Mutually Exclusive Formation of G‑Quadruplex and i‑Motif Is a General Phenomenon Governed by Steric Hindrance in Duplex DNA

G-Quadruplex and i-motif are tetraplex structures that may form in opposite strands at the same location of a duplex DNA. Recent discoveries have indicated that the two tetraplex structures can have conflicting biological activities, which poses a challenge for cells to coordinate. Here, by performing innovative population analysis on mechanical unfolding profiles of tetraplex structures in double-stranded DNA, we found that formations of G-quadruplex and i-motif in the two complementary strands are mutually exclusive in a variety of DNA templates, which include human telomere and promoter fragments of hINS and hTERT genes. To explain this behavior, we placed G-quadruplex- and i-motif-hosting sequences in an offset fashion in the two complementary telomeric DNA strands. We found simultaneous formation of the G-quadruplex and i-motif in opposite strands, suggesting that mutual exclusivity between the two tetraplexes is controlled by steric hindrance. This conclusion was corroborated in the BCL-2 promoter sequence, in which simultaneous formation of two tetraplexes was observed due to possible offset arrangements between G-quadruplex and i-motif in opposite strands. The mutual exclusivity revealed here sets a molecular basis for cells to efficiently coordinate opposite biological activities of G-quadruplex and i-motif at the same dsDNA location

Polyaniline Coated Ethyl Cellulose with Improved Hexavalent Chromium Removal

The ethyl celluloses (ECs) modified with 5.0, 10.0, and 20.0 wt % polyaniline (PANI) (PANI/ECs) prepared by homogeneously mixing the EC and PANI formic acid solutions have demonstrated a superior hexavalent chromium (Cr­(VI)) removal performance to that of pure EC. Having an increased Cr­(VI) removal percentage with increased PANI loading, the PANI/ECs with 20.0% PANI loading were noticed to remove 2.0 mg/L Cr­(VI) completely within 5 min, much faster than the pristine EC (>1 h). A chemical redox of Cr­(VI) to Cr­(III) by the active functional groups of PANI/ECs was revealed from the kinetic study. Meanwhile, isothermal study demonstrated a monolayer adsorption behavior following the Langmuir model with a calculated maximum absorption capacity of 19.49, 26.11, and 38.76 mg/g for the 5.0, 10.0, and 20.0 wt % PANI/ECs, much higher than that of EC (12.2 mg/g). The Cr­(VI) removal mechanisms were interpreted considering the functional groups of both PANI and EC, the valence state fates of Cr­(VI), and the variation of solution acidity