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Mimicking Photosystem II with Manganese Model Complexes to Approach Artificial Photosynthesis

Högblom, Joakim LU (2005)
Abstract (Swedish)
Popular Abstract in Swedish

Vi går mot ett mer och mer energikrävande samhälle. För att klara ett ökande behov av energi med de begränsande naturresurser som finns på jorden, har ett forskningsprojekt, som denna avhandling är en del av, som mål att utveckla artificiell fotosyntes. Den naturliga fotosyntesen använder sig av solljus för att omvandla koldioxid och vatten till energirika produkter såsom kolhydrater, och som restprodukt i denna process bildas syrgas. Det är från denna process, som startade för många hundra miljoner år sedan, som syret i atmosfären kommer.



Den naturliga fotosyntesen spjälkar (oxiderar) två vatten (H2O), till syrgas (O2) och fyra vätejoner (H+) (protoner) med hjälp av ljus,... (More)
Popular Abstract in Swedish

Vi går mot ett mer och mer energikrävande samhälle. För att klara ett ökande behov av energi med de begränsande naturresurser som finns på jorden, har ett forskningsprojekt, som denna avhandling är en del av, som mål att utveckla artificiell fotosyntes. Den naturliga fotosyntesen använder sig av solljus för att omvandla koldioxid och vatten till energirika produkter såsom kolhydrater, och som restprodukt i denna process bildas syrgas. Det är från denna process, som startade för många hundra miljoner år sedan, som syret i atmosfären kommer.



Den naturliga fotosyntesen spjälkar (oxiderar) två vatten (H2O), till syrgas (O2) och fyra vätejoner (H+) (protoner) med hjälp av ljus, enligt reaktion 1.



2 H2O + ljus --> O2 + 4 H+ + 4 e- 1



De fyra elektroner som frigörs ur denna reaktion, används för att göra energirika produkter. Fotosyntesen sker i växter, cyanobakterier och i alger. Solljus absorberas av klorofyll och den energi som ljuset för med sig, samlas i ett reaktionscentra där det sker en laddnings separation. I denna process övergår solenergin till kemisk energi och kan användas för att spjälka vatten, och det sker i Fotosystem II (PSII). PSII kan ses som ett D-S-A system, där D är en elektrondonator, ett mangankomplex som spjälkar vatten, S är en ljusabsorberande molekyl (sensiterare), som i den naturliga fotosyntesen är klorofyll, och A är en elektronacceptor.



Idéen med artificiell fotosyntes är att skapa ett liknande system som kan spjälka vatten till syrgas och vätejoner, vätejonerna ska sedan tillsammans med fyra elektroner bilda vätgas. I naturen finns det en enzymgrupp som kallas hydrogenaser, som katalyserar vätejoner till vätgas. Vad vi vill göra, är att skapa ett D-S-A komplex som består av (D) som är ett mangankomplex som kan spjälka vatten, en sensiterare (S) som kan absorbera ljus och göra en laddnings separation, och en elektronacceptor (A) som kan ta emot elektroner och katalysera protoner till vätgas på samma sätt som ett hydrogenas. När man förbränner vätgas tillsammans med syrgas bildas vatten och energi, det innebär att råmaterialet (vatten) återskapas och kan användas igen. Detta göra att ett sådant system producerar energi men inga farliga restprodukter så som koldioxid som bidrar till växthuseffekten. Detta gör det till en miljövänlig och förnyelsebar energiform.



Denna avhandling behandlar studien av donatormolekyler (D) och vi har valt att arbeta med tre olika mangankomplex innehållande två manganjoner vardera, i ett försök att efterlikna naturens mangankomplex. För att studera dessa metallkomplex har jag använt mig av en spektroskopisk teknik som kallas Elektron Paramagnetisk Resonans (EPR). Med den här tekniken kan man följa hur elektroner förflyttar sig, dvs. om någonting oxideras eller reduceras. Som sensiterare (S) använder vi ruteniumkomplex, och som elektronacceptor (A) ett koboltsalt. Vi har med hjälp av ljus och i lyckats oxidera våra mangankomplex till olika oxidations tillstånd. Vi har också visat att vatten är nödvändigt för att detta ska ske, och att syret i vatten bildar bryggor mellan manganerna under oxidationerna. Med mina resultat har vi fått en ökad förståelse för hur våra mangankomplex uppför sig vid oxidation och kan dra nytta av det i fortsatta studier på väg mot artificiell fotosyntes. (Less)
Abstract
The work in thesis aims for artificial photosyntesis, which could and will mimic natural photosynthesis, the process that uses light to create energy rich compounds from H2O and CO2. Water is oxidized in Photosystem II (PSII) by the Water Oxidizing Complex (WOC), which is a catalytic site on the lumenal side of the thylakoid membrane. The chlorophyll complex P680 absorbs light and electrons are transferred to the acceptor side of PSII. The electron-hole on P680 is filled by electrons from the Mn-cluster, which in turn oxidizes water. This can be viewed as a D (donor)-PS (photosensitizer)-A (acceptor) system. Artificial photosynthesis builds on the same principles, and in this work we have been focusing on redox reactions in dinuclear... (More)
The work in thesis aims for artificial photosyntesis, which could and will mimic natural photosynthesis, the process that uses light to create energy rich compounds from H2O and CO2. Water is oxidized in Photosystem II (PSII) by the Water Oxidizing Complex (WOC), which is a catalytic site on the lumenal side of the thylakoid membrane. The chlorophyll complex P680 absorbs light and electrons are transferred to the acceptor side of PSII. The electron-hole on P680 is filled by electrons from the Mn-cluster, which in turn oxidizes water. This can be viewed as a D (donor)-PS (photosensitizer)-A (acceptor) system. Artificial photosynthesis builds on the same principles, and in this work we have been focusing on redox reactions in dinuclear manganese complexes, using RuII(bpy)3 as photosensitizer.WOC in PSII is built up by four manganeses in a cluster, with ?-oxo bridges between the manganeses. On at least one of the manganeses, there is an open site for water binding.In this thesis I have investigated oxidation reactions of three different dinuclear manganese complexes, denoted complex 1, 2, and 3. 1 is a complex with N3O3 ligands coordination to each manganese, and is synthesised in Mn2II/II valance state. We have showed that it is possible to oxidize this complex three times to Mn2III/IV, which means that it forms four stable oxidation states. 2 has a N2O4 ligands coordinating each manganese, and is synthesised in the Mn2III/III valence state. This complex can be oxidized to what we think is Mn2IV/IV and Mn2IV/IVL?. 2 has five stable oxidation states. From electro chemistry we know that 2 is stable in Mn2II/II and Mn2II/III, and this means that 2 can be oxidized by photosensitizer three steps. 3 is an unymmetric manganese complex, with a mixture of the ligands in 1 and 2. On one side the manganese is coordinated to N3O3 ligands, and the other manganese is coordinated to N2O4 ligands. 3 is synthesised in the Mn2II/III valence state, and it is possible to oxidize this complex to, what we think is a Mn2IV/IV and a Mn2IV/IVL? oxidation state. By electro chemistry we can reduce 3 to Mn2II/II. This means that 3 can be oxidized four times with the photosensitizer and have five stable oxidation states. All three manganese complexes have acetate groups as bridging ligands to the manganeses. EXAFS measurements and from electro chemistry, indicate that the acetate groups can detach from the manganese, so that water can access the site and be converted into ligands to the manganese. From the X-ray absorption spectroscopy (EXAFS and K-edge), we could se that there was a shortening in the Mn-Mn distance of 0.5 Å for 1 in the Mn2III/III and at Mn2III/III for 2, which is an indication that a ?-oxo or a ?-hydroxo bond is formed. This means that the manganese cluster changes its conformation during oxidation.To oxidize our model complexes beyond Mn2III/III, we use CoIII(NH3)5Cl as sacrificial electron acceptor. When CoIII is reduced by Ru*(bpy)3 to CoII, an EPR signal appears in the g=5 region which belongs to CoII(H2O)6. After some time of illumination a new EPR signal appears in the same region, which is narrower and more symmetric in its shape, resulting from a photoreduction process in CoIII(NH3)5Cl. It is known that UV light can photoreduce CoIII(NH3)5Cl to CoII(NH3)4 and Cl?, but we have found that the same reaction occurs at 532 nm. We have interpreted this signal as an intermediate, when cobalt changes the ligands from NH3 and Cl- to H2O. (Less)
Please use this url to cite or link to this publication:
author
supervisor
opponent
  • Professor Andersson, K. Kristoffer, Department of Molecular Biosciences, University of Oslo
organization
publishing date
type
Thesis
publication status
published
subject
keywords
Metabolism, Hydrogen, EPR, Cobalt, Biokemi, metabolism, Biochemistry
pages
38 pages
publisher
Department of Biochemistry, Lund University
defense location
Centre for Chemistry and Chemical Engineering, Getingevägen 60
defense date
2005-11-10 10:30
ISBN
91-7422-095-0
language
English
LU publication?
yes
id
ed52e389-ef95-4fa9-b781-dad9a6b0dc0c (old id 545562)
date added to LUP
2007-10-13 12:25:11
date last changed
2016-09-19 08:45:02
@misc{ed52e389-ef95-4fa9-b781-dad9a6b0dc0c,
  abstract     = {The work in thesis aims for artificial photosyntesis, which could and will mimic natural photosynthesis, the process that uses light to create energy rich compounds from H2O and CO2. Water is oxidized in Photosystem II (PSII) by the Water Oxidizing Complex (WOC), which is a catalytic site on the lumenal side of the thylakoid membrane. The chlorophyll complex P680 absorbs light and electrons are transferred to the acceptor side of PSII. The electron-hole on P680 is filled by electrons from the Mn-cluster, which in turn oxidizes water. This can be viewed as a D (donor)-PS (photosensitizer)-A (acceptor) system. Artificial photosynthesis builds on the same principles, and in this work we have been focusing on redox reactions in dinuclear manganese complexes, using RuII(bpy)3 as photosensitizer.WOC in PSII is built up by four manganeses in a cluster, with ?-oxo bridges between the manganeses. On at least one of the manganeses, there is an open site for water binding.In this thesis I have investigated oxidation reactions of three different dinuclear manganese complexes, denoted complex 1, 2, and 3. 1 is a complex with N3O3 ligands coordination to each manganese, and is synthesised in Mn2II/II valance state. We have showed that it is possible to oxidize this complex three times to Mn2III/IV, which means that it forms four stable oxidation states. 2 has a N2O4 ligands coordinating each manganese, and is synthesised in the Mn2III/III valence state. This complex can be oxidized to what we think is Mn2IV/IV and Mn2IV/IVL?. 2 has five stable oxidation states. From electro chemistry we know that 2 is stable in Mn2II/II and Mn2II/III, and this means that 2 can be oxidized by photosensitizer three steps. 3 is an unymmetric manganese complex, with a mixture of the ligands in 1 and 2. On one side the manganese is coordinated to N3O3 ligands, and the other manganese is coordinated to N2O4 ligands. 3 is synthesised in the Mn2II/III valence state, and it is possible to oxidize this complex to, what we think is a Mn2IV/IV and a Mn2IV/IVL? oxidation state. By electro chemistry we can reduce 3 to Mn2II/II. This means that 3 can be oxidized four times with the photosensitizer and have five stable oxidation states. All three manganese complexes have acetate groups as bridging ligands to the manganeses. EXAFS measurements and from electro chemistry, indicate that the acetate groups can detach from the manganese, so that water can access the site and be converted into ligands to the manganese. From the X-ray absorption spectroscopy (EXAFS and K-edge), we could se that there was a shortening in the Mn-Mn distance of 0.5 Å for 1 in the Mn2III/III and at Mn2III/III for 2, which is an indication that a ?-oxo or a ?-hydroxo bond is formed. This means that the manganese cluster changes its conformation during oxidation.To oxidize our model complexes beyond Mn2III/III, we use CoIII(NH3)5Cl as sacrificial electron acceptor. When CoIII is reduced by Ru*(bpy)3 to CoII, an EPR signal appears in the g=5 region which belongs to CoII(H2O)6. After some time of illumination a new EPR signal appears in the same region, which is narrower and more symmetric in its shape, resulting from a photoreduction process in CoIII(NH3)5Cl. It is known that UV light can photoreduce CoIII(NH3)5Cl to CoII(NH3)4 and Cl?, but we have found that the same reaction occurs at 532 nm. We have interpreted this signal as an intermediate, when cobalt changes the ligands from NH3 and Cl- to H2O.},
  author       = {Högblom, Joakim},
  isbn         = {91-7422-095-0},
  keyword      = {Metabolism,Hydrogen,EPR,Cobalt,Biokemi,metabolism,Biochemistry},
  language     = {eng},
  pages        = {38},
  publisher    = {ARRAY(0xaa87630)},
  title        = {Mimicking Photosystem II with Manganese Model Complexes to Approach Artificial Photosynthesis},
  year         = {2005},
}