Damaged sections of photosynthetic protein complexes in plants and bacteria are repaired via an internal cellular system of recognition, removal, and repair.

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The molecular components of nature's photosynthetic machinery take a beating from high-energy photons, ultraviolet light, and highly oxidizing (i.e., degrading) byproducts. When damage occurs, cellular processes recognize and cut off the damaged part of photosynthetic protein complex, remove the intact section to the outer regions of the cell where it can be fully repaired, and then return to its original site and function.

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"Self-repair processes in plants, algae and photosynthetic bacteria use molecular recognition and metastable thermodynamic states to make protein complexes that can be continually repaired by partial disassembly and reassembly with new components, initiated by chemical signals alone. For example, the repair of protein D1 in photosystem II (PS II) when photodamaged is initiated by photoinactivation of the protein induced by both the acceptor side and the donor side. This results in peptide bond scissions that alter protein conformation and drive dissociation of the damaged complex from the large PS II assemblies embedded within membrane stacks inside the chloroplast of a plant cell1. The separated complex diffuses laterally from within the stacks of membranes and out towards the outer membrane regions, where it disassembles into peripheral lightharvesting complex II (LHC II) and a PS II core complex2. The damaged D1 component of the PS II core is then fully degraded and the depleted complex equilibrates with newly biosynthesized D1 protein, resulting in the self-assembly of a tightly bound, repaired complex. The repaired complex returns to within the membrane stacks, where it re-docks with the extended light-harvesting systems inside the membranes, thus completing the repair cycle1. Central to this self-repair process are molecular recognition of the components and thermodynamic metastability, which allow the system to reversibly transition between kinetically trapped and disassembled states." (Ham et al. 2010:992)

"The sun's rays can be brutal, even for a leaf that's harvesting them. When photosynthesis is going full blast, a leaf is constantly building new photosynthetic reaction centers to replace those damaged by harsh oxygen species and other destructive molecules generated by intense ultraviolet light.

"So rather than trying to make solar cells that are extremely durable, the team decided to take a literal leaf from nature's book and go the route of self-repair, says chemical engineer Michael Strano of MIT, who led the project. He and Stephen Sligar and Colin Wraight of the University of Illinois at Urbana-Champaign, along with other colleagues, designed a system where damaged parts could be easily replaced.

"The researchers began with light-harvesting reaction centers from a purple bacterium. Then they added some proteins and lipids for structure, and carbon nanotubes to conduct the resulting electricity.

"These ingredients were added to a water-filled dialysis bag--the kind used to filter the blood of someone whose kidneys don't work--which has a membrane that only small molecules can pass through. The soupy solution also contained sodium cholate, a surfactant to keep all the ingredients from sticking together.

"When the team filtered the surfactant out of the mix, the ingredients self-assembled into a unit, capturing light and generating an electric current.

"The spontaneous assembly occurs thanks to the chemical properties of the ingredients and their tendency to combine in the most energetically comfortable positions. The scaffolding protein wraps around the lipid, forming a little disc with the photosynthetic reaction center perched on top. These discs line up along the carbon nanotube, which has pores that electrons from the reaction center can pass through.

"Adding the sodium cholate back into the mix disassembles the complexes. But filtering it out again brings them right back together." (Ehrenberg 2010)

Journal article
Photoelectrochemical complexes for solar energy conversion that chemically and autonomously regenerateNature ChemistrySeptember 5, 2010
Moon-Ho Ham, Jong Hyun Choi, Ardemis A. Boghossian, Esther S. Jeng, Rachel A. Graff, Daniel A. Heller, Alice C. Chang, Aidas Mattis, Timothy H. Bayburt, Yelena V. Grinkova, Adam S. Zeiger, Krystyn J. Van Vliet, Erik K. Hobbie, Stephen G. Sligar, Colin A. Wraight, Michael S. Strano

Looking to leaves for solar technology: a new technique could yield self-assembling solar cells

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