Higher-order G-quadruplex structures and porphyrin ligands: Towards a non-ambiguous relationship

Herein, we evaluated the interaction of the tetracationic porphyrin H2TCPPSpm4 with three distinct DNA G-quadruplex (G4) models, i.e., the tetramolecular G4 d(TGGGGT)4 (Q1), the 5'-5' stacked G4-dimer [d(CGGAGGT)4]2 (Q2), and a mixture of 5'-5' stacked G-wires [d(5'-CGGT-3'-3'-GGC-5')4]n (Qn). The combined data obtained from UV-Vis, CD, fluorescence, PAGE, RLS, AFM, NMR, and HPLC-SEC experiments allowed us to shed light on the binding mode of H2TCPPSpm4 with the three G4 models differing for the type and the number of available G4 ending faces, the length of the G4 units, and the number of stacked G4 building blocks. Specifically, we found that H2TCPPSpm4 interacted with the shortest Q1 as an end-stacking ligand, whereas the groove binding mode was ascertained in the case of the Q2 and Qn G4 models. In the case of the interaction with Q1 and Qn, we found that H2TCPPSpm4 induces the formation of supramolecular aggregates at porphyrin/G4 ratios higher than 2:1, whereas no significant aggregation was observed for the interaction with Q2 up to the 5:1 ratio. These results unambiguously demonstrated the suitability of porphyrins for the development of specific G4 ligands or G4-targeting diagnostic probes, being H2TCPPSpm4 capable to distinguish between different G4s.

Temporal frequency dependence of the polarity inversion between upper and lower visual field in the pattern-onset steady-state visual evoked potential

PURPOSE

According to the cruciform model, the upper and lower halves of the visual field representation in the primary visual cortex are located mainly on the opposite sides of the calcarine sulcus. Such a shape would have consequences for the surface-recorded visual evoked potential (VEP), as V1 responses to stimulation of the upper and lower hemifield manifest with opposite polarity (i.e., polarity inversion). However, the steady-state VEP results from a complex superposition of response components from different cortical sources, which can obscure the inversion of polarity. The present study assesses the issue for different stimulation frequencies which result in different patterns of superposition in the steady-state response.

METHODS

Sequences of brief pattern-onset stimuli were presented at different stimulation rates ranging from 2 Hz (transient VEP) to 13 Hz (steady-state VEP). The upper and lower hemifields were tested separately and simultaneously. The data were assessed both in the time domain and in the frequency domain.

RESULTS

Comparing the responses to the stimulation of upper and lower hemifield, polarity inversion was present within a limited time interval following individual stimulus onsets. With increasing frequency, this resulted in an approximate inversion of the full steady-state response and consequently in a phase shift of approximately 180° in the time-domain response. Polarity inversion was more prominent at electrode Pz, also for transient responses. Our data also demonstrated that the sum of the hemifield responses is a good approximation of the full-field response.

CONCLUSION

While the basic phenomenon of polarity inversion occurs irrespective of the stimulus frequency, its relative impact on the steady-state response as a whole is the largest for high stimulation rates. We propose that this is because longer-lasting response components from other visual areas are not well represented in the steady-state VEP at higher frequencies.

A novel, rapidly preparable and easily maintainable biocathode electrochemical biosensor for the continuous and stable detection of nitrite in water

Nitrite is a highly toxic and pathogenic pollutant that is widely distributed in various nitrogenous wastewaters. Therefore, there is an urgent need for fast and stable nitrite detection to avoid water pollution and protect human health. In this study, we developed a novel rapidly preparable and easily maintainable biocathode electrochemical biosensor (BEB) using nitrite-reducing bacteria as the detectors to realize continuous nitrite monitoring in wastewater. The preparation of the biocathode was shortened by the polarity inversion method to less than 6 d. The BEB could detect nitrite solution samples in the range of 0.1- 16.0 mg NO₂^--N L^-1 within 1.7 min. The BEB was also successfully used to detect nitrite in real wastewater with a relative error < 4.0% and a relative standard deviation < 5.8%. In addition, the BEB could be easily maintained by an operation mode of microbial fuel cells and stably detected nitrite for at least 150 tests. Our study provided a feasible and convenient way to develop electrochemical biosensors based on the biocathode for continuous and stable monitoring of pollutants in wastewater.

Selectively recognizing extrahelical conformations of DNA trinucleotide repeats by a hydroxylated porphyrin ligand

Trinucleotide repeats (TRs) with abnormal lengths and atypical folding are implicated in various neurodegenerative diseases. The least stable cytosine-cytosine (C-C) mismatches in TRs when structuring into homoduplexes/hairpins have more chance in certain sequence contexts to preferentially adopt an extrahelical (E-motif) conformation with respect to those in polarity-inverted intrahelical counterparts. Herein, we designed a trihydroxyphenyl porphyrin ligand (POH3) to meet the challenge towards resolving the E-motif conformation. POH3 exhibited a specific 2:1 binding with DNAs adopting the E-motif cytosine conformation, independent of the TRs length. The trihydroxyl pattern was very crucial to gain the E-motif selectivity over the polarity-inverted counterparts via the complementary hydrogen bonding that occurred in the minor groove. Our work first elucidates the rationale in designing ligands to selectively resolve the E-motif nucleotides within TRs.

Utilisation of polarity inversion for phosphorus recovery in electrochemical precipitation with anaerobic digestion effluent

Phosphorus is a non-replaceable and limited resource, whose future drought is a matter of concern. Its recovery from wastewater has gained attention as a method of making the recovered phosphorus available for beneficial use. This study applied electrolysis with a platinum-coated titanium electrode to an actual anaerobic digestion effluent and investigated the phosphorus removal and precipitation characteristics with various current values. The separation of the phosphorus precipitate from the sludge residue and the usefulness of the polarity inversion to recover the phosphorus precipitate were evaluated. The availability of the recovered precipitate as a fertiliser was also demonstrated. The anaerobic digestion effluent after the centrifugal dehydration process with a coagulant was collected from a wastewater treatment plant. Phosphorus was precipitated as a form of calcium phosphate, including hydroxyapatite, which was confirmed by X-ray diffraction. The generated gases from the electrodes during the electrolysis floated and condensed the sludge residue, leading to successful separation of the phosphorus precipitate from the sludge residue. The polarity inversion successfully detached the phosphorus precipitate from the electrode, and at least 3 min polarity inversion was enough to detach the whole phosphorus precipitate. The submerging phosphorus precipitate contained abundant phosphorus (12.6%), in which citric-acid-soluble phosphorus accounted for 94.4%, and the utilisation of the precipitate as a slow-release phosphorus fertiliser was suggested.

Polarity inversion sensitized G-quadruplex metal sensors with K+ tolerance

Due to the high abundance of K⁺ in environments and K⁺-induced high stability of G-quadruplex (G4), developing a selective G4-based fluorescent sensor for other metal ions with K⁺ tolerance is a great challenge. Herein, we found that even in the presence of 15000-fold excess of K⁺, Ba²⁺ exhibits a highly specific binding with a human telomeric G4 (htG4) in comparison with other G4-binding metal ions such as Pb²⁺ and Sr²⁺. This specific binding event can be recognized by a natural fluorophore of hypericin with a lighting-up fluorescence response. Interestingly, inverting the polarity of the most 3' G in htG4 can sensitize the Ba²⁺ response with the retaining Ba²⁺ specificity and K⁺ tolerance. This polarity inversion of htG4 causes a G4 conformation change in K⁺ and the polarity-inverted htG4 tends to favorably dimerize in response to the Ba²⁺ specific binding. To our knowledge, this is the first report that polarity inversion of G4 can be applied to construct a selective metal sensor with K⁺ tolerance. Our findings will open a new way to conveniently regulate the G4 conformation and stability by polarity inversion towards developing high-performance sensors.

Improving biocathode community multifunctionality by polarity inversion for simultaneous bioelectroreduction processes in domestic wastewater

Bioelectrochemical systems (BESs) have been tentatively applied for wastewater treatment processes, but the complex composition of wastewater could lead to difficulties in establishing functional biofilm or result in performance instability. Few studies have investigated the enrichment of biocathode with domestic wastewater (DW) and the function. A biocathode with multi-pollutant removal capabilities was enriched based on polarity inverted bioanode, which was established with DW. The biocathode function was examined using model pollutants (nitrate, nitrobenzene and Acid Orange 7) supplemented as sole or mixed electron acceptors. When compared to the anaerobic control treatment, the biofilm demonstrated significantly enhanced reduction abilities in the open circuit. For the closed circuit, their removal efficiencies were further enhanced for both the sole and mixed substrates conditions. The bioanodes community structure and diversity markedly changed after operating for 50 d as biocathodes. The biocathode multifunctionality and stability could be related to the maintenance of organic matters fermentative bacteria (mainly belonging to Bacteroidetes, Firmicutes and Synergistetes) and the enrichment of versatile pollutant-reducing bacteria (e.g. Pseudomonas, Thauera and Comamonas from Proteobacteria). Other pollutants, such as perchlorate, sulfate, heavy metals, and halogenated organics, may also work as potential electron acceptors. This study provides a new strategy to improve the biocathode community multifunctionality for simultaneous bioelectroreduction, which can be combined with other wastewater treatment processes in actual application.

Polarity inversion of bioanode for biocathodic reduction of aromatic pollutants

The enrichment of specific pollutant-reducing consortium is usually required prior to the startup of biocathode bioelectrochemical system (BES) and the whole process is time consuming. To rapidly establish a non-specific functional biocathode, direct polar inversion from bioanode to biocathode is proposed in this study. Based on the diverse reductases and electron transfer related proteins of anode-respiring bacteria (ARB), the acclimated electrochemically active biofilm (EAB) may catalyze reduction of different aromatic pollutants. Within approximately 12 d, the acclimated bioanodes were directly employed as biocathodes for nitroaromatic nitrobenzene (NB) and azo dye acid orange 7 (AO7) reduction. Our results indicated that the established biocathode significantly accelerated the reduction of NB to aniline (AN) and AO7 to discolored products compared with the abiotic cathode and open circuit controls. Several microbes possessing capabilities of nitroaromatic/azo dye reduction and bidirectional electron transfer were maintained or enriched in the biocathode communities. Cyclic voltammetry highlighted the decreased over-potentials and enhanced electron transfer of biocathode as well as demonstrated the ARB Geobacter containing cytochrome c involved in the backward electron transfer from electrode to NB. This study offers new insights into the rapid establishment and modularization of functional biocathodes for the potential treatment of complicated electron acceptors-coexisting wastewaters.

Response of anodic bacterial community to the polarity inversion for chloramphenicol reduction

Chloramphenicol (CAP) is a frequently detected environmental pollutant. In this study, an electroactive biofilm for CAP reduction was established by initially in the anode and then inverting to the cathode. The established biocathode could enhance the reduction of CAP to the nitro-group reduced CAP (AMCl2) and further dechlorinated form (AMCl), both had lost the antibacterial activity. The phylogenetic diversity of the acclimated biofilm was decreased after the polar inversion. Proportions of functional bacterial genera, including Geobacter, Desulfovibrio and Pseudomonas responsible for the bidirectional electron transfer and nitroaromatics reduction, had increased 28%, 104% and 43% in the cathode. The relatively high abundances (over 50%) of Geobacter in anode and cathode were rarely detected for the nitroaromatics reduction. This study provides new insights into the electroactive biofilm structure improvement by the polarity inversion strategy for refractory antibiotics degradation.