物理化学全线上教学的策略与实践

受新型冠状病毒肺炎疫情影响,学校的教学活动都转到线上进行。本文以天津大学"物理化学"课程为例,介绍了全线上教学的策略、过程设计与实施效果。通过进一步丰富线上教学资源、完善教学手段、实行全过程精细化管理,使得物理化学这门原本不太适合完全线上学习的课程,取得了满意的教学效果。学习过程中,学生的主体地位和教师的主导作用得以完美体现,获得的教学经验对于未来课程教学有很好的借鉴意义。     

Detecting Counterion Dynamics in DNA–Protein Association

Due to a high density of negative charges on its surface, DNA condenses cations as counterions, forming the so-called “ion atmosphere”. Although the release of counterions upon DNA–protein association has been postulated to have a major contribution to the binding thermodynamics, this release remains to be confirmed through a direct observation of the ions. Herein, we report the characterization of the ion atmosphere around DNA using NMR spectroscopy and directly detect the release of counterions upon DNA–protein association. NMR-based diffusion data reveal the highly dynamic nature of counterions within the ion atmosphere around DNA. Counterion release is observed as an increase in the apparent ionic diffusion coefficient, which directly provides the number of counterions released upon DNA–protein association.     

Several types of bilayer smectic liquid crystals with ferroelectric and antiferroelectric properties in binary mixture of dimeric compounds

The mesomorphic behavior and phase structure were examined in the mixture of two kinds of dimeric compounds, alpha,omega-bis(4-alkoxyanilinebenzylidene-4'-carbonyloxy)pentane (mOAM5AMOm), by optical microscopy, X-ray diffraction, polarization switching, and second-harmonic generation measurements. One compound is 4OAM5AMO4 with a short terminal alkyl chain that forms a single-layer smectic phase (SmCAs) with a random mixing of spacer and tail groups. Another compound is 16OAM5AMO16 with a long terminal alkyl chain that forms a chiral, anticlinic, and antiferroelectric bilayer phase (SmCAb) with the bent molecules tilted to the bilayer. By mixing these two compounds, the SmCAs phase of 4OAM5AMO4 is easily destabilized, leading to the wide content region of the bilayer phases. In the bilayer regime, three other smectic phases are newly induced. Two of them are antiferroelectric and ferroelectric phases in which the molecules lie perpendicularly with respect to the layer. The other shows no polar response to an external electric field and behaves like a smectic A. The new appearance of these bilayer phases is discussed as a mixing effect of long and short tail groups.     

Highly efficient total synthesis of the marine natural products (+)-avarone, (+)-avarol, (-)-neoavarone, (-)-neoavarol and (+)-aureol

Biologically important and structurally unique marine natural products avarone (1), avarol (2), neoavarone (3), neoavarol (4) and aureol (5), were efficiently synthesized in a unified manner starting from (+)-5-methyl-Wieland-Miescher ketone 10. The synthesis involved the following crucial steps: i) Sequential BF(3)Et(2)O-induced rearrangement/cyclization reaction of 2 and 4 to produce 5 with complete stereoselectivity in high yield (2 --> 5 and 4 --> 5); ii) strategic salcomine oxidation of the phenolic compounds 6 and 8 to derive the corresponding quinones 1 and 3 (6 --> 1 and 8 --> 3); and iii) Birch reductive alkylation of 10 with bromide 11 to construct the requisite carbon framework 12 (10 + 11 --> 12). An in vitro cytotoxicity assay of compounds 1-5 against human histiocytic lymphoma cells U937 determined the order of cytotoxic potency (3 > 1 > 5 > 2 > 4) and some novel aspects of structure-activity relationships.     

Cloning and sequence analysis of the poly(3-hydroxyalkanoic acid)-synthesis genes ofPseudomonas acidophila

Pseudomonas acidophila can grow with CO₂ as a sole carbon source by the possession of a recombinant plasmid that clones genes that confer chemolithoautotrophic growth ability derived from the H₂-oxidizing bacteriumAlcaligenes hydrogenophilus. H₂-oxidizing bacteria produce poly(3-hydroxybutyric acid) (PHB) from CO₂, but recombinant P.acidophila can produce the more useful biopolymer poly(3-hydroxyalkanoic acid) (PHA). In this study, thepha genes ofP. acidophila were cloned and a sequence analysis was carried out. A gene library was constructed using the cosmid vector pVK102. A recombinant cosmid carrying thepha genes was selected by the complementation of a PHB-negative mutant ofAlcaligenes eutrophus H16. The resulting recombinant cosmid pIK7 contained a 14.8-kb DNA insert. Subcloning was done, and the recombinant plasmid pEH74 was selected by hybridization with theA. eutrophus H16pha genes.Escherichia coli possessing pEH74 produced PHB, indicating that pEH74 contained thepha genes ofP. acidophila. The nucleotide sequences of the PHA-synthesis genesphaA (3-ketothiolase),phaB (acetoacetyl-CoA reductase), andphaC (PHA synthase) in pEH74 were determined. The homologies ofphaA, phaB, andphaC betweenP. acidophila andA. eutrophus H16 were 64.7, 76.1, and 56.6%, respectively.

     

Controllable synthesis, magnetism and solubility enhancement of graphene nanosheets/magnetite hybrid material by covalent bonding

Hybrids of Fe(3)O(4) nanoparticles and surface-modified graphene nanosheets (GNs) were synthesized by a two-step process. First, graphene nanosheets were modified by SOCl(2) and 4-aminophenoxyphthalonitrile to introduce nitrile groups on their surface. Second, the nitrile groups of surface-modified graphene nanosheets were reacted with ferric ions on the surface of Fe(3)O(4) with the help of relatively high boiling point solvent ethylene glycol to form a GNs/Fe(3)O(4) hybrid. The covalent attachment of Fe(3)O(4) nanoparticles on the graphene nanosheet surface was confirmed by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), energy-dispersive X-ray spectrometer (EDS) and scanning electron microscopy (SEM). TEM and HRTEM observations indicated that the sizes of the nanoparticles and their coverage density on GNs could be easily controlled by changing the concentration of the precursor and the weight ratio to GNs. Magnetic measurements showed that magnetization of the hybrid materials is strongly influenced by the reaction conditions. Chemically bonded by phthalocyanine, the solubility of as-synthesized GNs/Fe(3)O(4) hybrid materials was greatly enhanced, which was believed to have potential for applications in the fields of composites, wastewater treatment and biomaterials.     

Effect of co-adsorbed CO and reaction temperature on the dynamics of N2 desorption under steady-state N2O-CO reaction on Rh(110)

We have investigated the effect of co-absorbed CO and reaction temperature on the angular distribution of N(2) desorption by N(2)O decomposition under the steady state of N(2)O-CO reaction on Rh(110). Spatial distributions of desorbing product N(2) emission have been measured at various surface temperatures and CO coverages. The decomposed N(2) collimates at 48°-61° off normal in the parallel plane to [001] and [110] directions, indicating that adsorbed N(2)O just before the decomposition is oriented along the [001] direction. Although the inclined and collimated N(2) desorption is always observed at any steady-state CO coverage and reaction temperature, the shape of the collimated N(2) distribution varied dependent on the co-adsorbed CO coverage. The distribution becomes sharp and shifts toward the surface normal direction with increasing CO coverage. These effects of adsorbed CO on the angular distribution of N(2) are interpreted by the collision of desorbed N(2) with co-adsorbed CO.     

A photocleavable rapamycin conjugate for spatiotemporal control of small GTPase activity

We developed a novel method to spatiotemporally control the activity of signaling molecules. A newly synthesized photocaged rapamycin derivative induced rapid dimerization of FKBP (FK-506 binding protein) and FRB (FKBP-rapamycin binding protein) upon UV irradiation. With this system and the spatially confined UV irradiation, we achieved subcellularly localized activation of Rac, a member of small GTPases. Our technique offers a powerful approach to studies of dynamic intracellular signaling events.