**1. Introduction**

Catalysts are essential in the fields of both synthetic applications and biological functions. Indeed, while they make thousands of chemical reactions possible for industry and research,

© 2016 The Author(s). Licensee InTech. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

they play a pivotal role in living systems. All along the years, nature develops strategies to catalyze biochemical reactions. The most representative catalysts are undoubtedly the enzymes, but they are not the only ones. Interestingly, ribonucleic acids (also known as RNA) are also able to play this role and to catalyze key reactions, like in the active site of the ribosome during the translation of messenger RNA (mRNA). Besides, it is strongly supposed that the origins of the prebiotic life were based on the use of RNA as both the carrier of the genetic information, and a catalyst: RNA was a self-sufficing molecule. This theory was termed "RNA world."[1–6]

Inspired by the role of RNA as a catalyst, chemists developed new catalytic systems based on deoxyribonucleic acids, also termed DNA.[7, 8] DNA offers more advantages, like its better stability compared to the RNA equivalent.[9] These aspects are developed in section 2 of this chapter.

Among all the DNA structures used as catalysts (*e.g*., canonical duplex structures or nonca‐ nonical triplexes, etc.), author would like to highlight the reader on the G-quadruplex struc‐ tures.[10] These noncanonical edifices, composed of a stacking of native G-quartets, are introduced in section 3. This presentation is followed by the story of the discovery of Gquadruplexes as catalysts, and then, by a rationalization of how this chemical mechanism works, and how chemists can modulate experimental conditions to obtain the efficiency desired. To complete the presentation, the large range of applications of these noncanonical structures is commented on and shows how versatile and effective quadruplex DNA are.

Based on the observation that the catalytic activity of G-quadruplex is mainly due to the presence of native G-quartets,[11] several groups designed new molecules able to form a synthetic G-quartet. The most representative examples are TASQ (for *template-assembled synthetic G-quartet*), [12–16] composed of four guanines grafted on a template, and able to selfassemble into an intramolecular G-quartet. In section 4 of this chapter, the concept of TASQ is first clarified and then their catalytic activity is specified. Finally, the very first applications proposed in the literature are described, and pave the way to the use of synthetic molecules to mimic natural enzymes, like peroxidases.
