Photochemical energy conversion in a helical oligoproline assembly.

Abstract

A biological photosynthetic reaction center contains a spatially ordered array of chromophores, electron donors, and electron acceptors that efficiently separate reductive and oxidative equivalents upon irradiation with visible light. This fundamental process of redox separation has been modeled in redox assemblies based on metal-bipyridyl complexes (1) or porphyrin-quinone systems (2-5). We have described the synthesis and photophysical characterization of lysine-based donorchromophore-acceptor triads that undergo light-induced redox separation (6-9). We describe here a general method for designing and constructing a helical oligoproline assembly having a spatially ordered array of redox or other functional groups protruding from a proline-II helical rod. Solid-phase peptide synthesis (10) is an efficient method for assembling a modular chain containing functional sites at specific positions. An a-helical peptide chain can serve as a molecular framework to display functional sites in a spatially ordered array (9). Alternatively, a chain of nine or more proline residues folds into a stable proline-II helix even when several of the proline residues have large functional sites on their side chains (11). The distance between two functional sites is determined by their helical location and orientation. The position of a modified proline residue in the proline chain determines its location on the helical surface. The configuration and conformation of the modified proline residue determine its orientation on the helical surface. To illustrate this general method, we have assembled an oligoproline assembly (triad 1) bearing three different redox sites (Fig. 1). Folding of the peptide triad 1 into a proline-II helix places the donor, chromophore, and acceptor sites in a linear array on one side of the helical rod.

Hierarchical Supramolecular Assembly of Sterically Demanding π‐Systems by Conjugation with Oligoprolines

“…We chose oligoprolines as peptidic scaffolds and alkynylated perylene monoimides (PMIs) bearing sterically demanding isopropyl groups, which are known to obstruct the p-stacking of PMIs, as chromophores ( Figure 1). [6,7] Herein, we show that conjugation of oligoprolines with PMI A leads to the formation of hierarchical supramolecular assemblies with tunable properties. [5] They adopt already at short chain lengths of six proline residues the conformationally well-defined left-handed polyproline II (PPII) helix, in which every third residue is stacked on top of each other in a distance of about 10 (Figure 1 a).…”

Modular synthesis of de novo- designed metalloproteins for light-induced electron transfer

“…The binding of heme groups to bundles of amphiphilic linear helices has been extensively investigated (9,10). For studies of the ET, different cofactors have been bound to a single helix (11) or bundles of two (12) and three helices (13). Light-induced ET through metalloproteins like cytochromes and myoglobins (14) has been investigated after modification of surface-exposed amino acid residues by ruthenium complexes.…”

Positional Isomers of Chromophore–Peptide Conjugates Self‐Assemble into Different Morphologies

“…This bundling mustb e driven by the oligoprolines in the outer shell of the columnar structures;afinding that is noteworthy because short oligoprolinesd on ot self-assemble on their own. [12,15,16] The packing is further supported by ad istinct out-of-plane wide-angle reflection( Gi nF igure 4b)t hat correspondst oad-spacing of 4.91 and is assignedt ot he distance between the oligoproline moieties of neighboring fibrils (Figure 4b).…”

“…[12][13][14] Also in contrast to other peptides,s hort oligoprolines do not aggregate with each other and have therefore been used as inert molecular rulers and scaffolds. [15,16] In conjugatesw ith p-systems, oligoprolines therefore serve as scaffolds that provide control over the distance and the orientation of chromophores. Conjugates betweeno ligoprolines and perylenem onoimides (PMIs) were found to self-assemble into highly ordered supramolecular architectures that extend into the micrometer regime.…”