The role of radicals in enzymatic processes

Research on the mechanism of action of coenzyme B12, adenosylcobalamin, as a graduate student introduced the author to the field of organic free radicals in enzymology. Twenty years later, related work on S-adenosylmethionine (SAM) as a "poor man's coenzyme B12" was initiated in a detailed analysis of the mechanism of action of lysine 2,3-aminomutase (LAM). The interconversion of L-lysine and L-beta-lysine is catalyzed by LAM, which requires SAM, pyridoxal-5'-phosphate (PLP), and a [4Fe-4S] cluster as coenzymes. The mechanism of this reaction has been delineated as a radical isomerization, in which radical formation is initiated by the [4Fe-4S]-dependent cleavage of the SAM into methionine and the 5'-deoxyadenosyl radical. The mechanism of this process is discussed, together with the role of this radical in hydrogen abstraction from lysine to initiate the substrate radical isomerization. The chemistry underlying the functions of SAM, PLP, and [4Fe-4S] in the action of LAM is novel in all respects, except for the formation of a lysine-PLP aldimine at the active site. Of the four free radicals in the mechanism, three have been characterized by EPR spectroscopy. In the suicide inactivation of adenosylcobalamin-dependent dioldehydrase (DDH) by glycolaldehyde, the formation of cob(II)alamin and 5'-deoxyadenosine is accompanied by the conversion of glycolaldehyde to cis-ethanesemidione radical at the active site. The cis-ethanesemidione radical has been characterized by EPR spectroscopy. Its exceptional stability at the active site is the basis for the inactivation of DDH by glycolaldehyde.     

基于氮化碳复合材料磁性固相萃取结合高效液相色谱法测定尿液中3种羟基多环芳烃

建立了测定尿液中3种羟基多环芳烃的氮化碳复合材料磁性固相萃取结合高效液相色谱-荧光检测法。所构筑的磁性氮化碳材料，经扫描电子显微镜、X射线衍射仪、振动样品磁强计和比表面积分析仪表征后，用于尿液中3种羟基多环芳烃的富集净化。考察了吸附剂用量、吸附时间、洗脱溶液和洗脱体积（单次洗脱体积×洗脱次数）对萃取效率的影响。结合高效液相色谱-荧光分析，在0.25~250 μg/L范围内线性关系良好（相关系数r=0.999），3种羟基多环芳烃的检出限和定量限分别为0.08和0.25 μg/L，回收率为90.1%~102%，日内和日间精密度分别为1.5%~7.7%和2.2%~8.7%。该本方法简单、快速、高效，可用于尿液中羟基多环芳烃的分析。     

戊二酸钛（Ⅲ）与氮化钛的制备

戊二酸钛（Ⅲ）与氮化钛的制备陈吉书，吕真国，徐其亨（曲靖师专化学系曲靖）（曲靖地区环境监测站曲靖）（云南大学化学系昆明６５００９１）关键词戊二酸钛，氮化钛，热分解氮化钛具有硬度大、熔点高、耐磨、耐腐蚀、导电性好、美观（金黄色）等优点，具有重要的工业用...     

[Fe4S4Cl4—n（S2CNEt2）n]^2—（n=0,1,2,4）：原子簇的电子结构研究

用CNDO/2(s,p,d)方法研究了类立方烷系列Fe-S簇合物[Fe_4S_4Cl_4(?)(S_2CNEt_2)_n]~(2-)(n=0,1,2,4)的电子结构。得出[S_2CN(Et)_2]-螯合配位Fe_4S_4~(2+)簇合物中存在两类不同价态铁原子的结论;骨架Fe_4S_4~(2+)中μ_3-S电子是非定域化的,同Mssbauer谱测定结果一致。讨论了簇合物Fe—Fe之间的成键作用、螫合配体的作用和氧化还原性质。     

Linear silver clusters in aqueous solution (clusterites): radiation-induced chemical synthesis and properties

The formation and properties of the products of the radiation-induced chemical reduction of silver ions in aqueous solutions containing sodium polyacrylate were investigated. The absorption spectra of these species exhibit a band at 290 mn and a band in the visible region. The latter shifts from 470 nm to -800 nm with time or after the addition of silver ions. The species obtained (clusterites) arc linear silver clusters bound to the carboxyl groups of the ion exchanger. The red shift of the visible band is due to the increase in their size. The mechanism of the formation and the structure of the clusterites are discussed.Translated fromIzvestiya Akademii Nauk. Seriya Khimicheskaya, No. 6, pp. 1429–1435, June, 1996.     

Phenol O–H bond dissociation energy in water clusters

We are reporting ab initio and density functional theory (DFT) calculations for the phenol O–H bond dissociation energy in the gas phase and in phenol–water clusters. We have tested a series of recently proposed functionals and verified that DFT systematically underestimates the O–H bond dissociation energy of phenol. However, O–H bond dissociation energies in water clusters are in reasonable agreement with experimental data for phenol in solution. We have evaluated electronic difference densities in phenol–water, phenoxy–water, and water, and we are suggesting that the representation of this quantity gives an interesting picture of the electronic density rearrangement induced by hydrogen bond interactions in phenol–water clusters. © 2001 John Wiley & Sons, Inc. Int J Quantum Chem, 2001     

Anab initio investigation of Cu2Se and Cu4Se2

Summary Experimentally known copper selenium clusters show extraordinary geometrical features, especially short Cu-Cu distances. We report the first theoretical investigation of Cu₂Se and Cu₄Se₂. Various quantum chemical methods (SCF, MP2, CPF, CCSD, CCSD(T), LDF) are applied to determine the importance of dynamic electron correlation. We find that inclusion of correlation does not essentially change the electronic structure of the clusters but has a strong influence on geometries. To reduce the computational effort we apply effective core potentials (ECPs) in combination with small, but carefully optimized basis sets. The applicability of simple modellings of correlation energies for approximate inclusion of correlation effects in SCF geometry optimizations is tested.     

Mixed-Metal Clusters of Palladium

The importance of palladium complexes in catalytic processes is well-demonstrated by its applications in hydrogenation processes in industry. Mixed metal clusters of palladium are of interest as the palladium ion can attain either a 16- or 18-electron configuration which is an important feature for catalytic processes. Recent developments in the preparation, characterization, chemical and physical properties of these clusters are discussed in this review.     

Self-Assembly of Gold Nanoclusters on Molecularly Modified GaAs

The elaboration of closed-packed monolayers of Au₅₅(PPh₃)₁₂Cl₆ clusters on oxidized and non-oxidized GaAs surfaces is reported. The first part of this work describes the use of silanethiol modified GaAs oxide surfaces to trap 18 nm gold colloids and Au₅₅(PPh₃)₁₂Cl₆ clusters. The surfaces characterized by AFM measurements present high-quality coverage on a quite long range for both metallic species. The second part is devoted to the elaboration of Au₅₅(PPh₃)₁₂Cl₆ cluster monolayers on non-oxidized p-type GaAs substrates, functionalized with dithiol molecules. AFM measurements demonstrate the presence of closed-packed two-dimensional arrangements of Au₅₅ clusters.     

Study of Structures and Electronic Properties of Pdn–1Pb and Pdn （n≤8） Clusters

Geometric and electronic properties of Pdn–1Pb and Pdn (n≤8) clusters have been studied by using density functional theory with effective core potentials, focusing on the differences between mono- and bimetallic clusters. The average bond length of Pdn–1Pb (n≤8) bimetallic clusters is longer than that of pure palladium clusters except for n = 2 and 3. The most stable structure of Pdn–1Pb (n≤7) is the singlet where there is at least a Pd or Pb atom on its excited state. The energy gaps of Pd–Pb binary clusters are narrower than those of Pdn clusters, and then the chemical activity is strengthened when Pdn clusters are doped with Pb.