Enhanced Photoinduced Electron Transfer at the Surface of Charged Lipid Bilayers

Abstract

Photocatalytic systems often suffer from poor quantum yields due to fast charge recombination: The energy-wasting annihilation of the photochemically created charge-separated state. In this report, we show that the efficiency of photoinduced electron transfer from a sacrificial electron donor to positively charged methyl viologen, or to negatively charged 5,5'-dithiobis(2-nitrobenzoate), increases dramatically upon addition of charged phospholipid vesicles if the charge of the lipids is of the same sign as that of the electron acceptor. Centrifugation and UV/Vis titration experiments showed that the charged photosensitizers adsorb at the liposome surface, that is, where the photocatalytic reaction takes place. The increased photoelectron transfer efficiency in the presence of charged liposomes has been ascribed to preferential electrostatic interactions between the photosensitizer and the membrane, which prevents the formation of photosensitizer-electron-acceptor complexes that are inactive towards photoreduction. Furthermore, it is shown that the addition of liposomes results in a decrease in photoproduct inhibition, which is caused by repulsion of the reduced electron acceptor by the photocatalytic site. Thus, liposomes can be used as a support to perform efficient photocatalysis; the charged photoproducts are pushed away from the liposomes and represent "soluble electrons" that can be physically separated from the place where they were generated.

Effect of Liposomes on the Kinetics and Mechanism of the Photocatalytic Reduction of Methyl Viologen

“…The photoinduced reduction of methyl viologen dichloride (MVCl 2 ) by cysteine (CysSH) in a phosphate buffer containing the photosensitizer sodium meso -tetrakis­(4-sulfonatophenyl)­porphinatozinc (Na 4 1 ) was (re)­investigated both in the absence and presence of positively charged liposomes. In liposome-containing solution, positively charged large unilamellar vesicles formed by standard extrusion techniques from a 1:1 mixture of 1,2-dimyristoyl- sn -glycero-3-ethylphosphocholine chloride (eDMPCCl, 83 μM) and 1,2-dimyristoyl- sn -glycero-3-phosphocholine (DMPC, 83 μM) and characterized by an average diameter of 128 nm and a polydispersity index of 0.12 (as measured by dynamic light scattering) were simply added to the solution containing MV 2+ (0.67 mM), 1 4– (1.7 μM), and CysSH (8.3 mM), as previously described by us with a 1 4– to lipid ratio of 1:100. The homogeneous or liposome-containing solutions were irradiated in the Soret band of the porphyrin (λ irr = 420 nm, Δλ 1/2 = 20 nm) resulting in the formation of the methyl viologen radical cation (MV •+ ), characterized by a blue color and an absorbance maximum at 605 nm.…”

“…This state is oxidatively quenched by MV 2+ , after which the oxidized photosensitizer 1 •3– is regenerated by CysSH. In addition, in homogeneous solution, the photosensitizer [ 1 ] 4– forms { 1 4– :MV 2+ } complexes in the ground state, which do not form any photoproduct (MV •+ ) upon excitation. , The reactions occurring with the photosensitizer are summarized in Scheme , and the corresponding rates are given in eqs – At the beginning of the reaction, where the concentration of MV •+ is negligible, it is assumed that CysSH •+ , formed by reductive regeneration ( r rr ) will dimerize to form CysSSCys •– , which subsequently reduces a second molecule of MV 2+ to MV •+ .…”

“…Methyl viologen (MV 2+ ) was recognized as a potential redox mediator due to its excellent electron-accepting properties, producing a cation radical MV •+ which is relatively stable in solution. − ,, Furthermore, MV •+ has desirable redox properties, as it can produce dihydrogen in the presence of a suitable catalyst. − For this reason, we recently reported the accelerating effect of adding positively charged liposomes on the photocatalytic reduction of MV 2+ by cysteine (Scheme ). Herein, we analyze in detail the mechanism and kinetics of this system in order to elucidate why charged liposomal bilayers have a beneficial effect on the efficiency of charge separation and on the stability of the charge-separated state.…”

Simultaneous Study of Anti-Ferroptosis and Antioxidant Mechanisms of Butein and (S)-Butin

“…As shown in Supplementary Materials S1, the two isomers increased the LPO inhibition percentages with increasing concentration. Both the inhibition and formation of LPO are reported to stem from electron-transfer (ET) reactions [37][38][39][40][41]. To explore ET, the two were further investigated using the Fe 3+ -reducing antioxidant power (FRAP) assay and Cu 2+ -reducing antioxidant power (CUPRAC) assay.…”

Mimicking Photosystem I with a Transmembrane Light Harvester and Energy Transfer‐Induced Photoreduction in Phospholipid Bilayers

“…In particular, phospholipid membranes and vesicles (e.g. liposomes) can serve as a scaffold for mimicking cellular compartmentalization, [9–11] light harvesting, [12] membrane interactions, [13, 14] transmembrane electron transfer, [10, 15–19] and co‐assembly of photosensitizers with electron relays and catalysts [20–23] . In very rare cases the assembly of chromophores at phospholipid membranes enabled for light‐induced energy and electron transfer [24] .…”