An Asymmetric Cationic Porphyrin As A New Good G-quadruplex Probe With Cancer-targeted Imaging Ability

G-quadruplex is a non-conventional DNA secondary structure that can be formed in many important regions of the human genome(e.g.telomere and promoters of many oncogenes).It has attracted attention in many fields including anticancer drug research,biosensor design and targeted delivery of drugs.Although some of them are reported to show excellent G-quadruplex recognition specificity under neutral and near neutral pH conditions,highly specific G-quadruplex recognition ligands that can work well under acidic pH conditions are rarely reported.Porphyrins are promising candidates for nucleic acid G-quadruplex-specific optical recognition.We previously demonstrated that G-quadruplex recognition specificity of porphyrins could be improved by introducing bulky side arm substituents,but enhanced protonation tendency limits their applications in some cases,such as under acidic conditions.Here,we demonstrated that the protonation tendency of porphyrin derivatives could be efficiently overcome by increasing molecular asymmetry.To validate this,an asymmetric,water-soluble,cationic porphyrin FA-TMPipEOPP(5-{4-[2-[[(2E)-3-[3-methoxy-4-[2-(1-methyl-1-piperidinyl)ethoxy]phenyl]-1-oxo-2-propenyl]oxy]ethoxy]phenyl},10,15,20-Tri{4-[2-(1-methyl-1-piperidinyl)ethoxy]-phenyl}porphyrin)was synthesized by introducing a ferulic acid(FA)unit at one side arm,and its structure was well characterized.Unlike its symmetric counterpart TMPipEOPP that has a tendency to protonation under acidic conditions,FA-TMPipEOPP remained in unprotonation monomeric form under the tested pH range of 2.0 – 8.0.Correspondingly,FA-TMPipEOPP showed better G-quadruplex recognition specificity than TMPipEOPP showed,and thus might be used as a specific optical probe for colorimetric and fluorescent recognition of G-quadruplexes under acidic conditions.The feasibility was demonstrated by two proof-of-concept studies:(1)probing structural competition between G-quadruplex and duplex;(2)label-free and wash-free cancer cell-targeted bioimaging under an acidic tumor microenvironment.As G-quadruplex optical probes,FA-TMPipEOPP works well under acidic conditions,while TMPipEOPP works well under neutral conditions.This finding provides useful information for G-quadruplex probe research.That is,porphyrin-based G-quadruplexprobes suitable for different pH conditions might be obtained by adjusting the molecular symmetry.