Relationship between the oxidation potential and electron spin density of the primary electron donor in reaction centers from Rhodobacter sphaeroides

The primary electron donor in bacterial reaction centers is a dimer of bacteriochlorophyll a molecules, labeled L or M based on their proximity to the symmetry-related protein subunits. The electronic structure of the bacteriochlorophyll dimer was probed by introducing small systematic variations in the bacteriochlorophyll–protein interactions by a series of site-directed mutations that replaced residue Leu M160 with histidine, tyrosine, glutamic acid, glutamine, aspartic acid, asparagine, lysine, and serine. The midpoint potentials for oxidation of the dimer in the mutants showed an almost continuous increase up to ≈60 mV compared with wild type. The spin density distribution of the unpaired electron in the cation radical state of the dimer was determined by electron–nuclear–nuclear triple resonance spectroscopy in solution. The ratio of the spin density on the L side of the dimer to the M side varied from ≈2:1 to ≈5:1 in the mutants compared with ≈2:1 for wild type. The correlation between the midpoint potential and spin density distribution was described using a simple molecular orbital model, in which the major effect of the mutations is assumed to be a change in the energy of the M half of the dimer, providing estimates for the coupling and energy levels of the orbitals in the dimer. These results demonstrate that the midpoint potential can be fine-tuned by electrostatic interactions with amino acids near the dimer and show that the properties of the electronic structure of a donor or acceptor in a protein complex can be directly related to functional properties such as the oxidation–reduction midpoint potential.

A multimer model for P680, the primary electron donor of photosystem II.

We consider a model of the photosystem II (PS II) reaction center in which its spectral properties result from weak (approximately 100 cm-1) excitonic interactions between the majority of reaction center chlorins. Such a model is consistent with a structure similar to that of the reaction center of purple bacteria but with a reduced coupling of the chlorophyll special pair. We find that this model is consistent with many experimental studies of PS II. The similarity in magnitude of the exciton coupling and energetic disorder in PS II results in the exciton states being structurally highly heterogeneous. This model suggests that P680, the primary electron donor of PS II, should not be considered a dimer but a multimer of several weakly coupled pigments, including the pheophytin electron acceptor. We thus conclude that even if the reaction center of PS II is structurally similar to that of purple bacteria, its spectroscopy and primary photochemistry may be very different.

Theoretical insights on electron donor-acceptor interactions involving carbon dioxide

Electron donor-acceptor (EDA) interactions are widely involved in chemistry and their understanding is essential to design new technological applications in a variety of fields ranging from material sciences and chemical engineering to medicine. In this work, we study EDA complexes of carbon dioxide with ketones using several ab initio and Density Functional Theory methods. Energy contributions to the interaction energy have been analyzed in detail using both variational and perturbational treatments. Dispersion energy has been shown to play a key role in explaining the high stability of a non-conventional structure, which can roughly be described by a cooperative EDA interaction.

光合放氧结构与机理的理论探索

光合放氧研究近十年来已有重要进展，但在该领域仍有很多重要问题待研究。本论文工作主要是对光系统氧化侧与光合放氧密切相关的组分的结构和性能进行理论和模拟研究，工作可概括如下： (一)、放氧中心结构和性能的探索。 1、对放氧中心Mn202单元与配体组氨酸、H2O和Cl等结合方式的理论研究显示：(a)、组氨酸和H20分子均可能与Mn202平面垂直，彼此保持较大的距离，且结合在不同的Mn离子上;(b)、2个H2O分子可能结合在不同Mn202单元上。 2、对Mn簇核心骨架的理论研究显示有必要引入新的Ca、Cl结合方式，以防止因两个Mn202单元线性化而导致2个H2O分子结合位点远离。 (二)、次级电子给体Tyr_z和Tyr_D的结构与功能。 对次级电子给体Tyr_z和Tyr_D进行精确量子化学研究显示：(a)、在中性条件下，Tyr_D和Tyr_z均只与组氨酸通过氢键作用;(b)、当失去电子后，结构发生明显变化，导致正电荷主要集中在组氨酸上，自由基主要集中在Tyr上;(c)、第三组分(H_20分子或羧基等)的引入使中性体系不容易给出电子;(d)、结合最新文献报导，推测Tyr_D~+和Tyr_z~+除与组氨酸作用外还可能分别与水和羧基作用。 (三)、原初电子给体的理论和模拟研究。 1、对紫细菌原初电子给体P_(870)的理论研究显示：(a)、双分子结构比单分子结构稳定;(b)、电荷分离之后，原初电子给体原有的空间结构不再是稳定的构型，它会向能量和化学活性均更低的构型转变。在光合细菌的原初电子给体P870中，这种转变可通过C3位的乙酰基旋转使其氧原子与另一个细菌叶绿素分子的镁原子相互作用使P870+•的总能量和化学活性明显降低。推测这种构型转变对于防止原初反应过程中的电荷重组、维持光能的高效转化有重要意义。提出了原初反应过程中结构动态变化的新观点，利用这一观点可对光合细菌原初反应动力学研究所观测到的慢过程及蛋白质微环境对原初电子给体和原初反应都有重要影响等实验现象给予较好的解释。 2、对光系统II原初电子给体P680的结构进行理论探讨，提出了两个叶绿素a分子平面间夹角为50.0±2.5°时能量最低的夹角模型。 3、采用N-甲基咪唑(C4H6N2)模拟生物体内的组氨酸，通过观测CCL4中的Chla与C4H6N2反应的吸收、CD和MCD光谱得到以下结论：(a)、在纯CCL4中，每个Chla处于5配位状态，Chla形成不对称的双聚体，彼此之间存在较强的偶合作用。提出两个Chla通过不等价的2个Mg-O配位键(O分别来自于C131位的酮基和C17位酯基的C=O)连接为紧密双体结构;(b)、当C4H6N2／Chla = 0.5和1时，其吸收、CD和MCD光谱均发生明显变化，两个Chla之间的偶合作用明显减弱，但此时仍为双聚体。推测C4H6N2首先取代原紧密双体结构中Mg-O酯键，进而取代Mg-O酮键，最后两个Chla分子通过两个Mg…O弱相互作用连接为松弛的双体结构，该模型与理论获得的P_(680)的结构相似。 在上述研究的基础上，提出了包括放氧中心外围配体和TyrZ在内的放氧中心结构新模型。在新结构模型中，2个H2O分子不对称地结合于“C”形结构开口端两个低价的Mn1II和Mn4III上，并保持较大距离;两个组氨酸的咪唑环通过N原子与两个高价的Mn2IV、Mn3IV结合;Cl结合于MB4TM，并与Ca相连;Ca通过O桥和COO-相连使两个Mn202单元保持特定空间构型。TyrZ通过组氨酸(D1-His190)与Mn簇作用。此外，新模型尝试着在O桥上引入质子。放氧中心结构及其邻近环境(包括TyrZ和TyrD)整体处于中性状态。 同时还提出了新的放氧机理，认为电子和质子的释放非同步进行，并首次明确提出两个水分子的不对称氧化和结构动态变化等观点。认为Ca在维持放氧中心的结构方面担负重要作用，C1与Mn离子之间的亲核作用变化是放氧中心结构变化的关键。

Effect of the Addition of a Fused Donor-Acceptor Ligand on a Ru(II) Complex: Synthesis, Characterization, and Photoinduced Electron Transfer Reactions of Ru(TTF-dppz)(2)(Aqphen) (2+)

The synthesis and the photophysical properties of the complex [Ru(TTF-dppz)(2)(Aqphen)](2+) (TTF = tetrathiafulvalene, dppz = dipyrido-[3,2-a:2',3'-c]phenazine, Aqphen = anthraquinone fused to phenanthroline via a pyrazine bridge) are described. In this molecular triad excitation into the metal ligand charge transfer bands results in the creation of a long-lived charge separated state with TTF acting as electron donor and anthraquinone as terminal acceptor. The lifetime of the charge-separated state is 400 ns in dichloromethane at room temperature. A mechanism for the charge separation involving an intermediate charge-separated state is proposed based on transient absorption spectroscopy.

Enhancement of excess electron transfer efficiency in DNA containing a phenothiazine donor and multiple stable phenanthrenyl base pairs

EET grown ohm: Excess electron transfer (EET) was observed within a DNA duplex containing π-stacked phenothiazine as an electron donor, phenanthrenes as electron carriers and 5-bromouracil as an electron trap. Increasing the number of phenanthrenyl base pairs increased EET efficiency.

Kinetics of photo-induced electron transfer from high-potential iron-sulfur protein to the photosynthetic reaction center of the purple phototroph Rhodoferax fermentans.

The kinetics of photo-induced electrontransfer from high-potential iron-sulfur protein (HiPIP) to the photosynthetic reaction center (RC) of the purple phototroph Rhodoferarfermentans were studied. The rapid photooxidation of heme c-556 belonging to RC is followed, in the presence of HiPIP, by a slower reduction having a second-order rate constant of 4.8 x 10(7) M(-1) x s(-1). The limiting value of kobs at high HiPIP concentration is 95 s(-1). The amplitude of this slow process decreases with increasing HiPIP concentration. The amplitude of a faster phase, observed at 556 and 425 nm and involving heme c-556 reduction, increases proportionately. The rate constant of this fast phase, determined at 425 and 556 nm, is approximately 3 x 10(5) s(-1). This value is not dependent on HiPIP concentration, indicating that it is related to a first-order process. These observations are interpreted as evidence for the formation of a HiPIP-RC complex prior to the excitation flash, having a dissociation constant of -2.5 microM. The fast phase is absent at high ionic strength, indicating that the complex involves mainly electrostatic interactions. The ionic strength dependence of kobs for the slow phase yields a second-order rate constant at infinite ionic strength of 5.4 x 10(6) M(-1) x s(-1) and an electrostatic interaction energy of -2.1 kcal/mol (1 cal = 4.184 J). We conclude that Rhodoferar fermentans HiPIP is a very effective electron donor to the photosynthetic RC.

A non-fullerene electron acceptor based on fluorene and diketopyrrolopyrrole building blocks for solution-processable organic solar cells with an impressive open-circuit voltage

A novel solution-processable non-fullerene electron acceptor 6,6′-(5,5′-(9,9-dioctyl-9H-fluorene-2,7-diyl)bis(thiophene-5,2-diyl))bis(2,5-bis(2-ethylhexyl)-3-(thiophen-2-yl)pyrrolo[3,4-c]pyrrole-1,4(2H,5H)-dione) (DPP1) based on fluorene and diketopyrrolopyrrole conjugated moieties was designed, synthesized and fully characterized. DPP1 exhibited excellent solubility and high thermal stability which are essential for easy processing. Upon using DPP1 as an acceptor with the classical electron donor poly(3-hexylthiophene), solution processable bulk-heterojunction solar cells afforded a power conversion efficiency of 1.2% with a high open-circuit voltage (1.1 V). As per our knowledge, this value of open circuit voltage is one of the highest values reported so far for a bulk-heterojunction device using DPP1 as a non-fullerene acceptor.

Preparation, Characterization And Properties Of C-60 And C-70 - Spectroscopy, Structure, Anions, Interaction With Electron-Donors And Superconductivity

Preparation and characterization of the fullerenes, C60 and C70, are described in detail, including the design of the generators fabricated locally. The characterization techniques employed are UV-visible, IR, Raman and C-13 NMR spectroscopies, scanning as well as transmission electron microscopy and mass spectrometry. The electron energy level diagram of C60 as well as the one-electron reductions of C60 and C70 leading to various anions are discussed. Electronic absorption spectra of C60- and C60(2-) are reported. Phase transitions from the plastic to the crystalline states of C60 and C70 are examined. Based on a C-13 NMR study in a mixture of nematic liquid crystals, it has been demonstrated that C60 retains its extraordinary symmetry in solution phase as well. Interaction of C60 and C70 with strong electron-donor molecules has been investigated employing cyclic voltammetry. Superconductivity of K(x)C60 has been studied by non-resonant microwave absorption; Na(x)C60 as well as K(c)C70 are shown to be non-superconducting. Doping C60 with iodine does not make it superconducting. Interaction of C60 with SbCl5 and liquid Br2 gives rise to halogenated products.

Influence of the highest occupied molecular orbital energy level of the donor material on the effectiveness of the anode buffer layer in organic solar cells

Efficiency of organic photovoltaic cells based on organic electron donor/organic electron acceptor junctions can be strongly improved when the transparent conductive Anode is coated with a Buffer Layer (ABL). Here, the effects of a metal (gold) or oxide (molybdenum oxide) ABL are reported, as a function of the Highest Occupied Molecular Orbital (HOMO) of different electron donors. The results indicate that a good matching between the work function of the anode and the highest occupied molecular orbital of the donor material is the major factor limiting the hole transfer efficiency. Indeed, gold is efficient as ABL only when the HOMO of the organic donor is close to its work function Phi(Au). Therefore we show that the MoO(3) oxide has a wider field of application as ABL than gold. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Non-covalent C-Cl center dot center dot center dot pi interaction in acetylene-carbon tetrachloride adducts: Matrix isolation infrared and ab initio computational studies

Non-covalent halogen-bonding interactions between n cloud of acetylene (C2H2) and chlorine atom of carbon tetrachloride (CCl4) have been investigated using matrix isolation infrared spectroscopy and quantum chemical computations. The structure and the energies of the 1:1 C2H2-CCl4 adducts were computed at the B3LYP, MP2 and M05-2X levels of theory using 6-311++G(d,p) basis set. The computations indicated two minima for the 1:1 C2H2-CCl4 adducts; with the C-Cl center dot center dot center dot pi adduct being the global minimum, where pi cloud of C2H2 is the electron donor. The second minimum corresponded to a C-H...Cl adduct, in which C2H2 is the proton donor. The interaction energies for the adducts A and B were found to be nearly identical. Experimentally, both C-Cl center dot center dot center dot pi and C-H center dot center dot center dot Cl adducts were generated in Ar and N2 matrixes and characterized using infrared spectroscopy. This is the first report on halogen bonded adduct, stabilized through C-Cl center dot center dot center dot pi interaction being identified at low temperatures using matrix isolation infrared spectroscopy. Atoms in Molecules (AIM) and Natural Bond Orbital (NBO) analyses were performed to support the experimental results. The structures of 2:1 ((C2H2)(2)-CCl4) and 1:2 (C2H2-(CCl4)(2)) multimers and their identification in the low temperature matrixes were also discussed. (C) 2015 Elsevier B.V. All rights reserved.

Correlation between optical and electrical properties in In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor structures on GaAs substrates

4.2 K photoluminescence (PL) and 77 K standard Hall-effect measurements were performed for In0.52Al0.48As/InxGa1-xAs metamorphic high-electron-mobility-transistor (HEMT) structures grown on GaAs substrates with different indium contents in the InxGa1-xAs well or different Si delta-doping concentrations. It was found that electron concentrations increased with increasing PL intensity ratio of the "forbidden" transition (the second electron subband to the first heavy-hole subband) to the sum of the "allowed" transition (the first electron subband to the first heavy-hole subband) and the forbidden transition. And electron mobilities decreased with increasing product of the average full width at half maximum of allowed and forbidden transitions and the electron effective mass in the InxGa1-xAs quantum well. These results show that PL measurements are a good supplemental tool to Hall-effect measurements in optimization of the HEMT layer structure. (c) 2006 American Institute of Physics.

Rapid thermal annealing effects on step-graded InAlAs buffer layer and In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor structures on GaAs substrates

A step-graded InAlAs buffer layer and an In0.52Al0.48As/In0.53Ga0.47As metamorphic high electron mobility transistor (MM-HEMT) structures were grown by molecular beam epitaxy on GaAs (001) substrates, and rapid thermal annealing was performed on them in the temperature range 500-800 degreesC for 30 s. The as-grown and annealed samples were investigated with Hall measurements, and 77 K photoluminescence. After rapid thermal annealing, the resistivities of step-graded InAlAs buffer layer structures became high. This can avoid leaky characteristics and parasitic capacitance for MM-HEMT devices. The highest sheet carrier density n(s) and mobility mu for MM-HEMT structures were achieved by annealing at 600 and 650degreesC, respectively. The relative intensities of the transitions between the second electron subband to the first heavy-hole subband and the first electron subband to the first heavy-hole subband in the MM-HEMT InGaAs well layer were compared under different annealing temperatures. (C) 2002 American Institute of Physics.