底物中的硅原子对酶反应的影响

在酶工程学的研究史上,人们一方面不断地研制开发新的酶种;一方面利用固定化、酶分子改造和修饰等技术来提高酶的活性和稳定性;另一方面,则不断地开拓酶的新用途。酶催化非天然化合物的生物合成和转化（正是这一方面研究的新进展）。由于有机硅化合物在有机合成,尤其...     

Kinetic resolution of organosilicon compounds by stereoselective dehydrogenation with horse liver alcohol dehydrogenase

Streoselective dehydrogenation of three isomers of trimethylsilypropanol was carried out with horse liver alcohol dehydrogenase (HLADH, EC 1.1.1.1.) and optically active organosilicon compounds were obtained in a water-organic solvent two-layer system with coenzyme regeneration. Furthermore, we examined the effects of the silicon atom on stereoselectivity of HLADH compared to the corresponding carbon compounds. Substitution of the silicon atom for the carbon atom was found to improve the stereoselectivity of HLADH. For example, the optical purity of the remaining 1-trimethylsilyl-2-propanol was higher than 99% enantiomeric excess (ee) at 50% conversion, whereas that of the carbon analogue was 84% ee. This phenomenon was probably ascribable to the bulkiness of the organosilicon compounds derived from their longer Si-C bond. Kinetic analysis in an aqueous monolayer system demonstrated that the specific properties of the silicon atom greatly affected the reactivity of these substrate compounds.Correspondence to: A. Tanaka     

Synthesis of cyclic silyl nucleosides having bulky groups on the silicon atom

Synthesis of cyclic silyl nucleosides having bulky tert-butyl groups on the silicon atom has been investigated. Cyclic silyl deoxyribonucleosides having one tert-butyl group on the silicon atom was obtained without difficulty in the standard silylation reaction using dichlorosilane and imidazole. However, under similar conditions the reaction with di-tert-butyldichloro-silane proceeded only slowly by virtue of steric hindrance. The reaction has been largely improved by the use of 1-hydroxy-benzotriazole as a catalyst for silyl transfer and silver salts of acids to generate silylating reagents of high reactivity.     

Bioconversion of organosilicon compounds by horse liver alcohol dehydrogenase: the role of the silicon atom in enzymatic reactions

Summary Bioconversion of three organosilicon compounds of different chain length between the silicon atom and the hydroxyl group (Me₃Si(CH₂)_nOH, n = 1–3) by horse liver alcohol dehydrogenase (HLADH, EC 1.1.1.1.) was studied. Furthermore, the effect of the silicon atom on the HLADH-catalysed reaction was examined in comparison with the corresponding carbon compounds. HLADH could catalyse the dehydrogenation of trimethylsilyeethanol (n = 2) and trimethylsilylpropanol (n = 3). Trimethylsilylethanol was a better substrate than both its carbon analogue, 3,3-dimethylbutanol, and ethanol. The improved activity of HLADH on trimethylsilylethanol could be accounted for by a higher affinity toward HLADH and a lower activation energy of the reaction by HLADH than those of the carbon counterpart. These are derived from physical properties of the silicon atom, that is, the lower electronegativity and the bigger radius than those of the carbon atom. In contrast, HLADH showed no activity on trimethylsilylmethanol (n = 1), whereas it catalysed the dehydrogenation of the carbon analogue, 2,2-dimethylpropanol, fairly well. The reason for the inactivity of HLADH in the case of trimethylsilylmethanol based on the electric effect of the silicon atom is also discussed. Offsprint requests to: A. Tanaka     

Thermal conductivity of silicon and carbon hybrid monolayers: a molecular dynamics study

Thermal conductivities of graphene-like silicon and carbon hybrid nanostructures with silicon atom percentages varying from 0?% (graphene) to 100?% (silicene) are investigated using the reserve non-equilibrium molecular dynamic (RNEMD) method and Tersoff bond order potentials. The thermal conductivity of graphene is dramatically reduced with increasing silicon concentration, and the reduction appears to be related more to the topological structures formed than the amount of doped silicon atoms present. The reduction is collectively contributed to by reduced phonon group velocities (v), phonon free paths (l ( ∞ )), and the specific heat capacity (c) of the material. For systems with high symmetry, thermal conductivity is mainly influenced by v and c. For systems with low symmetry, thermal conductivity is dominated by l ( ∞ ); such materials are also more direction-dependent on thermal flux than highly symmetric materials.     

Inhibitors of angiotensin-converting enzyme containing a tetrahedral arsenic atom.

A series of tetrahedral oxo acids of Group VA and VIA elements and of silicon and boron were examined as inhibitors of angiotensin-converting enzyme. Arsenate is a competitive inhibitor with a Ki of 27 +/- 1 mM, at least 10-fold more potent than phosphate. Dimethylarsinate is a competitive inhibitor with a Ki of 70 +/- 9 mM, 2-fold more potent than dimethylphosphinate. Oxo acids of boron, silicon, antimony, sulphur and selenium are not inhibitors. On the basis of these results and the strong inhibition of this zinc metallopeptidase by substrate analogues containing a tetrahedral phosphorus atom, two substrate analogues containing a tetrahedral arsenic atom were prepared. 2-Arsonoacetyl-L-proline is a competitive inhibitor with a Ki of 18 +/- 7 mM, more than 2000-fold weaker than that of its phosphorus analogue 2-phosphonoacetyl-L-proline. 4-Arsono-2-benzylbutanoic acid is a mixed inhibitor with a Ki of 0.5 +/- 0.2 mM, indistinguishable in potency from its phosphorus analogue 2-benzyl-4-phosphonobutanoic acid.     

The solvation study of carbon, silicon and their mixed nanotubes in water solution

Nanotubes are believed to open the road toward different modern fields, either technological or biological. However, the applications of nanotubes have been badly impeded for the poor solubility in water which is especially essential for studies in the presence of living cells. Therefore, water soluble samples are in demand. Herein, the outcomes of Monte Carlo simulations of different sets of multiwall nanotubes immersed in water are reported. A number of multi wall nanotube samples, comprised of pure carbon, pure silicon and several mixtures of carbon and silicon are the subjects of study. The simulations are carried out in an (N,V,T) ensemble. The purpose of this report is to look at the effects of nanotube size (diameter) and nanotube type (pure carbon, pure silicon or a mixture of carbon and silicon) variation on solubility of multiwall nanotubes in terms of number of water molecules in shell volume. It is found that the solubility of the multi wall carbon nanotube samples is size independent, whereas multi wall silicon nanotube samples solubility varies with diameter of the inner tube. The higher solubility of samples containing silicon can be attributed to the larger atomic size of silicon atom which provides more direct contact with the water molecules. The other affecting factor is the bigger inter space (the space between inner and outer tube) in the case of silicon samples. Carbon type multi wall nanotubes appeared as better candidates for transporting water molecules through a multi wall nanotube structure, while in the case of water adsorption problems it is better to use multi wall silicon nanotubes or a mixture of multi wall carbon/ silicon nanotubes.     

Efficient kinetic resolution of organosilicon compounds by stereoselective esterification with hydrolases in organic solvent

Stereoselective esterification of three isomers of trimethylsilylpropanol, 1-trimethylsilyl-2-propanol, 1-trimethylsilyl-1-propanol, and 2-trimethylsilyl-1-propanol, was systematically studied with five kinds of hydrolases in an organic solvent system in connection with the structure of the compounds. The hydrolases were found to be able to esterify these organosilicon compounds, even -hydroxyalkylsilanes, which are unstable under the conditions of acid-catalysed esterification, and the highly optically active organosilicon compounds were successfully prepared with the selected hydrolases. Even a primary alcohol, 2-trimethylsilyl-1-propanol, was stereoselectively esterified by lipase. Furthermore, comparative studies were made by using their carbon counterparts. The silicon atom in the substrates was found to enhance the enzyme stereoselectivity in some cases, but its effect on the substrate reactivity was dependent on the structure of the substrates. These results are discussed based on the specific characters of the silicon atom. Correspondence to: A. Tanaka     

Quantum chemical study of the mechanism of ethylene elimination in silylative coupling of olefins

Silylative coupling of olefins differs from olefin metathesis. Although in both these reactions ruthenium catalysts play a crucial role and ethylene product is detected, ruthenium-carbene intermediate is formed only in the course of the metathesis reaction. In this study quantum chemical calculations based on the density functional theory (DFT) have been carried out in order to examine the mechanism of the silylative coupling of olefins leading to ethylene elimination. In the first step of the catalytic cycle, a hydrogen atom from the ruthenium catalytic center is transferred preferentially to the carbon atom bound to Si in a vinylsilane. This H transfer is coupled with the formation of Ru-C bond. Next, the rotation around the newly formed C-C single bond occurs so that silicon atom is placed in the vicinity of the ruthenium center. The following step involves the migration of a silyl moiety, and leads to Ru-Si bond formation, coupled with ethylene elimination. The next reaction, that is the insertion of ethylene (alkene) into Ru-Si bond, has an activation barrier almost as high as the reaction of ethylene elimination. However, the posibility of removing gaseous ethylene from the reactive mixture together with the entropic fators suggests that the insertion of alkene that is larger than C₂H₄ is the rate limiting step in the silylative coupling of olefins. It also suggests that the substituents attached to the silicon atom or the carbon atoms of an alkene by electronic and steric effects may significantly affect silyl migration and thus the effectiveness of the catalytic reaction. Figure Insertion of alkene into Ru-Si bond seems to be rate limiting step in the silylative coupling of olefins Electronic supplementary material Supplementary material is available in the online version of this article at and is accessible for authorized users.     

Structural Determination of Silicon-Containing Oligonucleotides by 1H−29Si Long-Range Heteronuclear Multiple Quantum Correlation NMR Spectroscopy

Abstract

The technique of ¹H^?29Si Long-Range Heteronuclear Multiple Quantum Correlation NMR Spectroscopy was used to determine the structure of silicon-containing oligonucleotides. Trimers which contained silicon instead of phosphorus as part of the oligonucleotide link were synthesized through a synthetic route that required minimal hydroxyl protection. The resulting trimer could have one of two possible structures. Through the use of ¹H^?29Si HMQC NMR spectroscopy, it was possible to link the 3′-hydroxymethine proton of one sugar to the 5′-hydroxymethylene proton of an adjacent sugar by correlation to the same silicon atom, thus elucidating the final structure.     

Novel method of the synthesis and hybridization properties of an oligonucleotide containing non-ionic diisopropylsilyl internucleotide linkage

Efficient synthesis of a dithymidine dinucleotide analog bearing a diisopropylsilyl linkage instead of a phosphodiester linkage is described with respect to its incorporation into oligonucleotides. The diisopropylsilyl linkage was introduced into the oligonucleotide by preparation of the phosphoramidite derivative of a dithymidine dimer unit. The diisopropylsilyl-modified oligonucleotide exhibited hybridization behavior with both single strand and duplex DNA. The thermal stability of both the duplex and triplex showed a relative instability compared to the corresponding natural phosphodiester DNA, because of the steric hindrance of the isopropyl group on the silicon atom.     

Antimicrobial surfaces of viologen-quaternized poly((2-dimethyl amino)ethyl methacrylate)-Si(100) hybrids from surface-initiated atom transfer radical polymerization

To improve the antimicrobial ability of silicon-based bioelectronics and to tailor the silicon surfaces for inhibiting biofilm formation, well-defined functional polymer-Si(100) hybrids, consisting of nearly monodispersed poly((2-dimethylamino)ethyl methacrylate) (P(DMAEMA)) covalently tethered on the silicon surface and functionalized by viologen moieties, were prepared. P(DMAEMA)-Si hybrids were prepared via surface-initiated atom transfer radical polymerization (ATRP) of (2-dimethylamino)ethyl methacrylate (DMAEMA) on the hydrogen-terminated Si(100) surfaces (Si−H surfaces). The tertiary amino groups of the covalently immobilized (Si−C bonded) P(DMAEMA) brushes on the silicon substrates were quaternized by an alkyl halide to produce a high concentration of quaternary ammonium groups with biocidal functionality. Alternatively, covalent coupling of viologen moieties to the tertiary amino groups of P(DMAEMA) brushes produced the quaternized P(DMAEMA)-Si(100) hybrids with substantially enhanced antimicrobial capability, as well as capability to effectively inhibit biofilm formation. Thus, the viologen-quaternized P(DMAEMA)-Si(100) hybrids possess good antibacterial surface properties and are potentially useful to the silicon-based bioelectronics to ensure their efficiency, durability and reliability.     

Silanetriols as in vitro inhibitors for AChE

Three stable silanetriols with increasing steric protection of the silicon atom have been tested for inhibition of acetylcholinesterase (AChE). For all tested silanetriols we found reversible inhibition of the AChE activity at a 100 μM concentration. The highest inhibition rate was found for the sterically least hindered cyclohexylsilanetriol with 45% inhibition relative to galanthamine hydrobromide for which an IC₅₀ value of 121 ± 3 μM was determined as well. The cytotoxicity of the silanetriols used was found to be negligible at concentrations relevant for inhibition.     

Gas phase infrared spectra of small polyatomic ions as a benchmark for theory

Infrared multiple photon dissociation (IRMPD) provides a method for examining vibrational spectroscopy of ions in the gas phase. Comparison of such experiments with the IR spectra of neutral molecules shows that the strength of a bond between a trigonal center and an attached methyl group tends to diminish after fluoride attacks to make the central atom tetrahedral. For selected main group species, the change becomes more pronounced as the central atom becomes more electropositive. Trimethylboron displays a more dramatic alteration than do silicon analogs, which indicates an effect beyond that of electronegativity by itself. In comparing gaseous trimethylboron with the negative ions derived from it, CH hyperconjugation ought to operate to a much greater extent in the neutral than in the anions. Anharmonic vibrational calculations using Density Functional Theory (DFT) match the experimental vibrational spectra and confirm that interpretation, providing evidence in favor of substantial hyperconjugative stabilization. However, further simplification of vibrational spectroscopy in terms of “atomic force constants” does not show much promise. Atoms in molecules (AIM) comparison of C-C bond orders in cations with B-C bond orders in neutral boranes is consistent with hyperconjugation.     

Evaluation of absolute configuration of naphthylphenyl-substituted oligosilanes by CD exciton chirality method

Absolute configurations of methylnaphthylphenyl-substituted oligosilanes, MeNpPhSi*SiMeR(1)R(2) [2 (R(1), R(2)=Me), 3 (R(1)=Me, R(2)=Ph), 4 (R(1), R(2)=Ph), and 5 (R(1)=Me, R(2)=SiMe(3))] were predicted by circular dichroism (CD) exciton chirality method. The sigma-pi conjugation effect of oligosilylene units (sigma-linkage) with pi-electron systems caused an intense red-shift of (1)L(a,Ph) transition band of the oligosilanes as shown in UV/VIS and made it possible to observe clear CD exciton chirality between the two aromatic chromophores on chiral silicon atom.     

Density functional theory studies of the adsorption of hydrogen sulfide on aluminum doped silicane

First principles total energy calculations have been performed to study the hydrogen sulfide (H₂S) adsorption on silicane, an unusual one monolayer of Si(111) surface hydrogenated on both sides. The H₂S adsorption may take place in dissociative or non-dissociative forms. Silicane has been considered as: (A) non-doped with a hydrogen vacancy, and doped in two main configurations; (B) with an aluminum replacing a hydrogen atom and (C-n; n?=?1, 2, 3) with an aluminum replacing a silicon atom at a lattice site. In addition, three supercells; 4x4, 3x3 and 2x2 have been explored for both non-doped and doped silicane. The non-dissociative adsorption takes place in geometries (A), (C-1), (C-2) and (C-3) while the dissociative in (B). Adsorption energies of the dissociative case are larger than those corresponding to the non-dissociated cases. In the dissociative adsorption, the molecule is fragmented in a HS structure and a H atom which are bonded to the aluminum to form a H-S-Al-H structure. The presence of the doping produces some electronic changes as the periodicity varies. Calculations of the total density of states (DOS) indicate that in most cases the energy gap decreases as the periodicity changes from 4x4 to 2x2. The features of the total DOS are explained in terms of the partial DOS. The reported charge density plots explain quite well the chemisorptions and physisorptions of the molecule on silicane in agreement with adsorption energies.     

Germanium‐ and Silicon‐Substituted Donor–Acceptor Type Copolymers: Effect of the Bridging Heteroatom on Molecular Packing and Photovoltaic Device Performance

The effects of heteroatom substitution from a silicon atom to a germanium atom in donor‐acceptor type low band gap copolymers, poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]silole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PSiBTBT) and poly[(4,4′‐bis(2‐ethylhexyl)dithieno[3,2‐b:2′,3′‐d]germole)‐2,6‐diyl‐alt‐(2,1,3‐benzothiadiazole)‐4,7‐diyl] (PGeBTBT), are studied. The optoelectronic and charge transport properties of these polymers are investigated with a particular focus on their use for organic photovoltaic (OPV) devices in blends with phenyl‐C₇₀‐butyric acid methyl ester (PC₇₀BM). It is found that the longer C‐Ge bond length, in comparison to C‐Si, modifies the molecular conformation and leads to a more planar chain conformation in PGeBTBT than PSiBTBT. This increase in molecular planarity leads to enhanced crystallinity and an increased preference for a face‐on backbone orientation, thus leading to higher charge carrier mobility in the diode configuration. These results provide important insight into the impact of the heavy atom substitution on the molecular packing and device performance of polymers based on the poly[2,6‐(4,4‐bis‐(2‐ethylhexyl)‐4H‐cyclopenta[2,1‐b;3,4‐b]‐dithiophene)‐alt‐4,7‐(2,1,3‐benzothiadiazole) (PCPDTBT) backbone.     

3-Silaoxetane and 3-silathietane: structure and reactivity studies on the basis of spectroscopic data and theoretical calculations

The ab initio and DFT calculations (structural parameters, electron localization function (ELF)) on 3-silaoxetane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-oxa-3-silacyclobutane (1) and 3-silathietane 3,3-dimethyl-2,2,4,4-tetraphenyl-1-sila-3-thiacyclobutane (2) show the cyclobutane ring in 2 as being non-planar with a C-Si-C angle of 89.2° and a C-S-C angle of 93.3°, whereas the cyclobutane ring in 1 is planar with an unusual small bond angle at the silicon atom of 74.7°, which can only be explained by bent bonds. Since the synthesis was performed in water, small bent angles cannot be indicative for high reactivity. The Raman spectra of 1 and 2 were then recorded and analyzed in the 1800-200 cm⁻¹ spectral region at various temperatures (300-10 K) with the help of the DFT calculation results (harmonic vibrational wavenumbers, Raman scattering activities). Although the wavenumber shifts are quite small, the subtle changes in the spectral features of the 3-silaoxetane and phenyl rings vibrational modes may indicate a loss of symmetry in 1 (between 200 and 150 K) and a possible phase transition in 2 (at about 200 K). Furthermore, the Raman spectra of 1 and 2 confirmed the ELF calculation results, excluding any bond interaction between the silicon and the oxygen or sulfur atom.     

Donor-acceptor substituted disilanes with surprising nonlinear optical effects

The synthesis and characterization of a new class of organosilicon compounds with high transparency and nonlinear optical properties are presented. For silicon ended molecules, Si acts mostly as a donor. The presence of a neighbouring silicon does not significantly modify the intramolecular charge transfer (ICT). One Si atom in the middle of a conjugated chain breaks the ICT so that the overall charge transfer may be decomposed into the vectorial sum of each moiety's contribution. Two neighbouring Si-atoms allow more ICT but saturating at a much lower state of interaction between the side groups if compared to a C-C bond. This is the case because hyperconjugation is built up from orbitals instead of ones. Above a certain saturation threshold it deletes the electronic exchange between the two sides which is reflected in a decrease of ₀.     

Theoretical calculation on the reaction of alkene molecules on H-terminated Si(100)-3×1 surface

A study on mechanisms of radical initiated surface chain reaction of ethylene molecule on H-terminated Si(100)-3 × 1 has been carried out in a supercell approach by using density functional theory and ab initio molecular dynamic method. On the H-terminated Si(100)-3 × 1 surface, one of the crucial steps of the surface chain reaction, namely, the abstraction of a H atom from a nearby surface hydride unit, is found to have a somewhat smaller activation energy from the nearest silicon site than from the next-nearest silicon site. From the intermediate state to the final state, the transition state has bigger activation energy. Ab initio molecular dynamics (MD) shows that the H-abstraction on Si(100)-3 × 1 surface bound organic group with a carbon-centered radical is very easy to be obtained from the transition state, and it also shows that the C…H bond at methyl group is formed in a very short MD time, and the Si…C bond between the Si surface and the alkyl chain oscillates with time evolution on Si(100)-3 × 1 surface.