Determinants of unnatural nucleobase stability and polymerase recognition

Six new unnatural nucleobases have been synthesized and characterized in terms of stability and selectivity of self-pairing in duplex DNA and efficiency and fidelity of self-pairing during polymerase-mediated replication. Each nucleobase has a conserved ring structure but differs from the others in its specific pattern of substitution with oxygen and sulfur atoms. Heteroatom derivatization within the conserved scaffold is shown to have only moderate effects on unnatural self-pair synthesis by the polymerase; larger effects were observed on the thermal stability and polymerase-mediated extension of the self-pairs. The largest effects of heteroatom substitution were on the stability and synthesis of mispairs between the unnatural and natural bases. Certain heteroatom substitutions were found to have a general effect while others were found to have effects that were specific for a particular unnatural or natural base. The data are useful for designing stable and replicable third base pairs and for understanding the contributions of nucleobase shape, polarity, and polarizability to the stability and replication of DNA.     

Slow divergence integral and its application to classical Liénard equations of degree 5

The slow divergence integral is a crucial tool to study the cyclicity of a slow–fast cycle for singularly perturbed planar vector fields. In this paper, we deduce a useful form for this integral in order to apply it to various problems. As an example, we use it to prove that the slow divergence integral along any non-degenerate slow–fast cycle for singular perturbations of classical Liénard equations of degree 5 has at most one zero, and the zero is simple if it exists; hence the cyclicity of any non-degenerate slow–fast cycle in this class of equations is at most 2. Up to now there were many interesting results about Liénard equations of degree 3, 4 and ≥6, but almost nothing is known about degree 5. The result in this paper can be seen as a first stage to study the uniform property for classical Liénard equations of degree 5.     

化学教学中培养学生分析问题解决问题的研究

提出问题、解决问题是一类基本的学习方法。在学习过程中有各式各样的问题,需要学习者去发现、去思考。从广义上说学习就是解决问题的过程,也是学习怎样解决问题的过程。     

Periodic orbit of the pendulum with a small nonlinear damping

We study the pendulum with a small nonlinear damping, which can be expressed by a Hamiltonian system with a small perturbation. We prove that a unique periodic orbit exists for any initial position between the equilibrium point and the heteroclinic orbit of the unperturbed system, depending on the choice of the bifurcation parameter in the damping. The main tools are bifurcation theory and Abelian integral technique, as well as the Zhang''s uniqueness theorem on Li\''enard equations.     

The cyclicity of the elliptic segment loops of the reversible quadratic Hamiltonian systems under quadratic perturbations

Denote by Q^H and Q^R the Hamiltonian class and reversible class of quadratic integrable systems. There are several topological types for systems belong to Q^H∩Q^R. One of them is the case that the corresponding system has two heteroclinic loops, sharing one saddle-connection, which is a line segment, and the other part of the loops is an ellipse. In this paper we prove that the maximal number of limit cycles, which bifurcate from the loops with respect to quadratic perturbations in a conic neighborhood of the direction transversal to reversible systems (called in reversible direction), is two. We also give the corresponding bifurcation diagram.     

On Borromean links

n-Borromean links are nontrivial links in whichn rings,n 3, are combined in such a way that any two component rings form a trivial link. The symmetry of links withn = 3 is discussed, and it is shown that such links form a variety of series whose members are different isotopy types. Examples are adduced of 3-Borromean links that are topologically chiral: Novel constructions are described ofn-Borromean links with and without at least one nontrivial sublink.     

Differential-pulse polarography of amino acids in acetaldehyde-borax medium

A differential-pulse polarographic method for the determination of amino acids is reported, based on the formation of Schiff's base compounds in borax buffer solution (pH 10.10) containing acetaldehyde. The compounds are reduced at a mercury electrode with peak potentials of about ?1.5 V (vs. SCE) and well defined polarographic waves are obtained which can be used to determine amino acids in borax medium. The differential-pulse polarographic method was found to be the most sensitive and suitable for the determination of amino acids in the concentration range 1 × 10^?6–8 × 10^?4 M (lysine) and 2.8 × 10^?6–1 × 10^?3 M (arginine). The polarographic characteristics of these waves were studied by differential-pulse polarographic and cyclic voltammetric methods. The waves are ascribed to the reduction of the imido group in the Schiff's base compounds. The procedure was applied to the determination of lysine and arginine in foodstuffs and the total proteins in serum samples.     

直线度误差的光电非接触测量方法

基于光电检测技术,利用回转和轴向进给伺服系统,通过激光扫描检测系统测量刀口尺与被测零件素线之间的间隙,实现了对直线度误差的非接触自动测量。详细论述了直线度误差的测量方法和光电检测系统,通过实验对系统的测量精度进行了验证。     

Oligosaccharide Analogues of Polysaccharides. Part 5. Studies on the cross-coupling of alkynes and haloalkynes

The cross-coupling of the homopropargylic ether 1 and the halopropargylic ethers 2a and 2b was optimized, and aspects of the coupling mechanism were studied. Coupling promoted by Pd° and Cu¹ in the presence of an amine yielded a mixture of the heterodimer 3 and the homodimers 4 and 5 (Scheme 1). Optimizations were first directed at suppressing homo-coupling. Homo-coupling is partially due to a H/I exchange ( 1 + 2a ? 6 + 7 ) promoted by CuI and an amine. The exchange, but not the formation of homodimers, was largely suppressed in DMSO. The influence of phosphine ligands was also evaluated. Weaker σ-donors (with the exception of PPh₃) lead to a faster coupling and to a higher ratio of hetero- to homodimers, with P(fur)₃ leading to the cleanest reaction. Homodimers are also formed (together with I₂ · (i-Pr)₂NH) by reductive dimerization of the iodoalkyne 2a in the presence of [Pd₂(dba)₃], CuI, and (i-Pr)₂NH. Bulky and acceptor-substituted amines reduced the extent of the dimerization of 2a , but the bulkiest amines did not promote coupling. Better results were obtained by using the bromoalkyne 2b . Neither dimerization of 2b , nor H/Br exchange between 1 and 2b were observed. Coupling of 1 and 2b was slower than the one of 1 and 2a , but gave higher yields of the heterodimer 3 . The yield of 3 and the ratio of hetero-to homodimers was greatly improved by addition of LiI; no phosphine ligand is then required. While the oxidative addition of the iodoalkyne 2a to [Pd(PPh₃)₄] ( 2a → 8a ) was rapid, the one of the bromoalkyne 2b was much slower and proceeded via the η²-complex 9 as evidenced by ¹H-NMR spectroscopy. The rearrangement of 9 to the bromopalladium σ-complex 8b follows first-order kinetics (k = 0.014 min^?1). CuBr greatly increased the rate of this rearrangement. LiI caused rapid substitution of Br by I in the Pd σ-complex ( 8b → 8a ), but not in 9 , nor in 2b . The σ-complex 8a did not react with the alkyne 1 in the presence of (i-Pr)₂NH, unless Cu^I was added. The alkynes 10 or 1 did not react with CuI and either TMEDA or (i-Pr)₂NH to yield detectable amounts of the Cu-acetylides 11 or 12 . These observations are rationalized by the mechanism shown in Scheme 3, postulating the intermediacy of the binuclear alkyne-Pd-Cu complexes C and J , and some or all of E–H , and highlighting the role of Cu^I in this coupling.