Amphidinolides T2, T3, and T4, new 19-membered macrolides from the dinoflagellate Amphidinium sp. and the biosynthesis of amphidinolide T1.

Three new 19-membered macrolides, amphidinolides T2 (2), T3 (3), and T4 (4), structurally related to amphidinolide T1 (1) have been isolated from two strains of marine dinoflagellates of the genus Amphidinium. The structures of 2-4 were elucidated on the basis of spectroscopic data. The absolute configurations at C-7, C-8, and C-10 of 1-4 were determined by comparison of NMR data of their C-1-C-12 segments with those of synthetic model compounds for the tetrahydrofuran portion. The biosynthetic origins of amphidinolide T1 (1) were investigated on the basis of 13C NMR data of a 13C enriched sample obtained by feeding experiments with [1-(13)C], [2-(13)C], and [1,2-(13)C2] sodium acetates and 13C-labeled sodium bicarbonate in the cultures of the dinoflagellate. These incorporation patterns suggested that amphidinolide T1 (1) was generated from four successive polyketide chains, an isolated C1 unit formed from C-2 of acetates, and three unusual C2 units derived only from C-2 of acetates. Furthermore, it is noted that five oxygenated carbons of C-1, C-7, C-12, C-13, and C-18 were not derived from the C-1 carbonyl, but from the C-2 methyl of acetates.     

Use of isotopically labelled octanoic acid to assess the effect of meal size on gastric emptying

It has been proposed that the (13)C-octanoic acid breath test (OBT) provides a safe, non-radioactive means of measuring gastric emptying. However, deuterated octanoic acid provides a better marker when compared with scintigraphy, as the kinetics are less complex than those of the (13)C label. The appearance of (2)H in saliva is modelled as a two-compartment body water system, using an asymmetric triangular gastric emptying function. This study compared the (2)H-octanoic acid saliva test (OST) with the OBT in measuring altered states of gastric emptying in the nutritional context of diet manipulation. Gastric emptying was measured using the OST and OBT in a three-way crossover study involving 12 healthy male and female subjects (mean BMI = 23.4 kg/m(2), aged 24-57 years). Following an overnight fast, subjects were given an egg meal, labelled with 10 microL/kg body weight (2)H-octanoic acid and 100 microL (13)C-octanoic acid. The meal was nutritionally manipulated to provide a 1 MJ, 2 MJ or 3 MJ meal. Breath and saliva samples were collected at regular intervals for 6 h, with further saliva samples being collected over four subsequent days. (2)H isotopic enrichment in saliva and (13)C isotopic enrichment in breath were analysed using isotope ratio mass spectrometry and the data fitted to the respective gastric emptying models. The half excretion time (T(1/2) (D)), time to maximum emptying rate (T(1) (D)) and time when emptying is complete (T(2) (D)) were calculated from the (2)H saliva test data, and the lag time (T(lag) (C)), half excretion time (T(1/2) (C)), latency phase (T(lat) (C)) and ascension time (T(asc) (C)) were calculated from the (13)C breath test data. Overall, the OBT correlated well with the OST, with a significant relationship between T(1/2) (C) and T(1/2) (D), a significant relationship between T(lat) (C) and T(1) (D) and finally a significant relationship between T(asc) (C) and T(2) (D). Gastric emptying measured using the OST was significantly faster with the 1 MJ meal (DeltaT(1/2) (D) = -0.77 h vs. 2 MJ, p = 0.004). Increases were also seen when the meal size was increased from 2 MJ to 3 MJ (DeltaT(1/2) (D) = +0.44 h vs. 2 MJ), but these were not significant. These trends were mirrored in the OBT data, with significant differences between 1 MJ and 2 MJ (DeltaT(1/2) (C) = -0.63 h vs. 2 MJ, p = 0.013) and non-significant increases with the larger 3 MJ meal (DeltaT(1/2) (C) = +0.10 h vs. 2 MJ). Total meal calorie content was shown to have an effect on gastric emptying using both the OBT and the OST. The deuterium method allows the direct calculation of the gastric emptying function and could be used as an alternative to gamma scintigraphy, allowing further validation of the (13)C-octanoic acid breath test.     

Ab Initio/GIAO-CCSD(T) study of bicyclic and related strained olefins. Structures and 13C NMR chemical shifts

Bicyclic and related strained olefins were studied by the ab initio/GIAO-CCSD(T) method. Structures and (13)C NMR chemical shifts of the olefins were calculated using ab initio/GIAO-CCSD(T) method. The delta(13)C of the olefinic carbons of the yet unknown bicyclo[1.1.0]but-1,3-ene 1 and bicyclo[2.1.0]pent-1(4)-ene 2 were computed to be 69.4 and 212.4, respectively, at the GIAO-CCSD(T)/qzp/tzp//MP2/cc-pVTZ level. The delta(13)C of the olefinic carbons of the intriguing (larger and also yet unknown) tricyclo[3.3.1.0(3,7)]non-3(7)-ene 6 and cubene 7 were calculated to be 172.5 and 187.4, respectively, at the GIAO-CCSD(T)/tzp/dz//MP2/cc-pVTZ level. In a related study, the relative energies of the various conformers of ethylene were computed and were found to correlate extremely well with the (13)C NMR chemical shifts, reflecting the linear dependence of the (13)C NMR chemical shifts on the internal strain of the molecules.     

The indirect through-space F-F coupling in peri-difluoronaphthalene: is it anisotropic?

The (1)H, (19)F and (13)C spectra have been obtained of a sample of peri-difluoronaphthalene dissolved in the nematic liquid crystalline solvent ZLI 1695. The (13)C satellite spectra from the six, single-(13)C isotopomers at natural abundance in both the (1)H and (19)F spectra were identified and analysed to yield a set of residual total, anisotropic spin-spin couplings, T(ij). This was achieved by first obtaining residual (13)C-(19)F and (13)C-(1)H couplings from a proton-encoded, (13)C detected, local field 2D spectrum. The 45 values of T(HH), T(HF) and T(CH) were used to obtain the structure of the molecule, and then to estimate whether there is a significant contribution from the component along the magnetic field, J, of the anisotropic, electron-mediated, spin-spin coupling tensors for (13)C-(19)F and (19)F-(19)F pairs. It is found that there is strong evidence for a significant contribution of J to T(FF) but not for the (13)C-(19)F pairs.     

Elucidating connectivity and metal-binding structures of unlabeled paramagnetic complexes by 13C and 1H solid-state NMR under fast magic angle spinning

Characterizing paramagnetic complexes in solids is an essential step toward understanding their molecular functions. However, methodologies to characterize chemical and electronic structures of paramagnetic systems at the molecular level have been notably limited, particularly for noncrystalline solids. We present an approach to obtain connectivities of chemical groups and metal-binding structures for unlabeled paramagnetic complexes by 13C and 1H high-resolution solid-state NMR (SSNMR) using very fast magic angle spinning (VFMAS, spinning speed >or=20 kHz). It is experimentally shown for unlabeled Cu(II)(Ala-Thr) that 2D 13C/1H correlation SSNMR under VFMAS provides the connectivity of chemical groups and assignments for the characterization of unlabeled paramagnetic systems in solids. We demonstrate that on the basis of the assignments provided by the VFMAS approach multiple 13C-metal distances can be simultaneously elucidated by a combination of measurements of 13C anisotropic hyperfine shifts and 13C T1 relaxation due to hyperfine interactions for this peptide-Cu(II) complex. It is also shown that an analysis of 1H anisotropic hyperfine shifts allows for the determination of electron-spin states in Fe(III)-chloroprotoporphyin-IX in solid states.     

Ab initio/GIAO-CCSD(T) study of structures, energies, and 13C NMR chemical shifts of C4H7(+) and C5H9(+) ions: relative stability and dynamic aspects of the cyclopropylcarbinyl vs bicyclobutonium ions

The structures and energies of the carbocations C 4H 7 (+) and C 5H 9 (+) were calculated using the ab initio method. The (13)C NMR chemical shifts of the carbocations were calculated using the GIAO-CCSD(T) method. The pisigma-delocalized bisected cyclopropylcarbinyl cation, 1 and nonclassical bicyclobutonium ion, 2 were found to be the minima for C 4H 7 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level the structure 2 is 0.4 kcal/mol more stable than the structure 1. The (13)C NMR chemical shifts of 1 and 2 were calculated by the GIAO-CCSD(T) method. Based on relative energies and (13)C NMR chemical shift calculations, an equilibrium involving the 1 and 2 in superacid solutions is most likely responsible for the experimentally observed (13)C NMR chemical shifts, with the latter as the predominant equilibrating species. The alpha-methylcyclopropylcarbinyl cation, 4, and nonclassical bicyclobutonium ion, 5, were found to be the minima for C 5H 9 (+) at the MP2/cc-pVTZ level. At the MP4(SDTQ)/cc-pVTZ//MP2/cc-pVTZ + ZPE level ion 5 is 5.9 kcal/mol more stable than the structure 4. The calculated (13)C NMR chemical shifts of 5 agree rather well with the experimental values of C 5H 9 (+).     

Acceleration of carbon-13 spin-lattice relaxation times in amino acids by electrolytes

A series of amino acids and carboxylic acids were determined by 13C NMR spectroscopy.The results showed that addition of 3M MgCl2 led to the 13C NMR integral area of samples being well proportional to number of carbon atoms that produce the particular signal with reliability over 95%. Measurements of 13C spin-lattice relaxation times (T1's) are reported for a number of amino acids. T1's of all the carbons in amino acids generally tend to decrease with the increase of the concentration of electrolytes, and the presence of magnesium slats is of significant. Carboxylic carbons in amino acids are the most sensitive "acceptor" of the 13C spin-lattice relaxation accelerating effects in electrolytes, and the 13C spin-lattice relaxation accelerating ability of electrolytes is Mg(ClO4)2 ＞MgCl2 ＞CaCl2 ＞NaCl ＞KCl ＞LiClO4 ＞NaOH. In general, T1's of C1 carbons in nonpolar a-amino acids are higher than those in polar and basic a-amino acids both in aqueous and 3M MgCl2 medium. In aliphatic straight-chain amino acids, a-, a-, a-, ai- and a- amino acids, T1's of C1 carbons tend to reduce with the increase of inserted carbon numbers between amino and carboxylic groups compared with Gly. T1's can be decreased even more when amino acids are mixed in 3M MgCl2, but T1's of carbons in amino acids decrease slightly with increase of the concentration of amino acids in 3M MgCl2. The mechanisms of the observed phenomena are discussed in terms of intermolecular interaction and paramagnetic impurity in electrolytes, large contributions of intermolecular interaction which is enhanced in electrolytes concentrate on the incoming "unsaturation" of the primary solvation shell of cations with the increase of electrolytes concentration and complexes formation of amino acids with metal ions. In electrolytes, amino acids are "anchored" to cations and molecule tumbling is slowed down, molecular rigidity is increased and molecular size is "enlarged", all of these are helpful to accelerate the 13C spin-lattice relaxation. Atlast, MgCl2 is proposed as an efficient relaxation agent for analysis of amino acids and some carboxylic acids.Samples were dissolved with the aid of supersonic which has the effect of degassing, and they were degassed again with supersonic for 30 seconds right before determination. All of the 13C NMR was obtained with a Bruker DPX-300 NMR instrument, using NOE-suppressed inverse gated decoupling with a recycle delay 8.00s and a sweep width 30120.48Hz, experiment temperature is integral of the carbon with the smallest chemical shift is calibrated as 10.00. Spin-lattice relaxation times (T1's) were determined by using inversion recovery according to Bruker avance user's guide.The pulse sequence is (T-90.°-T-180°-o-90t°) n.     

13C NMR Quantitative Study-Part 1: Relationships between the Conformation of Amino Acids, Peptide, Carboxylic Acids and Integration Intensity of 13C NMR

In proton broad band decoupling 13C NMR, carbon atoms have different integration intensity because of NOE effects and their different relaxation time(T1), thus it makes a 13C NMR quantitative analyses very difficult. To acquire a 3C NMR quantitative analyses, a gated decoupling with suppressed NOE technology, i.e., an inversed gated decoupling pulse (IGDP), must be used. In IGDP relay time (tR) between two acquisition cycles must be more than 5T1, the time needed for a acquisition cycles is so long that makes the total 13C NMR quantitative analyses time much longer. For this reason, the 13C NMR quantitative analyses is paid less attention.     

Synthesis and testing of a p-H2 hyperpolarized 13C probe based on the pyrazolo[1,5-a]pyrimidineacetamide DPA-713, an MRI vector to target the peripheral benzodiazepine receptors

DPA-713 is the lead compound of a recently developed 2-phenylpyrazolo[1,5-a]pyrimidineacetamide series that has been shown to display a good targeting capability toward peripheral benzodiazepine receptors, recently renamed translocator protein (18 kDa) or in short TSPO. On the basis of this structure, a novel derivative bearing a [(13)C]butynoate moiety has been designed and synthesized (three steps-42% overall yield) providing, upon rapid and quantitative para-hydrogenation, the corresponding hyperpolarized [(13)C]alkene. Para-hydrogen-induced polarization effects have been detected in both (1)H and (13)C-NMR spectra. Upon applying a field cycling procedure, the spin order of para-H(2) added hydrogens is transferred on the (13)C carboxylate moiety yielding a signal enhancement of approximately 4500 times. T(1) of the carboxylate carbon atom is approximately 21.9 s (at 9.37 T). A (13)C-MR image has been acquired by using the (13)C RARE (Rapid Acquisition by Relaxation Enhancement) acquisition protocol on a 10-mM solution. The main limitation to the in vivo use of this novel para-hydrogenated [(13)C]derivative is its relatively low solubility in aqueous systems.     

Solid-state NMR studies and DFT calculations of flavonoids: baicalein, baicalin and wogonoside

Three flavonoids of pharmaceutical importance-baicalein, baicalin, and wogonoside-were isolated from a Chinese medicinal plant Scutellaria baicalensis Georgi and studied by 13C NMR in solution and solid state. Two-dimensional (2D) NMR spectroscopy in the liquid phase and dipolar dephasing (DD) experiments in magic-angle spinning (MAS) spectra enabled the assignment of 13C resonances. The cross-polarization (CP) time constants T(CH) and relaxation times T(H) (1rho) were obtained from the variable-contact time experiments. The principal elements of the 13C chemical shift tensor were determined in the spectra recorded under slow sample spinning (2 kHz) using phase-adjusted spinning sideband (PASS)-2D NMR technique, and were verified by density functional theory gauge-independent atomic orbital (DFT GIAO) calculations of shielding constants. Analysis of the 13C delta(ii) and comparison with shielding parameters calculated for different conformers of compounds 1-3 enabled the selection of the most reliable geometry in the solid phase. In all three compounds, an intramolecular hydrogen bond C5--OH...=C4 is formed; the existence of baicalein and baicalin with 'anticlockwise' orientation of OH groups is more probable.     

Novel Monomeric Phenanthroline—Thallium（Ⅲ） Complexes Multinuclear NMR Characterization in Organic Solvents

A novel complex of monomeric thallium (Ⅲ） with the nitrogen donor ligand phenanthroline (phen) has been prepared and characterized by multimuclear NMR(^1H,^13C,^205Tl). The three complexes exist in equilibria in DMSO and acetonitrile solution, which was proved by the ^205Tl NMR spectra. The ^1H and ^13C NMR spectra of tris-phen T1(Ⅲ） complex have been measured, where the spin-spin coupling between T1(I=1/2) and ^13C or ^1H signals were observed with the ^1H and ^13C NMR spectroscopy in acetonitrile. The coupling constants are presented and the chemical shifts of complexes are discussed in detail.     

End-functionalised 1-vinyl-2-pyrrolidinone oligomers bearing lactate functions at one end

1-Vinyl-2-pyrrolidinone (VP) oligomers bearing a lactate group at one end (PVP-L) were obtained by chain-transfer controlled radical polymerisation carried out in the presence of ethyl L-lactate as chain-transfer agent (CTA). Their number-average molecular weights were in the range 1500-4000 with molecular weight distributions ranging from 1.4 to 1.8. The chain transfer constant, C(T), of the ethyl L-lactate/VP system was determined by monitoring the variation of PVP-L number-average molecular weight on conversion. The C(T) value so obtained was 1.03 x 10(-2), which is by about one order of magnitude higher than the C(T) value previously determined for a seemingly similar system, namely methyl isobutyrate/1-vinyl-2-pyrrolidinone (1.64 x 10(-3)). The resultant PVP-L oligomers were thoroughly characterised by means of (1)H and (13)C NMR, in order to ascertain the regular presence of the lactate functions at one of their chain terminals. NMR characterisations gave results in full agreement with the proposed structure. Moreover, the molecular weight values determined by NMR very closely agreed with those obtained by SEC. Preliminary biological evaluations of the PVP-L oligomers showed a complete lack of toxicity.     

Hyperpolarized [1-13C]-ascorbic and dehydroascorbic acid: vitamin C as a probe for imaging redox status in vivo

Dynamic nuclear polarization (DNP) of (13)C-labeled metabolic substrates in vitro and their subsequent intravenous administration allow both the location of the hyperpolarized substrate and the dynamics of its subsequent conversion into other metabolic products to be detected in vivo. We report here the hyperpolarization of [1-(13)C]-ascorbic acid (AA) and [1-(13)C]-dehydroascorbic acid (DHA), the reduced and oxidized forms of vitamin C, respectively, and evaluate their performance as probes of tumor redox state. Solution-state polarization of 10.5 ± 1.3% was achieved for both forms at pH 3.2, whereas at pH 7.0, [1-(13)C]-AA retained polarization of 5.1 ± 0.6% and [1-(13)C]-DHA retained 8.2 ± 1.1%. The spin-lattice relaxation times (T(1)'s) for these labeled nuclei are long at 9.4 T: 15.9 ± 0.7 s for AA and 20.5 ± 0.9 s for DHA. Extracellular oxidation of [1-(13)C]-AA and intracellular reduction of [1-(13)C]-DHA were observed in suspensions of murine lymphoma cells. The spontaneous reaction of DHA with the cellular antioxidant glutathione was monitored in vitro and was approximately 100-fold lower than the rate observed in cell suspensions, indicating enzymatic involvement in the intracellular reduction. [1-(13)C]-DHA reduction was also detected in lymphoma tumors in vivo. In contrast, no detectable oxidation of [1-(13)C]-AA was measured in the same tumors, consistent with the notion that tumors maintain a reduced microenvironment. This study demonstrates that hyperpolarized (13)C-labeled vitamin C could be used as a noninvasive biomarker of redox status in vivo, which has the potential to translate to the clinic.     

Structural analysis of alanine tripeptide with antiparallel and parallel beta-sheet structures in relation to the analysis of mixed beta-sheet structures in Samia cynthia ricini silk protein fiber using solid-state NMR spectroscopy

The structural analysis of natural protein fibers with mixed parallel and antiparallel beta-sheet structures by solid-state NMR is reported. To obtain NMR parameters that can characterize these beta-sheet structures, (13)C solid-state NMR experiments were performed on two alanine tripeptide samples: one with 100% parallel beta-sheet structure and the other with 100% antiparallel beta-sheet structure. All (13)C resonances of the tripeptides could be assigned by a comparison of the methyl (13)C resonances of Ala(3) with different [3-(13)C]Ala labeling schemes and also by a series of RFDR (radio frequency driven recoupling) spectra observed by changing mixing times. Two (13)C resonances observed for each Ala residue could be assigned to two nonequivalent molecules per unit cell. Differences in the (13)C chemical shifts and (13)C spin-lattice relaxation times (T(1)) were observed between the two beta-sheet structures. Especially, about 3 times longer T(1) values were obtained for parallel beta-sheet structure as compared to those of antiparallel beta-sheet structure, which could be explicable by the difference in the hydrogen-bond networks of both structures. This very large difference in T(1) becomes a good measure to differentiate between parallel or antiparallel beta-sheet structures. These differences in the NMR parameters found for the tripeptides may be applied to assign the parallel and antiparallel beta-sheet (13)C resonances in the asymmetric and broad methyl spectra of [3-(13)C]Ala silk protein fiber of a wild silkworm, Samia cynthia ricini.     

Progress in 13C and 1H solid-state nuclear magnetic resonance for paramagnetic systems under very fast magic angle spinning

High-resolution solid-state NMR (SSNMR) of paramagnetic systems has been largely unexplored because of various technical difficulties due to large hyperfine shifts, which have limited the success of previous studies through depressed sensitivity/resolution and lack of suitable assignment methods. Our group recently introduced an approach using "very fast" magic angle spinning (VFMAS) for SSNMR of paramagnetic systems, which opened an avenue toward routine analyses of small paramagnetic systems by (13)C and (1)H SSNMR [Y. Ishii et al., J. Am. Chem. Soc. 125, 3438 (2003); N. P. Wickramasinghe et al., ibid. 127, 5796 (2005)]. In this review, we discuss our recent progress in establishing this approach, which offers solutions to a series of problems associated with large hyperfine shifts. First, we demonstrate that MAS at a spinning speed of 20 kHz or higher greatly improves sensitivity and resolution in both (1)H and (13)C SSNMR for paramagnetic systems such as Cu(II)(DL-alanine)(2)H(2)O (Cu(DL-Ala)(2)) and Mn(acac)(3), for which the spectral dispersions due to (1)H hyperfine shifts reach 200 and 700 ppm, respectively. Then, we introduce polarization transfer methods from (1)H spins to (13)C spins with high-power cross polarization and dipolar insensitive nuclei enhanced by polarization transfer (INEPT) in order to attain further sensitivity enhancement and to correlate (1)H and (13)C spins in two-dimensional (2D) SSNMR for the paramagnetic systems. Comparison of (13)C VFMAS SSNMR spectra with (13)C solution NMR spectra revealed superior sensitivity in SSNMR for Cu(DL-Ala)(2), Cu(Gly)(2), and V(acac)(3). We discuss signal assignment methods using one-dimensional (1D) (13)C SSNMR (13)C-(1)H rotational echo double resonance (REDOR) and dipolar INEPT methods and 2D (13)C(1)H correlation SSNMR under VFMAS, which yield reliable assignments of (1)H and (13)C resonances for Cu(Ala-Thr). Based on the excellent sensitivity/resolution and signal assignments attained in the VFMAS approach, we discuss methods of elucidating multiple distance constraints in unlabeled paramagnetic systems by combing simple measurements of (13)C T(1) values and anisotropic hyperfine shifts. Comparison of experimental (13)C hyperfine shifts and ab initio calculated shifts for alpha- and beta-forms of Cu(8-quinolinol)(2) demonstrates that (13)C hyperfine shifts are parameters exceptionally sensitive to small structural difference between the two polymorphs. Finally, we discuss sensitivity enhancement with paramagnetic ion doping in (13)C SSNMR of nonparamagnetic proteins in microcrystals. Fast recycling with exceptionally short recycle delays matched to short (1)H T(1) of approximately 60 ms in the presence of Cu(II) doping accelerated 1D (13)C SSNMR for ubiquitin and lysozyme by a factor of 7.3-8.4 under fast MAS at a spinning speed of 40 kHz. It is likely that the VFMAS approach and use of paramagnetic interactions are applicable to a variety of paramagnetic systems and nonparamagnetic biomolecules.     

两个特殊骨架的C_(20)－二萜生物碱的全部~1H(~(13)C)核磁共振信号指定

两个具有特殊骨架的C_(20)-二萜生物碱北乌灵（kusnezoline）和峨翠灵（ omeieline）分别分自中国特产毛茛科植物北草乌（Aconitum hkusnezoffii Paritz）、彭州岩乌头（A. racemulosum var. pengzhouense）和峨眉翠雀花（ Delphinium omeiense W. T. Wang）。应用2D NMR技术(~1H-~1H COSY, HMQC, HMBC, NOESY)全面归属了这两个生物碱的~1H和~(13)C信号的化学位移。     

PASADENA hyperpolarized 13C phospholactate

We demonstrate that potassium 1-(13)C-phosphoenolpyruvate becomes hyperpolarized potassium 1-(13)C-phospholactate with (13)C T(1) = 36 s after molecular hydrogenation by PASADENA (Parahydrogen and Synthesis Allows Dramatically Enhanced Nuclear Alignment). This proof-of-principle study was conducted with a fully protonated molecular precursor. (13)C was polarized to a level of 1%, corresponding to nearly 4000-fold sensitivity enhancement at 3 T. The relevant homo- and heteronuclear spin-spin couplings are reported.     

Cis-trans isomerizations of beta-carotene and lycopene: a theoretical study

The all-trans to mono-cis isomerizations of polyenes and two C40H56 carotenes, beta-carotene and lycopene, at the ground singlet (S0) and triplet (T1) states are studied by means of quantum chemistry computations. At the S0 state of polyenes containing n acetylene units (Pn), we find that the energy barrier of the central C=C rotation decreases with n. In contrast, however, at the T 1 state, the rotational barrier increases with n. For the C40H56 carotenes, the rotational barriers of lycopene are lower than those of their beta-carotene counterparts. This difference renders the rotational rates of lycopene to be 1-2 orders of magnitude higher than those of beta-carotene at room temperature. For both these carotenes, the barrier is lowest for the rotation toward the 13-cis isomer. The relative abundances are in the following order: all-trans > 9-cis > 13-cis > 15-cis. Although the 5-cis isomer of lycopene has the lowest energy among the cis isomers, its formation from the all-trans form is restricted, owing to a very large rotational barrier. The possible physiological implications of this study are discussed.     

Synthesis and characterization of ordered polyurethanes from nonsymmetric diisocyanate and ethylene glycol

An ordered polyurethane with a head‐to‐head (H‐H) or tail‐to‐tail (T‐T) content over 95% was prepared by polyaddition reaction of a nonsymmetric monomer, p‐isocyanatobenzyl isocyanate (1) with a symmetric monomer, ethylene glycol (2). The model reactions were studied in detail to demonstrate the feasibility of polymer formation. The polymerization was conducted in THF in the presence of triethylamine (TEA) at 0 °C by slow addition of a half amount of 2 to 1, followed by removing THF and then adding the rest of 2 in DMF at once at 30 °C in the presence of dibutyltin dilaurate (DBTL). The microstructure of the polymer obtained was investigated by ¹³C NMR spectroscopy, and it was found that the polymer had the expected structural regularity. The constitutional regularity of polymers influenced their thermal properties. © 2000 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 38: 2106–2114, 2000     

Two Novel Dibutyltin Complexes with Trimers and Hexanuclear Based on the Bis(5-Cl/Me-salicylaldehyde) Carbohydrazide: Syntheses,Structures, Fluorescent Properties and Herbicidal Activity

Two novel bis(2-hydroxy-5-R-benzylidene) carbohydrazide dibutyltin complexes(R: Cl(T1), Me(T2)) with hexanuclear and triangular frame structures were obtained by microwave-assisted solvothermal reaction of dibutyltin oxide precursor with the ligands in methanol environments, However, their structures are characterized by elemental analysis, IR,(1 H, 13 C) NMR spectra, and the molecular structures of T1 and T2 were confirmed by X-ray diffraction. The crystal of T1 belongs to triclinic system, space group P1, and T2 is of monoclinic system, space group C2/c. Five-coordinated distorted triangular bipyramids and six-coordinated distorted octahedral configurations were formed by the coordination of oxygen and nitrogen atoms of ligands with two dibutyltins, respectively, and trimer hexanuclear dibutyltin complexes were constructed by the cross-coordination of enol imines. The two complexes exhibit fluorescence emission in DMF solvents and DMF-water mixtures. When the volume fraction of water content is between 0 and 10%(V/V), the aggregation fluorescence enhancement effect is good, and fluorescence quenching occurs when the water content is more than 10%(V/V). The T1 can inhibit the growth of target plants, such as Portulaca oleracea, Cassia tora L., Brassica campestris L.ssp.chinensis var.utilis Tsen et Lee and Amaranthus tricolor L. The T2 can selectively inhibit the growth of Portulaca oleracea L., Amaranthus spinosus L. and Amaranthus tricolor L. It can be used as a candidate herbicide for Portulaca oleracea L., Amaranthus spinosus L. and Amaranthus tricolor L.