Guanine-rich telomeric sequences fold into G-quadruplex conformation and are known to bind a variety of ligands including potential drug candidates. By means of CD spectroscopy and fluorescence lifetime measurements we demonstrate that putative anticancer therapeutic sanguinarine (SGR) exhibits two distinct interactions with human telomere d[(TTAGGG)(4)] (H24) in presence of K+. Up to about 1:2 M ratio of H24:SGR (10 mu M H24), two molecules of SGR bind H24. Above this molar ratio, SGR induces a conformational transition in H24 from the K+-form to the Na+-form. The demonstration of SGR-induced conformational transition in a G-quadruplex formed by a human telomeric sequence could provide new insights into interaction of drugs with quadruplex DNA structure. (C) 2010 Elsevier Inc. All rights reserved.