A Pb²⁺-specific DNA G-quadruplex mimic of red fluorescent protein with a monovalent cation ion tolerance

Nucleic acid mimics of fluorescent proteins (FPs) currently find wide applications in pursuing high-performance sensors and advanced biomedical tools. However, compared to the mimics of green FPs (GFP), the advancement of mimics of red FPs (RFP) remains limited despite their distinct advantages. Herein, we tried to develop a DNA mimic of RFPs based on an originally screened Bibb Lettuce (BL) and a chromophore originally developed for an RNA mimic of RFPs. Pb²⁺ causes BL to fold into a G-quadruplex (G4) structure that can accommodate the phenolate form of DMHBO⁺. We found that activation of the red fluorescence of BL/DMHBO⁺ is Pb²⁺ specific. Furthermore, this Pb²⁺-driven G4 formation confers the BL/DMHBO⁺/Pb²⁺ ensemble with a higher tolerance to monovalent cations compared to the previously developed BL/DFHBI-1T/Mg²⁺ ensemble. Notably, the BL/DMHBO⁺/Pb²⁺ ensemble also exhibits improved photostability. Consequently, this ensemble is promising to establish a selective and sensitive method for Pb²⁺ detection. By engineering BL with the splitting strategy, the BL/DMHBO⁺/Pb²⁺ ensemble can detect the pathogenic SARS-COVID19 DNA with a single-base mismatch resolution. In addition, the BL/DMHBO⁺/Pb²⁺ ensemble can also find potential applications in analyzing metabolic small molecules, monitoring enzyme activity, and screening enzymes inhibitors. Our work provides a new avenue in developing functional nucleic acid mimics of FPs by crosstalking the previously screened RNAs/DNAs and chromophores.