Tuning the conformation of G-quadruplexes by sodium and potassium ions: application to photometric and fluorometric determination of amyloid β(1-40).

A dual channel method is described for the determination of the amyloid-β peptide Aβ(1-40) that is associated with Alzheimer's disease. The method exploits (a) conformational changes of a G-quadruplex that are triggered by Na+ and K+ ions and (b) the strong affinity between Aβ(1-40) and Cu2+. A G-quadruplex DNA forms an antiparallel structure in the presence of Na+ and can catalyze the oxidation of tetramethylbenzidine by H2O2 system in the presence of Cu2+ to form a visible blue color. If, however, Cu2+ binds to Aβ(1-40), the blue color is no longer formed. Measuring the absorption decrease at 452 nm, the determination of Aβ(1-40) is realized. If K+ is added to the Na+-containing buffer, the antiparallel G-quadruplex DNA is transformed to parallel. This leads to the insertion of protoporphyrin IX (PPIX) into the G-quadruplex and generates enhanced fluorescent signal, with excitation/emission wavelength at 410/630 nm. The G-quadruplex then catalyzes the metalation of PPIX by Cu2+, and the fluorescence intensity decreases. In the presence of Aβ(1-40), the formation of Aβ(1-40)-Cu2+ triggers the recovery of the fluorescence. The Na+/K+-induced tuning of the conformation of the G-quadruplex with the same sequence enables dual (colorimetric and fluorometric) determination of Aβ(1-40), with detection limits of 4.9 pM and 2.3 pM, respectively. The cost is quite low since the developed strategy is label free and enzyme free by using low-cost DNA and Cu2+. More importantly, the dual channel determination operation is very simple without any further modification process. Tuning the conformation of G-quadruplexes by sodium(I) and potassium(I): application to photometric and fluorometric determination of amyloid β(1-40).