# Impact of Thymidine Loop Modifications on Telomeric G-Quadruplex Catalytic Systems for Asymmetric Sulfoxidation

## TL;DR

This study explores how modifying thymidine loops in G-quadruplex DNA affects their ability to catalyze asymmetric chemical reactions.

## Contribution

The paper reveals how specific thymidine loop modifications impact enantioselectivity in G4-mediated catalysis.

## Key findings

- Thymidine-based modifications in G4 loops reduced enantioselectivity compared to β-L-2′-deoxyadenosine modifications.
- Loop alterations critically affect the chiral microenvironment of G4 structures.
- Single substitutions within the same loop can lead to different enantioselectivities.

## Abstract

G-quadruplex (G4) DNA structures have recently emerged as promising chiral scaffolds for enantioselective catalysis. This study investigates how thymidine loop modifications influence the catalytic performance of the telomeric G4 sequence HT21 in the asymmetric sulfoxidation of thioanisole. To this end, several singly or doubly modified HT21 derivatives were synthesized by using β-L-2′-deoxythymidine, 5-hydroxymethyl-2′-deoxyuridine, and 5-bromo-2′-deoxyuridine instead of a T residue, or β-L-2′-deoxyadonesine instead of an A residue, in specific positions within the TTA loops. The catalytic activity of these analogues was evaluated in the Cu(II)-mediated oxidation of thioanisole using hydrogen peroxide as oxidant. All modified sequences maintained complete substrate conversion, but their enantioselectivities varied markedly. Whereas the highest enantiomeric excess (84% ee) had previously been achieved with the HT21 analogue bearing a β-L-2′-deoxyadenosine in the first loop, the thymidine-based modifications, either alone or in combination, resulted in lower ee values, suggesting that loop alterations critically affect the chiral microenvironment, not all loop positions are functionally equivalent, and single substitutions within the same loop can result in different enantioselectivities. These findings highlight new insights on how individual loop residues contribute to asymmetric induction and offer further details for tuning G4-based catalytic scaffolds.