We experimentally investigated the equilibrium conformation and dynamics of single DNA molecules in slitlike nanochannels. We measured the in-plane radius of gyration (R ) ), diffusivity (D), and longest relaxation time (τ) of λ-DNA (48.5 kbp) as functions of the slit height using fluorescence microscopy. Our results show that the in-plane radius of gyration increases monotonically with decreasing slit height, in contrast to results from Bonthuis et al. 15 but in agreement with our simulations and those of other groups. In strong confinement (slit height <100 nm), the scaling of D, τ, and R ) with slit height does not show an evident change, suggesting that the transition from the de Gennes regime to the Odijk regime is gradual and broad.

We experimentally study the effects of a uniform electric field on the conformation of single DNA molecules. We demonstrate that a moderate electric field (∼200 V∕cm) strongly compresses isolated DNA polymer coils into isotropic globules. Insight into the nature of these compressed states is gained by following the expansion of the molecules back to equilibrium after halting the electric field. We observe two distinct types of expansion modes: a continuous molecular expansion analogous to a compressed spring expanding, and a much slower expansion characterized by two long-lived metastable states. Fluorescence microscopy and stretching experiments reveal that the metastable states are the result of intramolecular self-entanglements induced by the electric field. These results have broad importance in DNA separations and single molecule genomics, polymer rheology, and DNA-based nanofabrication.DNA conformation | electrophoresis | polymer physics | knots | microfluidic T he use of electric fields to transport DNA has become an essential technique for a variety of research areas including molecular biology, gene therapy, and fundamental studies of polyelectrolytes (1). In most applications, nonlinear electrokinetics (2) due to the interplay of negatively charged DNA and the electric field are neglected. However, Viovy and coworkers (3, 4) demonstrated the ability of an electric field on order of 100 V∕cm to induce strong intermolecular DNA aggregation in a standard electrophoresis buffer in the absence of a sieving matrix. Follow-up studies (5, 6) have attributed the aggregation to an electrohydrodynamic instability triggered by a coupling of macroion (DNA) concentration fluctuations and electric field induced flows beyond the Debye length scale in a moderately concentrated DNA solution (around the DNA overlap concentration c à ). These results shed light onto why it is so difficult to separate large DNA molecules using capillary electrophoresis.Over the last decade there has been a shift from techniques measuring ensemble-averaged molecular properties (e.g., the aforementioned capillary electrophoresis experiments), to techniques that manipulate single, dilute DNA molecules in micro/ nanofluidic devices or nanopores (7-9). Electric fields are a convenient mode to transport DNA as they scale favorably with device dimensions. The long standing belief in such dilute, single molecule experiments is that uniform DC (direct current) electric fields (up to a few hundred V∕cm in standard electrophoresis buffers) do not greatly perturb the conformation of large DNA unless some sort of sieving matrix is used (e.g., gel or microfabricated post array) (1). Numerous fluorescence microscopy experiments confirm this belief to be true [see (10) for a recent review], but are typically limited to field strengths of a few tens of V∕cm to avoid image blur. Much larger electric fields give rise to some chain orientation and possibly some slight chain stretching (11), though the stretching is inconclusive because it is not directly meas...

A nanofluidic cross-slot device is designed and fabricated to investigate the effects of slitlike confinement on the electrophoretic stretching of single DNA molecules. The device is capable of trapping and stretching single DNA molecules at the stagnation point of a homogeneous planar elongational electric field. Different from studies of unconfined DNA, the longest relaxation time in slitlike confinement is extensiondependent, and we find the higher extension relaxation time allows better prediction of the drastic increase of extension with applied strain rate in confinement. The low extension relaxation time is important in polymer rotation and small deviations from equilibrium.

We report a highly enantioselective Cu(I)-catalyzed azide-alkyne cycloaddition via asymmetric desymmetrization of oxindole-based 1,6-heptadiynes, which furnishes quaternary oxindoles bearing a 1,2,3-triazole-containing moiety with 84-98% ee.

A remarkable fluorine effect on "on water" reactions is reported. The CF⋅⋅⋅HO interactions between suitably fluorinated nucleophiles and the hydrogen-bond network at the phase boundary of oil droplets enable the formation of a unique microstructure to facilitate on water catalyst-free reactions, which are difficult to realize using nonfluorinated substrates. Accordingly, a highly efficient on water, catalyst-free reaction of difluoroenoxysilanes with aldehydes, activated ketones, and isatylidene malononitriles was developed, thus leading to the highly efficient synthesis of a variety of α,α-difluoro-β-hydroxy ketones and quaternary oxindoles.

The unprecedented title reaction between glycine derivatives and indoles, as well as the auto-oxidative Povarov/aromatization tandem reaction of glycine derivatives with olefins are described. The reactions were performed in the absence of redox-active catalysts and chemical oxidants under mild reaction conditions. Only simple organic solvents and air (or O2 ) were required.

The poly(1,2,3-triazolium) vitrimer adhesive has particularly high lap-shear strength (23.7 MPa) and can self-heal 20 times after being damaged.

By impregnating polyethylenimine (PEI) into silica mesocellular foam with the template remaining (MCF(a)), a novel sorbent with both high CO 2 adsorption capacity and high thermal stability was obtained. The remaining P123 template in the MCF played a great role in promoting the CO 2 adsorption capacity, which could be 4.5 mmol•g −1 (adsorbent) when the amount of amine loading and the adsorption temperature were optimized as 60% and at 70 °C for the sample MCF(a)/PEI. Meanwhile, MCF(a)/PEI had a high thermal stability and selectivity, after 10 adsorption−desorption cycles, MCF(a)/PEI almost held a constant adsorption capacity; for different compositions of CO 2 and N 2 mixed gases, it always kept a high adsorption selectivity of CO 2 /N 2 . The mechanism of the template synergistic effect was elucidated by the result of a secondorder rate law through CO 2 adsorption kinetic studies. Moreover, as predicted by the Langmuir adsorption model with n = 2 (two active adsorption sites for one CO 2 molecule), the adsorption enthalpy was calculated as about −85 kJ•mol −1 , a value which belonged to typical chemical adsorption.