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A comparative study on human telomeric DNA G-quadruplex binding of meso-5,10,15,20-tetrakis(N-methyl-4-pyridyl)porphyrin(TMPyP4)between its two salt forms,i.e.,tetratosylate and tetrachloride,was conducted by using ESI-TOF-MS 1-3,UV-melting measurement 4 and molecular modeling methods.Besides cation TMPyP4,the tosyl anion was found to bind to human telomeric DNA G-quadruplex with multiple binding stoichiometries from 1:1 to 3:1 observed in ESI-TOF-MS spectra,indicating that the stabilization activity of TMPyP4 tetratosylate on G-quadruplex is derived from a synergetic effect of both TMPyP4 cation and tosyl anion.A molecular modeling study suggests that a tosyl anion fills up the vacant space between TMPyP4 cation and DNA G-quadruplex and thus stabilizes the complex by 3.8 kcal/mol(Fig.1).Therefore,it is estimated that TMPyP4 tetratosylates activity might not reflect the real effect of TMPyP4 cation in some bioassays related to G-quadruplex stabilization.This was verified by the results of less binding affinity of TMPyP4 tetrachloride with DNA G-quadruplex obtained from ESI-TOF-MS measurement,and of 2.27℃ less thermal stabilization of TMPyP4 tetrachloride for DNA G-quadruplex,compared to its tetratosylate under the same conditions.Our study demonstrated the influence of counter ions of TMPyP4 on G-quadruplex binding,which shed light on the proper usage of TMPyP4 salt in the chemical and biological research associated with G-quadruplex binding.This work was financially supported by Macao Science and Technology Development Fund,MSAR(039/2011/A2 to ZHJ,and 056/2013/A2 to LPB).