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1.
J Chem Phys ; 152(3): 035103, 2020 Jan 21.
Artigo em Inglês | MEDLINE | ID: mdl-31968956

RESUMO

The molecular dynamics (MD) computer simulation technique is powerful for the investigation of conformational equilibrium properties of biomolecules. In particular, free energy surfaces of the torsion angles (those degrees of freedom from which the geometry mostly depends) allow one to access conformational states, as well as kinetic information, i.e., if the transitions between conformational states occur by simple jumps between wells or if conformational regions close to these states also are populated. The information obtained from MD simulations may depend substantially on the force field employed, and thus, a validation procedure is essential. NMR relaxation data are expected to be highly sensitive to the details of the torsional free energy surface. As a case-study, we consider the disaccharide α-l-Rhap-(1 → 2)-α-l-Rhap-OMe that features only two important torsion angles, ϕ and ψ, which define the interglycosidic orientation of the sugar residues relative to each other, governed mainly by the exo-anomeric effect and steric interactions, respectively. In water, a ψ- state is preferred, whereas in DMSO, it is a ψ+ state, suggesting inherent flexibility at the torsion angle. MD simulations indicated that bistable potentials describe the conformational region well. To test whether a unimodal distribution suffices or if a bimodal distribution better represents molecular conformational preferences, we performed an alchemical morphing of the torsional free energy surface and computed T1, T2, and NOE13C NMR relaxation data that were compared to experimental data. All three NMR observables are substantially affected by the morphing procedure, and the results strongly support a bimodal Boltzmann equilibrium density with a major and a minor conformational state bisected at ψ ≈ 0°, in accord with MD simulations in an explicit solvent.

2.
Phys Chem Chem Phys ; 22(6): 3455-3465, 2020 Feb 12.
Artigo em Inglês | MEDLINE | ID: mdl-31984980

RESUMO

We propose an approach to the evaluation of kinetic rates of elementary chemical reactions within Kramers' theory based on the definition of the reaction coordinate as a linear combination of natural, pseudo Z-matrix, internal coordinates of the system. The element of novelty is the possibility to evaluate the friction along the reaction coordinate, within a hydrodynamic framework developed recently [J. Campeggio et al., J. Comput. Chem. 2019, 40, 679-705]. This, in turn, allows to keep into account barrier recrossing, i.e. the transmission coefficient that is employed in correcting transition state theory evaluations. To test the capabilities and the flaws of the approach we use as case studies two archetypal SN2 reactions. First, we consider to the standard substitution of chloride ion to bromomethane. The rate constant at 295.15 K is evaluated to k/c⊖ = 2.7 × 10-6 s-1 (with c⊖ = 1 M), which compares well to the experimental value of 3.3 × 10-6 s-1 [R. H. Bathgate and E. A. Melwyn-Hughes, J. Chem. Soc 1959, 2642-2648]. Then, the method is applied to the SN2 reaction of methylthiolate to dimethyl disulfide in water. In biology, such an interconversion of thiols and disulfides is an important metabolic topic still not entirely rationalized. The predicted rate constant is k/c⊖ = 7.7 × 103 s-1. No experimental data is available for such a reaction, but it is in accord with the fact that the alkyl thiolates to dialkyl disulfides substitutions in water have been found to be fast reactions [S. M. Bachrach, J. M. Hayes, T. Dao and J. L. Mynar, Theor. Chem. Acc. 2002, 107, 266-271].

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.
J Chem Phys ; 150(18): 184108, 2019 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-31091922

RESUMO

In Paper I [Polimeno et al., J. Chem. Phys. 150, 184107 (2019)], we proposed a general approach for interpreting relaxation properties of a macromolecule in solution, derived from an atomistic description. A simple scheme (the semiflexible Brownian, SFB, model) has been defined for the case of limited internal flexibility, but retaining full coupling with external degrees of freedom, inclusion of all of the momenta, and dissipation. Here we discuss the application of the SFB model to the practical evaluation of orientation spectral densities, based on two complementary computational treatments.

5.
J Chem Phys ; 150(18): 184107, 2019 May 14.
Artigo em Inglês | MEDLINE | ID: mdl-31091939

RESUMO

A framework for the stochastic description of relaxation processes in flexible macromolecules, including dissipative effects, is introduced from an atomistic point of view. Projection-operator techniques are employed to obtain multidimensional Fokker-Planck operators governing the relaxation of internal coordinates and global degrees of freedom and depending upon parameters fully recoverable from classic force fields (energetics) and continuum models (friction tensors). A hierarchy of approaches of different complexity is proposed in this unified context, aimed primarily at the interpretation of magnetic resonance relaxation experiments. In particular, a model based on a harmonic internal Hamiltonian is discussed as a test case.

6.
Am J Physiol Cell Physiol ; 316(5): C722-C730, 2019 05 01.
Artigo em Inglês | MEDLINE | ID: mdl-30865515

RESUMO

Electron paramagnetic resonance (EPR), coupled with site-directed spin labeling, has been proven to be a particularly suitable technique to extract information on the fraction of myosin heads strongly bound to actin upon muscle contraction. The approach can be used to investigate possible structural changes occurring in myosin of fiber s altered by diseases and aging. In this work, we labeled myosin at position Cys707, located in the SH1-SH2 helix in the myosin head cleft, with iodoacetamide spin label, a spin label that is sensitive to the reorientational motion of this protein during the ATPase cycle and characterized the biochemical states of the labeled myosin head by means of continuous wave EPR. After checking the sensitivity and the power of the technique on different muscles and species, we investigated whether changes in the fraction of strongly bound myosin heads might explain the contractile alterations observed in atrophic and hypertrophic murine muscles. In both conditions, the difference in contractile force could not be justified simply by the difference in muscle mass. Our results showed that in atrophic muscles the decrease in force generation was attributable to a lower fraction of strongly bound cross bridges during maximal activation. In contrast in hypertrophic muscles, the increase in force generation was likely due to several factors, as pointed out by the comparison of the EPR experiments with the tension measurements on single skinned fibers.


Assuntos
Contração Muscular/fisiologia , Músculo Esquelético/patologia , Músculo Esquelético/fisiologia , Atrofia Muscular/patologia , Atrofia Muscular/fisiopatologia , Animais , Espectroscopia de Ressonância de Spin Eletrônica/métodos , Humanos , Hipertrofia/patologia , Hipertrofia/fisiopatologia , Camundongos , Camundongos Knockout , Camundongos Transgênicos , Técnicas de Cultura de Órgãos , Coelhos
7.
J Comput Chem ; 40(4): 697-705, 2019 02 05.
Artigo em Inglês | MEDLINE | ID: mdl-30549075

RESUMO

We report on an extended hydrodynamic modeling of the friction tensorial properties of flexible molecules including all types of natural, Z-Matrix like, internal coordinates. We implement the new methodology by extending and updating the software DiTe [Barone et al. J. Comput. Chem. 30, 2 (2009)]. DiTe (DIffusion TEnsor) implements a hydrodynamic modeling of the generalized translational, rotational, and configurational friction and diffusion tensors of flexible molecules in which flexibility is described in terms of dihedral angles. The new tool, DiTe2, has been renewed to include also stretching and bending types of internal mobility. Furthermore, DiTe2 is able to calculate the friction and diffusion tensors along collective (or reaction) coordinates defined as linear combinations of the internal natural ones. A number of tests are reported to show the new features of DiTe2. As leitmotiv for the tests, the calmodulin protein is taken into consideration, described both at all-atom and coarse-grained levels. © 2018 Wiley Periodicals, Inc.

8.
Phys Chem Chem Phys ; 21(7): 3662-3668, 2019 Feb 13.
Artigo em Inglês | MEDLINE | ID: mdl-30417189

RESUMO

We show that under proper assumptions it is possible to estimate with good precision the principal values of the rotational diffusion tensor of proteins from the analysis of short (up to 2-3 ns) molecular dynamics trajectories. We apply this analysis to a few model cases: three polyalanine peptides (2, 5, and 10 aminoacids), the fragment B3 of protein G (GB3), the bovine pancreatic trypsin inhibitor (BPTI), the hen egg-white lysozyme (LYS), the B1 domain of plexin (PB1), and thrombin. The protocol is based on the analysis of the global angular momentum autocorrelation functions, complementing the standard approach based on rotational autocorrelation functions, which requires much longer trajectories. A comparison with values predicted by hydrodynamic modeling and available experimental data is presented.

9.
J Phys Chem B ; 122(8): 2287-2294, 2018 03 01.
Artigo em Inglês | MEDLINE | ID: mdl-29385337

RESUMO

Among biomolecules, carbohydrates are unique in that not only can linkages be formed through different positions, but the structures may also be branched. The trisaccharide ß-d-Glcp-(1→3)[ß-d-Glcp-(1→2)]-α-d-Manp-OMe represents a model of a branched vicinally disubstituted structure. A 13C site-specific isotopologue, with labeling in each of the two terminal glucosyl residues, enabled the acquisition of high-quality 13C NMR relaxation parameters, T1 and T2, and heteronuclear NOE, with standard deviations of ≤0.5%. For interpretation of the experimental NMR data, a diffusive chain model was used, in which the dynamics of the glycosidic linkages is coupled to the global reorientation motion of the trisaccharide. Brownian dynamics simulations relying on the potential of mean force at the glycosidic linkages were employed to evaluate spectral densities of the spin probes. Calculated NMR relaxation parameters showed a very good agreement with experimental data, deviating <3%. The resulting dynamics are described by correlation times of 196 and 174 ps for the ß-(1→2)- and ß-(1→3)-linked glucosyl residues, respectively, i.e., different and linkage dependent. Notably, the devised computational protocol was performed without any fitting of parameters.


Assuntos
Glicosídeos/química , Simulação de Dinâmica Molecular , Oligossacarídeos/química , Configuração de Carboidratos , Espectroscopia de Ressonância Magnética Nuclear de Carbono-13/normas , Padrões de Referência , Processos Estocásticos
10.
J Phys Chem B ; 121(17): 4379-4387, 2017 05 04.
Artigo em Inglês | MEDLINE | ID: mdl-28422504

RESUMO

We address the interpretation, via an integrated computational approach, of the experimental continuous-wave electron paramagnetic resonance (cw-EPR) spectra of a complete set of conformationally highly restricted, stable 310-helical peptides from hexa- to nonamers, each bis-labeled with nitroxide radical-containing TOAC (4-amino-1-oxyl-2,2,6,6-tetramethylpiperidine-4-carboxylic acid) residues. The usefulness of TOAC for this type of analysis has been shown already to be due to its cyclic piperidine side chain, which is rigidly connected to the peptide backbone α-carbon. The TOAC α-amino acids are separated by two, three, four, and five intervening residues. This set of compounds has allowed us to modulate both the radical···radical distance and the relative orientation parameters. To further validate our conclusion, a comparative analysis has been carried out on three singly TOAC-labeled peptides of similar main-chain length.


Assuntos
Óxidos N-Cíclicos/química , Óxidos de Nitrogênio/química , Peptídeos/química , Teoria Quântica , Espectroscopia de Ressonância de Spin Eletrônica , Marcadores de Spin
11.
J Chem Theory Comput ; 13(1): 309-319, 2017 01 10.
Artigo em Inglês | MEDLINE | ID: mdl-28068775

RESUMO

In this article, we present a clustering method of atoms in proteins based on the analysis of the correlation times of interatomic distance correlation functions computed from MD simulations. The goal is to provide a coarse-grained description of the protein in terms of fewer elements that can be treated as dynamically independent subunits. Importantly, this domain decomposition method does not take into account structural properties of the protein. Instead, the clustering of protein residues in terms of networks of dynamically correlated domains is defined on the basis of the effective correlation times of the pair distance correlation functions. For these properties, our method stands as a complementary analysis to the customary protein decomposition in terms of quasi-rigid, structure-based domains. Results obtained for a prototypal protein structure illustrate the approach proposed.


Assuntos
Simulação de Dinâmica Molecular , Proteínas/química
12.
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.

13.
Biochemistry ; 55(28): 3984-94, 2016 07 19.
Artigo em Inglês | MEDLINE | ID: mdl-27347732

RESUMO

Thrombin exists as an ensemble of active (E) and inactive (E*) conformations that differ in their accessibility to the active site. Here we show that redistribution of the E*-E equilibrium can be achieved by perturbing the electrostatic properties of the enzyme. Removal of the negative charge of the catalytic Asp102 or Asp189 in the primary specificity site destabilizes the E form and causes a shift in the 215-217 segment that compromises substrate entrance. Solution studies and existing structures of D102N document stabilization of the E* form. A new high-resolution structure of D189A also reveals the mutant in the collapsed E* form. These findings establish a new paradigm for the control of the E*-E equilibrium in the trypsin fold.


Assuntos
Eletricidade Estática , Trombina/química , Biocatálise , Cristalografia por Raios X , Humanos , Simulação de Dinâmica Molecular , Conformação Proteica , Trombina/metabolismo
14.
J Phys Chem A ; 120(25): 4389-400, 2016 Jun 30.
Artigo em Inglês | MEDLINE | ID: mdl-27267296

RESUMO

The General AMBER Force Field (GAFF) has been extended to describe a series of selenium and tellurium diphenyl dichalcogenides. These compounds, besides being eco-friendly catalysts for numerous oxidations in organic chemistry, display peroxidase activity, i.e., can reduce hydrogen peroxide and harmful organic hydroperoxides to water/alcohols and as such are very promising antioxidant drugs. The novel GAFF parameters are tested in MD simulations in different solvents and the (77)Se NMR chemical shift of diphenyl diselenide is computed using structures extracted from MD snapshots and found in nice agreement with the measured value in CDCl3. The whole computational protocol is described in detail and integrated with in-house code to allow easy derivation of the force field parameters for analogous compounds as well as for Se/Te organocompounds in general.


Assuntos
Derivados de Benzeno/química , Modelos Moleculares , Compostos Organometálicos/química , Compostos Organosselênicos/química , Conformação Molecular , Teoria Quântica , Termodinâmica
15.
Phys Chem Chem Phys ; 18(4): 3086-96, 2016 Jan 28.
Artigo em Inglês | MEDLINE | ID: mdl-26741055

RESUMO

The monosaccharide L-rhamnose is common in bacterial polysaccharides and the disaccharide α-L-Rhap-α-(1 → 2)-α-L-Rhap-OMe represents a structural model for a part of Shigella flexneri O-antigen polysaccharides. Utilization of [1'-(13)C]-site-specific labeling in the anomeric position at the glycosidic linkage between the two sugar residues facilitated the determination of transglycosidic NMR (3)JCH and (3)JCC coupling constants. Based on these spin-spin couplings the major state and the conformational distribution could be determined with respect to the ψ torsion angle, which changed between water and dimethyl sulfoxide (DMSO) as solvents, a finding mirrored by molecular dynamics (MD) simulations with explicit solvent molecules. The (13)C NMR spin relaxation parameters T1, T2, and heteronuclear NOE of the probe were measured for the disaccharide in DMSO-d6 at two magnetic field strengths, with standard deviations ≤1%. The combination of MD simulation and a stochastic description based on the diffusive chain model resulted in excellent agreement between calculated and experimentally observed (13)C relaxation parameters, with an average error of <2%. The coupling between the global reorientation of the molecule and the local motion of the spin probe is deemed essential if reproduction of NMR relaxation parameters should succeed, since decoupling of the two modes of motion results in significantly worse agreement. Calculation of (13)C relaxation parameters based on the correlation functions obtained directly from the MD simulation of the solute molecule in DMSO as solvent showed satisfactory agreement with errors on the order of 10% or less.


Assuntos
Dimetil Sulfóxido/química , Glicosídeos/química , Espectroscopia de Ressonância Magnética Nuclear de Carbono-13 , Conformação Molecular , Simulação de Dinâmica Molecular , Processos Estocásticos
16.
Free Radic Biol Med ; 87: 1-14, 2015 Oct.
Artigo em Inglês | MEDLINE | ID: mdl-26163004

RESUMO

Glutathione peroxidases (GPxs) are enzymes working with either selenium or sulfur catalysis. They adopted diverse functions ranging from detoxification of H(2)O(2) to redox signaling and differentiation. The relative stability of the selenoenzymes, however, remained enigmatic in view of the postulated involvement of a highly unstable selenenic acid form during catalysis. Nevertheless, density functional theory calculations obtained with a representative active site model verify the mechanistic concept of GPx catalysis and underscore its efficiency. However, they also allow that the selenenic acid, in the absence of the reducing substrate, reacts with a nitrogen in the active site. MS/MS analysis of oxidized rat GPx4 complies with the predicted structure, an 8-membered ring, in which selenium is bound as selenenylamide to the protein backbone. The intermediate can be re-integrated into the canonical GPx cycle by glutathione, whereas, under denaturing conditions, its selenium moiety undergoes ß-cleavage with formation of a dehydro-alanine residue. The selenenylamide bypass prevents destruction of the redox center due to over-oxidation of the selenium or its elimination and likely allows fine-tuning of GPx activity or alternate substrate reactions for regulatory purposes.


Assuntos
Glutationa Peroxidase/química , Glutationa/química , Oxirredução , Selenocisteína/química , Animais , Catálise , Glutationa/metabolismo , Glutationa Peroxidase/metabolismo , Peróxido de Hidrogênio/química , Cinética , Teoria Quântica , Ratos , Selênio/química , Selenocisteína/metabolismo , Enxofre/química , Espectrometria de Massas em Tandem
17.
J Magn Reson ; 246: 94-103, 2014 Sep.
Artigo em Inglês | MEDLINE | ID: mdl-25117152

RESUMO

The analysis of NMR relaxation data is revisited along the lines of a Bayesian approach. Using a Markov Chain Monte Carlo strategy of data fitting, we investigate conditions under which relaxation data can be effectively interpreted in terms of internal dynamics. The limitations to the extraction of kinetic parameters that characterize internal dynamics are analyzed, and we show that extracting characteristic time scales shorter than a few tens of ps is very unlikely. However, using MCMC methods, reliable estimates of the marginal probability distributions and estimators (average, standard deviations, etc.) can still be obtained for subsets of the model parameters. Thus, unlike more conventional strategies of data analysis, the method avoids a model selection process. In addition, it indicates what information may be extracted from the data, but also what cannot.


Assuntos
Algoritmos , Interpretação Estatística de Dados , Espectroscopia de Ressonância Magnética/métodos , Cadeias de Markov , Modelos Estatísticos , Método de Monte Carlo , Simulação por Computador , Reconhecimento Automatizado de Padrão/métodos , Reprodutibilidade dos Testes , Sensibilidade e Especificidade
18.
Phys Chem Chem Phys ; 16(33): 17515-22, 2014 Sep 07.
Artigo em Inglês | MEDLINE | ID: mdl-25023568

RESUMO

Oxo-dipeptides and thio-dipeptides are built via condensation between couples of amino acids and amino thioacids, the latter with the carbonyl oxygen replaced by an sp(2) sulfur. We explored via in silico methods (PBE0/6-31G(d,p) and PBE0/6-311G(d,p)) all the possible combinations and built 800 dipeptides, whose structures were fully optimized. Maps of condensation energies are presented to highlight optimal partners leading to stable dipeptides and critical situations for which lower stability or instability is predicted in terms of Gibbs reaction free energies. To validate the feasibility of our computational investigation, we synthesized and compared the stabilities of two thionated dimers, namely -Gly[Ψ(CSNH)]Gly- and -Phe[Ψ(CSNH)]Phe-, characterized by diverging physico-chemical properties. To the best of our knowledge, this is the first systematic analysis reported for dipeptides built from natural amino acids as well as for their corresponding thio-analogs.


Assuntos
Aminoácidos/química , Dipeptídeos/química , Modelos Químicos , Modelos Moleculares , Oxigênio/química , Compostos de Sulfidrila/química , Simulação por Computador , Teste de Materiais , Transição de Fase , Conformação Proteica
19.
Biochem Biophys Res Commun ; 445(1): 10-5, 2014 Feb 28.
Artigo em Inglês | MEDLINE | ID: mdl-24468086

RESUMO

Connexin hemichannels are regulated by several gating mechanisms, some of which depend critically on the extracellular Ca(2+) concentration ([Ca(2+)]e). It is well established that hemichannel activity is inhibited at normal (∼1 mM) [Ca(2+)]e, whereas lowering [Ca(2+)]e to micromolar levels fosters hemichannel opening. Atomic force microscopy imaging shows significant and reversible changes of pore diameter at the extracellular mouth of Cx26 hemichannels exposed to different [Ca(2+)]e, however, the underlying molecular mechanisms are not fully elucidated. Analysis of the crystal structure of connexin 26 (Cx26) gap junction channels, corroborated by molecular dynamics (MD) simulations, suggests that several negatively charged amino acids create a favorable environment for low-affinity Ca(2+) binding within the extracellular vestibule of the Cx26 hemichannel. In particular a highly conserved glutammic acid, found in position 47 in most connexins, is thought to undergo post translational gamma carboxylation (γGlu47), and is thus likely to play an important role in Ca(2+) coordination. γGlu47 may also form salt bridges with two conserved arginines (Arg75 and Arg184 in Cx26), which are considered important in stabilizing the structure of the extracellular region. Using a combination of quantum chemistry methods, we analyzed the interaction between γGlu47, Arg75 and Arg184 in a Cx26 hemichannel model both in the absence and in the presence of Ca(2+). We show that Ca(2+) imparts significant local structural changes and speculate that these modifications may alter the structure of the extracellular loops in Cx26, and may thus account for the mechanism of hemichannel closure in the presence of mM [Ca(2+)]e.


Assuntos
Ácido 1-Carboxiglutâmico/metabolismo , Cálcio/metabolismo , Conexinas/metabolismo , Canais Iônicos/metabolismo , Ácido 1-Carboxiglutâmico/química , Animais , Arginina/química , Arginina/metabolismo , Cálcio/química , Cálcio/farmacologia , Conexina 26 , Conexinas/química , Humanos , Ativação do Canal Iônico/efeitos dos fármacos , Canais Iônicos/química , Microscopia de Força Atômica , Modelos Moleculares , Simulação de Dinâmica Molecular , Estrutura Molecular , Ligação Proteica , Estrutura Terciária de Proteína , Termodinâmica
20.
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
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