Computational studies of electron‐transfer processes in old yellow enzyme

Old Yellow Enzyme (OYE) is a flavoenzyme that was first isolated from brewer's bottom yeast. Homologues have been identified in other strains of yeast, bacteria, and plants. In plants, the OYE homologue functions enzymatically in the synthesis of plant hormones, but the biological function of OYE in yeast is still unknown. Flavin mononucleotide (FMN) is the cofactor that is noncovalently bound in the enzyme. OYE binds several phenolic ligands that serve as models for reactive biological substrates. These complexes have broad long‐wavelength absorption bands, which have been ascribed to charge‐transfer interactions, with the phenolate anion acting as the electron donor and the FMN as the acceptor [Abramovitz, A. S.; Massey, V. J Bio Chem 1976, 251, 5327–5336]. The computational characterization of these electronic transitions in the active site will help in understanding the biological processes in the enzyme. It was found that at several levels of computational methods, and through computationally mutating relevant amino acids, a charge‐transfer process is occurring. This result agrees with previous experimental work and is consistent with all ultraviolet–visible spectrophotometric data. The preliminary results for the computational studies of these electron‐transfer processes will be presented. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

钼磷酸和2-吡啶-偶氮-2萘酚电荷转移配合物的合成、表征及性质研究

杂多化合物具有很强的接受电子能力,是一类优秀的受体分子,它可以与有机分子以氢键或配位键作用形成电荷转移配合物。由于这类配合物具有特殊的光、电、磁性质在催化、功能材料及药物化学等领域受到研究者的广泛关注[1]。有大量文献报道了杂多酸作为电子受体与有机给体相结合形成电子给一受型配合物,并在性能方面进行了探讨[2-3]。为了考察有机配体的位阻效应对配合物形成的影响,我们选择体积大,含有偶氨基及共轭π键的2-吡啶- 偶氮-2-萘酚（简写为PAN）为电子给体,12-钼磷酸（简写为PMo12）为电子受体,制备了新型标题化合物,并对…     

钨磷酸［C2H10NO］3［PW12O40］CH3CN电荷转移盐的合成、表征、光色性及非线性光学性质

杂多化合物为一类重要的多核配合物,因其优良的催化性能及其在其他领域的应用前景而受到广泛的重视［１－３］。研究发现,Ｋｅｇｇｉｎ型阴离子具有很强的接受电子能力［４］,可作为电子受体与有机给体相结合形成电荷转移盐,该类盐在光激发下可以发生电荷转移,从而表...     

Sensitized emission of luminescent lanthanide complexes based on 4-naphthalen-1-yl-benzoic acid derivatives by a charge-transfer process.

The photophysical properties of 4-naphthalen-1-yl-benzoic acid ligands and their Eu(III)-cored complexes were systematically investigated to elucidate the effective energy-transfer pathway in luminescent lanthanide complexes. A series of 4-naphthalen-1-yl-benzoic acid ligands, such as 4-naphthalen-1-yl-benzoic acid (NA-1), 4-[4-(4-methoxyphenyl)-naphthalen-1-yl]-benzoic acid (NA-2), and 4-{4-[4-(4-methoxyphenyl)-naphthalen-1-yl]-benzyloxy}-benzoic acid (NA-3), were synthesized and utilized for the synthesis of their Eu(III)-cored complexes, corresponding to NAC-1, NAC-2, and NAC-3. The fluorescence spectra of NA-1 and NA-2 show large Stokes shifts with increasing solvent polarity. These large Stokes shifts might be dominantly due to the formation of an intramolecular charge transfer (ICT) complex in the excited state. Also, the intensive luminescence of the Eu(III) ions by the photoexcitation of the ligand in NAC-1 and NAC-2 in polar solvents supports that the energy transfer from the ligand to the Eu(III) ion takes place efficiently. In the case of NA-3, which has a -CH2OPh- group that acts as a blocking group, there is no dependence of the fluorescence spectrum on the solvent nature and no luminescence of the Eu(III) ions by the photoexcitation of the ligand, indicating no formation of the ICT state. This can be due to the fact that the formation of the ICT state in NA-3 was prevented because the -OCH2- group acts as a blocking group by interrupting the pi-conjugation between the benzoic acid and the naphthalene unit. From these photophysical studies, we suggest that the ICT state plays a very important role in the energy-transfer pathway from the ligand to the Eu(III) ion. To our best knowledge, this is the first demonstration of sensitized emission of luminescent lanthanide complexes based on 4-naphthalen-1-yl-benzoic acid derivatives by the charge-transfer process.     

胺类吸收剂模拟烟气脱硫

以醇胺和乙二胺为原料,配制0.3 mol/L的水溶液,用于实验室模拟烟气脱硫的研究。通过碘量法测定脱硫率,采用离子色谱测定SO₂吸收容量、吸收液中SO₂的氧化率和解吸率,同时探讨了循环次数对脱硫率和富液中SO₂含量的影响。结果表明,吸收进行30 min时,吸收液的脱硫率均保持在99 %以上,60 min时,乙二胺类溶液的脱硫率仍达99 %,醇胺吸收液脱硫效果明显下降。乙二胺溶液对SO₂ 的吸收容量为455 mg/L,远远高于醇胺类吸收液。若乙二胺溶液中加入硼酸,则其对SO₂的吸收容量略低(450 mg/L),但提高了溶液的抗氧化性,吸收液中SO₂的氧化率由1.14 %减至0.29 %,同时解吸率由38.0 %增至59.0 %。10次吸收-解吸循环实验数据显示,乙二胺/硼酸溶液的平均脱硫率均在99 %以上,富胺液中SO₂含量由0.44 mol/L减至0.40 mol/L,表明吸收液的脱硫性能良好,可以作为烟气脱硫剂。     

氯冉酸可见分光光度法快速测定酮康唑

建立了快速测定酮康唑含量的可见分光光度法。实验结果表明:酮康唑与氯冉酸在甲醇-丙酮混合溶剂中发生电荷转移反应,产物的最大吸收波长是524 nm,表观摩尔吸光系数为1.04×103L·mol-1·cm-1。酮康唑质量浓度在20～400 mg·L-1范围内服从比耳定律,相关系数为0.9995。当酮康唑浓度为200 mg·L-1时,五次测定结果的相对标准偏差为1.03%。测得荷移络合物的组成比和稳定常数分别为1∶1和3.9×103。回收率在98.0%以上。本方法简便、快速、准确,可作为制剂的质量控制方法。     

尿素与Keggin结构磷钼酸电荷转移盐的合成与表征

在水中合成了两种新的有机 金属氧酸盐电荷转移配合物H3 PMo12 O4 0 ·4.5CO(NH2 ) 2 ·2 .5H2 O和H3 PMo12O4 0 ·12CO (NH2 ) 2 ,用IR、UV、TG、XRD及元素分析对其进行了表征。结果表明两种化合物中均存在[(H2 N) 2 COH]+ ,阴离子保持Keggin结构 ,但Keggin结构由于尿素的有机给体作用发生了不同程度的畸变。H3 PMo12 O4 0 ·4.5CO(NH2 ) 2 ·2 .5H2 O在固态 ,H3 PMo12 O4 0 ·12CO(NH2 ) 2 的水溶液具有光致变色效应     

Thermochromic Organoaminomodified Silica Composite Films Containing Phosphomolybdic Acid

A novel thermochromic sol-gel film has been prepared for the first time through entrapping phosphomolybdic acid into a kind of inorganic-organic matrix co-hydrolyzed from tetraethylorthosilicate and 3-aminopropyltriethoxysilane. The thus-obtained amorphous nanocomposite film was characterized by IR spectra, UV-vis absorption spectra, XRD, TG-DTA, and ESR. Results show that phosphomolybdate polyanions interact with R-NH⁺₃ cations strongly and disperse homogeneously in the sol-gel matrix. The sol-gel film exhibits thermochromic properties. Annealed from 120 to 180°C, the transparent film changes from pale-yellow to blue. A possible charge-transfer process has been suggested to explain the thermochromism.     

Hyperpolarizabilities of Chelidamic Acid Complexes M_m(C_7H_3O_5N)_n (M=Cu, Ag): Theoretical Analysis

The frequency‐dependent hyperpolarizabilities of chelidamic acid complexes M_m(C₇H₃O₅N)_n (M?Cu, Ag) were investigated under the time dependent density functional theory (TDDFT) combined with the sum‐over‐states method (SOS). The relationship between molecular orbitals and nonlinear optical (NLO) properties has been explored. The results show that the charge transitions of π‐π* and 3d_M‐π* are very important to the second‐order polarizabilities, and the largest component of dynamic β is 3.84×10^?25 cm⁵·esu^?1 at 0.74 eV for Ag₂Cu₂(C₇H₃O₅N)₄. The charge transition between π‐π* is also highly crucial to the third‐order polarizabilities, and the largest component of dynamic γ is ?4.46×10^?29 esu at 0.50 eV for Ag₂Cu₂(C₇H₃O₅N)₄. The central Cu ion, as electron bridge, extends the range of delocalization and leads to an interesting phenomenon of spiroconjugation.     

有机溶剂中微量水的荷移光谱法测定

研究了氯冉酸与有机溶剂中水的反应，确定了反应条件，氯冉酸与水的络合物的最大吸收波长为530nm。在乙醇、甲醇，正丙醇，异丙醇、正丁醇，正庚醇，正己醇溶剂中，水的含量分别在0－5％（V／V）或0－8％（V／V）范围内呈线性关系，相关系数为0．998或0．999；该方法重现性好，灵敏度高，操作简单，用于测定乙醇中微量水的含量，与标准值相符；加入回收法测得甲醇中水的回收率分别为98％－103％，结果令人满意。     

Synthesis of water‐soluble monotosylated ethylenediamines and their application in ruthenium and iridium‐catalyzed transfer hydrogenation of aldehydes

Synthesis of novel water‐soluble monotosylated ethylenediamines and their application in ruthenium and iridium‐catalyzed transfer hydrogenation of aldehydes are described. Various aldehydes, including electron‐deficient and electron‐rich variants, were reduced with high conversions and chemoselectivity using sodium formate as a hydrogen source in neat water. Copyright © 2011 John Wiley & Sons, Ltd.     

Computational Insights on the Hydride and Proton Transfer Mechanisms of D-Arginine Dehydrogenase

D-Arginine dehydrogenase from Pseudomonas aeruginosa (PaDADH) is an amine oxidase which catalyzes the conversion of D-arginine into iminoarginine. It contains a non-covalent FAD cofactor that is involved in the oxidation mechanism. Based on substrate, solvent, and multiple kinetic isotope effects studies, a stepwise hydride transfer mechanism is proposed. It was shown that D-arginine binds to the active site of enzyme as α-amino group protonated, and it is deprotonated before a hydride ion is transferred from its α-C to FAD. Based on a mutagenesis study, it was concluded that a water molecule is the most likely catalytic base responsible from the deprotonation of α-amino group. In this study, we formulated computational models based on ONIOM method to elucidate the oxidation mechanism of D-arginine into iminoarginine using the crystal structure of enzyme complexed with iminoarginine. The calculations showed that Arg222, Arg305, Tyr249, Glu87, His 48, and two active site water molecules play key roles in binding and catalysis. Model systems showed that the deprotonation step occurs prior to hydride transfer step, and active site water molecule(s) may have participated in the deprotonation process.     

Charge redistribution effect on the properties of charge transfer complexes HnR⋅XY and HnR⋅X2 (X,Y = F,Cl, Br,I; R = O,S, N,P)

Systematic studies on structures, energies, charge transfer, dipole moments, and ionic character of a series of weakly bonded charge transfer (CT) complexes (D⋅AB, D = H₂O, H₂S, NH₃, PH₃, AB = F₂, Cl₂, Br₂, I₂, BrCl, IBr, ClF, ICl, BrF, IF) have been carried out by the hybrid Hartree–Fock density functional theory (HF‐DFT) method, where those results are validated by available experimental and theoretical investigations. Employing the Hohenberg–Kohn theorem, the property of a multicomponent system is formulated with contributions from both component properties and the charge redistribution (CR) effect, which describes the electronic coupling between components. For any property of a multicomponent system, provided that the intercomponent coupling is weak enough, the first‐order approximation can be applied, which yields a linear correlation of the component contribution to the CR effect. In fact, this kind of linear relationship can be evidenced by all the studied properties including the geometry, energy, charge transfer, dipole moment, and ionic character of all 40 complexes. This approximation quantitatively describes the relative contribution of the components to a given property, which shows the same tendency in a series of complexes. Based on the investigations of the CT effect on the intermolecular bond energy and the total dipole moment, it has been found that the principal bonding character of the title complexes was ascertained to be ionic with the exception of the F₂ complexes, which agrees well with the calculated ionic character. The CT effect, though small in a quantitative aspect, is directly connected to various kinds of system properties. The effectiveness and consistency of the present type of calculations in multicomponent systems may allow their wider applications in the study of intermolecular interactions. © 2001 John Wiley & Sons, Inc. J Comput Chem 22: 327–338, 2001     

Synthesis and Characterization of Charge-transfer Complexes of Aminoethylphosphono-and Dimethylaminoethylphosphono-heteropolytungstic Acids of Keggin Type Structure

The charge-transfer complexes of heteropoly acids (HPA) possess special properties of light, electricity and magnetism, and have attracted widespread attention by researchers in the fields of functional materials and catalysis1. But until now, the electron acceptors of the charge-transfer complexes are the heteropolyanions with inorganic phosphoric acid, silicic acid etc. as the coordinate center2,3, the charge-transfer complexes of Keggin type structure of the organophosphonoheteropolytungs…     

First experimental evaluation of partial charge transfer during anion adsorption

The partial charge transfer during anion adsorption cannot be measured in electrochemical experiments or reliably estimated by ab initio quantum-chemical and DFT calculations. However, it is directly involved in the adsorbate charge that is experimentally accessible by in situ contact electric resistance technique. We present the first quantitative evaluation of charge transfer during halides adsorption on silver from aqueous solutions in dependence on the electrode potential. Partial charge transfer from adsorbed bromide and iodide ions to the silver electrode increases with the positive potential shift and may reach complete charge transfer resulting in adatom creation. Chloride adsorption on silver occurs with much lower charge transfer, but on some stainless superalloys it proceeds with substantial charge transfer owing to their specific electronic structure.     

Study of Spectrophotometric Characteristics of the Charge Transfer Reaction of Aminomethylbenzoic Acid with 7,7,8,8‐Tetracyanoquinodimethane

A new spectrophotometric method was developed for the determination of aminomethylbenzoic acid (PAMBA) using 7,7,8,8‐tetracyanoquinodimethane (TCNQ). The method was based on the formation of charge transfer (CT) complex of this drug as n‐electron donor with the π‐acceptor TCNQ. TCNQ was found to react with PAMBA to produce a kind of yellow complex. The CT reaction proceeded quantitatively in pH 8.5 buffer solution. Different variables affecting the reaction were carefully studied and optimized. Under optimal reaction conditions, the stoichiometric ratio of the reaction, maximum absorption wavelength and the value of molar absorptivity were measured to be 1:1, 425 nm, and 1.9×10⁴ L·mol^?1·cm^?1, respectively. Beer′s law was obeyed in the range of 1–9 µg·mL^?1 of PAMBA. The data have been filled to a linear regression equation A=?0.2612+0.1123c (µg·mL^?1), with a correlation coefficient of 0.9996. The detection limit was 0.4 µg·mL^?1, R.S.D. was less than 1.9%, and average recovery was over 97.6%. The formation of the CT complex was also confirmed by both infrared and ¹H NMR measurements. The thermodynamic property, kinetic property and reaction mechanism have also been discussed. The method developed was applied successfully to the determination of the subject drug in its pharmaceutical dosage forms with good precision and accuracy compared to official method revealed by t‐ and F‐tests.     

Catalytic photooxidation of 4-methoxybenzyl alcohol with a flavin-zinc(II)-cyclen complex

Flavin-zinc(II)-cyclen 10 contains a covalently linked substrate binding site (zinc(II)-cyclen) and a chromophore unit (flavin). Upon irradiation, compound 10 effectively oxidizes 4-methoxybenzyl alcohol (11-OCH3) to the corresponding benzaldehyde both in water and in acetonitrile. In the presence of air, the reduced flavin 10-H2 is reoxidized, and so catalytic amounts of 10 are sufficient for alcohol conversion. The mechanism of oxidation is based on photoinduced electron transfer from the coordinated benzyl alcohol to the flavin chromophore. This intramolecular process provides a much higher photooxidation efficiency, with quantum yields 30 times those of the comparable intermolecular process with a flavin chromophore without a binding site. For the reaction in buffered aqueous solution a quantum yield of Phi = 0.4 is observed. The turnover number in acetonitrile is increased (up to 20) by high benzyl alcohol concentrations. The results show that the covalent combination of a chromophore and a suitable binding site may lead to photomediators more efficient than classical sensitizer molecules.     

Investigation of the pathways of excess electron transfer in DNA with flavin-donor and oxetane-acceptor modified DNA hairpins

Oxetane is a potential intermediate that is enzymatically formed during the repair of (6-4) DNA lesions by special repair enzymes (6-4 DNA photolyases). These enzymes use a reduced and deprotonated flavin to cleave the oxetane by single electron donation. Herein we report synthesis of DNA hairpin model compounds containing a flavin as the hairpin head and two different oxetanes in the stem structure of the hairpin. The data show that the electron moves through the duplex even over distances of 17 A. Attempts to trap the moving electron with N2O showed no reduction of the cleavage efficiency showing that the electron moves through the duplex and not through solution. The electron transfer is sequence dependent. The efficiency is reduced by a factor of 2 in GC rich DNA hairpins.