Identification of Intermediates in the Biosynthesis of PR Toxin by Penicillium roqueforti

The sesquiterpenoid 7‐epi‐neopetasone was synthesized via the Wieland–Miescher ketone. The compound was identical to a previously tentatively identified headspace constituent of Penicillium roqueforti. Feeding experiments with ¹³C‐labeled mevalonolactone isotopomers demonstrated that oxidation at C12 and an isomerization of the C11?C12 to a C7?C11 double bond must occur independently and not via a C7‐C11‐C12 allyl radical in one step. Feeding with (11,12,13‐¹³C₃)‐7‐epi‐neopetasone resulted in labelling of the PR toxin, thus establishing this compound as a newly identified pathway intermediate.     

2-甲基-6-乙酰基萘结构的表征

2-甲基-6-乙酰基萘是一种有发展前景的化工产品。本实验在合成和精制的基础上,采用气相色谱、质谱、^1H、^13C及二维核磁共振技术表征了2-甲基-6-乙酰基萘的结构,首次给出了该化合物的化学位移数据。     

Line shape analysis of two-colour photoassociation spectra on the example of the Cs ground state

Two-photon photoassociation spectra in a Λ-type excitation scheme are analysed under the systematically varied experimental conditions of frequency detunings and laser intensities. Line shape fits are presented as well as the investigation of intensity and detuning dependent line shifts. From both we determine the attained spectroscopic precision, that is corrected for a systematic line shift due to the thermal distribution of atoms in the trap. An energy correction for this effect is given. Information about the feasibility of generating translationally cold molecules in a well defined rotational and vibrational level by the photoassociation process is derived from the analysis. Received 21 May 2002 Published online 15 October 2002 RID="a" ID="a"Current address e-mail: christian.lisdat@ptb.de RID="b" ID="b"e-mail: pierre.pillet@lac.u-psud.fr     

Ab initio molecular modeling of 13C NMR chemical shifts of polymers. 1. Ethylene–norbornene copolymers

Cycloolefin copolymers (COC) have recently raised much interest because of their excellent thermal and optical properties, largely determined by the chain composition and stereochemistry. Previous force‐field computations allowed us to define the main conformational characteristics of ethylene–norbornene (E–N) copolymers and to contribute to the elucidation of their microstructure on the basis of empirical relationships between conformation and ¹³C nuclear magnetic resonance (NMR) chemical shifts. A thorough test of ab initio ¹³C chemical shifts computations [gauge‐invariant atomic orbitals (GIAO)] on known cases shows that the agreement with experimental data is quite good, especially with the MPW1PW91 density functional theory (DFT), using the 6‐311+G(2d,p) basis set on properly energy‐minimized structures. We applied this method on proper model compounds to confirm the signal assignment of the spectra of E–N copolymers in the presence of norbornene microblocks, where strong effects arising from ring distortions are expected to occur. The results nicely confirm the latest assignment of norbornene signals belonging to ENNE sequences. This shows the great potentialities of GIAO/DFT computations with regard to complex spectra interpretation and polymer microstructural investigations. © 2002 Wiley Periodicals, Inc. Int J Quantum Chem, 2002     

Efficient Microscale Preparation of Isotopically Enriched 1‐[79Br]Bromo‐2‐Fluoroethylene, [79Br]BrHC˭CHF

Abstract

An efficient preparation of 1‐[⁷⁹Br]bromo‐2‐fluoroethylene, [⁷⁹Br]BrHC?CHF, was carried out by a three‐step procedure: (a) natural 1‐bromo‐2‐fluoroethylene, BrHC?CHF, was iodinated to 1‐fluoro‐2‐iodoethylene, FHC?CHI; (b) 1‐fluoro‐2‐iodoethylene was ⁷⁹Br₂‐brominated to 1,2‐di[⁷⁹Br]bromo‐1‐fluoro‐2‐iodoethane, [⁷⁹Br]BrFCHCH[⁷⁹Br]BrI; and (c) 1,2‐di[⁷⁹Br]bromo‐1‐fluoro‐2‐iodoethane was dehalogenated to 1‐[⁷⁹Br]bromo‐2‐fluoroethylene, [⁷⁹Br]BrHC?CHF. The yield of isolated product, on a 2‐mmol scale, was 62% with respect to ⁷⁹Br₂.     

Protegrin‐1 orientation and physicochemical properties in membrane bilayers studied by potential of mean force calculations

Protegrin‐1 (PG‐1) belongs to the family of antimicrobial peptides. It interacts specifically with the membrane of a pathogen and kills the pathogen by releasing its cellular contents. To fully understand the energetics governing the orientation of PG‐1 in different membrane environments and its effects on the physicochemical properties of the peptide and membrane bilayers, we have performed the potential of mean force (PMF) calculations as a function of its tilt angle at four distinct rotation angles in explicit membranes composed of either DLPC (1,2‐dilauroylphosphatidylcholine) or POPC (1‐palmitoyl‐2‐oleoylphosphatidylcholine) lipid molecules. The resulting PMFs in explicit lipid bilayers were then used to search for the optimal hydrophobic thickness of the EEF1/IMM1 implicit membrane model in which a two‐dimensional PMF in the tilt and rotation space was calculated. The PMFs in explicit membrane systems clearly reveal that the energetically favorable tilt angle is affected by both the membrane hydrophobic thickness and the PG‐1 rotation angle. Local thinning of the membrane around PG‐1 is observed upon PG‐1 tilting. The thinning is caused by both hydrophobic mismatch and arginine‐lipid head group interactions. The two‐dimensional PMF in the implicit membrane is in good accordance with those from the explicit membrane simulations. The ensemble‐averaged Val16 ¹⁵N and ¹³CO chemical shifts weighted by the two‐dimensional PMF agree fairly well with the experimental values, suggesting the importance of peptide dynamics in calculating such ensemble properties for direct comparison with experimental observables. © 2010 Wiley Periodicals, Inc. J Comput Chem, 2010     

An NMR, IR and theoretical investigation of (1)H chemical shifts and hydrogen bonding in phenols

The change in (1)H NMR chemical shifts upon hydrogen bonding was investigated using both experimental and theoretical methods. The (1)H NMR spectra of a number of phenols were recorded in CDCl(3) and DMSO solvents. For phenol, 2- and 4-cyanophenol and 2-nitrophenol the OH chemical shifts were measured as a function of concentration in CDCl(3). The plots were all linear with concentration, the gradients varying from 0.940 (phenol) to 7.85 (4-cyanophenol) ppm/M because of competing inter- and intramolecular hydrogen bonding. Ab initio calculations of a model acetone/phenol system showed that the OH shielding was linear with the H...O=C distance (R) for R < 2.1 A with a shielding coefficient of - 7.8 ppm/A and proportional to cos(2)phi where phi is the H...O=C--C dihedral angle. Other geometrical parameters had little effect. It was also found that the nuclear shielding profile is unrelated to the hydrogen bonding energy profile. The dependence of the OH chemical shift on the pi density on the oxygen atom was determined as ca 40 ppm/pi electron. This factor is similar to that for NH but four times the value for sp(2) hybridized carbon atoms. The introduction of these effects into the CHARGE programme allowed the calculation of the (1)H chemical shifts of the compounds studied. The CHARGE calculations were compared with those from the ACD database and from GIAO calculations. The CHARGE calculations were more accurate than other calculations both when all the shifts were considered and also when the OH shifts were excluded. The calculations from the ACD and GIAO approaches were reasonable when the OH shifts were excluded but not as good when all the shifts were considered. The poor treatment of the OH shifts in the GIAO calculations is very likely due to the lack of explicit solvent effects in these calculations.     

TIMING OF COLLISION BETWEEN THE NORTH AND SOUTH CHINA BLOCKS——THE Sm-Nd ISOTOPIC AGE EVIDENCE

Sm-Nd isotopic ages for C-type eclogite (243.9±5.6 Ma) and mafic and ultramafic rocks(230.6±30.7 Ma and 402.6±17.4 Ma) from the Qinling-Dabieshan orogenic belt are reported.These ages suggest that at the early Triassic, the North and South China Blocks have beenunited along the Qinling-Dabieshan collision zone, and before the final collision, a se-ries of ophiolite obductions took place successively away from the continental marginduring the Paleozoic.     

Al and Cu NMR study in UCu5Al

We have studied the microscopic properties of the tetragonal UCu₅Al Kondo compound by ²⁷Al and ^63,65Cu NMR in the paramagnetic state. NMR and susceptibility measurements performed on the powdered sample, but oriented along the applied field, showed χ>χ. Plots of K(T) against χ(T) at temperatures T≥100 K yield the transferred hyperfine fields of +5.9 kOe/μ_B for ²⁷Al nuclei, and +5.3 and −7.0 kOe/μ_B for ⁶⁵Cu nuclei in crystallographically inequivalent Cu(2) and Cu(1) sites, respectively. The Knight shift vs. susceptibility plots for T<100 K exhibit a deviation from the linear behaviour (absolute values of shifts become smaller than expected). We attribute this finding to the crystalline electric field effect in similar way as it was reported for several Ce-based compounds. The random distribution of the Al and Cu(2) atoms in the crystal lattice we consider as a reason of an unusual broadening of the NMR spectra, particularly at low temperatures.