A 270 MHz 1H-NMR study of three “branched” ribonucleotides A2'p5'U3'p5'g,A2'p5'C3'p5'G and A2'p5'G3'p5'G the importance of being branched with a 2'→5' phosphodiester bond in the pre-mRNA processing reactions (splicing)

Conformational studies with three analogues 3, 4 and 5 of naturally-occurring branched ribonucleotides 1 and 2 have shown that they adopt a secondary structure which is overwhelmingly controlled by a stacking between adenine(2' → 5')nuc1eobase residues while the 3' → 5' linked guanine residue is apart. This observation along with the fact that all four nucleosides can form lariats in nuclear mRNA splicing, but it is guanosine as the 2' → 5' nucleotide that can drive the splicing reaction to completion, suggests a biological significance of the additional 2' → 5' phosphodiester bond formation in the splicing reaction. It is likely that the final implication of formation of such a 2' → 5' linked lariat is that it provides a pathway to assume a free energy minimum conformation through the 2' → 5' stacking, especially in the case of guanosine, to drive the splicing reaction to completion (stacklng-driven-energy-pump).     

《大学化学教程——原理·应用·前沿》简介

由清华大学丁廷桢教授主编的《大学化学教程——原理·应用·前沿》(2003年，高等教育出版社)是工科高等学校普通化学课程的一本新教材。该书是教育部“新世纪网络课程建设工程”的研究项目“网络普通化学课程”(化学远程教育系统)的配套教材。普通化学是一门     

Primary photophysical and photochemical processes upon UV excitation of PtBr<Subscript>6</Subscript><Superscript>2–</Superscript> and PtCl<Subscript>6</Subscript><Superscript>2–</Superscript> complexes in water and methanol

Primary photophysical and photochemical processes were studied for Pt^IVBr₆ ^2– and Pt^IVCl₆ ^2– complexes in water and methanol by ultrafast kinetic spectroscopy upon excitation in the band region of charge transfer from the ligand-centered group π-orbitals to the e_g*-orbital of Pt^IV complex anion (LMCT bands). The data obtained earlier upon excitation in the region of d—d bands were compared. Irrespective of the excitation wavelength, the photochemical properties of complexes are caused by the reactions of intermediates proceeding in the picosecond time range. These intermediates were identified as Pt^IVBr₅ ^– upon photolysis of Pt^IVBr₆ ^2– and, presumably, the Adamson radical pair [Pt^IIICl₅ ^2–(C _4v )...Cl?] upon photolysis of Pt^IVCl₆ ^2–. The difference in the exciting light wavelengths has an impact only on the first step of these processes, i.e., transition from the Franck—Condon excited state to intermediates.     

Composite Eu-MOF@CQDs “off & on” ratiometric luminescent probe for highly sensitive chiral detection of l-lysine and 2-methoxybenzaldehyde

The high amount of l-lysine can increase the potential risk of cardiovascular disease. Additionally, 2-methoxy benzaldehyde (2-MB) has high toxicity and can easily pollute the environment. In this work, carbon quantum dots (CQDs) can be encapsulated into Eu-BTB (H₃BTB = 1,3,5-tri(4-carboxyphenyl)benzene), forming the multi-emission composite material Eu-BTB@CQDs. It has two emissions peaks (617 nm for Eu and 470 nm for CQDs). Eu-BTB@CQDs can be applied as bi-functional ratiometric “off & on” luminescent sensor for l-lysine and 2-MB with high sensitivity and selectivity, the low limit of detection (LOD) for l-lysine is 3.68 µmol/L and for 2-MB is 0.54 µmol/L, respectively. Additionally, Eu-BTB@CQDs can quantitatively discriminate l-lysine in the mixed d- and l-lysine water solutions (five different concentrations ratio of l/d-lysine has been set) makes the chiral detection of l-lysine are more meaningful. On the other hand, Eu-BTB@CQDs also can detect 2-MB over 4-methoxybenzaldehyde (4-MB) with high selectivity. Further the detection of 2-MB and l-lysine in the lake water real samples with the reasonable recovery rate. Finally, the detection mechanisms for l-lysine and 2-MB were also investigated and discussed in detail.     

Intercalation and controlled release of vitamin B<Subscript>6</Subscript> from montmorillonite–vitamin B<Subscript>6</Subscript> hybrid

Controlled release of a therapeutic agent to patients is gaining enormous importance during the recent past. The present paper focused on the intercalation of vitamin B₆ (pyridoxine, VB₆) into montmorillonite (MMT) as a controlled release drug carrier. MMT used in this study is of Indian origin. Intercalation of VB₆ into MMT at different times, temperatures, pH values, and initial concentration is illustrated. The MMT–VB₆ hybrid was characterized by X-ray diffraction, Fourier transform infrared, and thermogravimetric analysis. VB₆ was successfully adsorbed on the surface of MMT and also intercalated into the interlayer of MMT. The release processes were monitored under in vitro conditions using simulated gastric fluid (pH 1.2) and simulated intestinal fluid (pH 7.4) at 37 ± 0.5 °C. In vitro release experiments revealed that VB₆ was released from the MMT–VB₆ hybrid steadily and was pH-dependent.     

Structural-dimensional effects and their application in the &quot;core—nanoshell&quot; systems synthesized by the molecular layering

In connection with the development of nanotechnology in the last 15—20 years, the method of molecular layering (ML), created in the middle of the last century in the USSR, has attracted increasing attention. The features of structural-dimensional effects in the products obtained by new chemical nanotechnology and promising directions of their practical application in "core—nanoshell" compositions are considered in the review. In accordance with the synthetic capabilities of the ML method, the functional properties of the "core—nanoshell" type materials are affected by the "monolayer effect" and the substrate shielding effect, as well as the multicomponent nature of the system, and mutual structural coordination effect of the substrate and the build-up nanolayer. The review presents theoretical and experimental data on the application of the observed effects in the creation of shell pigments and fillers, modified sorbents, catalytic membrane reactors, nanodoped ceramic materials, polymers, quartz fibers with adjustable optical characteristics, electrets, etc.     

双金属配合物PrBi（edta）（N03）2&#183;6&#183;5H2O的合成、表征与热分解

采用NH4Bi（edta）&#183;2H2O和硝酸镨为原料,二者按摩尔比为1：1在水溶液中合成了含铋（Ⅲ）和镨（Ⅲ）的双金属配合物PrBi（edta）（NO3）2&#183;6．5H2O,产率为45％,该配合物在空气中稳定,不吸湿。用元：素分析、FT-IR、XRD和TG—DSC测试技术对产物进行了组成和结构表征。结果表明,配合物属于单斜晶系,相应晶胞参数为α=0．9885nm,b=2．3472nm,c=1．3756nm,β=93．375&#176;。通过红外光谱和热分析共同研究了热分解过程,配合物在热分解时,经过脱水、配体热分解、金属盐分解,最后在600℃失重恒定,残余物为BiPrO3。     

Photophysics and photochromism of Δ2,2&#x27; -BI-(2H-1,4-benzothiazine)

Δ^2,2' -bi-(2H-1,4-benzothiazine) shows striking photochromism and thermochromism. The color change is thermally and photochemically reversible. The photochromism can be ascribed to the cis-trans isomerization about the central double bond, the yellow trans isomer being more stable. The quantum yield of formation ø_CP of the cis-benzothiazine from trans in dioxan was found to be 0.035 at 355 nm excitation. Laser flash photolysis of the trans-benzothiazine revealed that the excited singlet state of π, π^* character is the originating state for the photochromism since there is no experimental evidence of a triplet intermediate.     

[（C6n5NO2）Pr（H2O）4]（CrMo6O24H6）&#183;（C6H5NO2）&#183;2．5H2O的合成与晶体结构

用常规合成方法制备了基于Anderson结构阴离子的二维层状化合物[（C6n5NO2）Pr（H2O）4]（CrMo6O24H6）&#183;（C6H5NO2）&#183;2．5H2O,通过红外光谱和X射线单晶衍射对其进行了表征．结果表明,该化合物属于单斜晶系,C2/c空间群．a=2．3442（9）nm,b=1．3291（5）nm,c=2．458（1）nm,β=103．08（1）&#176;,V=7．460（5）nm^3,R1=0．0727,wR2=0．1903．结构分析表明,[CrMo6O24H6]^3-阴离子通过端氧担载一个配位的Pr^3＋离子形成中性的（C6H5NO2）Pr（H2O）4（CrMo6O24H6）基团,相邻的中性基团在O1-Pr-O1桥联下形成一维链,链与链又通过异烟酸的桥联形成二维层状结构．     

介质阻挡放电等离子体中&#183;OH和HO2&#183;自由基的数值模拟计算

在介质阻挡放电等离子体N2／O2／H2O／HCHO体系中通过解Boltzmann方程,得到电子能量分布函数,利用得到的电子能量分布函数计算电子。分子碰撞反应速率常数。然后把有关的反应速率常数带入速率方程,计算得到该体系在介质阻挡放电时,&#183;OH、HO2&#183;和电子的浓度随时间的演变以及&#183;OH、HO2&#183;浓度随H2O、O2摩尔分数的变化,并将模拟结果与实验值进行了对比,两者符合得较好。     

（&#177;）-HypoglycinA的全合成

采用Wittig试剂与环氧氯丙烷和多聚甲醛构建亚甲基环丙烷结构单元，利用2，5-二乙氧基-3，6-二氢吡嗪引入氨基酸结构单元，通过6步反应成功实现了天然植物毒素HypoglycinA外消旋体的全合成，总收率3．2％。中间体和Hypoglycin A的结构经^1HNMR表征。对反应中的问题进行了分析和讨论。     

Sm（Val）C13&#183;6H20低温热容及热化学性质

采用精密绝热量热计测定了稀土氨基酸配合物[Sm（Val）C13&#183;6H2O]在80-376K温区的热容,从实验热容值计算出了热力学函数（HT-H298.15和ST-S298.15）。在308K附近,配合物的热容出现一个大的跳跃,可能是其玻璃化转变所致。对该配合物进行热重测试,得到了其可能的分解机理。     

Synthese Fluorierter Kanamycin-A-derivate,Modifizierung der Positionen “4 and 6”

Abstract

Starting from the kanamycin A 4″,6″-ditriflate 6 und -6″-brosy-late-4″-triflate 8, respectively, the following derivatives were prepared: 4″,6″-dideoxy-4″,6″-difluoro-4″-epi- (20), 4″,6″-dideoxy-4″-fluoro-4″-epi- (22), 6″-deoxy-6″-fluoro-4″-epi- (19), and 6″-deoxy-4″-epi-kanamycin A (21). C NMR and antibacterial data are given.     

Photo-induced reduction of Cr<Superscript>6+</Superscript> by the hybrid systems “Cu<Superscript>II</Superscript> complex with Schiff base and TiO<Subscript>2</Subscript>”: dependence on irradiation wavelength

The synthesis of novel Cu^II complexes with a Schiff base obtained by condensation of salicylaldehyde and an l-aspartic acid ester are described. The physicochemical properties of the complexes were compared with those of related Cu^II complexes obtained earlier. All the complexes studied were characterized by elemental analysis as well as by IR, UV-Vis, and EPR spectroscopies. The activity of the complexes and their hybrid systems (HS) with TiO₂ in visible-light-driven photocatalysis in organic solvents was investigated. After irradiation with visible light, the complexes and corresponding HS reduce Cr⁶⁺ to Cr³⁺ more efficiently than bare TiO₂. To determine the molecular orbital compositions and energies and to explain the electronic spectra and redox properties of the systems studied, density functional calculations of the optimized structures of representative model complexes were performed.     

6‐Chloroisocytosine and 5‐bromo‐6‐methylisocytosine: again,one or two tautomers present in the same crystal?

It is well known that pyrimidin‐4‐one derivatives are able to adopt either the 1H‐ or the 3H‐tautomeric form in (co)crystals, depending on the coformer. As part of ongoing research to investigate the preferred hydrogen‐bonding patterns of active pharmaceutical ingredients and their model systems, 2‐amino‐6‐chloropyrimidin‐4‐one and 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4‐one have been cocrystallized with several coformers and with each other. Since Cl and Br atoms both have versatile possibilities to interact with the coformers, such as via hydrogen or halogen bonds, their behaviour within the crystal packing was also of interest. The experiments yielded five crystal structures, namely 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (1/3), C₅H₇N₂⁺·C₄H₃ClN₃O⁻·3C₄H₄ClN₃O, (Ia), 2‐aminopyridin‐1‐ium 2‐amino‐6‐chloro‐4‐oxo‐4H‐pyrimidin‐3‐ide–2‐amino‐6‐chloropyrimidin‐4(3H)‐one–2‐aminopyridine (2/10/1), 2C₅H₇N₂⁺·2C₄H₃ClN₃O⁻·10C₄H₄ClN₃O·C₅H₆N₂, (Ib), the solvent‐free cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one (1/1), C₅H₆BrN₃O·C₅H₆BrN₃O, (II), the solvate 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–N‐methylpyrrolidin‐2‐one (1/1/1), C₅H₆BrN₃O·C₅H₆BrN₃O·C₅H₉NO, (III), and the partial cocrystal 2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(3H)‐one–2‐amino‐5‐bromo‐6‐methylpyrimidin‐4(1H)‐one–2‐amino‐6‐chloropyrimidin‐4(3H)‐one (0.635/1/0.365), C₅H₆BrN₃O·C₅H₆BrN₃O·C₄H₄ClN₃O, (IV). All five structures show R₂²(8) hydrogen‐bond‐based patterns, either by synthon 2 or by synthon 3, which are related to the Watson–Crick base pairs.     

Non-empirical investigation of the geometrical structure and energy stability of dimers of yttrium β-diketonates Y<Subscript>2</Subscript>(MDA)<Subscript>6</Subscript> and Y<Subscript>2</Subscript>(HFA)<Subscript>6</Subscript>

The structure of tris-complexes Y(MDA)₃, Y(HFA)₃ and their dimeric forms of Y₂(MDA)₆ and Y₂(HFA)₆ (MDA = C₃O₂H₃; HFA = C₅O₂F₆H) are investigated by the non-empirical Hartree-Fock method and also taking into account the correlation of electrons in the context of density functional theory (DFT/B3LYP) using effective pseudopotentials to describe the atomic core and double-exponential valence basis sets supplemented with polarization functions. For the first time the structure of Y(HFA)₃, Y₂(MDA)₆, and Y₂(HFA)₆ molecules has been studied in this work. The equilibrium configuration of Y(HFA)₃ monomer is a structure with a coordination polyhedron of [YO₆] that has the form of a distorted octahedron with planar chelate fragments and CF₃ substituents in which the C-F bond screens the C-C bond in the chelate fragment. The barrier of the internal rotation of one CF₃ group is ～3.0 kJ/mole (DFT/B3LYP). It is shown that rotations of CF₃ groups in Y(HFA)₃ molecule could be considered as mutually independent. The structures of C ₁ and C ₂ symmetry are the equilibrium configurations of Y₂(MDA)₆ and Y₂(HFA)₆ dimers respectively (DFT/B3LYP). The structure of the coordination polyhedron of [Y₂O₁₂] in C ₁ and C ₂ symmetry configurations can be represented as a combination of eight-and seven-coordinated polyhedra or two seven-coordinated polyhedra respectively. The energy stability of dimeric complexes is studied. According to DFT/B3LYP data, the dimerization energy equals ?80.8 kJ/mole and ?62.3 kJ/mole for Y₂(MDA)₆ and Y₂(HFA)₆ respectively. The force fields and frequencies of normal vibrations are calculated for the equilibrium configurations. The trends of changes in the vibrational spectra in Y(MDA)₃→Y(HFA)₃ and monomer→dimer transitions are discussed. The comparison with available experimental data is made.