Na⁺-Induced Conformational Change of Pb²⁺-Stabilized G-Quadruplex and Its Influence on Pb²⁺ Detection

Here, we first find that Na⁺ can induce Pb²⁺-stabilized T30695 undergoing conformational transition from partly parallel to completely parallel, and further forming a dimeric G-quadruplex, which was characterized by circular dichroism (CD) spectroscopy, matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF-MS), and native polyacrylamide gel electrophoresis (PAGE). Thermal denaturation experiments show that the transforming process is a thermodynamics-driven process. Furthermore, the presence of Na⁺ further improves the binding efficiency of Pb²⁺-stabilized T30695 with the fluorescent probe (such as ZnPPIX). Based on the fact, with a partially hybridized double-stranded DNA (ds-DNA) containing T30695 as a sensing probe and ZnPPIX as a fluorescence probe, the effect of Na⁺ on Pb²⁺ detection is subsequently investigated. The presence of Na⁺ (varied from 0.3 mM to 500 mM) simultaneously increases the read-out and background fluorescence, which results in a decreased signal-to-noise ratio and further leads to a decreased sensing performance (detection limits is increased to 120 nM). In order to avoid Na⁺ interference, a fully matched ds-DNA containing T30695 is utilized as a sensing probe to fix the background fluorescence, regardless of whether Na⁺ is present or not. Thus, a relatively lower detection limit (10 nM) in all Na⁺-containing real samples is achieved, respectively. Therefore, the paper provides a novel insight into the conformational changes in G-quadruplex and presents an efficient step to resolve the challenging problem about Pb²⁺ detection in Na⁺-containing real samples.