Sequence and Structure of Peptoid Oligomers Can Tune the Photoluminescence of an Embedded Ruthenium Dye

Abstract

The understanding of structure–function relationships within synthetic biomimetic systems is a fundamental challenge in chemistry. Herein we report the direct correlation between the structure of short peptoid ligands—N‐substituted glycine oligomers incorporating 2,2′‐bipyridine groups—varied in their monomer sequence, and the photoluminescence of RuII centers coordinated by these ligands. Based on circular dichroism and fluorescence spectroscopy we demonstrate that while helical peptoids do not affect the fluorescence of the embedded RuII chromophore, unstructured peptoids lead to its significant decay. Transmittance electron microscopy (TEM) revealed significant differences in the arrangements of metal‐bound helical versus unstructured peptoids, suggesting that only the latter can have through‐space interactions with the ruthenium dye leading to its quenching. High‐resolution TEM enabled the remarkable direct imaging of singular ruthenium‐bound peptoids and bundles, supporting our explanation for structure‐depended quenching. Moreover, this correlation allowed us to fine‐tune the luminescence properties of the complexes simply by modifying the sequence of their peptoid ligands. Finally, we also describe the chiral properties of these Ru–peptoids and demonstrate that remote chiral induction from the peptoids backbone to the ruthenium center is only possible when the peptoids are both chiral and helical.

Strengthening Peptoid Helicity through Sequence Site-Specific Positioning of Amide cis-Inducing NtBu Monomers

“…There are many reasons for this, ranging from their ease of synthesis with unique side chain diversity 1 to their conformational versatility which opens up a wide range of potential applications relevant to biomaterial science 23 , therapeutics development, 45 and catalysis. 7,8,9 Peptoids should be regarded as biotic foldamers since their backbone closely resembles that of peptides with the side chains attached to the amide nitrogen atoms rather than the C carbons, and also because single-chain peptoids may exhibit biopolymer-like conformational behaviours.…”

“…33 (S)-N-(1-phenylethyl)glycine monomers are, for example, commonly used to induce right-handed helices. 9,34,35 Sequence requirements and chain length effects were carefully studied which established the rules for the formation and stabilisation of chiral PPI-type helical secondary structures from aromatic-containing monomers. 36,37,38 In contrast, the control of peptoid helicity from non-aromatic monomers has been much less investigated.…”

Metal‐Binding Foldamers

“…Exploring this idea in the context of peptoids, Maayan has shown that suitably designed sequences bearing 2,2′‐bipyridine moieties can fold and associate in the presence of metal ions including Ru 2+ [39] . Chiral induction from the peptoid chain to the metal center was observed, and later work demonstrated alterations in the peptoid sequence could be used to tune photoluminescent properties of the complex [40] . Related bipyridine‐functionalized peptoids were shown to form complexes with other metals, including Cu 2+ , Co 2+ , and Ni 2+ [41] …”

“…[39] Chiral induction from the peptoid chain to the metal center was observed, and later work demonstrated alterations in the peptoid sequence could be used to tune photoluminescent properties of the complex. [40] Related bipyridine-functionalized peptoids were shown to form complexes with other metals, including Cu 2 + , Co 2 + , and Ni 2 + . [41] While most metalloproteins involve a single metal ion at each binding site, multimetallic clusters are found in some systems.…”

Using Fluorescence to Enable Innovative Functions of Foldamers

“…A number of oligomers with well‐defined spatial arrangements of functional groups have been designed and employed to coordinate to metals, and detection of the foldamer‐metal binding has been evaluated by monitoring changes to fluorescence features. In an excellent example, the Maayan laboratory detailed the importance of peptoid structural features to modulate the luminescence of bound Ru(II) in acetonitrile [49] . Coordinating 2,2’‐bipyridine groups were incorporated into peptoids with helical ( 16 ) or flexible structures.…”