pH-Mediated Fluorescence and G-Quadruplex Binding of Amido Phthalocyanines

A new family of G-quadruplex ligands termed "amido phthalocyanines" (APcs) was synthesized by reacting variable amino acids with tetraamino zinc phthalocyanine. Variation in the number of methylene units separating the A Pc scaffold from terminal ammonium groups systematically modulated ammonium pK(a) values that, in turn, mediated APc aggregation and DNA binding. Certain APcs exhibited nearly 1000-fold enhancements in fluorescence quantum yield upon binding G-quadruplex DNA under physiological conditions of pH and ionic strength. G-quadruplexes derived from the c-myc and c-kit promoters and the human telomeric repeat were evaluated for A Pc affinity and specificity using two complementary and direct fluorescence binding assays that revealed apparent dissociation constants ranging from 20 to 200 nM. Approximately 500-fold lower affinities for duplex and single-stranded DNAs were observed. Interestingly. APc-quadruplex binding was relatively insensitive to ionic strength (0.03-1 M KCl) but highly dependent on the pH of the solution. Our results provide a mechanism for the "turn-on" fluorescence properties exhibited by these compounds that will assist in future rational design of new G-quadruplex-specific fluorescent probes and drug. candidates.