Polar solvent dynamics and electron-transfer reactions

Polar solvents often exert a dramatic influence on reactions in solution. Equilibrium aspects of this influence involve differential solvation of reactants compared to the transition state that lead to alteration of the free-energy barrier to reaction. Such effects are well known, and often give rise changes in reaction rates of many orders of magnitude. Less well understood are effects arising from non-equilibrium, dynamical aspects of solvation. During the course of reaction, charge is rapidly redistributed among reactants. How the reaction couples to its solvent environment depends critically on how fast the solvent can respond to these changes in reactant charge distribution. In this article the dynamics of solvation in polar liquids and the influence of this dynamics on electron-transfer reactions are discussed. A molecular picture suggests that polar solvation occurs on multiple time scales as a result of the involvement of different types of solvent motion. A hierarchy of models from a homogeneous continuum model to one incorporating molecular aspects of solvation, combined with computer simulations, gives insight into the underlying dynamics. Experimental measures of solvation dynamics from picosecond and subpicosecond time-dependent Stokes shift studies are compared with the predictions of theoretical models. The implication of these results for electron-transfer reactions in solution are then briefly considered.     

Gas-phase ionic reactions: dynamics and mechanism of nucleophilic displacements

Nucleophilic displacement reactions (the SN2 reaction) of ions in the gas phase are a prototypical reaction system that allows a study of dynamics, mechanisms, and structure-energy relations. This article reviews aspects of the kinetics (especially the applicability of statistical reaction rate theory), the relation of structure and reactivity, and the effects of small numbers of solvent molecules on the reaction and compares the behavior of the ionic reaction in the gas phase with that in solution.     

Dynamics of water molecules in aqueous solvation shells

We report on the direct measurement of the dynamics of water molecules in the solvation shell of an ion in aqueous solution. The hydrogen-bond dynamics of water molecules solvating a Cl-, Br-, or I- anion is slow compared with neat liquid water, indicating that the aqueous solvation shells of these ions are rigid. This rigidity can play an important role in the overall dynamics of chemical reactions in aqueous solution. The experiments were performed with femtosecond midinfrared nonlinear spectroscopy, because this technique allows the spectral response of the water molecules in the solvation shell to be distinguished clearly from that of the other water molecules in the solution.     

Phosphadioxirane: a peroxide from an ortho-substituted arylphosphine and singlet dioxygen

We prepared the primary adduct for the reaction of singlet dioxygen (1O2) with an arylphosphine by using the sterically hindered arylphosphine tris(o-methoxyphenyl)phosphine. The resulting phosphadioxirane has a dioxygen molecule triangularly bound to the phosphorus atom. Olefin trapping experiments show that the phosphadioxirane can undergo nonradical oxygen atom-transfer reactions. Under protic conditions, two different intermediates are formed during the reaction of singlet dioxygen with tris(o-methoxyphenyl)phosphine, namely, the corresponding hydroperoxy arylphosphine and a hydroxy phosphorane. Experiments with other arylphosphines possessing different electronic and steric properties demonstrate that the relative stability of the tris(o-methoxyphenyl)phosphadioxirane is due to both steric and electronic effects.     

Electron solvation in finite systems: femtosecond dynamics of iodide. (Water)n anion clusters

Electron solvation dynamics in photoexcited anion clusters of I-(D2O)n=4-6 and I-(H2O)4-6 were probed by using femtosecond photoelectron spectroscopy (FPES). An ultrafast pump pulse excited the anion to the cluster analog of the charge-transfer-to-solvent state seen for I- in aqueous solution. Evolution of this state was monitored by time-resolved photoelectron spectroscopy using an ultrafast probe pulse. The excited n = 4 clusters showed simple population decay, but in the n = 5 and 6 clusters the solvent molecules rearranged to stabilize and localize the excess electron, showing characteristics associated with electron solvation dynamics in bulk water. Comparison of the FPES of I-(D2O)n with I-(H2O)n indicates more rapid solvation in the H2O clusters.     

Reactions of oriented molecules

Beams of oriented molecules have been used to directly study geometrical requirements in chemical reactions. These studies have shown that reactivity is much greater in some orientations than others and demonstrated the existence of steric effects. For some reactions portions of the orientation results are in good accord with traditional views of steric hindrance, but for others it is clear that our chemical intuition needs recalibrating. Indeed, the information gained from simultaneously orienting the reactants and observing the scattering angle of the products may lead to new insights about the detailed mechanism of certain reactions. Further work must be done to extend the scope and detail of the studies described here. More detailed information is needed on the CH(3)I reaction and the CF(3)I reaction. The effects of alkyl groups of various sizes and alkali metals of various sizes are of interest. In addition, reactions where a long-lived complex is formed should be studied to see if orientation is important. Finally, it would be of interest to apply the technique to the sort of reactions that led to our interest in the first place: the S(N)2 displacements in alkyl halides where the fascinating Walden inversion occurs.     

Synthesis of carbenes through substitution reactions at a carbene center

An (amino)(phosphino) carbene can be transformed into (amino)(phosphonio) carbenes, which undergo nucleophilic intermolecular as well as intramolecular substitution reactions at the carbene center. A variety of carbenes can be synthesized starting from a single carbene precursor. The resulting gamut of electronic and steric effects possible should open the way not only to a detailed study of the mechanism, but also to the subsequent improvement of catalytic reactions that involve carbene-transition metal complexes.     

根瘤菌接种方式对复播大豆干物质积累与产量的影响

探讨不同根瘤菌接种方式的实施效果,以前期分离、鉴定、纯化的3株根瘤菌菌株为供试材料,选用南疆地区复播大豆绥农35为供试品种,比较根瘤菌拌种、做种肥、随水滴施等接种方式的效果,测定大豆结瘤数量、根瘤干重、植株地上部干物质积累分配、产量及其构成因素等指标,探讨接种方式和根瘤菌对复播大豆干物质积累及产量的影响。结果表明,3种接种方式均能促进大豆根系结瘤,其中,以SN7-2拌种、SMH12做种肥,SN7-2做种肥和T6随水滴施大豆根瘤菌结瘤效果最好;SN7-2拌种,SMH12做种肥,SN7-2做种肥,T6随水滴施能显著促进大豆干物质积累,SN7-2拌种或者SN7-2做种肥、T6随水滴施均促进干物质向生殖器官分配;不同接种方式通过增加主茎节数、单株荚数、单株粒数和百粒重提高大豆籽粒产量,SN7-2适合以拌种或做种肥的方式接种,T6适合随水滴施。     

Vibrational promotion of electron transfer

By using laser methods to prepare specific quantum states of gas-phase nitric oxide molecules, we examined the role of vibrational motion in electron transfer to a molecule from a metal surface free from the complicating influence of solvation effects. The signature of the electron transfer process is a highly efficient multiquantum vibrational relaxation event, where the nitrogen oxide loses hundreds of kilojoules per mole of energy on a subpicosecond time scale. These results cannot be explained simply on the basis of Franck-Condon factors. The large-amplitude vibrational motion associated with molecules in high vibrational states strongly modulates the energetic driving force of the electron transfer reaction. These results show the importance of molecular vibration in promoting electron transfer reactions, a class of chemistry important to molecular electronics devices, solar energy conversion, and many biological processes.     

稳定氮肥用量对黄山贡菊产量和氮肥利用率的影响

[目的]比较稳定氮肥和普通氮肥在黄山贡菊上施用效果的差异。[方法]2016年通过在安徽歙县布置黄山贡菊不施氮(CK)、100%N量普通氮肥(CN)、100%N量稳定氮肥(SN1)和80%N量稳定氮肥(SN2)田间试验,研究稳定氮肥用量对黄山贡菊产量、氮肥利用率及经济效益的影响。[结果]80%N量稳定氮肥与100%N量普通氮肥施用效果相当,能够达到节本增效目标。100%N量稳定氮肥能提高贡菊产量,显著提高贡菊吸氮量、氮肥表观利用率、农学效率和经济效益,较普通氮肥增产6.9%,氮肥表观利用率显著提高6.08百分点,农学效率显著提高0.35 kg/kg,收益增加6.5%。[结论]综合产量、氮肥利用率和经济效益来看,100%N量稳定氮肥在黄山贡菊上施用效果最佳。     

棉花秸秆纤维素降解菌的筛选鉴定 与降解棉秆效果研究

【目的】 利用微生物学和分子生物学手段筛选和鉴定棉秆表面附着的真菌,获得具有高效纤维素分解能力的真菌。【方法】 通过将棉花秸秆接种至不同选择性培养基中,分离挑选水解圈较大的菌株,进行镜检和分子生物学鉴定;并同时对菌株进行滤纸酶活性(FPA)和羧甲基纤维素钠(CMC-Na)糖化活力的测定,以及连续(25 d)观测棉秆纤维的失重率。【结果】 主要4株菌种水解圈直径大小排序为,SN-1＞SN-4＞SN-3＞SN-4(P=0.013)。对于CMC-Na糖化活力,24~96 h时SN-1菌种显著低于其它3个菌种(P＜0.05),120 h时,各菌种处理间差异不显著(P＞0.05)。对于FPA活性,第24和120 h时,SN-1和SN-4菌种显著高于SN-2和SN-3菌种(P＜0.05);其它时间点,各菌种按活性高低排序为:SN-1、SN-4＞SN-3＞SN-2(P＜0.05)。通过镜检和分子生物学鉴定,得到4株菌种分别为:SN-1为黑曲霉(Aspergillus niger),SN-2为赤霉(Gibberella moniliformis),SN-3为链格孢霉(Alternaria nees),SN-4为青霉菌(Penicillium italicum)。【结论】 黑曲霉降解棉花秸秆的效果最为显著。对于CMC-Na糖化酶活力、滤纸酶活性以及棉花秸秆的实际失重率(降解率),接种黑曲霉或青霉菌时表现均最佳。     

小麦白粉病拮抗细菌的分离筛选

从沈阳、丹东、东港、吉林等地采集土样,采用土壤稀释法分离得到73株细菌,经孢子萌发抑制试验、离体叶药效试验筛选出8株对小麦白粉病菌有拮抗作用的菌株。通过盆栽试验研究了8个菌株对小麦白粉病的生防效果,其中菌株SN1、SN11的治疗效果分别为83.81%、73.53%,预防效果分别为91.72%、87.78%。     

Linear response breakdown in solvation dynamics induced by atomic electron-transfer reactions

The linear response (LR) approximation, which predicts identical relaxation rates from all nonequilibrium initial conditions that relax to the same equilibrium state, underlies dominant models of how solvation influences chemical reactivity. We experimentally tested the validity of LR for the solvation that accompanies partial electron transfer to and from a monatomic solute in solution. We photochemically prepared the species with stoichiometry Na0 in liquid tetrahydrofuran by both adding an electron to Na+ and removing an electron from Na-. Because atoms lack nuclear degrees of freedom, ultrafast changes in the Na0 absorption spectrum reflected the solvation that began from our two initial nonequilibrium conditions. We found that the solvation of Na0 occurs more rapidly from Na+ than Na-, constituting a breakdown of LR. This indicates that Marcus theory would fail to describe electron-transfer processes for this and related chemical systems.     

Medium effects in antibody-catalyzed reactions

Catalytic antibody technology has been used to explore the contribution of medium effects to the overall rate of an enzyme-catalyzed reaction. An antibody generated against a derivative of 2-acetamido-1,5-napthalenedisulfonate efficiently catalyzes the decarboxylation of 5-nitro-3-carboxybenzisoxazole. This unimolecular reaction is not susceptible to general acid-base catalysis but is highly sensitive to microenvironment; thus, it provides a simple chemical model for biologically important decarboxylations. The 10(4)-fold rate acceleration observed for the antibody reflects the kinetic advantage of the low-dielectric environment of the binding pocket acting to destabilize the substrate by desolvation and to stabilize the charge-delocalized transition state through dispersion interactions. These results are pertinent to an understanding of solvent effects in enzymic reactions in general and suggest approaches for developing antibody catalysts for numerous other reactions that involve large changes in charge distribution as the reaction coordinate is traversed.     

氮素调控对玉米氮素同化过程及产量的影响

氮素调控措施与作物氮素吸收利用和产量密切相关,但目前关于不同氮素调控措施对玉米主要生育期氮素同化过程的影响仍不清楚。以郑单958为试验材料,设置不施氮肥（CK）、传统施肥（CN）、氮肥+生物炭（SN）和氮肥+硝化抑制剂DMPP（DN）4个处理,分析不同氮素调控对玉米氮素同化过程中铵态氮和硝态氮含量、硝酸还原酶（NR）和谷氨酰胺合成酶（GS）活性、游离氨基酸和可溶性蛋白含量以及氮素利用率和产量的影响。结果表明：DN和SN处理较传统施肥处理均可以提高玉米植株体内硝态氮和铵态氮含量、NR和GS活性;DN和SN处理显著提高灌浆期谷氨酸、游离氨基酸和可溶性蛋白含量;DN处理成熟期籽粒的氮素积累量显著高于SN和CN处理,分别显著增加18.4%和30.0%;DN处理产量最高,SN次之,二者并无显著差异,但相较CN处理分别显著增产1 483.0和1 154.2 kg·hm^-2。两种氮素调控均促进了玉米对氮素的吸收,显著提高氮肥吸收利用率,其中硝化抑制剂处理氮肥吸收利用率最高且显著高于其他处理。综上,生物炭或硝化抑制剂配施氮肥,可以促进玉米氮素同化和转运过程,显著提高玉米产量和氮肥利用效率,综合产量、籽粒氮素积累量和氮肥吸收利用率,硝化抑制剂配施氮肥可作为淮河流域玉米高产高效的栽培措施。     

磷氮配施对小麦籽粒蛋白质组分含量和面团特性的影响

 【目的】试图阐明磷氮配施对小麦籽粒蛋白质品质的影响。【方法】以蛋白质品质差异较大的小麦强筋型品种藁城8901和弱筋型品种山农1391为材料,进行磷氮配施田间试验,测定籽粒蛋白质及其组分含量的动态变化和面团的流变学特性。【结果】藁城8901在施225 kg•ha-1氮肥水平下,与不施磷肥相比,增施磷肥的清蛋白、球蛋白、醇溶蛋白、谷蛋白和总蛋白及谷蛋白大聚合体含量提高,吸水率、面团形成时间、稳定时间和评价值增加,公差指数降低。施P2O5 240 kg•ha-1处理较施P2O5 120 kg•ha-1处理的相应效果降低。藁城8901在不施氮肥、山农1391在不施氮肥及施225 kg•ha-1氮肥水平下,随磷肥用量增加清蛋白和球蛋白含量提高,醇溶蛋白、谷蛋白、总蛋白和谷蛋白大聚合体含量降低;吸水率、面团形成时间、稳定时间和评价值下降,公差指数升高。【结论】不同品种在不同供氮水平条件下,增施磷肥时,小麦籽粒蛋白质及其各组分含量、面团流变学特性等变化的趋势不同。对强筋型品种藁城8901,在保证氮肥供应的基础上适量增施磷肥可提高其加工品质;对弱筋型品种山农1391,增加磷肥用量有利于其加工品质的改善。     

菌株MY02发酵培养基的优化设计

以龟裂链霉菌MY02菌株为试材,采用单因子试验与均匀试验相结合的方法,通过二次多项式回归分析,筛选出活性组分SN06最佳发酵培养基配方,并建立了多元二次回归数学模型。菌株MY02优化发酵培养基的最佳配方:淀粉质量分数为2.69%,花生饼粉质量分数为1.39%,(NH4)2SO4质量分数为0.18%,CaCO3质量分数为0.14%,NaCl质量分数为0.16%。根据回归模型计算出SN06理论效价与实测效价相比较,二者非常接近,拟合误差小。     

Direct Br?nsted analysis of the restoration of activity to a mutant enzyme by exogenous amines

A true Br?nsted analysis of proton transfer in an enzyme mechanism is made possible by the chemical rescue of an inactive mutant of aspartate aminotransferase, where the endogenous general base, Lys258, is replaced with Ala by site-directed mutagenesis. Catalytic activity is restored to this inactive mutant by exogenous amines. The eleven amines studied generate a Br?nsted correlation with beta of 0.4 for the transamination of cysteine sulfinate, when steric effects are included in the regression analysis. Localized mutagenesis thus allows the classical Br?nsted analysis of transition-state structure to be applied to enzyme-catalyzed reactions.     

STERIC FACTORS IN THE CHEMISTRY OF POLYPEPTIDES, POLY-ALPHAAMINO ACIDS AND PROTEINS

An application of the rule of six for the estimation of steric hindrance in reactions of dipeptides and polypeptides, and conformational preferences of polypeptides and proteins, is described.     

Three-dimensional correlation of steric and electronic free energy relationships guides asymmetric propargylation

Chemical reaction outcomes are often rationalized on the basis of independent analyses of steric and electronic effects. We applied three-dimensional free energy relationships correlating steric and electronic effects to design and optimize a ligand class for the enantioselective Nozaki-Hiyama-Kishi propargylation of ketones. The resultant mathematical model describing the steric and electronic parameter relationship is highly reliant on the synergistic interactions of these two effects.