**4.1. Concept and structure of TASQ**

Based on the idea that G-quadruplexes, mimicking the natural *horseradish peroxidase*, lead to an increase of the range of applications, mainly due to the higher stability of the DNA compared to the protein and permitting a use in a bigger range of experimental conditions (temperature, buffer, ion strength, etc.),[7, 23, 176] few research teams decided to develop Gquadruplex-mimicking systems.

Because the pivotal step of the catalytic cycle is based on the activation of hemin by interaction with one of the external G-quartet,[11] the group of Dr. D. Monchaud decided to synthesize the very first example of a water-soluble molecule, composed of four guanine residues, and able to form, intramolecularly, a synthetic G-quartet.[14]

**i.** Interestingly, the DNAzyme methodology was also used to detect bigger living systems, like the bacteria *Escherichia coli* O157:H7[162] or *Alicyclobacillus acidoterrestris*.[163] In the first example, authors used graphene oxide/thionine/gold nanoparticles coated SiO2 nanocomposites to immobilize DNA, while for the second one, a more classic approach was used, in which G-quadruplex–hemin complexes oxidize the colorless guaiacol,

**j.** Developed to target G-quadruplexes *in vivo*, *in cellulo*, or *in vitro* for biological applications, G-quadruplex ligands are molecules able to interact with G-quadruplexes and to stabilize them.[164–166] This ability can be evaluated, thanks to the DNAzyme process, because of a competition between the ligands researchers want to try, and the hemin. In other words, a good ligand takes the place of the hemin; hemin is subsequently not activated and, consequently, leads to a decrease of the signal intensity (measured by UV–Vis or by

**k.** To finish this laundry list, it is essential to mention other ingenious applications for the G-quadruplex DNAzymes, like the development of logic gates[170–173] as the INHIBIT one published by T. Li *et al.*,[174] or as the AND one proposed by J. Chen *et al*., described

To summarize, all these cases, which represent the range from the more applied to the more conceptual scientific applications of the same DNAzyme catalysis, illustrate how using DNA instead of enzyme to catalyze a reaction puts out a new avenue in terms of polyvalence. It is believed that this list will increase more and more in the next years. But the precise under‐ standing of the mechanism constitutes also an exciting challenge. On the one hand, this progress should offer scientists the possibility to fine-tune the experimental conditions (*e.g*., sequence of the G-quadruplex DNA strand(s), length of the loop(s), addition of a boosting agent, etc.). On the other hand, a better comprehension should help to enlighten chemists about the mechanistic aspect of the oxidation states of the hemin, in both biological and DNAzyme

**4. Synthetic G-quartet-based DNAzymes: template-assembled synthetic G-**

Based on the idea that G-quadruplexes, mimicking the natural *horseradish peroxidase*, lead to an increase of the range of applications, mainly due to the higher stability of the DNA compared to the protein and permitting a use in a bigger range of experimental conditions (temperature, buffer, ion strength, etc.),[7, 23, 176] few research teams decided to develop G-

Because the pivotal step of the catalytic cycle is based on the activation of hemin by interaction with one of the external G-quartet,[11] the group of Dr. D. Monchaud decided to synthesize

produced by the bacteria, to tetraguaiacol, which is amber.

456 Advanced Catalytic Materials - Photocatalysis and Other Current Trends

fluorescence).[61, 167–169]

systems.

**quartets (or TASQ)**

**4.1. Concept and structure of TASQ**

quadruplex-mimicking systems.

as a keypad lock security system.[175]

Historically, the very first observation of synthetic G-quartets was made by I. Bang in 1910, who was able to form gels from a concentrate solution of guanosine monophosphate. However, the hypothesis of the self-assembly of guanine derivatives to G-quartet arrangements was only published more than half a century later by M. Gellert, M. Lipsett, and D. Davies in 1962. The dried fibers obtained could be analyzed by X-ray diffraction and helped the authors to propose the initial supramolecular structure shown in Figure 1.[177] Interestingly, a recent study proved that the length of the fibers can reach from 8 nm to 30 nm, corresponding to from 24 to 87 stacked G-quartets, respectively.[178]

During the last decades, plenty of examples using synthetic G-quartets as a supramolecular motif were developed and well summarized in several reviews.[10, 179, 180] The applications of these systems concern pH-sensitive hydrogel probes,[181] synthetic transmembrane Na+ transporter,[182] and other ionic channels,[183–185] enantioselective systems controlled by the cation used,[186] combinatorial chemistry,[187] and also molecular electronics and liquid crystals.[179] Among all these examples, in which the elementary brick is made of one or two guanines, only few stem from a four-guanine-based compound. This assessment is inquisitive and nonintuitive because G-quartets are composed of four guanines. It was why the team of J. Davis developed in 2000 and 2003 1,3-alternate calix[4]arene derivatives functionalized by four guanines.[188, 189] This smart system in which two guanines are on one side, while the two others are on the other side of the calixarene template, was used as both cation (inside the G-quartet) and anion (thanks to the H-bonds between protons of the amide groups and the anion) receptors.[188] The formation of the G-quartet is intermolecular between two guanines from one molecule, and two guanines from another one. It took the scientific community until 2008 to propose the first intramolecular synthetic G-quartet molecules, termed TASQ.

The name "TASQ," for *template-assembled synthetic G-quartet*, was introduced by the research group of J. C. Sherman to describe molecules built around a template and functionalized by four guanines, able to interact each other to self-assemble into an intramolecular G-quartet.[12] This concept probably derived from a modification of the TASP (for *template-assembled synthetic peptide*) development,[190] in which four peptide sequences were used instead of DNA bases. The aim of these models was the understanding of protein interactions, thanks to their spatial proximity when grafted to the same scaffold.

Thus, the first TASQ were synthesized from a highly lipophilic calixarene moiety substituted by four 2′,3′-*O*-isopropylideneguanosine.[12] The intramolecular formation of the G-quartet was demonstrated by 1 H NMR, NOESY, COSY, and HMQC and was definitively proved by X-ray diffraction in 2012.[191] Notwithstanding, no application of these systems was publish‐ ed, and their hydrophobic properties were probably the reason for that. Interestingly, they rectified this point using first phosphate groups to functionalize the calixarene on the opposite side of the guanosines[192] and then, subsequently, with the phosphate group intercalating between the guanine moieties and the scaffold (due to the use of 3′-monophosphate guano‐ sines).[193]

**Figure 3.** Chemical structures of the first TASQ developed by J. C. Sherman (**A**), [12] RAFT-G4 (**B**), [13] DOTASQ-C5 in equilibrium between its open (left) and closed conformations (right) (**C**), [14] PNADOTASQ (**D**), [15] and PyroTASQ (E). [16]

In parallel, the group of E. Defrancq was focused on the functionalization of a cyclodecapeptide termed RAFT (for *regioselectivity addressable functionalized template*) by DNA strands.[127, 194– 196] This research led the team to synthesize a TASQ from a RAFT equipped with four guanosines.[13] Thanks to a collaboration with D. Monchaud's team, this molecule was studied as a DNAzyme (see section 4.2) and showed that the G-quartet intramolecular formation (confirmed by circular dichroism, and NMR studies) was able to catalyze the hemin oxidation/ reduction cyclic reaction.[197]

However, the very first example of a water-soluble TASQ was proposed by L. Stefan *et al*. in 2011.[14] The key point was the choice of the *ad hoc* template, able to drive the G-quadruplex formation, and being fully soluble in water. It was why the cyclen macrocycle was chosen, for its high solubility, its ability of metal chelation, and its C4-type symmetry, identical to the one of a G-quartet.[198, 199] Even if this last criteria was not a *sine qua non* condition, it seemed to be favorable, like in the calixarene-templated TASQ developed by J. C. Sherman's group. Thus, DOTASQ were born from a DOTA (for 1,4,7,11-tetraazacyclododecane-*N,N*′*,N″,N'''*-tetraace‐ tic acid) moiety in which the four "arms" were functionalized by four alkylguanine groups. The term DOTASQ is a portmanteau word created from both acronyms DOTA (*i.e*., the template) and TASQ (*i.e*., the supramolecular property of the molecule). For the sake of comparison, four DOTASQ were synthesized: DOTASQ-C1 and DOTASQ-C5 (C1 and C5 suffixes indicating the length of the alkyl chain), and the terbium(III) equivalents Tb.DOTASQ-C1 and Tb.DOTASQ-C5, respectively.[14]

Furthermore, another TASQ was developed with peptide nucleic acid guanine (also termed PNA guanines) arms and was called PNADOTASQ.[15]

The peroxidase-mimicking catalytic activities of these compounds were evaluated and are described in the next sections. As a remark, other TASQ were developed by D. Monchaud's team like PyroTASQ (pyrene as a template),[16] NaphtoTASQ (naphthalene as a template) [200], and also PorphySQ and PNAPorphySQ (porphyrin moieties as templates).[201, 202]
