硅表面嫁接有机单分子膜的研究近况

由于在微电子、化学 /生物化学传感器、纳米技术及太阳能等领域具有潜在的应用价值 ,通过 Si— C键在硅表面上直接嫁接有机单分子膜 ,已成为近几年新开展的研究热点 .对这一研究领域进行了概要综述     

Morphological and structural characteristics of diazo dyes at the air-water interface: in situ Brewster angle microscopy and polarized UV/vis analysis

A morphological analysis is presented for Langmuir films of the diazo dyes Sudan 4 (S4), Sudan 3 (S3), and Sudan red (SR), using Brewster angle microscopy. Stable nonmonomolecular structures are formed at the air-water interface denoted as a plateau in the pressure-area isotherms. Monolayer domains are evident by the contrastless image even before the pressure onset, which grow in size until it reached a condensed monolayer. This behavior resembles that of Langmuir films from simple aromatic fatty acids. Films from all the azo dyes display similar features, according to the surface potential isotherms and in situ polarized UV/vis spectroscopy except for the larger area per molecule occupied by S4 and SR. This is attributed to the presence of CH(3) groups that cause steric hindrance modifying the organization of diazo dye molecules at the air-water interface. UV/vis polarized absorption spectroscopy showed preferential orientation of S4 and S3 on the water surface, while SR molecules lie isotropically. For these three diazo dyes, film absorption was negligible at very large areas per molecule, becoming nonzero only at a critical area coinciding with the onset of surface potential. The critical area is ascribed to the formation of a H-bonded network between water molecules and diazo dye headgroups.     

Chemo- and Stereoselective Coordination of 5,6-Dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene by d8- and d6-Metal Carbonyls. Diels-Alder reactivity of Dienes Perturbed by Remote Olefin Complexation

The endocyclic double bond C(2), C(3) in 5,6-dimethylidene-7-oxabicyclo[2.2.1]-hept-2-ene ( 1 ) can he coordinated selectively on its exo-face before complexation of the exocyclic s-cis-butadiene moiety. Irradiation of Ru₃(CO)₁₂ or Os₃(CO)₁₂ in the presence of 1 gave tetracarbonyl [(1R,2R, 3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]-hept-2-ene)]ruthenium ( 6 ) or -osmium ( 8 ). Similarly, irradiation of Cr(CO)₆ or W(CO)₆ in the presence of 1 gave pentacarbonyl[(1R, 2R, 3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene)]chromium (10) or -tungsten (11) . Irradiation of complexes 6 and 11 in the presence of 1 led to further CO substitution giving bed-tricarbonyl-ae-bis[(1R,2R,3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hept-2-ene)]ruthenium ( 7 ) and trans-tetracarbonyl[(1R,2R,3S,4S)-2,3-η-(5,6-dimethylidene-7-oxabicyclo-[2.2.1]hept-2-ene)]tungsten (12) , respectively. The diosmacyclobutane derivative cis-m?-[(1R,3R,3S,4S)-(5,6-dimethylidene-7-oxabicyclo[2.2.1]hepta-2,3-diyl)]bis(tetracarbonyl-osmium) (Os-Os) (9) wa also obtained. The Diels-Alder reactivity of the exocyclic s-cis-butadiene moiety in complexs 7 and 8 was found to be significantly higher than that of the free triene 1 .     

Use of molecular dynamics in the design and structure determination of a photoinducible beta-hairpin

The study presented here consists of three parts. In the first, the ability of a set of differently substituted diazobenzene-based linkers to act as photoswitchable beta-turn building blocks was assessed. A 12-residue peptide known to form beta-hairpins was taken as the basis for the modeling process. The central (beta-turn) residue pair was successively replaced by six symmetrically ((o,o), (m,m), or (p,p)) substituted (aminomethyl/carboxymethyl or aminoethyl/carboxyethyl) diazobenzene derivatives leading to a set of peptides with a photoswitchable backbone conformation. The folding behavior of each peptide was then investigated by performing molecular dynamics simulations in water (4 ns) and in methanol (10 ns) at room temperature. The simulations suggest that (o,o)- and (m,m)-substituted linkers with a single methylene spacer are significantly better suited to act as photoswitchable beta-turn building blocks than the other linkers examined in this study. The peptide containing the (m,m)-substituted linker was synthesized and characterized by NMR in its cis configuration. In the second part of this study, the structure of this peptide was refined using explicit-solvent simulations and NOE distance restraints, employing a variety of refinement protocols (instantaneous and time-averaged restraining as well as unrestrained simulations). We show that for this type of systems, even short simulations provide a significant improvement in our understanding of their structure if physically meaningful force fields are employed. In the third part, unrestrained explicit-solvent simulations starting from either the NMR model structure (75 ns) or a fully extended structure (25 ns) are shown to converge to a stable beta-hairpin. The resulting ensemble is in good agreement with experimental data, indicating successful structure prediction of the investigated hairpin by classical explicit-solvent molecular dynamics simulations.     

Synthesis of podophyllotoxin analogues: δ-lactone-containing picropodophyllin, podophyllotoxin and 4′-demethyl-epipodophyllotoxin derivatives

Non-epimerizable cis and trans δ-lactone analogues of podophyllotoxin have been prepared. Thus the synthesis of the cis isomer 4 has been achieved in 8 steps and 4% overall yield from podophyllotoxin 1 via the reduction of the γ lactone ring into the trans diol, selective protection of the 4-OH and 11-OH as a benzylidene acetal, and Wittig elongation at C-13 with inversion of configuration at C-2. Same elongation at C-13 but via the formation of a mesylate and introduction of a cyano group, led to the trans δ-lactone 5 (7 steps from 1 and 6% overall yied) with a small amount of its C-4 epimer 6. The synthesis of non-epimerizable δ-lactone analogues of 4′-demethyl-epipodophyllotoxin 7 and of 4-demethyl podophyllotoxin 8 are also reported. The synthesis of 7 and 8 was based upon the reduction of the γ-lactone ring of 4′-demethyl-4-epipodophyllotoxin followed by selective protection at C-11 and elongation at C-13. (8-15% and 4% overall yields). Compounds 4, 5 and 7 did not display relevant cytotoxicity in vitro against L1210 murine leukemia.     

Three-dimensional mesomeric networks assembled from helix-linked sheets: syntheses, structures, and magnetisms

Two new three-dimensional nickel coordination polymers, [Ni3(BTC)2(4,4'-bpy)2(H2O)5].1.5H2O (1) and [Ni(PDB)(4,4'-bpy)].0.5H2O (2)(4,4'-bpy = bipyridine, BTC = 1,2,4-benzenetricarboxylate, PDB = pyridine-3,4-dicarboxylate), have been synthesized under hydrothermal conditions and characterized by elemental analysis, IR, TGA, and single crystal X-ray diffraction. Both compounds contain 2D scaffolding motifs, and most interestingly adjacent scaffolds are connected by infinite helical chains into unique three-dimensional mesomeric networks. Moreover, temperature-dependent magnetic susceptibilities for the two compounds were studied, and ferromagnetic interactions through syn-anti carboxylate bridges between Ni sites have been observed.     

Detection of genetically modified corn (Bt176) in spiked cow blood samples by polymerase chain reaction and immunoassay methods

The fate of DNA and protein transgenic sequences in products derived from animals fed transgenic crops has recently raised public interest. Sensitive molecular tests targeting the Bt176 genetic construct and the transgenic Cry1Ab protein were developed to determine whether plant sequences, especially transgenic sequences, are present in animal products. A protocol for total DNA extraction and purification from cow whole blood samples was first drawn up and assessed by spiking with known amounts of DNA from Bt176 maize. The limit of detection for transgenic sequences (35S promoter and Bt176-specific junction sequence) was determined by both the polymerase chain reaction-enzyme-linked immunosorbent assay (PCR-ELISA) and the 5'-nuclease PCR assay. Four additional PCR systems were built to substantiate the results. The first detects a mono-copy maize-specific sequence (ADH promoter). Two others target multi-copy sequences from plant nucleus (26S rRNA gene) and chloroplast (psaB gene). The last one, used as a positive control, targets a mono-copy animal sequence (alpha(s1)-casein gene). Both methods detected a minimum spiking at 25 copies of Bt176 maize/mL in 10 mL whole blood samples. The sandwich ELISA kit used detected down to 1 ng transgenic Cry1Ab protein/mL spiked whole blood.     

Behaviour of water-soluble dinuclear rhodium complexes in the hydroformylation reaction of oct-1-ene

The biphasic hydroformylation reaction of oct-1-ene, has been investigated by using the water-soluble dinuclear complex [Rh₂(μ-S^tBu)₂(CO)₂(TPPTS)₂] as precursor. Addition of ethanol as a cosolvent dramatically improved the yields but the good regioselectivity in linear aldehyde observed for neat oct-1-ene—water systems (97%) decreased to 83% (for 22% ethanol w/w). It is shown that the dinuclear framework cannot be maintained, that the mononuclear complex [RhH(CO)(TPPTS)₃] is formed, and that thiol and significant amounts of [Rh₂(μ-S^tBu)₂(CO)₄] move into the organic phase. This reaction from the dinuclear species requires the simultaneous presence of water and carbon monoxide. Introduction of the water-soluble thiol HS(CH₂)₃NMe₂ in the bridging positions affords the complex [Rh₂(μ-S(CH ₂)₃NHMe₂)₂(CO)₂(TPPTS)₂]Cl₂ which can be kept in the aqueous hase but has a low level of catalytic activity.     

Photoinduced processes within compact dyads based on triphenylpyridinium-functionalized bipyridyl complexes of ruthenium(II)

As an alternative to conventional charge-separation functional molecular models based on long-range ET within redox cascades, a "compact approach" has been examined. To this end, spacer elements usually inserted between main redox-active units within polyad systems have been removed, allowing extended rigidity but at the expense of enhanced intercomponent electronic communication. The molecular assemblies investigated here are of the P-(theta (1))-A type, where the theta (1) twist angle is related to the degree of conjugation between the photosensitizer (P, of {Ru(bpy)(3)}(2+) type) and the electron-acceptor (A). 4-N- and 4-N-,4'-N-(2,4,6-triphenylpyridinio)-2,2'-bipyridine ligands (A(1)-bpy and A(2)-bpy, respectively) have been synthesized to give complexes with Ru(II), 1-bpy and 2-bpy, respectively. Combined solid-state analysis (X-ray crystallography), solution studies ((1)H NMR, cyclic voltammetry) and computational structural optimization allowed verifying that theta (1) angle approaches 90 degrees within 1-bpy and 2-bpy in solution. Also, anticipated existence of strong intercomponent electronic coupling has been confirmed by investigating electronic absorption properties and electrochemical behavior of the compounds. The capability of 1-bpy and 2-bpy to undergo PET process was evaluated by carrying out their photophysical study (steady state emission and time-resolved spectroscopy at both 293 and 77 K). The conformational dependence of photoinduced processes within P-(theta (1))-A systems has been established by comparing the photophysical properties of 1-bpy (and 2-bpy) with those of an affiliated species reported in the literature, 1-phen. A complementary theoretical analysis (DFT) of the change of spin density distribution within model [1-bpy(theta (1))](-) mono-reduced species as a function of theta (1) has been undertaken and the possibility of conformationally switching emission properties of P was derived.     

Does charge carrier dimensionality increase in mixed-valence salts of tetrathiafulvalene-terminated dendrimers?

[structure: see text] In four new dendrimers terminated by 12 electroactive tetrathiafulvalenyl substituents, the tridimensional character of the inter- and intradendrimeric charge and electron transfer, and hence of the electroconductivity, is evidenced by examination of the electronic spectra of their corresponding neutral state and cation radical, dication, and mixed-valence salts, including a closed-shell anion.