Guanine rich sequences present in the promoter region of oncogenes could fold into G-quadruplexes and modulate transcription. Equilibrium between folding and unfolding of the quadruplexes in these regions play important role in disease processes. We have studied the effect of a putative anticancer agent chelerythrine on G-rich NHE III1 present in the promoter region of c-myc oncogene. We have demonstrated the ability of chelerythrine, a telomerase inhibitor, to block the hybridization of Pu27 with its complementary strand via folding it into a quadruplex structure. Calorimetry shows that the association of Pu27 with chelerythrine is primarily enthalpy driven with high binding affinity (similar to 10(5) M-1). The association does not lead to any major structural perturbation of Pu27. The resulting 2:1 complex has enhanced stability as compared to free Pu27. Another notable feature is that the presence of molecular crowding agent like ficoll 70 does not change the mode of recognition though the binding affinity decreases. We suggest that the anticancer activity of chelerythrine could be ascribed to its ability to stabilize the quadruplex structure in the c-myc promoter region thereby downregulating its transcription. (C) 2015 Elsevier Inc. All rights reserved.