Telomeric i-motifs and C-strands inhibit parallel G-quadruplex extension by telomerase.

Telomeric C-rich repeated DNA sequences fold into tetrahelical i-motif structures in vitro at acidic pH. While studies have suggested that i-motifs may form in cells, little is known about their potential role in human telomere biology. In this study, we explore the effect of telomeric C-strands and i-motifs on the ability of human telomerase to extend G-rich substrates. To promote i-motif formation at neutral pH, we use telomeric sequences where the cytidines have been substituted with 2'-fluoroarabinocytidine. Using FRET-based studies, we show that the stabilized i-motifs resist hybridization to concomitant parallel G-quadruplexes, implying that both structures could exist simultaneously at telomeric termini. Moreover, through telomerase activity assays, we show that both unstructured telomeric C-strands and telomeric i-motifs can inhibit the activity and processivity of telomerase extension of parallel G-quadruplexes and linear telomeric DNA. The data suggest at least three modes of inhibition by C-strands and i-motifs: direct hybridization to the substrate DNA, hybridization to nascent product DNA resulting in early telomerase dissociation, and interference with the unique mechanism of telomerase unwinding and extension of a G-quadruplex. Overall, this study highlights a potential inhibitory role for the telomeric C-strand in telomere maintenance.

Stabilization of i-motif structures by 2'-ß-fluorination of DNA.

i-Motifs are four-stranded DNA structures consisting of two parallel DNA duplexes held together by hemi-protonated and intercalated cytosine base pairs (C:CH(+)). They have attracted considerable research interest for their potential role in gene regulation and their use as pH responsive switches and building blocks in macromolecular assemblies. At neutral and basic pH values, the cytosine bases deprotonate and the structure unfolds into single strands. To avoid this limitation and expand the range of environmental conditions supporting i-motif folding, we replaced the sugar in DNA by 2-deoxy-2-fluoroarabinose. We demonstrate that such a modification significantly stabilizes i-motif formation over a wide pH range, including pH 7. Nuclear magnetic resonance experiments reveal that 2-deoxy-2-fluoroarabinose adopts a C2'-endo conformation, instead of the C3'-endo conformation usually found in unmodified i-motifs. Nevertheless, this substitution does not alter the overall i-motif structure. This conformational change, together with the changes in charge distribution in the sugar caused by the electronegative fluorine atoms, leads to a number of favorable sequential and inter-strand electrostatic interactions. The availability of folded i-motifs at neutral pH will aid investigations into the biological function of i-motifs in vitro, and will expand i-motif applications in nanotechnology.

Antibiotic removal from water: elimination of amoxicillin and ampicillin by microscale and nanoscale iron particles.

Zerovalent iron powder (ZVI or Fe(0)) and nanoparticulate ZVI (nZVI or nFe(0)) are proposed as cost-effective materials for the removal of aqueous antibiotics. Results showed complete removal of Amoxicillin (AMX) and Ampicillin (AMP) upon contact with Fe(0) and nFe(0). Antibiotics removal was attributed to three different mechanisms: (i) a rapid rupture of the beta-lactam ring (reduction), (ii) an adsorption of AMX and AMP onto iron corrosion products and (iii) sequestration of AMX and AMP in the matrix of precipitating iron hydroxides (co-precipitation with iron corrosion products). Kinetic studies demonstrated that AMP and AMX (20 mg L(-1)) undergo first-order decay with half-lives of about 60.3+/-3.1 and 43.5+/-2.1 min respectively after contact with ZVI under oxic conditions. In contrast, reactions under anoxic conditions demonstrated better degradation with t(1/2) of about 11.5+/-0.6 and 11.2+/-0.6 min for AMP and AMX respectively. NaCl additions accelerated Fe(0) consumption, shortening the service life of Fe(0) treatment systems.