Unidirectional Transmembrane Photoinduced Electron Transfer with Artificial Metallopeptides

Abstract

The ability to drive unidirectional photoinduced electron transfer through an insulating lipid membrane is one of the most striking features of the thylakoid membrane, and recreating this property artificially is considered as one of the holy grails of artificial photosynthesis. Here, we report two artificial metallopeptides, WALP23-Re ₂ and WALP23-Ru ₂ , that were embedded in the membrane of dissymmetric liposomes containing an electron donor in the inner compartment and an electron acceptor in the bulk aqueous phase. Upon light irradiation under air, unidirectional electron transfer was observed from one side of the membrane to the other one with both metallopeptides. However, the mechanism of photoinduced electron transfer strongly depended on the metal center: the neutral WALP23-Re ₂ peptide achieved genuine electron transfer through the membrane, but with the tetracationic WALP23-Ru ₂ peptide the transmembrane electron transfer appeared to be the result of light-induced leakage of the electron donor molecules through the lipid bilayer, followed by electron transfer on the same side of the membrane by peptides assembled parallel to the membrane. These results highlight both the unique potential of the neutral metallopeptide WALP23-Re₂ to drive transmembrane photoinduced electron transfer in artificial photosynthetic systems, as well as the importance of membrane-leakage studies to validate the working mechanism of such supramolecular systems.

Keywords: artificial photosynthesis; leakage; lipid bilayers; photoelectron transfer; transmembrane.

1

Transmembrane electron transfer with metallopeptides in the lipid bilayer. (A) Two pathways for electron transfer are possible; (I) electron transfer across the lipid bilayer via a light-activatable metallopeptide from an electron donor in the inner compartment of liposomes to an electron acceptor located in the aqueous bulk, and (II) transmembrane leakage of the electron donor to the aqueous bulk resulting in electron transfer via a light-activatable metallopeptide on the same side of the membrane. (B) Molecular structure of the metallopeptides investigated in this study. A = alanine, A_bpy = bipyridylalanine, Ac = acetyl, G = glycine, L = leucine, and W = tryptophan.

2

Molecular dynamics simulations to study the orientation of the metallopeptides. (A) Illustration of the procedure that was used to determine the orientation of the metallopeptide with respect to the lipid membrane. θ is the angle between the metal–metal vector (V⃗ _M–M) and the normal vector of the lipid membrane (V⃗ _memb). r _cutoff is the cutoff range. (B) φ and θ values of WALP 2 3-Ru ₂ and WALP 2 3-Re ₂ after 200 ns MD simulation time. φ is the ratio between the number of DPPC molecules and the total number of molecules within r _cutoff. (C) MD snapshots showing a parallel orientation of WALP 2 3-Ru ₂ with respect to the lipid bilayer and (D) a transmembrane WALP 2 3-Re ₂ .

3

Electron transfer studies of the metallopeptides in liposomes. (A) Electron transfer processes initiated by light irradiation; (I) transmembrane electron transfer from the sacrificial electron donor HEDTA^3– located in the inner compartment of the liposome via a membrane-embedded peptide-photosensitizer conjugate to the sacrificial electron acceptor WST1^– located in the bulk forming the colored Fz1^2–, which can be followed by UV–vis, and (II) externally added Zn²⁺ ions form a complex with HEDTA^3–, thereby inactivating the electron transfer pathway on the outside of the membrane. (B) The formation of Fz^2– as monitored by a change in absorbance (λ_max = 438 nm) versus irradiation time for liposome systems containing either WALP23-Ru ₂ (blue circles) or WALP23-Re ₂ (red squares) with (closed symbols) or without Zn­(OAc)₂ (open symbols). Blue light irradiation (λ_irr = 450 nm, P = 15.8 mW, Φ₀ = 39.1 nmol/s for WALP23-Ru ₂ and λ_irr = 385 nm, P = 7.3 mW, Φ₀ = 13.1 nmol/s for WALP23-Re ₂ ) was started after 0.5 h in the dark. Experimental conditions: [DPPC] = 2.5 mM, [DSPE-PEG2K] = 25 μM, [WALP23-Ru ₂ or WALP23-Re ₂ ] = 25 μM, [HEDTA^3–] = 0.125 M (before SEC column), [WST1^–] = 0.33 mM, [Zn­(OAc)₂] = 0 or 5 mM in 0.1 M NH₄OAc (pH = 7.0, p = 0.42 Osm). The measurements were performed at 25 °C under an air atmosphere.

4

Phosphorescence decay curves (590 nm) of WALP-Re ₂ in liposomes with and without quenchers. (A) HEDTA^3– added, excitation at 380 nm, Na₂B₄O₇ buffer pH 8.2. (B) WST1^– added, excitation at 420 nm, NH₄OAC buffer pH 6.8.

5

(A) Redox potentials of the different couples involved in photoinduced transmembrane electron transfer with WALP23-Re ₂ , according to literature data. The red arrows marked 2.36 eV indicate the energy difference between the ground state and the most stable triplet (T₁) state. (B–D) Different hypotheses for the mechanism of photoinduced transmembrane electron transfer with WALP23-Re ₂ in DPPC:DSPE-PEG2K 99:1 liposomes (this work). CR = charge recombination; CS = charge separation; CM = charge migration; CT = (thermal) charge transfer. (B) Diffusive encounter of oppositely charged WALP23-Re ₂ formed by reductive and oxidative quenching. (C) Oxidative quenching of Re* by WST1^– followed by transmembrane hole transfer and oxidation of HEDTA^3–. (D) Reductive quenching of Re* by HEDTA^3–, followed by transmembrane electron transfer and reduction of WST1^–.