Dispersed polaron simulations of electron transfer in photosynthetic reaction centers

A microscopic method for simulating quantum mechanical, nuclear tunneling effects in biological electron transfer reactions is presented and applied to several electron transfer steps in photosynthetic bacterial reaction centers. In this "dispersed polaron" method the fluctuations of the protein and the electron carriers are projected as effective normal modes onto an appropriate reaction coordinate and used to evaluate the quantum mechanical rate constant. The simulations, based on the crystallographic structure of the reaction center from Rhodopseudomonas viridis, focus on electron transfer from a bacteriopheophytin to a quinone and the subsequent back-reaction. The rates of both of these reactions are almost independent of temperature or even increase with decreasing temperature. The simulations reproduce this unusual temperature dependence in a qualitative way, without the use of adjustable parameters for the protein's Franck-Condon factors. The observed dependence of the back-reaction on the free energy of the reaction also is reproduced, including the special behavior in the "inverted region."     

Electron tunneling paths in proteins

One of the crucial issues in biological electron transfer is the determination of the role of spatially intermediate amino acid residues in controlling or directing the electronic tunneling interaction between redox sites. A quantum path integral Monte Carlo method is developed for the analysis of electronic tunneling pathways in a highly structured environment. This path integral method is applied to intramolecular electron transfer in a ruthenium-modified myoglobin that contains a tryptophan in the "line-of-flight." A principal result is the identification of the relevant cylindrical zone swept out by the tunneling electron.     

Dielectric asymmetry in the photosynthetic reaction center

Although the three-dimensional structure of the bacterial photosynthetic reaction center (RC) reveals a high level of structural symmetry, with two nearly equivalent potential electron transfer pathways, the RC is functionally asymmetric: Electron transfer occurs along only one of the two possible pathways. In order to determine the origins of this symmetry breaking, the internal electric field present in the RC when charge is separated onto structurally characterized sites was probed by using absorption band shifts of the chromophores within the RC. The sensitivity of each probe chromophore to an electric field was calibrated by measuring the Stark effect spectrum, the change in absorption due to an externally applied electric field. A quantitative comparison of the observed absorption band shifts and those predicted from vacuum electrostatics gives information on the effective dielectric constant of the protein complex. These results reveal a significant asymmetry in the effective dielectric strength of the protein complex along the two potential electron transfer pathways, with a substantially higher dielectric strength along the functional pathway. This dielectric asymmetry could be a dominant factor in determining the functional asymmetry of electron transfer in the RC.     

Partial symmetrization of the photosynthetic reaction center

The bacterial photosynthetic reaction center (RC) is a pigmented intrinsic membrane protein that performs the primary charge separation event of photosynthesis, thereby converting light to chemical energy. The RC pigments are bound primarily by two homologous peptides, the L and M subunits, each containing five transmembrane helices. These alpha helices and pigments are arranged in an approximate C2 symmetry and form two possible electron transfer pathways. Only one of these pathways is actually used. In an attempt to identify nonhomologous residues that are responsible for functional differences between the two branches, homologous helical regions that interact extensively with the pigments were genetically symmetrized (that is, exchanged). For example, replacement of the fourth transmembrane helix (D helix) in the M subunit with the homologous helix from the L subunit yields photosynthetically inactive RCs lacking a critical photoactive pigment. Photosynthetic revertants have been isolated in which single amino acid substitutions (intragenic suppressors) compensate for this partial symmetrization.     

Methylhydroxycarbene: tunneling control of a chemical reaction

Chemical reactivity is conventionally understood in broad terms of kinetic versus thermodynamic control, wherein the decisive factor is the lowest activation barrier among the various reaction paths or the lowest free energy of the final products, respectively. We demonstrate that quantum-mechanical tunneling can supersede traditional kinetic control and direct a reaction exclusively to a product whose reaction path has a higher barrier. Specifically, we prepared methylhydroxycarbene (H(3)C-C-OH) via vacuum pyrolysis of pyruvic acid at about 1200 kelvin (K), followed by argon matrix trapping at 11 K. The previously elusive carbene, characterized by ultraviolet and infrared spectroscopy as well as exacting quantum-mechanical computations, undergoes a facile [1,2]hydrogen shift to acetaldehyde via tunneling under a barrier of 28.0 kilocalories per mole (kcal mol(-1)), with a half-life of around 1 hour. The analogous isomerization to vinyl alcohol has a substantially lower barrier of 22.6 kcal mol(-1) but is precluded at low temperature by the greater width of the potential energy profile for tunneling.     

Conformationally controlled chemistry: excited-state dynamics dictate ground-state reaction

Ion imaging reveals distinct photodissociation dynamics for propanal cations initially prepared in either the cis or gauche conformation, even though these isomers differ only slightly in energy and face a small interconversion barrier. The product kinetic energy distributions for the hydrogen atom elimination channels are bimodal, and the two peaks are readily assigned to propanoyl cation or hydroxyallyl cation coproducts. Ab initio multiple spawning dynamical calculations suggest that distinct ultrafast dynamics in the excited state deposit each conformer in isolated regions of the ground-state potential energy surface, and, from these distinct regions, conformer interconversion does not effectively compete with dissociation.     

A functional link between RuBisCO-like protein of Bacillus and photosynthetic RuBisCO

The genomes of several nonphotosynthetic bacteria, such as Bacillus subtilis, and some Archaea include genes for proteins with sequence homology to the large subunit of ribulose bisphosphate carboxylase/oxygenase (RuBisCO). We found that such a RuBisCO-like protein (RLP) from B. subtilis catalyzed the 2,3-diketo-5-methylthiopentyl-1-phosphate enolase reaction in the methionine salvage pathway. A growth-defective mutant, in which the gene for this RLP had been disrupted, was rescued by the gene for RuBisCOfrom the photosynthetic bacterium Rhodospirillum rubrum. Thus, the photosynthetic RuBisCO from R. rubrum retains the ability to function in the methionine salvage pathway in B. subtilis.     

Hydrogen tunneling in enzyme reactions

Primary and secondary protium-to-tritium (H/T) and deuterium-to-tritium (D/T) kinetic isotope effects for the catalytic oxidation of benzyl alcohol to benzaldehyde by yeast alcohol dehydrogenase (YADH) at 25 degrees Celsius have been determined. Previous studies showed that this reaction is nearly or fully rate limited by the hydrogen-transfer step. Semiclassical mass considerations that do not include tunneling effects would predict that kH/kT = (kD/kT)3.26, where kH, kD, and kT are the rate constants for the reaction of protium, deuterium, and tritium derivatives, respectively. Significant deviations from this relation have now been observed for both primary and especially secondary effects, such that experimental H/T ratios are much greater than those calculated from the above expression. These deviations also hold in the temperature range from 0 to 40 degrees Celsius. Such deviations were previously predicted to result from a reaction coordinate containing a significant contribution from hydrogen tunneling.     

Regulation of protein secretion through controlled aggregation in the endoplasmic reticulum

A system for direct pharmacologic control of protein secretion was developed to allow rapid and pulsatile delivery of therapeutic proteins. A protein was engineered so that it accumulated as aggregates in the endoplasmic reticulum. Secretion was then stimulated by a synthetic small-molecule drug that induces protein disaggregation. Rapid and transient secretion of growth hormone and insulin was achieved in vitro and in vivo. A regulated pulse of insulin secretion resulted in a transient correction of serum glucose concentrations in a mouse model of hyperglycemia. This approach may make gene therapy a viable method for delivery of polypeptides that require rapid and regulated delivery.     

Spin-dependent tunneling in self-assembled cobalt-nanocrystal superlattices

Self-assembled devices composed of periodic arrays of 10-nanometer-diameter cobalt nanocrystals display spin-dependent electron transport. Current-voltage characteristics are well described by single-electron tunneling in a uniform array. At temperatures below 20 kelvin, device magnetoresistance ratios are on the order of 10%, approaching the maximum predicted for ensembles of cobalt islands with randomly oriented preferred magnetic axes. Low-energy spin-flip scattering suppresses magnetoresistance with increasing temperature and bias-voltage.     

Picosecond resolution in scanning tunneling microscopy

A method has been developed for performing fast time-resolved experiments with a scanning tunneling microscope. The method uses the intrinsic nonlinearity in the microscope's current versus voltage characteristics to resolve optically generated transient signals on picosecond time scales. The ability to combine the spatial resolution of tunneling microscopy with the time resolution of ultrafast optics yields a powerful tool for the investigation of dynamic phenomena on the atomic scale.     

Female specific protein: biosynthesis controlled by corpus allatum in Leucophaea maderae

A specific protein is Present in serums of all of Leucophaea maderae that mature eggs but not in serums of males, nymphs of either sex, or females that are reproductively inactive. Ablations, microsurgery and remplantation showed that this protein is produced de novo under the influence of the corpus allatum hormone. This protein, essential for egg maturation, is synthesized in isolated abdimens of allatectomized females treated with 0.08 to 0.12 microgram of the t,t,t-isomer of the authentic juvenile hormone. The crops allatum hormone also stimulates toa similar degree the synthesis of other proteins which appear to be essential for egg maturation as well; these are not specific to females.     

多效唑对大豆植株光合机构和光合速率的影响

以大豆(cv.Q_(14-6-9),秋7-1)为材料研究了不同浓度(50,100,200,300 ppm)多效唑(MET)对光合机构、叶片光合速率和呼吸速率的影响.其中,100ppm处理效应最佳,其叶绿素a、总叶绿素含量和叶绿素a/b比值显著地提高,处理叶的叶面积较对照有所减少,但增加了叶片厚度,叶重/叶面积比值和叶片栅栏组织厚度,叶绿体形状由椭圆变成圆形,叶绿体内基粒较大,片层增加,垛叠增厚且排列紧密,淀粉粒明显较小,叶片光合速率显著提高而叶片呼吸速率却明显降低.     

优化碳同化实现作物高光效研究进展

随着全球人口的持续增长和耕地面积的不断减少,人类生存所面临的粮食危机越来越严重。进一步提高作物产量是保障我国粮食生产安全的重要途径。光合作用是作物产量形成的物质基础,采用现代育种技术以提高作物光合效率为中心的作物改良被认为是新一轮的“绿色革命”。本文从提高Rubisco羧化活性、将C₄光合途径引入C₃作物、降低光呼吸碳耗损等方面,介绍了优化改进植物光合碳同化领域的研究进展、存在的瓶颈问题,以及提高作物光合效率的实践应用;对当前改善植物光合碳同化的研究重点和方向进行了展望。     

Maximal photosynthetic rates in nature

It seems likely that turbulence under natural conditions, both aquatic and terrestrial, is higher than it is in the bottles or leaf chambers used when photosynthesis is measured experimentally. Most of the maximal photosynthetic rates reported in the literature are probably lower than those which occur in nature.     

Spin-polarized resonant tunneling in magnetic tunnel junctions

Insertion of a thin nonmagnetic copper Cu(001) layer between the tunnel barrier and the ferromagnetic electrode of a magnetic tunnel junction is shown to result in the oscillation of the tunnel magnetoresistance as a function of the Cu layer thickness. The effect is interpreted in terms of the formation of spin-polarized resonant tunneling. The amplitude of the oscillation is so large that even the sign of the tunnel magnetoresistance alternates. The oscillation period depends on the applied bias voltage, reflecting the energy band structure of Cu. The results are encouraging for the development of spin-dependent resonant tunneling devices.