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DNA/RNA mimics of fluorescent proteins (DMFPs and RMFPs) have been exogenously screened for inspiring variant applications by specifically binding to chromophore analogues to activate their fluorescence. Considerable DMFPs and RMFPs form G-quadruplex (G4) or G4-like structures to accommodate FP chromophore analogues. Therefore, efforts ought to be made to investigate whether these FP chromophore analogues have distinctive interactions with endogenous G4 structures found in natural sequences, such as human telomeric G4s (htG4s). Herein, we found that htG4s can specifically interact with one of the hydroxyethylamino-benzylidene cyanophenyl chromophore derivatives (HBC514) to serve as DNA mimics of green FPs (DMGFPs). Importantly, Na + is required to remodel the K + -favored hybrid htG4s into antiparallel htG4s-based DMGFPs with the synergic binding of HBC514. Furthermore, HBC514 most likely aggregates on DMGFPs into an AIEgen with suppressed intramolecular rotation to cause a 2 orders of magnitude enhancement in its fluorescence. Based on the Na + -specific DMGFPs, a selective Na + sensor was developed with the potential to tolerate high K + abundance using HBCs as the bifunctional G4-inducer and fluorescent reporter. The DMGFPs conquer the long-standing challenge of developing practical Na + sensors that can be used in physiological environments (for example, serums) with a K + -instigated high G4 stability.

Abnormal amplification of trinucleotide repeats (TNRs) is associated with neurodegenerative diseases by forming a particular hairpin bulge. It is well known that the polarity and parity of TNRs can regulate the formed hairpin structures. Therefore, there is a great challenge to efficiently discriminate the hairpin structures of TNRs with substantial selectivity. Herein, we developed a fluorescent ligand of pseudohypericin (Pse) with a beyond-size-matching (BSM) geometry to selectively sense hairpin structures of GTC and CTG TNRs. The GTC hairpin structures can bind with Pse dominantly at extreme T-T mismatches by the virtue of their most extrahelical conformations, while there is no binding event to occur with the polarity-inverted counterpart CTG hairpin structures because of the limited space provided by their intrahelical T-T mismatches. In addition, this all-or-none response with the polarity-dependent folding (PoDF) is independent of the length of these TNRs. Interestingly, the parity-dependent folding (PaDF) of GTC hairpin structures can also be resolved. Besides pure TNRs, the competency of this BSM ligand to sense the PoDF and PaDF effects was also generalized to DNAs with TNRs occurring at loop and stem end regions. To our knowledge, this is the first experimental observation with the state-of-the-art performance over the fluorescence measurement of PoDF and PaDF in TNRs. Our work provides an expedient way to elucidate the TNR folding by designing ligands having BSM features.

The i-motif structure (iM) has attracted much attention, because of its in vivo bioactivity and wide in vitro applications such as DNA-based switches. Herein, the length-dependent folding of cytosine-rich repeats of the human telomeric 5'-(CCCTAA) n-1 CCC-3' (iM- n , where n = 2-8) was fully explored. We found that iM-4, iM-5, and iM-8 mainly form the intramolecular monomer iM structures, while a tetramolecular structure populates only for iM-3. However, iM-6 and iM-7 have the potential to fold as well into the dimeric iM structures besides the monomer ones. The natural hypericin (Hyp) was used as the polymorphism-selective probe to recognize the iM structures. Interestingly, only iM-3, iM-6, and iM-7 can efficiently switch on the Hyp fluorescence by specifically binding with the outmost C-C + base pairs that are exposed directly to solution. However, other iM structures that fold in a way with a coverage of the outmost C-C + pairs by loop sequences are totally unavailable for the Hyp binding. Theoretical modeling indicates that adaptive π-π and cation-π interactions contribute to the Hyp recognition toward the exposed C-C + pairs. This specific iM recognition can be boosted by a photocatalytic DNAzyme construct. Our work provides a reliable fluorescence method to selectively explore the polymorphism of iM structures.

Activated carbon, an amorphous carbon material with a high specific surface area and void fraction, is widely used as an economical adsorbent in many fields. In this work, a kind of new activated carbon composite for saccharide column separation was prepared by zinc chloride impregnating and microwave heating. The structural characterizations validate the increase in porosity and the specific surface area of the activated carbon as well as the change of the activated carbon crystallite lattice. The chemical characterizations validate the increase in the number of oxygen-containing functional groups and structural bonding of zinc with the activated carbon surface. Compared with the blank control, the surface Zn element improves the adsorption selectivity of the activated carbon to the target saccharides. Under the special mechanism of microwaves, the pores created by expansion from the inside to the outside facilitate the free flow of the mobile phase. The eight saccharides can be separated by the columns packed with the activated carbon impregnated with 40% and 70% zinc chloride., Competing Interests: The authors declare no competing financial interest., (Copyright © 2020 American Chemical Society.)

Selective nucleotide recognition for biosensor evolution requires rational probe design toward the binding-pattern-susceptible readout but without serious poison in selectivity from the context sequences. In this work, we synthesized a dual-function (trihydroxyphenyl)porphyrin (POH 3 ) to target the abasic site (AP site) in ds-DNA using the trihydroxyphenyl substituent and the tetrapyrrole macrocycle as the recognition unit (RU) and the fluorescent signal unit (SU), respectively. RU and SU are separated from each other but are prototropically allosteric. We found that an appropriate pH favors formation of the nonfluorescent quinine/pyrrole (O-NH) conformer of POH 3 . However, the complementary hydrogen bonding of RU in O-NH with the target cytosine opposite the AP site switches on the SU fluorescence through prototropic allostery toward the phenol/isopyrrole (OH-N) conformer, while the bases thymine, guanine, and adenine totally silence this allostery, suggesting a superb selectivity in single-nucleotide polymorphism (SNP) analysis. The role of the prototropic allostery in achieving such SNP selectivity is also evidenced using porphyrins with other hydroxyl substituent patterns. Because of the SU separation from RU, SU is not directly involved in the interaction with the AP site, and thus, the turn-on selectivity is also realized for DNA with flanking guanine, the most easily oxidized base in DNA. This tolerance to the flanking base identity has seldom been achieved in previous studies. Additionally, other DNA structures cannot bring this allostery, indicating that the combination recipe of the AP site design and the prototropically allosteric probe will find wide applications in DNA-based sensors.

The triplex DNA has received much interest due to its various applications in gene regulation, molecular switch, and sensor development. However, realizing a highly selective recognition using a fluorescence probe specific only for the triplex topology is still a great challenge. Herein, we found that relative to the structural analogues of natural robinetin, myricetin, quercetin, kaempferol, morin, rutin, baicalin, luteolin, naringenin, genistein, chrysin, galangin, isorhamnetin, and several synthetic flavonoids, fisetin (FIS) is the brightest emitter when targeting the triplex DNA in contrast to binding with ss-DNA, ds-DNA (with or without an abasic site), i-motif, and DNA/RNA G-quadruplexes. Only the triplex association triggers the FIS green fluorescence that is relaxed from the tautomer favorable for excited-state intramolecular proton transfer (ESIPT). FIS can stabilize the triplex structure and primarily interact with the two terminals of the triplex via a 2:1 binding mode. This work demonstrates the potential of FIS as a DNA structure-selective switch-on ESIPT probe when evolving the triplex-forming oligonucleotides and developing the novel triplex-based sensors and switches.

Nonionic Fluorosurfactant Zonyl FSN self-assembly on Au(111) is investigated with scanning tunneling microscopy under ambient conditions. STM reveals that the FSN forms SAMs on Au(l11) with very large domain size and almost no defects. A (mean square root of 3 x mean square root of 3)R3 degree arrangement of the FSN SAM on Au(111) is observed. The SAMs show excellent chemical stability and last for at least a month in atmospheric conditions. The structure and stability of the FSN SAMs are compared with those of alkanethiols SAMs. It is expected that FSN may serve as a new kind of molecule to form SAMs for surface modification, which would benefit wider applications for various purposes.