Human telomere d[(TTAGGG)4] undergoes a conformational transition to the Na+-form upon binding with sanguinarine in presence of K+

► Sanguinarine (SGR) - human telomere (H24) interaction was studied by CD, NMR and time-resolved fluorescence. ► SGR exhibits two distinct interactions with H24 in presence of potassium. ► Up to about 1:2 M ratio of H24:SGR (10 μM H24), two molecules of SGR bind H24. ► Beyond 1:2 M ratio, SGR induces a conformational transition in H24 from the physiologically relevant K+-form to the non-canonical Na+-form.