Electron transfer in Me-blocked heterodimeric α,γ-peptide nanotubular donor–acceptor hybrids

Abstract

Bio-inspired cyclopeptidic heterodimers built on ␤-sheet-like hydrogen-bonding networks and bearing photoactive and electroactive chromophores on the outer surface have been prepared. Different cross-strand pairwise relationships between the side chains of the cyclic ␣,␥-peptides afford the heterodimers as three nonequivalent dimeric species. Steady-state and time-resolved spectroscopies clearly show an electron transfer process from -extended tetrathiafulvalene, covalently attached to one of the cyclopeptides, to photoexcited [60]fullerene, located on the complementary cyclopeptide. The charge-separated state was stabilized for up to 1 s before recombining and repopulating the ground state. Our current example shows that cyclopeptidic templates can be successfully used to form light-harvesting/lightconverting hybrid ensembles with a distinctive organization of donor and acceptor units able to act as efficient artificial photosystems.cyclic peptide nanotubes ͉ fullerenes ͉ self-assembly P eptide-based tubular systems are being widely used to mimic Nature's channel-forming structures (1). Stable architectures ranging from ␤-helices to stacked macrocycles can self-assemble through multiple hydrogen-bonding interactions between and/or within peptide units with backbones capable of adopting an appropriate curvature. However, although a variety of versatile strategies have been developed to produce peptide tubes (2, 3), comparatively little has been done in the way of controlled functionalization of their inner and/or outer surfaces so as to fit them for specific tasks (4).Peptide nanotubes (PNs) or nanotube segments formed by the self-stacking of two or more cyclic peptides (2, 3, 5) are notable examples of the ''bottom-up'' approach to functional nanostructures. These self-assembled PNs have found application in biological and medical research and materials science (6-10). Their self-assembly is brought about by hydrogen bonding between their constituent cyclic peptides (CPs). The chirality of the amino acid residues of the CPs is such that the CP backbone forms an essentially flat ring with its CAO and NOH groups oriented nearly perpendicular to the ring plane and its side chains radiating outward. This conformation allows each face of the CP to take part in a ␤-sheet-like hydrogen-bond array with another CP that, depending on the sequence of amino acid residues in the CPs, must be oriented either parallel or antiparallel to the first (11-13). Scheme 1 shows an example of the antiparallel stacking of CPs formed of alternating ␣-and ␥-amino acid residues (␣,␥-CPs). Note that the precise orientation of side chains in planes perpendicular to the CP rings depends on backbone interactions within each ring.Within the nanobiomaterials field, one of the most actively pursued goals is the design of highly efficient and highly directional electron transfer mimics of the photosynthetic systems of plants and bacteria. In principle, self-assembled PNs bearing an appropriate array of photoactive and electroactive units might ac...

Peptides with regularly alternating enantiomeric sequence: From ion channel models to bioinspired nanotechnological applications

“…In particular, hybrid heterodimeric assemblies were achieved by self‐complementary cyclic α,γ‐peptides . The conjugation with hybrid functionalities allowed the design of novel materials to build biosensors, artificial photosystems, delivery and release nanomedicine systems, transmembrane channel transporters, nanotools for electronics, and responsive molecular machines containing properly designed reversible covalent bonds that are sensitive to environmental stimuli …”

“…Such cyclic peptides are structurally similar to the homologous l , d‐ α‐peptides. However, contrast to the cyclic l , d ‐α‐peptides, the insertion of γ‐units allows the functionalization of the inner pore of the cycle whose polarity and size can be tuned, generating more selective and specific systems …”

Self‐Assembling Cyclic Peptide‐Based Nanomaterials

“…With a view to favoring adoption of the required all-trans flat conformation, our group have recently been working on the design, synthesis, characterization, and application of a new class of cyclic peptides in which α-amino acid residues alternate with cis-3 aminocycloalkanecarboxylic acids (γ -Acas) [41][42][43][44][45][46][47][48][49]. The cycloalkane rings of these peptide units not only direct a hydrophobic, functionalizable methylene toward the interior of the cyclic peptide ring (thus allowing manipulation of the behavior of the inner cavity of the corresponding nanotubular structure) but also ensure the flatness and rigidity of the cycloalkane segments of the peptide backbone.…”

“…In principle, self-assembled peptide nanostructures bearing an appropriate array of photoactive and electroactive units might achieve this goal. In this context, our group described the synthesis and physicochemical properties of a novel class of nanotubular heterodimers in which a cyclic peptide bearing an electron-donor unit (extended tetrathiafulvalene (exTTF)) is coupled by a β-sheet-like hydrogen-bond system to another bearing a photoactive electron-acceptor unit (C 60 ) (Figure 1.15) [48]. Photoexcitation of the fullerene moieties to their 1.76 eV excited state is followed by a charge separation process generating a 1.15 eV radical ion pair state.…”

Understanding and designing head‐to‐tail cyclic peptides

“…Cyclic peptides (CPs) are a unique class of molecules with interesting properties. Notably, CPs have been developed for various nanotechnologies, including alternatives to carbon nanotubes, transmembrane transport channels, and artificial photosynthetic systems . Additionally, CPs have emerged as important players in drug development .…”