLISHER]