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1.
Phys Chem Chem Phys ; 22(35): 19982-19991, 2020 Sep 16.
Artigo em Inglês | MEDLINE | ID: mdl-32869045

RESUMO

We report the observation of electron spin polarization transfer from the triplet state of a porphyrin to a weakly coupled nitroxide radical in a mutant of human neuroglobin (NGB). The native iron-containing heme substrate of NGB has been substituted with Zn(ii) protoporphyrin IX and the nitroxide has been attached via site-directed spin labeling to the Cys120 residue. A reference synthetic polypeptide with free base tetraphenylporphyrin and a nitroxide bound to it is also studied. In both systems the nitroxide and the porphyrin are held at a fixed distance of approximately 2.4 nm. The transient EPR data of the NGB sample show that the triplet state of Zn(ii) protoporphyrin acquires significant net polarization, which is attributed to the dynamic Jahn-Teller effect. As the spin polarization of the protoporphyrin triplet state decays, a polarized EPR signal of the nitroxide arises. In contrast, the free base porphyrin in the reference polypeptide does not acquire net polarization and no polarization of the nitroxide label is observed. This is likely a result of the fact that the porphyrin is not Jahn-Teller active because of its lower symmetry. A perturbation theory treatment suggests that in the NGB sample, the polarization of the radical occurs by the transfer of net polarization from the triplet state. This process is also enhanced by the spectral broadening caused by the back and forth transitions associated with the dynamic Jahn-Teller effect. We propose that the novel transfer of polarization to the radical could be exploited to enhance the sensitivity of light-induced dipolar spectroscopy experiments.

2.
J Chem Phys ; 152(3): 034201, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31968969

RESUMO

The nature of the photoexcited triplet state of free-base 5,10,15,20-tetrakis(4-sulfonatophenyl)porphyrin (H2TPPS4-) has been investigated by advanced Electron Paramagnetic Resonance (EPR) techniques combined with quantum chemical calculations. The zero-field splitting (ZFS) parameters, D and E, the orientation of the transition dipole moment in the ZFS tensor frame, and the proton hyperfine couplings have been determined by magnetophotoselection-EPR and pulse electron-nuclear double resonance spectroscopy. Both time-resolved and pulse experiments exploit the electron spin polarization of the photoexcited triplet state. Comparison of the magnetic observables with computational results, including CASSCF calculations of the ZFS interaction tensor, provides an accurate picture of the triplet-state electronic structure. The theoretical investigation has been integrated with a systematic analysis on the parent free-base porphyrin molecule to assess the effect of the sulfonatophenyl substituents on the magnetic tensors. Additionally, the magnetophotoselection effects are discussed in terms of tautomerization in the excited singlet state of H2TPPS4-.

3.
J Phys Chem B ; 123(39): 8232-8239, 2019 10 03.
Artigo em Inglês | MEDLINE | ID: mdl-31502466

RESUMO

An investigation of the photoexcited triplet state of chlorophyll (Chl) b has been carried out by means of electron nuclear double resonance, both in a frozen organic solvent and in a protein environment provided by the water-soluble chlorophyll protein of Lepidium virginicum. Density functional theory calculations have allowed the complete assignment of the observed hyperfine couplings corresponding to the methine protons and the methyl groups, leading to a complete picture of the spin density distribution of the triplet state in the tetrapyrrole macrocycle. The triplet-state properties of Chl b are found to be similar, in many respects, to those previously reported for Chl a, although some specificities have been highlighted. Concerning the spin density distribution, the differences are mainly localized on the carbon atoms close to the formyl group which, in Chl b, replaces the methyl group of Chl a.

4.
Chemphyschem ; 20(7): 931-935, 2019 04 02.
Artigo em Inglês | MEDLINE | ID: mdl-30817078

RESUMO

Light-induced pulsed EPR dipolar spectroscopic methods allow the determination of nanometer distances between paramagnetic sites. Here we employ orthogonal spin labels, a chromophore triplet state and a stable radical, to carry out distance measurements in singly nitroxide-labeled human neuroglobin. We demonstrate that Zn-substitution of neuroglobin, to populate the Zn(II) protoporphyrin IX triplet state, makes it possible to perform light-induced pulsed dipolar experiments on hemeproteins, extending the use of light-induced dipolar spectroscopy to this large class of metalloproteins. The versatility of the method is ensured by the employment of different techniques: relaxation-induced dipolar modulation enhancement (RIDME) is applied for the first time to the photoexcited triplet state. In addition, an alternative pulse scheme for laser-induced magnetic dipole (LaserIMD) spectroscopy, based on the refocused-echo detection sequence, is proposed for accurate zero-time determination and reliable distance analysis.


Assuntos
Neuroglobina/química , Óxidos N-Cíclicos/química , Cisteína/química , Espectroscopia de Ressonância de Spin Eletrônica , Humanos , Luz , Mesilatos/química , Estrutura Molecular , Mutação , Neuroglobina/genética , Protoporfirinas/química , Protoporfirinas/efeitos da radiação , Marcadores de Spin
5.
Biochim Biophys Acta Bioenerg ; 1859(8): 612-618, 2018 08.
Artigo em Inglês | MEDLINE | ID: mdl-29782823

RESUMO

The peridinin-chlorophyll-a protein (PCP) is a water-soluble light harvesting protein of the dinoflagellate Amphidinium carterae, employing peridinin (Per) as the main carotenoid to fulfil light harvesting and photo-protective functions. Per molecules bound to the protein experience specific molecular surroundings which lead to different electronic and spectral properties. In the refolded N89 L variant PCP (N89 L-RFPCP) a significant part of the intensity on the long wavelength side of the absorption spectrum is shifted to shorter wavelengths due to a significant change in the Per-614 site energy. Since Per-614 has been shown to be the main chlorophyll (Chl) triplet quencher in the protein, and the relative geometry of pigments is not affected by the mutation as verified by X-ray crystallography, this variant is ideally suited to study the dependence of the triplet-triplet energy transfer (TTET) mechanism on the pigment site energy. By using a combination of Optically Detected Magnetic Resonance (ODMR), pulse Electron Paramagnetic Resonance (EPR) and Electron Nuclear DOuble Resonance (ENDOR) we found that PCP maintains the efficient Per-614-to-Chl-a TTET despite the change of Per-614 local energy. This shows the robustness of the photoprotective site, which is very important for the protection of the system.


Assuntos
Carotenoides/química , Clorofila/química , Transferência de Energia , Fotossíntese , Proteínas de Protozoários/química , Spiroplasma/química , Dinoflagelados/metabolismo , Espectroscopia de Ressonância de Spin Eletrônica , Modelos Moleculares , Conformação Proteica
6.
Phys Chem Chem Phys ; 19(40): 27173-27177, 2017 Oct 18.
Artigo em Inglês | MEDLINE | ID: mdl-28991960

RESUMO

In this work, the electronic structure of the triplet state of self-assembled J-aggregates of tetrakis(4-sulfonatophenyl)porphyrin (TPPS) has been characterized by means of time-resolved electron paramagnetic resonance spectroscopy. Several insights into the triplet properties of the aggregate have been gained through comparison with the corresponding monomeric unit in both free base and acidified forms. Molecular distortions in the monomeric acidified TPPS cause variation in its zero-field splitting parameters and a redirection of triplet spin sublevel activity. The aggregation process does not alter the mechanism of triplet state population compared to the acidified monomer but it is accompanied by a further reduction in the zero-field splitting parameter D, which is possibly indicative of the formation of a delocalized triplet state species. The detection of a long-lived spin-polarized radical species also proves polaron generation and movement to a trap site in the J-aggregates.

7.
Chemistry ; 22(48): 17204-17214, 2016 Nov 21.
Artigo em Inglês | MEDLINE | ID: mdl-27868323

RESUMO

We present a novel pulsed electron paramagnetic resonance (EPR) spectroscopic ruler to test the performance of a recently developed spin-labeling method based on the photoexcited triplet state (S=1). Four-pulse electron double resonance (PELDOR) experiments are carried out on a series of helical peptides, labeled at the N-terminal end with the porphyrin moiety, which can be excited to the triplet state, and with the nitroxide at various sequence positions, spanning distances in the range 1.8-8 nm. The PELDOR traces provide accurate distance measurements for all the ruler series, showing deep envelope modulations at frequencies varying in a progressive way according to the increasing distance between the spin labels. The upper limit is evaluated and found to be around 8 nm. The PELDOR-derived distances are in excellent agreement with theoretical predictions. We demonstrate that high sensitivity is acquired using the triplet state as a spin label by comparison with Cu(II)-porphyrin analogues. The new labeling approach has a high potential for measuring nanometer distances in more complex biological systems due to the properties of the porphyrin triplet state.

8.
Biochim Biophys Acta ; 1857(12): 1909-1916, 2016 12.
Artigo em Inglês | MEDLINE | ID: mdl-27659505

RESUMO

Triplet-triplet energy transfer from chlorophylls to carotenoids is the mechanism underlying the photoprotective role played by carotenoids in many light harvesting complexes, during photosynthesis. The peridinin-chlorophyll-a protein (PCP) is a water-soluble light harvesting protein of the dinoflagellate Amphidinium carterae, employing peridinin as the main carotenoid to fulfil this function. The dipolar coupling of the triplet state of peridinin, populated under light excitation in isolated PCP, to the MTSSL nitroxide, introduced in the protein by site-directed mutagenesis followed by spin labeling, has been measured by Pulse ELectron-electron DOuble Resonance (PELDOR) spectroscopy. The triplet-nitroxide distance derived by this kind of experiments, performed for the first time in a protein system, allowed the assignment of the triplet state to a specific peridinin molecule belonging to the pigment cluster. The analysis strongly suggests that this peridinin is the one in close contact with the water ligand to the chlorophyll a, thus supporting previous evidences based on ENDOR and time resolved-EPR.


Assuntos
Carotenoides/efeitos da radiação , Espectroscopia de Ressonância de Spin Eletrônica , Luz , Fotossíntese/efeitos da radiação , Proteínas de Protozoários/efeitos da radiação , Carotenoides/química , Carotenoides/metabolismo , Transferência de Energia , Ligantes , Modelos Moleculares , Ligação Proteica , Conformação Proteica , Proteínas de Protozoários/química , Proteínas de Protozoários/metabolismo , Marcadores de Spin , Relação Estrutura-Atividade , Água/química , Água/metabolismo
9.
Chemistry ; 22(26): 8745-50, 2016 Jun 20.
Artigo em Inglês | MEDLINE | ID: mdl-27123774

RESUMO

Gaining detailed information on the structural rearrangements associated with stimuli-induced molecular movements is of utmost importance for understanding the operation of molecular machines. Pulsed electron-electron double resonance (PELDOR) was employed to monitor the geometrical changes arising upon chemical switching of a [2]rotaxane that behaves as an acid-base-controlled molecular shuttle. To this aim, the rotaxane was endowed with stable nitroxide radical units in both the ring and axle components. The combination of PELDOR data and molecular dynamic calculations indicates that in the investigated rotaxane, the ring displacement along the axle, caused by the addition of a base, does not alter significantly the distance between the nitroxide labels, but it is accompanied by a profound change in the geometry adopted by the macrocycle.

10.
Faraday Discuss ; 185: 121-41, 2015.
Artigo em Inglês | MEDLINE | ID: mdl-26400662

RESUMO

The increasing global energy demand has stimulated great recent efforts in investigating new solutions for artificial photosynthesis, a potential source of clean and renewable solar fuel. In particular, according to the generally accepted modular approach aimed at optimising separately the different compartments of the entire process, many studies have focused on the development of catalytic systems for water oxidation to oxygen. While in recent years there have been many reports on new catalytic systems, the mechanism and the active intermediates operating the catalysis have been less investigated. Well-defined, molecular catalysts, constituted by transition metals stabilised by a suitable ligand pool, could help in solving this aspect. However, in some cases molecular species have been shown to evolve to active metal oxides that constitute the other side of this catalysis dichotomy. In this paper, we address the evolution of tetracobalt(III) cubanes, stabilised by a pyridine/acetate ligand pool, to active species that perform water oxidation to oxygen. Primary evolution of the cubane in aqueous solution is likely initiated by removal of an acetate bridge, opening the coordination sphere of the cobalt centres. This cobalt derivative, where the pristine ligands still impact on the reactivity, shows enhanced electron transfer rates to Ru(bpy)3(3+) (hole scavenging) within a photocatalytic cycle with Ru(bpy)3(2+) as the photosensitiser and S2O8(2-) as the electron sink. A more accentuated evolution occurs under continuous irradiation, where Electron Paramagnetic Resonance (EPR) spectroscopy reveals the formation of Co(ii) intermediates, likely contributing to the catalytic process that evolves oxygen. All together, these results confirm the relevant effect of molecular species, in particular in fostering the rate of the electron transfer processes involved in light activated cycles, pivotal in the design of a photoactive device.

11.
J Phys Chem B ; 119(43): 13680-9, 2015 Oct 29.
Artigo em Inglês | MEDLINE | ID: mdl-25978307

RESUMO

The catalytic site of [FeFe]-hydrogenase, the "H-cluster", composed of a [4Fe-4S] unit connected by a cysteinyl residue to a [2Fe] center coordinated by three CO, two CN(-), and a bridging dithiolate, is assembled in a complex maturation pathway, at present not fully characterized, involving three conserved proteins, HydG, HydE, and HydF. HydF is a complex enzyme, which is thought to act as a scaffold and carrier for the [2Fe] subunit of the H-cluster. This maturase protein contains itself a [4Fe-4S] cluster binding site, with three conserved cysteine residues and a noncysteinyl fourth ligand. In this work, we have exploited 3p-ESEEM and HYSCORE spectroscopies to get insight into the structure and the chemical environment of the [4Fe-4S] cluster of HydF from the hyperthermophilic organism Thermotoga neapolitana. The nature of the fourth ligand and the solvent accessibility of the active site comprising the [4Fe-4S] cluster are discussed on the basis of the spectroscopic results obtained upon H/D exchange. We propose that the noncysteinyl ligated Fe atom of the [4Fe-4S] cluster is the site where the [2Fe] subcluster precursor is anchored and finally processed to be delivered to the hydrogenase (HydA).


Assuntos
Hidrogenase/química , Proteínas com Ferro-Enxofre/química , Thermotoga neapolitana/enzimologia , Espectroscopia de Ressonância de Spin Eletrônica , Hidrogenase/genética , Hidrogenase/metabolismo , Proteínas com Ferro-Enxofre/metabolismo , Solventes/química
12.
J Am Chem Soc ; 136(18): 6582-5, 2014 May 07.
Artigo em Inglês | MEDLINE | ID: mdl-24735449

RESUMO

This work demonstrates, for the first time, the feasibility of applying pulsed electron-electron double resonance (PELDOR/DEER) to determine the interspin distance between a photoexcited porphyrin triplet state (S = 1) and a nitroxide spin label chemically incorporated into a small helical peptide. The PELDOR trace shows deep envelope modulation induced by electron-electron dipole interaction between the partners in the pair, providing an accurate distance measurement. This new labeling approach has a high potential for measuring nanometer distances in more complex biological systems due to the sensitivity acquired from the spin polarization of the photoexcited triplet state spectrum.

13.
Curr Protein Pept Sci ; 15(4): 332-50, 2014.
Artigo em Inglês | MEDLINE | ID: mdl-24678668

RESUMO

Peridinin-Chlorophyll-α-Proteins (PCPs) are water-soluble light harvesting complexes from dinoflagellates. They have unique light-harvesting and energy transfer properties which have been studied in details in the last 15 years. This review aims to give an overview on all the main aspects of PCPs photophysics, with an emphasis on some aspects which have not been reviewed in details so far, such as vibrational spectroscopy studies, theoretical calculations, and magnetic resonance studies. A paragraph on the present development of PCPs towards technological applications is also included.


Assuntos
Carotenoides/química , Carotenoides/metabolismo , Dinoflagelados/química , Proteínas de Protozoários/química , Proteínas de Protozoários/metabolismo , Dinoflagelados/metabolismo , Processos Fotoquímicos
14.
Biochim Biophys Acta ; 1837(8): 1235-46, 2014 Aug.
Artigo em Inglês | MEDLINE | ID: mdl-24704151

RESUMO

Violaxanthin-chlorophyll a binding protein (VCP) is the major light harvesting complex (LHC) of the Heterokonta Nannochloropsis gaditana. It binds chlorophyll a, violaxanthin and vaucheriaxanthin, the last in the form of 19' deca/octanoate esters. Photosynthetic apparatus of algae belonging to this group have been poorly characterized in the past, but they are now receiving an increasing interest also because of their possible biotechnological application in biofuel production. In this work, isolated VCP proteins have been studied by means of advanced EPR techniques in order to prove the presence of the photoprotective mechanism based on the triplet-triplet energy transfer (TTET), occurring between chlorophyll and carotenoid molecules. This process has been observed before in several light harvesting complexes belonging to various photosynthetic organisms. We used Optically Detected Magnetic Resonance (ODMR) to identify the triplet states populated by photo-excitation, and describe the optical properties of the chromophores carrying the triplet states. In parallel, time-resolved EPR (TR-EPR) and pulse EPR have been employed to get insight into the TTET mechanism and reveal the structural features of the pigment sites involved in photoprotection. The analysis of the spectroscopic data shows a strong similarity among VCP, FCP from diatoms and LHC-II from higher plants. Although these antenna proteins have differentiated sequences and bind different pigments, results suggest that in all members of the LHC superfamily there is a protein core with a conserved structural organization, represented by two central carotenoids surrounded by five chlorophyll a molecules, which plays a fundamental photoprotective role in Chl triplet quenching through carotenoid triplet formation.


Assuntos
Proteínas de Ligação à Clorofila/genética , Clorofila/genética , Fotossíntese/genética , Sequência de Aminoácidos , Carotenoides/química , Carotenoides/genética , Clorofila/química , Clorofila/metabolismo , Clorofila A , Proteínas de Ligação à Clorofila/química , Transferência de Energia , Complexos de Proteínas Captadores de Luz/genética , Conformação Proteica , Estramenópilas/genética , Estramenópilas/crescimento & desenvolvimento , Xantofilas/química , Xantofilas/genética
15.
Biochim Biophys Acta ; 1837(1): 85-97, 2014 Jan.
Artigo em Inglês | MEDLINE | ID: mdl-23871938

RESUMO

Experimental and theoretical studies indicate that water molecules between redox partners can significantly affect their electron-transfer and possibly also the triplet-triplet energy transfer (TTET) properties when in the vicinity of chromophores. In the present work, the interaction of an intervening water molecule with the peridinin triplet state in the peridinin-chlorophyll a-protein (PCP) from Amphidinium carterae is studied by using orientation selective (2)H electron spin echo envelope modulation (ESEEM) spectroscopy, in conjunction with quantum mechanical calculations. This water molecule is located at the interface between the chlorophyll and peridinin pigments involved in the photoprotection mechanism (Chl601(602)-Per614(624), for nomenclature see reference [1]), based on TTET. The characteristic deuterium modulation pattern is observed in the electron spin-echo envelopes for the PCP complex exchanged against (2)H2O. Simulations of the time- and frequency-domain two-pulse and three-pulse ESEEM require two types of coupled (2)H. The more strongly coupled (2)H has an isotropic coupling constant (aiso) of -0.4MHz. This Fermi contact contribution for one of the two water protons and the precise geometry of the water molecule at the interface between the chlorophyll and peridinin pigments, resulting from the analysis, provide experimental evidence for direct involvement of this structured water molecule in the mechanism of TTET. The PCP antenna, characterised by a unity efficiency of the process, represents a model for future investigations on protein- and solvent-mediated TTET in the field of natural/artificial photosynthesis.


Assuntos
Carotenoides/química , Proteínas de Ligação à Clorofila/química , Transferência de Energia , Fotossíntese , Água/química , Carotenoides/metabolismo , Proteínas de Ligação à Clorofila/metabolismo , Dinoflagelados/química , Elétrons , Complexos de Proteínas Captadores de Luz/química , Modelos Moleculares
16.
Chem Commun (Camb) ; 49(85): 9941-3, 2013 Nov 04.
Artigo em Inglês | MEDLINE | ID: mdl-24036577

RESUMO

A salophen cobalt(II) complex enables water oxidation at neutral pH in photoactivated sacrificial cycles under visible light, thus confirming the high appeal of earth abundant single site catalysis for artificial photosynthesis.


Assuntos
Cobalto/química , Complexos de Coordenação/química , Luz , Salicilatos/química , Água/química , Catálise , Modelos Biológicos , Estrutura Molecular , Oxirredução
17.
Biochim Biophys Acta ; 1827(10): 1226-34, 2013 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-23856166

RESUMO

Although the major light harvesting complexes of diatoms, called FCPs (fucoxanthin chlorophyll a/c binding proteins), are related to the cab proteins of higher plants, the structures of these light harvesting protein complexes are much less characterized. Here, a structural/functional model for the "core" of FCP, based on the sequence homology with LHCII, in which two fucoxanthins replace the central luteins and act as quenchers of the Chl a triplet states, is proposed. Combining the information obtained by time-resolved EPR spectroscopy on the triplet states populated under illumination, with quantum mechanical calculations, we discuss the chlorophyll triplet quenching in terms of the geometry of the chlorophyll-carotenoid pairs participating to the process. The results show that local structural rearrangements occur in FCP, with respect to LHCII, in the photoprotective site.


Assuntos
Proteínas de Ligação à Clorofila/química , Clorofila/metabolismo , Diatomáceas/metabolismo , Xantofilas/metabolismo , Clorofila A , Proteínas de Ligação à Clorofila/metabolismo , Espectroscopia de Ressonância de Spin Eletrônica , Transferência de Energia , Luz
18.
Biochem Biophys Res Commun ; 427(3): 637-41, 2012 Oct 26.
Artigo em Inglês | MEDLINE | ID: mdl-23026044

RESUMO

In this work we present an optically detected magnetic resonance (ODMR) study on the triplet states populated under illumination in the isolated fucoxanthin-chlorophyll light-harvesting complex from the diatom Cyclotella meneghiniana. Evidence for the quenching of chlorophyll triplet states by fucoxanthin is provided, showing that this carotenoid is able to perform the photoprotective role. For the first time, the magnetic parameters characterizing the fucoxanthin triplet state have been determined. The results reveal analogies but also differences with respect to the triplet-triplet energy transfer process, which involves chlorophylls a and carotenoids in the LHC complex from dinoflagellates and LHCII from higher plants. The degree of efficiency of the photoprotection mechanism, in these light harvesting complexes, is discussed in terms of pigment-protein structure.


Assuntos
Proteínas de Ligação à Clorofila/química , Clorofila/química , Diatomáceas/enzimologia , Xantofilas/química , Fluorescência , Espectroscopia de Ressonância Magnética , Conformação Proteica
19.
Biochim Biophys Acta ; 1817(12): 2149-57, 2012 Dec.
Artigo em Inglês | MEDLINE | ID: mdl-22985598

RESUMO

[FeFe] hydrogenases are key enzymes for bio(photo)production of molecular hydrogen, and several efforts are underway to understand how their complex active site is assembled. This site contains a [4Fe-4S]-2Fe cluster and three conserved maturation proteins are required for its biosynthesis. Among them, HydF has a double task of scaffold, in which the dinuclear iron precursor is chemically modified by the two other maturases, and carrier to transfer this unit to a hydrogenase containing a preformed [4Fe-4S]-cluster. This dual role is associated with the capability of HydF to bind and dissociate an iron-sulfur center, due to the presence of the conserved FeS-cluster binding sequence CxHx(46-53)HCxxC. The recently solved three-dimensional structure of HydF from Thermotoga neapolitana described the domain containing the three cysteines which are supposed to bind the FeS cluster, and identified the position of two conserved histidines which could provide the fourth iron ligand. The functional role of two of these cysteines in the activation of [FeFe]-hydrogenases has been confirmed by site-specific mutagenesis. On the other hand, the contribution of the three cysteines to the FeS cluster coordination sphere is still to be demonstrated. Furthermore, the potential role of the two histidines in [FeFe]-hydrogenase maturation has never been addressed, and their involvement as fourth ligand for the cluster coordination is controversial. In this work we combined site-specific mutagenesis with EPR (electron paramagnetic resonance) and HYSCORE (hyperfine sublevel correlation spectroscopy) to assign a role to these conserved residues, in both cluster coordination and hydrogenase maturation/activation, in HydF proteins from different microorganisms.


Assuntos
Espectroscopia de Ressonância de Spin Eletrônica , Hidrogenase/química , Hidrogenase/metabolismo , Proteínas com Ferro-Enxofre/química , Proteínas com Ferro-Enxofre/metabolismo , Thermotoga neapolitana/enzimologia , Sítios de Ligação , Domínio Catalítico , Hidrogênio/metabolismo , Hidrogenase/genética , Proteínas com Ferro-Enxofre/genética , Mutagênese Sítio-Dirigida , Ligação Proteica , Conformação Proteica , Thermotoga neapolitana/crescimento & desenvolvimento
20.
Phys Chem Chem Phys ; 14(35): 12238-51, 2012 Sep 21.
Artigo em Inglês | MEDLINE | ID: mdl-22864767

RESUMO

Carotenoid molecules are essential for the life of photosynthetic organisms in that they protect the cell from the photo-oxidative damage induced by light-stress conditions. One of the photo-protective mechanisms involves triplet-triplet energy transfer from the chlorophyll molecules to the carotenoids: a process that is strongly dependent on the electronic properties of the triplet states involved. Here, we obtain a clear description of the triplet state of lutein in LHCII from higher plants for the first time by density functional theory (DFT) calculations. DFT predictions have been validated by comparison with hyperfine couplings obtained with pulsed-ENDOR spectroscopy. Knowledge of the spin density distribution, the frontier orbitals and orbital excitations forms a basis for discussing the requirements for an efficient triplet-triplet energy transfer. The results obtained for the lutein in LHCII are compared with those of the highly-substituted carotenoid peridinin in PCP from Amphidinium carterae [Di Valentin et al., Biochim. Biophys. Acta, 2008, 1777, 295-307]. The presence of substituents in the peridinin molecule does not alter significantly the triplet state electronic structure compared to lutein. Despite the unusual spectroscopic behaviour of the peridinin excited singlet state, lutein and peridinin have similar triplet state properties. In both molecules the unpaired spins are delocalized uniformly over the whole π-conjugated system in an alternating even-odd pattern.


Assuntos
Complexos de Proteínas Captadores de Luz/química , Luteína/química , Spinacia oleracea/química , Spinacia oleracea/enzimologia , Carotenoides/química , Espectroscopia de Ressonância de Spin Eletrônica , Elétrons , Modelos Moleculares , Conformação Molecular , Teoria Quântica
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