Unnatural Cytosine Bases Recognized as Thymines by DNA Polymerases by the Formation of the Watson–Crick Geometry

The emergence of unnatural DNA bases provides opportunities to demystify the mechanisms by which DNA polymerases faithfully decode chemical information on the template. It was previously shown that two unnatural cytosine bases (termed “M‐fC” and “I‐fC”), which are chemical labeling adducts of the epigenetic base 5‐formylcytosine, can induce C‐to‐T transition during DNA amplification. However, how DNA polymerases recognize such unnatural cytosine bases remains enigmatic. Herein, crystal structures of unnatural cytosine bases pairing to dA/dG in the KlenTaq polymerase‐host–guest complex system and pairing to dATP in the KlenTaq polymerase active site were determined. Both M‐fC and I‐fC base pair with dA/dATP, but not with dG, in a Watson–Crick geometry. This study reveals that the formation of the Watson–Crick geometry, which may be enabled by the A‐rule, is important for the recognition of unnatural cytosines.     

Synthesis,Structures, and Single‐Molecule Magnetic Properties of Three Dy2 Complexes

To explore the influences of the subtle structural variations in the ligand backbones on the single‐molecule magnetic properties of dinuclear dysprosium(III) complexes, three ligands—H₂L₁ (H₂L₁=N₁,N₃‐bis(salicylaldehyde)diethylenetriamine), H₂L₂ (H₂L₂=N₁,N₃‐bis(3‐methoxysalicylidene)diethylenetriamine), and H₂L₃ (H₂L₃=N₁,N₃‐bis(5‐chlorosalicyladehyde)diethylenetriamine)—were synthesized and employed to prepare the expected dinuclear dysprosium(III) complexes. The three ligands differ in terms of the substituents at the benzene rings of the salicylaldehyde moieties. The reactions of Dy(NO₃)₃ ? 6 H₂O, pivalic acid, and the ligands H₂L₁, H₂L₂, and H₂L₃ generated complexes with the formulae [Dy₂(L₁)₂(piv)₂] ( 1 ), [Dy₂(L₂)₂(piv)₂] ( 2 ), and [Dy₂(L₃)₂(piv)₂] ? 2 MeCN ( 3 ), respectively. The purposeful attachment of the functional groups with varied sizes at the benzene rings of the salicylaldehyde backbones resulted in slight differences in the Dy‐O‐Dy bond angles and the Dy ??? Dy bond lengths in 1 – 3 ; consequently, the three complexes exhibited distinct magnetic properties. They all showed slow magnetization relaxation with energy barriers of 40.32 ( 1 ), 31.67 ( 2 ), and 33.53 K ( 3 ). Complete active space self‐consistent field (CASSCF) calculations were performed on complexes 1 – 3 to rationalize the slight discrepancy observed in the magnetic behavior. The calculated results satisfactorily explained the experimental outcomes.     

Single‐Molecule Magnetism in Three Dy2 Complexes from the Use of a Pentadentate Schiff Base Ligand and Different Benzoates

Three dinuclear dysprosium(III) complexes, [Dy₂L₂(O₂CPh)₂]?2 MeOH ( 1 ), [Dy₂L₂{(2‐NO₂)O₂CPh}₂] ( 2 ), and [Dy₂L₂{(2‐OH)O₂CPh}₂] ? MeOH ? MeCN ( 3 ) (H₂L=N₁,N₃‐bis(4‐chlorosalicyladehyde)diethylenetriamine), have been synthesized and structurally characterized. Complexes 1 – 3 possess similar Ln₂ cores and differ in substituents at the benzyl rings of benzoates. Direct current (dc) magnetic susceptibility studies in the 2–300 K range showed weak antiferromagnetic interactions between two dysprosium(III) ions in 1 – 3 . The alternating current (ac) magnetic susceptibility measurements indicated that they all exhibited SMM behavior. The strategic attachment of the ?NO₂ group (in 2 ) and the ?OH functionality (in 3 ) on the skeleton of the benzoic acid led to subtle variations of the bond lengths and bond angles in the coordination environments of the central dysprosium(III) ions, consequently resulting in the enhancement of the energy barriers for 2 and 3 . Complete‐active‐space self‐consistent field (CASSCF) calculations were employed to rationalize the experimental outcomes. Theoretical calculations confirm the existence of antiferromagnetic interactions in 1 – 3 , and the calculated dc magnetic susceptibility data agree well with those obtained experimentally. The computational results reveal more axial g tensors, as well as higher first excited Kramers doublets in 2 and 3 ; thus resulting in higher energy barriers in compounds 2 and 3 .     

Synthesis,structure and antiproliferative and optical activities of two new biphenyl‐derived Schiff bases

Two novel Schiff bases derived from indole and biphenyl have been designed and synthesized, namely 3‐((E)‐{(E)‐[1‐(biphenyl‐4‐yl)ethylidene]hydrazinylidene}methyl)‐1‐methyl‐1H‐indole ( 3‐BEHMI ) acetonitrile monosolvate, C₂₄H₂₁N₃·CH₃CN, and 3‐((E)‐{(E)‐[1‐(biphenyl‐4‐yl)ethylidene]hydrazinylidene}methyl)‐1‐methyl‐1H‐indole ( 3‐BEHEI ) acetonitrile monosolvate, C₂₄H₂₁N₃·CH₃CN. Their structures were characterized by elemental analysis, quadrupole time‐of‐flight MS, NMR and UV–Vis spectroscopy. The single‐crystal packing structure of 3‐BEHMI is largely dominated by C—H…π interactions and weak van der Waals interactions. The in vitro cytotoxicity of the two title compounds have been evaluated against two tumour cell lines (A549 human lung cancer and 4T₁ mouse breast cancer) and two normal cell lines (MRC‐5 normal lung cells and NIH 3T3 fibroblasts) by MTT assay. The results indicate that 3‐BEHEI exhibits a slightly weaker antiproliferative capability (IC₅₀ = ～50 µM) than the previously reported similar Schiff base 3‐BEHI (IC₅₀ = ～20 µM). This is in line with docking results. 3‐BEHMI demonstrates a weak cytotoxic activity, with IC₅₀ values around 110 µM, which disagrees with its docking results. Overall, the tested compounds manifest relevant cytotoxicities on the selected cancer cell lines and normal cell lines. The UV–Vis and fluorescence spectra were recorded and reproduced through the TD‐DFT method with four types of hybrid density functionals, including B3LYP, M062X, PBE1PBE and WB97XD.     

Recent Advances in Chiral Guanidine‐Catalyzed Enantioselective Reactions

Chiral guanidines have been widely used as Brønsted base catalysts and phase transfer catalysts in enantioselective reactions. Due to their amendable structure and powerful catalytic ability, they have attracted much interest. Several new catalysts containing a guanidinium moiety have been reported over the past decade and many promising outcomes have been achieved. This article illustrates the progress of chiral guanidine catalysis in asymmetric synthesis from 2009 to 2018. It is an update of a review of the same title published in 2009.     

Enantioselective reactions catalyzed by phosphine oxides

The development of chiral small organic molecules that serve as Lewis base catalysts promoting highly stereoselective transformations has been the subject of intense research over the past decades. As a matter of fact, among the plethora of molecules used as Lewis bases, chiral phosphine oxides have thoroughly been overlooked by the organic synthetic community. Thus, this review focuses exclusively on Lewis base catalysis mediated by chiral phosphine oxides with emphasis on mechanistic aspects, covering most of the publications related to this field since their first use as organocatalyst in 2005 until the end of March 2019.     

4-硫脱氧胸苷及类似物在UVA光辅助治疗抗癌药物方面的研究

本文叙述了含硫碱基DNA的化学和医疗应用。硫碱基（Thio-base）有其独特的性质,比如,易烷基化,易氧化和强的紫外长波(400-315 nm,UVA)吸收。这些特定化学和物理性质存于含硫碱基、含硫核苷和含硫碱基DNA中,它们对制备修饰的DNA和对硫碱基的DNA与其它生物大分子之间进行光诱导的交联反应是非常有用的。功能化的含有硫碱基的DNA及其类似物可用于对DNA的修复的研究。本文也介绍和评述了利用紫外光/含硫脱氧胸（腺嘧啶脱氧核）苷（UVA/Thiothymidine）的抗癌疗法,同时,探讨了含硫核苷类似物和紫外线光之间的协同作用,为癌症和其它疾病治疗提供一种新的方法。     

一种新型的极弱酸碱分析方法——固定pH滴定法

固定pH滴定法是一种新型的酸碱滴定方法。它的理论基础是弱酸弱碱在水溶液中各种存在形式的分布系数与溶液pH的关系。滴定时不需要确定等当点,也不需要标定滴定剂浓度,可用单标准比较法、标准曲线法或线性回归法对试样定量分析。用本法测定苯酚和甘氨酸,回收率在99.4～101%)。     

New chelated complexes of bis-β-carboalkoxyethyltin(IV) dichloride with S-benzyldithiocarbazate Schiff bases

The Schiff bases [H₂SBSaD], [H₂SBVD] and [H₂SBND], derived by the condensation of S-benzyldithiocarbazate and salicylaldehyde, 2-hydroxy-3-methoxybenzaldehyde and 2-hydroxy-1-naphthaldehyde respectively, react with diestertin dichlorides, R₂SnCl₂ [R=? CH₂CH₂CO₂CH₃, ? CH₂CH₂CO₂C₂H₅ or ? CH₂CH₂CO₂C₄H₉] in 1:1 molar proportion to yield chlorine-substituted complexes of the type R₂Sn(Schiff base), the base being tridentate. The complexes are characterized on the basis of their elemental analyses, IR and ¹H NMR spectral studies. The ¹³C and ¹¹⁹Sn NMR and the tin-carbon coupling constant data reveal the structures of the complexes to be octahedral with trans ester grouping, and bidentate ester linkages. The pentacoordinated complex (CH₃)₂Sn(SBSaD) was prepared by the reaction of dimethyltin oxide with H₂SBSaD in equimolar proportions.     

Novel acetal-containing calix[4]resorcinolarene-based Mannich bases

The reaction of calix[4]resorcinolarene with methylaminoacetaldehyde dimethyl acetal and formalin gave novel acetal-containing calix[4]resorcinolarene derivatives aminomethylated at the upper rim.