From alkylating to shape-shifting G-quadruplex ligands: the RHAU peptide story.

G-quadruplexes (G4s) are non-canonical secondary nucleic acid structures with important biological implications in telomere elongation and gene expression. A large number of small molecules have been developed to bind and even covalently target these structures, enhancing the potency and duration of binding. Alternatively, peptide-based ligands have been studied and shown to offer several advantages, including high specificity, a modular design, and ease of synthesis. In this work, we describe a peptide-based methodology for covalent G4-targeting, based on the introduction of two photoactivatable moieties in a peptide derived from the RHAU helicase. Rational insertion of crosslinkers at different positions yielded nine different peptides, which were evaluated for their G4-stabilizing effect and alkylation potential. Moderate to high alkylation yields towards G4s were obtained. The G4 stabilizing potential drastically increased for N-terminal modifications of the RHAU18 peptide. This led to the design of a further series of peptides with varying N-terminal residues to gain insight in the stabilization potential of each single amino acid modification and provided a comprehensive study of the binding behaviour of modified RHAU peptides.