Química Física

URI permanente para esta comunidadhttps://hdl.handle.net/11441/10913

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A Comparative Study between Hydrogen Peroxide Amperometric Biosensors Based on Different Peroxidases Wired by Os-Polymer: Applications in Water, Milk and Human Urine
(Multidisciplinary Digital Publishing Institute (MDPI), 2017) Bollella, Paolo; Marcaccio, Massimo; Poloznikov, Andrey; Hushpulian, Dimitry; Andreu Fondacabe, Rafael Jesús; Leech, Donal; Gorton, Lo; Antiochia, Riccarda; Universidad de Sevilla. Departamento de Química Física
A convenient decontraction procedure of internally contracted state-specific multireference algorithms
(American Institute of Physics Publising LLC, 2006) Angeli, Celestino; Jiménez Calzado, Carmen; Cimiraglia, Renzo; Malrieu, J.P.; Universidad de Sevilla. Departamento de Química Física
Internally contracted state-specific multireference MR algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave function
A convenient decontraction procedure of internally contracted state-specific multireference algorithms
(American Institute of Physics, 2006) Angeli, Celestino; Jiménez Calzado, Carmen; Cimiraglia, Renzo; Malrieu, Jean Paul; Universidad de Sevilla. Departamento de Química Física
Internally contracted state-specific multireference (MR) algorithms, either perturbative such as CASPT2 or NEVPT2, or nonperturbative such as contracted MR configuration interaction or MR coupled cluster, are computationally efficient but they may suffer from the internal contraction of the wave function in the reference space. The use of a low dimensional multistate model space only offers limited flexibility and is not always practicable. The present paper suggests a convenient state-specific procedure to decontract the reference part of the wave function from a series of state-specific calculations using slightly perturbed zero-order wave functions. The method provides an orthogonal valence bond reading of the ground state and an effective valence Hamiltonian, the excited roots of which are shown to be relevant. The orthogonal valence bond functions can be considered quasidiabatic states and the effective valence Hamiltonian gives therefore the quasidiabatic energies and the electronic coupling among the quasidiabatic states. The efficiency of the method is illustrated in two case problems where the dynamical correlation plays a crucial role, namely, the LiF neutral/ionic avoided crossing and the F2 ground state wave function.
A coupled EXAFS-molecular dynamics study on PuO
(American Chemical Society, 2022) Raposo Hernández, Gema; Martínez Fernández, José Manuel; Rodríguez Pappalardo, Rafael; den Auwer, Christophe; Sánchez Marcos, Enrique; Universidad de Sevilla. Departamento de Química Física; Ministerio de Ciencia e Innovación (MICIN). España; Agencia Estatal de Investigación. España; European Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER); Junta de Andalucía; Universidad de Sevilla
The physicochemical properties of the monovalent actinyl cations, PuO2+and NpO2+, in water have been studied by means of classical molecular dynamicsimulations. A specific set of cation-water intermolecular potentials based on ab initiopotential energy surfaces has been built on the basis of the hydrated ion concept. TheTIP4P water model was adopted. Given the paramagnetic character of these actinyls, thecation−water interaction energies were computed from highly correlated wave functionsusing the NEVPT2 method. It is shown that the multideterminantal character of the wavefunction has a relevant effect on the main distances of the hydrated molecular cations.Several structural, dynamical, and energetic properties of the aqueous solutions have beenobtained and analyzed. Structural RDF analysis gives An−Oyldistances of 1.82 and 1.84 Åand An−O(water) distances of 2.51 and 2.53 Å for PuO2+and NpO2+in water,respectively. Experimental EXAFS spectra from dilute aqueous solutions of PuO2+and NpO2+are revisited and analyzed, assumingtetra- and pentahydration of the actinyl cations. Simulated EXAFS spectra have been computed from the snapshots of the MDsimulations. Good agreement with the experimental information available is found. The global analysis leads us to conclude that bothPuO2+and NpO2+cations in water are stable pentahydrated aqua ions.
A general procedure to evaluate many-body spin operator amplitudes from periodic calculations: application to cuprates
(2007) Malrieu, J.P.; Illas, Francesc; Jiménez Calzado, Carmen; Moreira, I. P. R.; Universidad de Sevilla. Departamento de Química Física
A general purpose acetonitrile interaction potential to describe its liquid, solid and gas phases
(Elsevier, 2020) Hernández Cobos, Jorge; Martínez Fernández, José Manuel; Rodríguez Pappalardo, Rafael; Ortega Blake, Iván; Sánchez Marcos, Enrique; Universidad de Sevilla. Departamento de Química Física; Ministerio de Ciencia e Innovación (MICIN). España; Universidad Autónoma de México (UNAM)
A flexible and polarizable force field to describe acetonitrile in its three states of matter has been developed on the basis of an ab initio potential energy surface at the MP2 level. The acetonitrile molecule is represented by a sophisticated twelve-site model: the six nuclei plus six mobile charges. Three thousands structures have been employed for the fitting including monomers, dimers and trimers. Gas phase behavior has been tested by analyzing aggregates from dimers to 27-mer which give accurate reproducibility of the experimental and quantummechanical results. Classical MD simulations have been performed for the liquid and its crystalline α- and βforms. An analysis of liquid in a wide range of structural, dynamic, energetics and spectroscopic properties has been performed, as well as neutron diffraction data. Comparison with experimental information is satisfactory. Due to the first principles nature and the polarizable character of this force field properties of the three states of matter are properly reproduced.
A general study of actinyl hydration by molecular dynamics simulations using ab initio force fields
(AIP Publishing, 2019-03) Pérez Conesa, Sergio; Torrico Perdomo, Francisco Miguel; Martínez Fernández, José Manuel; Rodríguez Pappalardo, Rafael; Sánchez Marcos, Enrique; Universidad de Sevilla. Departamento de Química Física; Sánchez Marcos, Enrique; Junta de Andalucía
A set of new ab initio force fields for aqueous [AnO2] 2+/+ (An = Np(VI,V), Pu(VI), Am(VI)) has been developed using the Hydrated Ion (HI) model methodology previously used for [UO2] 2+. Except for the non-electrostatic contribution of the HI-bulk water interaction, the interaction potentials are individually parameterized. Translational diffusion coefficients, hydration enthalpies, and vibrational normal mode frequencies were calculated from the MD simulations. Physico-chemical properties satisfactorily agree with experiments validating the robustness of the force field strategy. The solvation dynamics and structure for all hexavalent actinoids are extremely similar and resemble our previous analysis of the uranyl cation. This supports the idea of using the uranyl cation as a reference for the study of other minor actinyls. The comparison between the NpO2+ 2 and NpO+ 2 hydration only provides significant differences in first and second shell distances and second-shell mean residence times. We propose a single general view of the [AnO2] 2+/+ hydration structure: aqueous actinyls are amphiphilic anisotropic solutes which are equatorially conventional spherically symmetric cations capped at the poles by clathrate-like water structures.
A hydrated ion model of [UO2]2+ in water: Structure, dynamics, and spectroscopy from classical molecular dynamics
(AIP Publishing, 2016-12) Pérez Conesa, Sergio; Torrico Perdomo, Francisco Miguel; Martínez Fernández, José Manuel; Rodríguez Pappalardo, Rafael; Sánchez Marcos, Enrique; Universidad de Sevilla. Departamento de Química Física; Sánchez Marcos, Enrique; Junta de Andalucía
A new ab initio interaction potential based on the hydrated ion concept has been developed to obtain the structure, energetics, and dynamics of the hydration of uranyl in aqueous solution. It is the first force field that explicitly parameterizes the interaction of the uranyl hydrate with bulk water molecules to accurately define the second-shell behavior. The [UO2(H2O)5 ] 2+ presents a first hydration shell U–O average distance of 2.46 Å and a second hydration shell peak at 4.61 Å corresponding to 22 molecules using a coordination number definition based on a multisite solute cavity. The second shell solvent molecules have longer mean residence times than those corresponding to the divalent monatomic cations. The axial regions are relatively de-populated, lacking direct hydrogen bonding to apical oxygens. Angle-solved radial distribution functions as well as the spatial distribution functions show a strong anisotropy in the ion hydration. The [UO2(H2O)5 ] 2+ solvent structure may be regarded as a combination of a conventional second hydration shell in the equatorial and bridge regions, and a clathrate-like low density region in the axial region. Translational diffusion coefficient, hydration enthalpy, power spectra of the main vibrational modes, and the EXAFS spectrum simulated from molecular dynamics trajectories agree fairly well with the experiment.
A local fingerprint for hydrophobicity and hydrophilicity: From methane to peptides
(AIP Publishing, 2019) Pérez Conesa, Sergio; Piaggi, Pablo M.; Parrinello, Michele; Universidad de Sevilla. Departamento de Química Física; Ministerio de Educación, Cultura y Deporte (MECD). España
An important characteristic that determines the behavior of a solute in water is whether it is hydrophobic or hydrophilic. The traditional classification is based on chemical experience and heuristics. However, this does not reveal how the local environment modulates this important property. We present a local fingerprint for hydrophobicity and hydrophilicity inspired by the two body contribution to the entropy. This fingerprint is an inexpensive, quantitative, and physically meaningful way of studying hydrophilicity and hydrophobicity that only requires as input the water-solute radial distribution functions. We apply our fingerprint to octanol, benzene, and 20 proteinogenic amino acids. Our measure of hydrophilicity is coherent with chemical experience, and moreover, it also shows how the character of an atom can change as its environment is changed. Finally, we use the fingerprint as a collective variable in a funnel metadynamics simulation of a host-guest system. The fingerprint serves as a desolvation collective variable that enhances transitions between the bound and unbound states.
A Non-Viral Plasmid DNA Delivery System Consisting on a Lysine-Derived Cationic Lipid Mixed with a Fusogenic Lipid
(MDPI, 2019) Martínez Negro, María; Sánchez Arribas, Natalia; Guerrero Martínez, Andrés; Moyá Morán, María Luisa; Tros de Llarduya, Conchita; Mendicuti, Francisco; Aicart, Emilio; Junquera, Elena; Universidad de Sevilla. Departamento de Química Física
The insertion of biocompatible amino acid moieties in non-viral gene nanocarriers is an attractive approach that has been recently gaining interest. In this work, a cationic lipid, consisting of a lysine-derived moiety linked to a C12 chain (LYCl) was combined with a common fusogenic helper lipid (DOPE) and evaluated as a potential vehicle to transfect two plasmid DNAs (encoding green fluorescent protein GFP and luciferase) into COS-7 cells. A multidisciplinary approach has been followed: (i) biophysical characterization based on zeta potential, gel electrophoresis, small-angle X-ray scattering (SAXS), and cryo-transmission electronic microscopy (cryo-TEM); (ii) biological studies by fluorescence assisted cell sorting (FACS), luminometry, and cytotoxicity experiments; and (iii) a computational study of the formation of lipid bilayers and their subsequent stabilization with DNA. The results indicate that LYCl/DOPE nanocarriers are capable of compacting the pDNAs and protecting them efficiently against DNase I degradation, by forming Lα lyotropic liquid crystal phases, with an average size of ~200 nm and low polydispersity that facilitate the cellular uptake process. The computational results confirmed that the LYCl/DOPE lipid bilayers are stable and also capable of stabilizing DNA fragments via lipoplex formation, with dimensions consistent with experimental values. The optimum formulations (found at 20% of LYCl content) were able to complete the transfection process efficiently and with high cell viabilities, even improving the outcomes of the positive control Lipo2000*
A novel biocompatible polymer derived from D-mannitol used as a vector in the field of genetic engineering of eukaryotic cells
(Elsevier Science BV, 2023-02-23) Pérez, David; Moyá Morán, María Luisa; Bautista, María; León, Rosa; Molina-Márquez, Ana; Vila, Marta; Romero Azogil, Lucía; Benito Hernández, Elena María; García Martín, María de Gracia; Valle Rosado, Iván; Romero Balestra, Fernando; Huertas Sánchez, Pablo; López-Cornejo, María del Pilar; Universidad de Sevilla. Departamento de Química Física; Consejería de Conocimiento, Innovación y Universidades de la Junta de Andalucía; Universidad de Sevilla; European Commission (EC). Fondo Europeo de Desarrollo Regional (FEDER)
The design and preparation of new vectors to transport genetic material and increase the transfection efficiency continue being an important research line. Here, a novel biocompatible sugar-based polymer derived from Dmannitol has been synthesized to be used as a gene material nanocarrier in human (gene transfection) and microalga cells (transformation process). Its low toxicity allows its use in processes with both medical and industrial applications. A multidisciplinary study about the formation of polymer/p-DNA polyplexes has been carried out using techniques such as gel electrophoresis, zeta potential, dynamic light scattering, atomic force microscopy, and circular dichroism spectroscopy. The nucleic acids used were the eukaryotic expression plasmid pEGFP-C1 and the microalgal expression plasmid Phyco69, which showed different behaviors. The importance of DNA supercoiling in both transfection and transformation processes was demonstrated. Better results were obtained in microalga cells nuclear transformation than in human cells gene transfection. This was related to the plasmid’s conformational changes, in particular to their superhelical structure. It is noteworthy that the same nanocarrier has been used with eukaryotic cells from both human and microalga.
A theoretical insight into the catalytic effect of a mixed-metal oxide at the nanometer level: The case of the highly active metal/CeOx/TiO2(110) catalysts
(AIP Publishing, 2010-03) Graciani Alonso, Jesús; Plata Ramos, José Javier; Fernández Sanz, Javier; Liu, Ping; Rodríguez, José A.; Universidad de Sevilla. Departamento de Química Física; Fernández Sanz, Javier; Gobierno de España. Ministerio Ciencia e Innovación (MICINN); Junta de Andalucía; Universidad de Sevilla. FQM132: Química Teórica
The structural and electronic properties of CeOx species supported on the rutile TiO2 110 surface have been examined by means of periodic density-functional calculations that use a generalized gradient approximation functional including a Hubbard-like type correction. Deposition of Ce atoms leads in a first step to Ce3+ ions bound to the surface through bridge and in-plane oxygen atoms, the released electrons occupying the Ti 3d empty orbitals. Further addition of Ce and molecular oxygen gives place to Ce2O3 dimers diagonally arranged on the surface, in agreement with the spots observed in the scanning tunnel microscope images. The formation process of CeOx nanoparticles NPs on the TiO2 surface is highly exothermic and our calculations show that the redox properties of the Ce III-Ce IV couple are significantly altered when it is supported on TiO2. In particular the reactivity against CO/O2 indicates that on the surface the presence of Ce III is favored over Ce IV species. Our results also indicate that the CeOx /TiO2 interface should be seen like a real mixed-metal oxide rather than a supported NP of ceria. Finally, in the context of the high catalytic activity of the M /CeOx /TiO2 M =Au,Cu,Pt systems in the water-gas shift reaction, we have examined the dissociation of water on the CeOx /TiO2 surface and estimated a barrier as small as 0.04 eV, i.e. 8 times smaller than that computed for a TiO2 oxygen vacancy. This result agrees with the experimental superior catalytic activity of the M /CeOx /TiO2 systems over M /TiO2.
A Theoretical Study on the Inclusion of Fe, Cu, and Zn in Illite Clays
(Hindawi, 2019) Sánchez Coronilla, Antonio; Martín Fernández, Elisa Isabel; Fernández de Córdova Martín, Francisco José; Santos, Francisco Javier; Hidalgo Toledo, José; Universidad de Sevilla. Departamento de Química Física; Universidad de Sevilla. Departamento de Ingeniería Química
The inclusion of Fe, Cu, and Zn in (1, 0, 0), (0, 0.5, 0), and (0.5, 0.5, 0.5) sites of an illite with the KAl2Si4O12H2 structure has been studied. For the inclusion of the metals, their common oxidation states were chosen, that is, 0, +2, +3 and 0, +1, +2, for Fe and Cu, respectively, while 0 and +2 for Zn. Periodic DFT calculations were performed to know the most favourable site of incorporation of the ions. Energetically the most favourable site for the inclusion corresponds to the (1, 0, 0) coordinate for all the ions independently of their oxidation state. However, the highest oxidation state of the metals (Fe3+, Cu2+, and Zn2+) was the most favoured for being incorporated into the illite structure and was the selected ion for the discussion. In those structures, metal oxygen interaction plays an important role in stabilizing the systems. Structural and energetic results indicate that illite presents good adsorption characteristic of those Fe3+, Cu2+, and Zn2+ in the (1, 0, 0) site. Thus, those ions may be available for plants for its extraction by phytoextraction techniques and the consequent soil regeneration. The inclusion of a second metallic ion revealed the most favourable inclusion corresponding to the inclusion of Fe3+ ion. The inclusion of this ion modifies the coordination sphere around the first metal being available for subsequent extraction by phytoremediation or other techniques for clean-up of the soil and its regeneration.
Ab initio determination of an extended Heisenberg Hamiltonian in CuO2 layers
(Springer Nature, 2001) Jiménez Calzado, Carmen; Malrieu, Jean Paul; Universidad de Sevilla. Departamento de Química Física; European Commission (EC); European Community (EC)
Accurate ab initio calculations on embedded Cu4O12 square clusters, fragments of the La2CuO4 lattice, confirm a value of the nearest neighbor antiferromagnetic coupling (J = 124 meV) previously obtained from ab initio calculations on bicentric clusters and in good agreement with experiment. These calculations predict non negligible antiferromagnetic second-neighbor interaction (J′ = 6.5 meV) and four-spin cyclic exchange (K = 14 meV), which may affect the thermodynamic and spectroscopic properties of these materials. The dependence of the magnetic coupling on local lattice distortions has also been investigated. Among them the best candidate to induce a spin-phonon effect seems to be the movement of the Cu atoms, changing the Cu-Cu distance, for which the variation of the nearest neighbor magnetic coupling with the Cu-O distance is ΔJ/ΔdCu-O ∼ 1700 cm-1 Å-1.
Ab initio group model potentials including electron correlation effects
(AIP Publishing, 2000-10) Cruz Hernández, Norge; Fernández Sanz, Javier; Universidad de Sevilla. Departamento de Química Física; Dirección General de Enseñanza Superior (DGES). España; Junta de Andalucía; Universidad de Sevilla. FQM132: Química Teórica
A method for determination of ab initio group model potentials, with the intention of describing the effects of a whole molecule or a chemical group within a density functional theory framework, is reported. The one-electron part of the Kohn–Sham equations is modified by incorporation of a Coulomb operator, which accounts for the classical electron interaction arising from the group. Exchange and correlation effects are introduced by a suitable modification of the exchange-correlation functionals. The strong orthogonality condition, usually required by the theory of separability of many electron systems, is written in terms of first order reduced density matrices. In order to check the method a group model potential for H2O (environment) was obtained and employed in the calculation of HF⋯H2O and H2O⋯H2O complexes using several functionals. Equilibrium intergroup distances and binding energies are compared with all-electron calculations.
ABP y tics adaptados a los laboratorios de prácticas de química física: su inserción e implementación
(Universidad de Sevilla, 2010) Sánchez Coronilla, Antonio; Universidad de Sevilla. Departamento de Química Física
El trabajo que se presenta está basado en la experiencia docente llevada a cabo en los grupos de laboratorio de las asignaturas Termodinámica Química y Experimentación en Química Física, de primer y segundo curso de la Licenciatura en Química. El objetivo principal de este trabajo ha sido demostrar que el hecho de emplear e integrar el Aprendizaje Basado en Problemas (ABP) como estrategia didáctica con las Tecnologías de la Información y Comunicación (TICs), es válido y aplicable a cualquier asignatura universitaria de índole práctica que implique a la Química Física.
Accurate ab initio determination of magnetic interactions and hopping integrals in La2-xSrxCuO4 systems
(American Institute of Physics Publising LLC, 2000) Jiménez Calzado, Carmen; Fernández Sanz, Javier; Malrieu, J.P.; Universidad de Sevilla. Departamento de Química Física
The nature of magnetic interactions and electron transfer processes in La22xSrxCuO4 systems are studied, by means of an ab initio embedded cluster model approach, using a difference dedicated configuration interaction ~DDCI! procedure. For the undoped system, the crucial role played by the ligand to metal charge transfer ~LMCT! configurations in the magnetic process makes necessary the use of an enlarged DDCI space, which explicitly takes account of the relaxation of these LMCT configurations. This procedure allows us to approach the experimental magnetic coupling constant value. In hole-doped systems, the value obtained for the electron transfer integral, t, is of 0.54–0.57 eV. The extra hole, characterized from the nature and occupation of differential natural orbitals, has a strong p character ~approximately 50%! and is essentially localized in CuO2 planes. These results are in agreement with the experimental evidence about these kinds of compounds. Neither the value of t nor the nature of the extra hole are seriously affected by the optimization of the orbitals used in the CI expansion. This suggests that a t – J effective Hamiltonian is an adequate model to study the electronic properties of these systems
Acoplamiento de procesos de transferencia de carga en sistemas biomiméticos de electrodos proteicos
(2019-12-20) Márquez Escudero, Inmaculada; Calvente Pacheco, Juan José; Olloqui Sariego, José Luis; Universidad de Sevilla. Departamento de Química Física
En esta tesis se ha llevado a cabo el estudio de la respuesta electroquímica de dos porfirinas de hierro (hemina y octaetilporfirina de hierro) adsorbidas sobre un electrodo de grafito en presencia de diferentes entornos químicos que mimetizan la envoltura peptídica de las proteínas, utilizando para ello ligandos de diferente acidez, hidrofobicidad y adsortividad. El estudio voltamétrico de las dos porfirinas de hierro en las disoluciones tampón más frecuentemente utilizadas (acetato y fosfato) ha puesto de manifiesto que su transferencia electro-protónica está acoplada con equilibrios de asociación entre el centro redox y las formas ionizadas del tampón. Dicha asociación está restringida a la forma acuohierro(III) de la metaloporfirina, por lo que su impacto en la respuesta electroquímica varía con el pH y la concentración de la disolución tampón, dando lugar a una amplia variedad de curvas de valoración voltamétricas. Se ha desarrollado una estrategia de dos etapas para cuantificar la constante de equilibrio intrínseca de asociación de dichas porfirinas con las formas aniónicas de ligandos ionizables, poniendo de manifiesto que i ) el Fe(III) porfirínico presenta una mayor afinidad por el acetato que por el dihidrógenofosfato, y ii) la sustitución de los residuos β de la hemina (propiónicos, vinilos y metilos) por grupos etilos posibilita la asociación del Fe(III) con el dianión monohidrógenofosfato. Por otro lado, se ha puesto de manifiesto que la asociación entre la especie acuohierro(III) de las porfirinas de hierro y los aniones carboxilatos procedentes de ácidos orgánicos lineales aumenta con la longitud de la cadena alquílica, como consecuencia de la interacción entre la cadena hidrocarbonada del ácido y el anillo de porfirina. A partir de la dependencia lineal de la energía libre de asociación con la longitud de la cadena alquílica, se han cuantificado las contribuciones individuales los grupos carboxilato, metilo y metileno, aumentando dichas contribuciones en la secuencia metileno < metilo << carboxilato. Basados en estos resultados, se ha desarrollado un modelo teórico para cuantificar la dependencia de la formación de la β-hematina con el pH descrita en la bibliografía. Dicho modelo ha permitido determinar la constante de equilibrio para la formación del dímero cíclico de la hemina μ-propionato. La comparación de esta constante de equilibrio con la correspondiente a la asociación Fe(III)-propionato ha puesto de manifiesto que el bajo contenido de agua de las interfases acuo-orgánicas impulsa la formación de la hemozoina (pigmento de la malaria) en los medios fisiológicos. Por último, el estudio electroquímico de las dos porfirinas de hierro en presencia de cantidades crecientes de n-hexanol ha puesto de manifiesto la existencia de dos poblaciones adsorbidas de dicho alcohol sobre el electrodo de grafito modificado con las porfirinas de hierro, que difieren en su energía de adsorción e interacción con la metaloporfirina. Una de las poblaciones está fuertemente adsorbida sobre la superficie de grafito y no modifica las propiedades redox de la porfirina de hierro, mientras que la otra se encuentra adsorbida menos fuertemente sobre la porfirina de hierro y estabiliza su forma reducida. Asimismo, se ha puesto de manifiesto que la energía libre de adsorción de la segunda población varía linealmente con la longitud de la cadena hidrocarbonada del alcohol, lo que ha permitido cuantificar la contribución del grupo metileno a dicha energía de adsorción, obteniéndose un valor similar a los descritos para la transferencia de dicho grupo desde un medio acuoso a uno hidrofóbico.
Activation Volumes for Peroxodisulphate Oxidation of Low-spin Iron(II) Complexes in Aqueous Salt Solutions
(Croatian Chemical Society, 2001-08) Benko, Ján; Burgess, John; López-Cornejo, María del Pilar; Vollárová, Olga; Universidad de Sevilla. Departamento de Química Física
Activation volumes are reported for peroxodisulphate oxidation of the four complexes of the Fe2+-2,2′-bipyridyl-cyanide sequence, viz. [Fe(bipy)3]2+, [Fe(bipy)2(CN)2], [Fe(bipy)(CN)4]2- and [Fe(CN)6]4- (abbr. bipy for 2,2′-bipyridyl), in aqueous solutions of a selection of nitrate, sulphate, and perchlorate salts of Li+, Na+, K+, and Mg2+ containing up to 6 mol dm-3 of added salt. The dependences of activation volumes on the nature and concentration of the added salt, and on the nature of the iron(II) complex, are established and discussed. Ancillary information relevant to hydration of the substrate complexes in the initial state for peroxodisulphate oxidation has been obtained from solvatochromism (halochromism) of [Fe(bipy)2(CN)2] and of [Fe(bipy)(CN)4]2-, and from the solubility of [Fe(bipy)2(CN)2], in some of the electrolyte solutions used in the high pressure kinetic studies.
Active role of the buffer in the proton-coupled electron transfer of immobilized iron porphyrins
(American Chemical Society, 2021-01) Márquez Escudero, Inmaculada; Olloqui Sariego, José Luis; Molero Casado, Miguel; Andreu Fondacabe, Rafael Jesús; Roldán González, Emilio; Calvente Pacheco, Juan José; Universidad de Sevilla. Departamento de Química Física; Ministerio de Economía y Competitividad (MINECO). España; Universidad de Sevilla. FQM128: Cinética Electródica e Instrumentación
Evaluation of the proton-coupled electron transfer thermodynamics of immobilized hemin is challenging due to the disparity of its electrochemical titration curves reported in the literature. Deviations from the one-electron, one-proton transfer at circumneutral pHs have been commonly ascribed to either the formation of dimeric species or the ionization of a second iron-bound water molecule. Herein, however, we report on non-idealities in the more acidic region, whose onset and extent vary with the nature and concentration of the commonly used phosphate and acetate buffers. It is shown that these deviations originate in the ligand-exchange binding between the oxidized aquo-hemin complex and the anionic components of the buffer, so that they are restricted to the pH interval where these forms coexist. A stepwise approach was developed to quantify unambiguously the apparent and intrinsic binding equilibrium constants. The apparent binding equilibrium constant exhibits a peak-shaped pH dependence, whose maximum is located at approximately the midpoint between the pKaof the iron-bound water and the first pKaof the buffer, and its magnitude is greater for the phosphate than for the acetate buffer. But strikingly, the opposite trend was found for the magnitude of the intrinsic binding equilibrium constants determined from the apparent ones, due to the different relative locations of the phosphoric and acetic pKavalues with respect to that of the oxidized aquo-hemin. To probe the role of the heme propionic residues, a similar study was carried out with a propionic-free iron porphyrin containing eight ethyl residues. These substituents decrease the acidity of the iron-bound water, strengthen the iron(III)-acetate binding, weaken the iron(III)-dihydrogen phosphate binding, and enable the binding between iron(III) and monohydrogen phosphate, which was hampered in hemin by the presence of the negatively charged propionate residues. Overall, this work provides a more complete speciation of immobilized iron porphyrins under acidic conditions than previously considered, showing the substitutional lability of the aqua ligand in the oxidized state of the iron center and the reluctance of its hydroxyl counterpart to anion exchange. Knowledge of these redox- and pH-dependent bindings with the buffer components is crucial for a rigorous quantification of the proton-coupled electron transfer and the electrocatalytic activity of iron porphyrins.

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