Abstract
Guanine-rich quadruplex (G-QD) are formed by conversion of nucleotides with specific sequences by stabilization of positively charged K+ or Na+. These G-QD structures differentially absorb two-directional (right- and left-handed) circularly polarized light, which can discriminate the parallel or anti-parallel structures of G-QDs. In this study, G-QDs stabilized by Pb2+ were analyzed by a circular dichroism (CD) spectroscopy to determine Pb2+ concentration in water samples. Thrombin aptamer (TBA), PS2.M, human telomeric DNA (HTG), AGRO 100, and telomeric related sequence (T2) were studied to verify their applicability as probes for platform- and label-free detection of Pb2+ in environmental as well as laboratory samples. Among these nucleotides, TBA and PS2.M exhibited higher binding constants for Pb2+, 1.20–2.04 × 106/M at and 4.58 × 104–1.09 × 105/M at 100 micromolar and 100 mM K+ concentration, respectively. They also exhibited excellent selectivity for Pb2+ than for Al3+, Cu2+, Ni2+, Fe3+, Co2+, and Cr2+. When Pb2+ was spiked into an effluent sample from a wastewater treatment plant (WWTP), its existence was detected by CD spectroscopy following a simple addition of TBA or PS2.M. By the addition of TBA and PS2.M, the Pb2+ signals were observed in effluent samples over 0.5 micromolar (100 ppb) concentration. Furthermore, PS2.M caused a Pb2+-specific absorption band in the effluent sample without spiking of Pb2+, and could be induced to G-QD structure by the background Pb2+ concentration in the effluent, 0.159 micromolar concentration (3.30 ppb). Taken together, we propose that TBA and PS2.M are applicable as platform- and label-free detection probes for monitoring Pb2+ in environmental samples such as discharged effluent from local WWTPs, using CD spectroscopy.
Highlights
Among toxic metals, lead is a well-known anthropogenic contaminant, which is discharged into the environment during the disposal of lead-containing consumer products, metal manufacturing processes, fossil fuel combustion, and sewage treatment and disposal p rocesses[1,2,3,4]
For the detection of Pb2+, we considered the use of self-reacting guanine-rich quadruplexes (G-QDs) as a simple probe that does not require any sensing platform and labeling reagents
This result indicated that the switched form of AGRO 100 induced by K+ ions had the same peak wavelength as the denatured AGRO 100, and the circular dichroism (CD) signal intensity was enhanced by complexation with K+ at 100 mM concentration
Summary
Lead is a well-known anthropogenic contaminant, which is discharged into the environment during the disposal of lead-containing consumer products, metal manufacturing processes, fossil fuel combustion, and sewage treatment and disposal p rocesses[1,2,3,4]. For their biological relevance, including their participation in the protection of chromosomes by telomeres and in the control of gene e xpression[13] For these biological processes of G-QDs, the unfolded DNA with guaninerich sequences must be stabilized by the binding of cations (typically K + or Na+) and switched to quadruplex secondary structures (Fig. 1A,B)[14]. Various nucleotides that can be switched to G-QDs by K + stabilization have been reported, such as thrombin aptamer (TBA), PS2.M, human telomeric DNA (HTG), AGRO 100, and telomeric related sequences (T2)[17,18,19,20,21]. Pb2+ and hemin were co-stabilized in the AGRO 100 quadruplex structure, and P b2+ could be detected by fluorescent, colorimetric, and electrochemical methods because of the optical and electrical properties of h emin[26]. The TAQ [d(TAG3 T)3TAG3] sequence was reported as a B a2+ sensor with excellent tolerance to highly concentrated K+ by addition of a fluorophore, hypericin[29]
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