Light-driven molecular hinge: a new molecular machine showing a light-intensity-dependent photoresponse that utilizes the trans-cis isomerization of azobenzene

[reaction: see text] A new class of molecular machine exhibits a hingelike motion upon photoirradiation. The motion (close and open) can be operated by alternate irradiation with UV and visible light. The trans/trans and cis/cis isomers are thermally stable at 40 degrees C, and the photochemical closure reaction (from trans/trans to cis/cis isomer) is dependent on the intensity of the light used because of the short-lived intermediate (trans/cis isomer).     

Diastereocontrol by a hydroxyl auxiliary in the synthesis of pyrrolidines via radical cyclization

[reaction: see text] Organoselenium precursors of 3-aza-5-hexenyl radicals carrying a 1-hydroxyalkyl group in the 2-position were prepared by addition of organometallic reagents to N-allyl-2-aziridinecarbonitrile, reduction of the resulting aziridine ketone, and regioselective benzeneselenol ring opening of the aziridine. Reductive radical cyclization was highly selective, affording the corresponding trans-2,4-disubstituted pyrrolidine (cis/trans ca. 1/10) as the major diastereomer. Recrystallization afforded material that was substantially more enriched in the trans isomer (cis/trans < 1/25).     

Highly stereoselective radical carbonylations of gem-dihalocyclopropane derivatives with CO

A couple of radical carbonylations of gem-dihalocyclopropanes 1 using CO and Bu3SnH (formylation) or Bu3Sn(CH2CH=CH2) (allylacylation) successfully proceeded to give trans and cis adducts (2 and 3) with good to excellent stereoselectivity (trans/cis = >99/1-75/25 or 17/83-1/99). The formylation of 2,3-cis-disubstituted 1,1-dihalocyclopropanes enhanced trans selectivity (trans/cis = >99/1-95/5), whereas both 2,3-cis-disubstituted and 2-monosubstituted 1,1-dihalocyclopropanes underwent allylacylation with nearly complete trans selectivity (trans/cis = >99/1). Inherently less reactive gem-dichloro- and bromochlorocyclopropanes than gem-dibromocyclopropanes served as favorable substrates. [reaction: see text].     

Theoretical study of the mechanism of NO2 production from NO + ClO

The reaction of NO with ClO has been studied theoretically using density-functional and wave function methods (B3LYP and CCSD(T)). Although a barrier for cis and trans additions could be located at the RCCSD(T) and UCCSD(T) levels, no barrier exists at the B3LYP/6-311+G(d) level. Variational transition state theory on a CASPT2(12,12)/ANO-L//B3LYP/6-311+G(d) surface was used to calculate the rate constants for addition. The rate constant for cis addition was faster than that for trans addition (cis:trans 1:0.76 at 298 K). The rate constant data summed for cis and trans addition in the range 200-1000 K were fit to a temperature-dependent rate in the form kdi) = 3.30 x 10(-13)T(0.558) exp(305/T) cm3.molecule(-1).s(-1), which is in good agreement with experiment. When the data are fit to an Arrhenius plot in the range 200-400 K, an activation barrier of -0.35 kcal/mol is obtained. The formation of ClNO2 from ONOCl has a much higher activation enthalpy from the trans isomer compared to the cis isomer. In fact, the preferred decomposition pathway from trans-ONOCl to NO2 + Cl is predicted to go through the cis-ONOCl intermediate. The trans --> cis isomerization rate constant is kiso = 1.92 x 10(13) exp(-4730/T) s(-1) using transition state theory.     

Theoretical Study of Halogen Effect in Isomerization of 2-Halo-[9]-annulen Anion at the DFT Level

Cis-trans isomerization of [9]-annulenanion(1) and its 2-fluoro-,2-chloro-and 2-bromo-derivatives(2,3 and 4,respectively) were investigated at the HF/6-31G* and B3LYP/6-311++G** levels of theory.Cis,cis,cis,cis structures appear more stable than their corresponding cis,cis,cis,trans-isomers.The relative height of energy barriers for cis-trans isomerization is:2cis > 1cis > 3cis > 4cis.This trend for the reverse trans-cis isomerization follows the electronegativity of the substituent at C-2(2trans > 3trans > 4trans > 1trans).     

Principles of RNA base pairing: structures and energies of the trans Watson-Crick/sugar edge base pairs

Due to the presence of the 2'-OH hydroxyl group of ribose, RNA molecules utilize an astonishing variability of base pairing patterns to build up their structures and perform the biological functions. Many of the key RNA base pairing families have no counterparts in DNA. In this study, the trans Watson-Crick/sugar edge (trans WC/SE) RNA base pair family has been characterized using quantum chemical and molecular mechanics calculations. Gas-phase optimized geometries from density functional theory (DFT) calculations and RIMP2 interaction energies are reported for the 10 crystallographically identified trans WC/SE base pairing patterns. Further, stable structures are predicted for all of the remaining six possible members of this family not seen in RNAs so far. Among these novel six base pairs, the computations substantially refine two structures suggested earlier based on simple isosteric considerations. For two additional trans WC/SE base pairs predicted in this study, no arrangement was suggested before. Thus, our study brings a complete set of trans WC/SE base pairing patterns. The present results are also contrasted with calculations reported recently for the cis WC/SE base pair family. The computed base pair sizes are in sound correlation with the X-ray data for all WC/SE pairing patterns including both their cis and trans isomers. This confirms that the isostericity of RNA base pairs, which is one of the key factors determining the RNA sequence conservation patterns, originates in the properties of the isolated base pairs. In contrast to the cis structures, however, the isosteric subgroups of the trans WC/SE family differ not only in their H-bonding patterns and steric dimensions but also in the intrinsic strength of the intermolecular interactions. The distribution of the total interaction energy over the sugar-base and base-base contributions is controlled by the cis-trans isomerism.     

Novel stereoselective control over cis vs trans opening of benzo[c]phenanthrene 3,4-diol 1,2-epoxides by the exocyclic N(2)-amino group of deoxyguanosine in the presence of hexafluoropropan-2-ol

We describe a novel and efficient synthesis (62-84% yields) of the eight possible, diastereomerically pure, cis and trans, R and S O(6)-allyl-protected N(2)-dGuo phosphoramidite building blocks derived through cis and trans opening of (+/-)-3alpha,4beta-dihydroxy-1beta,2beta-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-1 (1)] and (+/-)-3alpha,4beta-dihydroxy-1alpha,2alpha-epoxy-1,2,3,4-tetrahydrobenzo[c]phenanthrene [BcPh DE-2 (2)] by hexafluoropropan-2-ol (HFP)-mediated addition of O(6)-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3) at C-1 of the epoxides. Simply changing the relative amount of HFP used in the reaction mixture can achieve a wide ratio of cis/trans addition products. Thus, the observed cis/trans adduct ratio for the reaction of DE-1 (1) in the presence of 5 equiv of 3 varied from 17/83 to 91/9 over the range of 5-532 equiv of HFP. The corresponding ratios for DE-2 (2) varied from 2/98 to 61/39 under the same set of conditions. When 1 or 2 was fused with a 20-fold excess of 3 at 140 degrees C in the absence of solvent HFP, almost exclusive trans addition (>95%) was observed for the both DEs. Through the use of varying amounts of HFP in the reaction mixture as described above, each of the eight possible phosphoramidite oligonucleotide building blocks (DE-1/DE-2, cis/trans, R/S) of the BcPh DE N(2)-dGuo adducts can be prepared in an efficient fashion. To rationalize the varying cis-to-trans ratio, we propose that the addition of 3 to 1 or 2 in the absence of solvent or in the presence of small amounts of HFP proceeds primarily via an S(N)2 mechanism to produce mainly trans-opened adducts. In contrast, increasing amounts of HFP promote increased participation of an S(N)1 mechanism involving a relatively stable carbocation with two possible conformations. One of these conformations reacts with 3 to give mostly trans adduct, while the other conformation reacts with 3 to give mostly cis adduct.     

Translational diffusion in sucrose benzoate near the glass transition: probe size dependence in the breakdown of the Stokes-Einstein equation

The translational diffusion coefficient D(trans) for rubrene, 9,10-bis(phenylethynyl)anthracene (BPEA), and tetracene in the fragile molecular glass-former sucrose benzoate (SB) (Tg=337 K) was studied as a function of temperature from Tg+3 K to Tg+71 K by use of the holographic fluorescence recovery after photobleaching technique. The values of D(trans) vary by five to six orders of magnitude in this temperature range. Contrary to the predictions of the Stokes-Einstein equation, the temperature dependence of probe diffusion in SB over the temperature range of the measurements is weaker than that of T/eta, where eta is the shear viscosity. In going from the crossover temperature Tx approximately 1.2Tg to Tg, D(trans)eta/T increases by factors of 2.4+/-0.2 decades for rubrene, 3.4+/-0.2 decades for BPEA, and 3.8+/-0.4 decades for tetracene. The decoupling between probe diffusion in SB and viscosity is characterized by the scaling law D(trans) approximately T/eta(xi), with xi=0.621 for tetracene, 0.654 for BPEA, and 0.722 for rubrene. Data for probe diffusion in SB are combined with data from the literature for probe diffusion in ortho-terphenyl and alphaalphabeta-tris(naphthyl)benzene in a plot of enhancement versus the relative probe size parameter rho(m)=(m(p)m(h))(1/3), where m(p) and m(h) are, respectively, the molecular weights of the probe and host solvent. The plot clearly shows a sharp increase in enhancement of translational diffusion at rho(m) approximately 1. By applying temperature shifts, D(trans) for probe diffusion in SB and the dielectric relaxation time tau(D) can be superimposed on a single master curve based on the Williams-Landel-Ferry equation. This suggests that the dynamics of probe diffusion in SB is described by the scaling relationship D(trans) approximately 1/tau(D)(T+DeltaT), where tau(D)(T+DeltaT) is the temperature-shifted dielectric relaxation time. The results from this study are discussed within the context of dynamic heterogeneity in glass-forming liquids.     

Theoretical studies on the conformations and properties of phenylazonaphthalenes

The conformations of phenylazo‐2‐naphthalene (I) and phenylazo‐1‐naphthalene (II) have been studied using ab initio methods and density functional theory. The rotational potential energy surfaces about the C N bonds were calculated for both the trans and the cis forms at the PM3 and HF/STO‐3G levels. The PM3 method was found to be incapable for locating the energy minima correctly. The geometries of rotamers obtained from the HF/STO‐3G surface were fully optimized at the HF/6‐31G* and B3LYP/6‐31G* levels. Single‐point MP2/6‐31G* calculations have also been carried out at the HF/6‐31G* geometries. The vibrational spectra, standard thermodynamic functions, heats of formation, and equilibrium molar fractions have been obtained. According to the calculated results, I is comparatively more stable than II. The trans forms have much lower energies than the cis forms, implying that I and II mainly exist in the trans planar forms. The energy gap (ΔE) between the frontier molecular orbitals (MOs) of the cis forms are about 0.7 eV higher than those of the trans ones, indicating that change in geometry from trans to cis form will cause a hypsochromic effect. © 2000 John Wiley & Sons, Inc. Int J Quant Chem 79: 25–33, 2000     

Formation of adenine-N3/guanine-N7 cross-link in the reaction of trans-oriented platinum substrates with dinucleotides

The reactions of the anticancer complex trans-[PtCl2{(E)-HN=C(OMe)Me}2] (trans-EE) with a series of ribo and deoxyribodinucleotides have been studied by HPLC and 2D [1H, 15N] HMQC NMR spectroscopy and compared with those of the inactive trans isomer of cisplatin, trans-[PtCl2(NH3)2] (trans-DDP). Reactions of trans-EE with r(ApG) and d(ApG) take place through solvolysis of the starting substrate and subsequent formation of trans G-N7/monochloro and G-N7/monoaqua adducts. Slowly, the monofunctional adducts evolve to a bifunctional adduct forming an unprecedented and unexpected A-N3/G-N7 platinum cross-link spanning two trans positions. For stereochemical reasons, trans platinum complexes cannot form N7/N7 cross-links between adjacent purines in di- or polynucleotides. For the reverse sequence r(GpA), no chelate structure was formed even after a two-week reaction. The reaction of trans-DDP with r(ApG) produces many more products than the analogous reaction with trans-EE. One of these products was identified as the A-N3/G-N7 trans-chelate.     

Conformation of drawn poly(trimethylene terephthalate) studied by solid-state 13C NMR

The change in the conformation of the flexible O-CH2-CH2-CH2-O segment of poly(trimethylene terephthalate) (PTT) monofilament caused by drawing was investigated by means of the gamma-gauche effect on the 13C solid-state NMR chemical shift of the internal methylene carbon, combined with the NMR relaxations. The conformation around the O-CH2 and CH2-O bonds for as-spun fiber was trans/trans. On drawing, followed by heat treatment, the conformation changed to gauche/gauche. The ratio of gauche/gauche to trans/trans for the drawn PTT fiber was determined quantitatively.     

NMR and theoretical calculations: a unified view of the cis/trans isomerization of 2-substituted thiazolidines containing peptides

The cis/trans isomerization of peptides containing the pseudoproline (4R)-thiazolidine-4-carboxylic acid Cys(Psi (R1,R2) pro) is investigated from both an experimental and a theoretical point of view by NMR and DFT calculations. A series of Ac-Cys(Psi(R1,R2) pro)-OCH3 and Ac-Cys(Psi(R1,R2) pro)-NHCH3 peptides were prepared to assess the influence of the substitution at the C2 position as well as of the amide following the thiazolidine residue. For each compound, the cis/trans ratio along with free energy, the puckering of the thiazolidine ring and the free rotational energy barrier are reported and discussed. We observe there is a pronounced effect of the C2 substituents and of the chirality upon the cis/trans ratio with the population of the cis content in the order (2R)-Cys(Psi(CH3,H) pro)<(2S)-Cys(Psi(H,CH3) pro)相似文献   

Molecular structure of the trans and cis isomers of metal-free phthalocyanine studied by gas-phase electron diffraction and high-level quantum chemical calculations: NH tautomerization and calculated vibrational frequencies

The molecular structure of the trans isomer of metal-free phthalocyanine (H2Pc) is determined using the gas electron diffraction (GED) method and high-level quantum chemical calculations. B3LYP calculations employing the basis sets 6-31G**, 6-311++G**, and cc-pVTZ give two tautomeric isomers for the inner H atoms, a trans isomer having D2h symmetry and a cis isomer having C2v symmetry. The trans isomer is calculated to be 41.6 (B3LYP/6-311++G**, zero-point corrected) and 37.3 kJ/mol (B3LYP/cc-pVTZ, not zero-point corrected) more stable than the cis isomer. However, Hartree-Fock (HF) calculations using different basis sets predict that cis is preferred and that trans does not exist as a stable form of the molecule. The equilibrium composition in the gas phase at 471 degrees C (the temperature of the GED experiment) calculated at the B3LYP/6-311++G** level is 99.8% trans and 0.2% cis. This is in very good agreement with the GED data, which indicate that the mole fraction of the cis isomer is close to zero. The transition states for two mechanisms of the NH tautomerization have been characterized. A concerted mechanism where the two H atoms move simultaneously yields a transition state of D2h symmetry and an energy barrier of 95.8 kJ/mol. A two-step mechanism where a trans isomer is converted to a cis isomer, which is converted into another trans isomer, proceeds via two transition states of C(s) symmetry and an energy barrier of 64.2 kJ/mol according to the B3LYP/6-311++G** calculation. The molecular geometry determined from GED is in very good agreement with the geometry obtained from the quantum chemical calculations. Vibrational frequencies, IR, and Raman intensities have been calculated using B3LYP/6-311++G**. These calculations indicate that the molecule is rather flexible with six vibrational frequencies in the range of 20-84 cm(-1) for the trans isomer. The cis isomer might be detected by infrared matrix spectroscopy since the N-H stretching frequencies are very different for the two isomers.     

Stereochemically general approach to adjacent bis(tetrahydrofuran) cores of annonaceous acetogenins

A series of six 2,5-disubstituted adjacent bis(tetrahydrofuran) stereoisomers with trans/erythro/cis, trans/threo/trans, or cis/threo/cis relative stereochemistry have been synthesized from known dihydroxycyclooctenes via ring opening/cross metathesis and Pd(0)-mediated asymmetric double cycloetherification. The stereochemistry of four of these isomers has been found in the biologically active annonaceous acetogenin natural products. [reaction: see text].     

Synthesis and Surface-Active Properties of New Photosensitive Surfactants Containing the Azobenzene Group

Several water-soluble cationic surfactants, 4-alkylazobenzene-4'-(oxy-2-hydroxypropyl)trimethylammonium methylsulfate (AZMS) (AZMS-0, AZMS-1, AZMS-2, AZMS-4, and AZMS-8), containing alkylglycidylether and azoarene have been synthesized with high yields of 63-78% and their surface-active properties have been investigated upon irradiation with UV/vis light. All of the trans-AZMS surfactants are isomerized to cis-trans mixtures containing 92.5% cis isomer by UV light irradiation at 350 nm. The cis isomers in the mixtures are reverted to trans isomers by visible light irradiation (lambda>445 nm). Such photoisomerization induces changes in the surface activity of each surfactant. The critical micelle concentration (cmc) of the trans form of AZMS-8 surfactant is about 1.28x10(-4) mol/l. At the photostationary state, 92.5% of the trans form is changed to the cis form which exhibits a slightly higher cmc (3.41x10(-4) mol/l). The new cmc of AZMS surfactants upon photoisomerization is similar to that of the ideal mixed micellar system. In particular, the ratio of cmc(cis) to cmc(trans) of AZMS derivatives is about 1.87-2.85 which increases proportionally with the chain length of alkyl group. The minimum average area per molecule (A(min)(a/w)) for the trans and cis isomers of AZMS-8 is 0.60 and 0.74 nm(2), respectively. The difference in the A(min)(a/w) may originate from the structural differences in the two isomers. These values are quite different as compared to those of the conventional azobenzene surfactants. Copyright 2000 Academic Press.     

周边含己氧基的三代光致变色液晶树枝状大分子的光化学

报道了新化合物含108个己氧基端基的三代(G3)碳硅烷光致变色液晶树状大分子在溶液中的反-顺光异构化反应速率常数k_p, 光回复异构化正/逆反应速率常数k_t和k_c, 热回复异构化反应速率常数k_H, 光回复异构化反应平衡常数k_t/k_c, 活化能E, 异构转化率A/A₀及组分比A'/A₀. G3的光致变色反应速率常数的数量级为10^－1 s^－1, 而侧链含偶氮基元的光致变色聚硅氧烷的光致变色反应速率常数的数量级为10^－8 s^－1, 因此G3的光响应速率比后者快10⁷倍.     

Factors influencing the autoxidation of fatty acids: effect of olefin geometry of the nonconjugated diene

Autoxidations of cis,cis, cis,trans, and trans,trans nonconjugated octadecadienoates and pentadecadienes were carried out in the presence of alpha-tocopherol to investigate the effect of olefin geometry on this oxidation process and provide insight into the factors that influence the autoxidation of fatty acids. We have found that as the trans character of the diene increases, the amount of O(2) trapping at the central (bis-allylic) position of the pentadienyl radical also increases. In addition, the rate constant for beta-fragmentation (k(beta) approximately 10(6) s(-1)) of the bis-allylic peroxyl radical decreased on going from the cis,cis to the trans,trans diene. We have also found that for the cis,trans nonconjugated dienes, there is a preference for trapping of the pentadienyl radical by O(2) at the transoid end, generating the cis,trans conjugated hydroperoxide as the major product.     

Fluorinated alcohol mediated displacement of the C10 acetoxy group of benzo[a]pyrene-7,8,9,10-tetrahydrotetraol tetraacetates: a new route to diol epoxide-deoxyguanosine adducts

We describe a novel trifluoroethanol (TFE) or hexafluoropropan-2-ol (HFP) mediated substitution reaction of the bay-region C10 acetoxy group in four stereoisomeric 7,8,9,10-tetraacetoxy-7,8,9,10-tetrahydrobenzo[a]pyrenes (tetraol tetraacetates, two pairs of cis and trans isomers at the 9,10 positions) by the exocyclic N2-amino group of O6-allyl-3',5'-di-O-(tert-butyldimethylsilyl)-2'-deoxyguanosine (3). The tetraacetates are derived from cis and trans hydrolysis of (+/-)-7beta,8alpha-dihydroxy-9beta,10beta-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P DE-1) and of (+/-)-7beta,8alpha-dihydroxy-9alpha,10alpha-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (B[a]P DE-2) at C-10 followed by acetylation. Excellent yields and high regioselectivity were observed. Similar cis/trans product ratios were observed for each set of cis and trans tetraol tetraacetates derived from DE-1 ( approximately 75/25) and from DE-2 (approximately 67/33) in HFP. This strongly suggests that the substitution proceeds via an SN1 mechanism involving a carbocation intermediate that is common to the cis and trans tetraacetates. Since it is likely that the cis and trans products from 3 arise from different conformations of the carbocation, its lifetime must be sufficiently long to permit conformational equilibration before its capture by the purine nucleophile. The corresponding reaction of (+/-)-9alpha-bromo-7beta,8alpha,10beta-triacetoxy-7,8,9,10-tetrahydrobenzo[a]pyrene with 3 in HFP was highly regio- and stereoselective and gave exclusively trans 10beta-adducts. This newly developed substitution reaction provides an attractive alternative synthetic strategy for the preparation of polycyclic hydrocarbon adducted oligonucleotide building blocks.     

31P-NMR-SPEKTROSKOPISCHER NACHWEIS VON SUPERMESITYL-SUBSTITUIERTEM TRIPHOSPHAALLYLLITHIUM UND 2-ARSA-1,3-DIPHOSPHAALLYLLITHIUM

Abstract

The compounds 1.3-bis(2,4,6-tri-tert.butylphenyl)triphosphaallyllithium (5) and 1.3-bis(2,4,6-tritert.butylphenyl)-2-arsa-1,3-diphosphaallyllithium (7) are prepared by deprotonation of a phosphino-substituted diphosphene and arsaphosphene, resp. The presence of trans/trans and cis/trans isomers has been proven by ³¹P-NMR spectroscopy.

Die Verbindungen 1.3-Bis(2,4,6-tri-tert.butylphenyl)triphosphaallyllithium (5) und 1.3-Bis-(2,4,6-tri-tert.butylphenyl)-2-arsa-1,3-diphosphaallyllithium (7) werden durch Deprotonierung eines phosphino-substituierten Diphosphens bzw. Arsaphosphens dargestellt. Das Vorliegen von trans/trans und cis/trans-Isomeren wird durch ³¹P-NMR-Spektroskopie nachgewiesen.     

Solvent-assisted conformational isomerization and the conformationally-pure REMPI spectrum of 3-aminophenol

3-Aminophenol (3AP) has two conformers, cis and trans, depending on the orientation of the OH group relative to the NH(2) group. While both conformers are found in the jet-cooled spectra of 3AP, only the trans isomer was found in the REMPI spectrum of the 3AP(NH(3))(1) cluster. It was suggested that the cis conformer of the cluster isomerizes to the more stable trans conformer in the ground state during supersonic expansion. Solvent-assisted conformational isomerization (SACI) is believed to drive the population into the more stable trans isomer. SACI also occurs for the 3AP monomer, reducing 50% of the cis/trans ratio when the ammonia concentration in the expansion is higher than 0.1%. Depending on the expansion condition, the cis conformer can be completely depleted. When other solvents were introduced in the expansion, SACI occurred with only certain solvents whose binding energy is higher than the isomerization barrier. SACI can be used as a means to prepare the most stable conformer of gas phase biomolecules.