Anisotropic local motions and location of amide protons in proteins

A new method has been developed to obtain dynamic and structural information about peptide planes in proteins by a combination of measurements of weak short-range cross-correlation rates R(H(N)N/NC') that are due to concerted fluctuations of the H(N)-N and N-C' dipole-dipole interactions and stronger long-range cross-correlation rates R(C'H(N)/H(N)N) and R(NH(N)/H(N)C(alpha)). The rates were interpreted using the axially symmetric Gaussian axial fluctuation model (GAF). The oscillation amplitudes as well as the positions of H(N) atoms with respect to peptide planes in ubiquitin were determined. Most N-H(N) bonds were found not to lie exactly along the bisector of the N-C' and N-C(alpha) bonds but to be slightly tilted toward the carbon-terminal side of the peptide.     

Facile Preparation of (±)-12-Epiprostaglandins from 7-Oxabicyclo[2.2.1]hept-5-en-2-one via an all-cis-formyllactone related to Corey lactone

The bicyclic monoselenoacetal 7 , easily obtained from (±)-7-oxabicyclo[2.2.1]hept-5-en-2-one ( 6 ) via a radical addition-acyl migration sequence, was converted to racemic 12-epiprostaglandins 3 and 4 . The key intermediate was the all-cis-formyllactone 2b related to Corey lactone (see 12 ; Scheme 1). The presence of a (tert-butyl)-dimethylsilyl protective group for the 11-OH substituent (prostaglandin numbering) was found to be crucial in avoidingβ -elimination and epimerization during the Wittig-Horner reaction (Scheme 2). Epimerization at C(12) at the formyllactone stage (see 2b ) was also possible and gave the known precursor 1b of naturally occurring prostaglandins and analogs.     

Countercurrent chromatographic isolation of lolitrem B from endophyte-infected ryegrass (Lolium perenne L.) seed

This paper describes a new method of purification of the Lolitrem B, a tremorgenic mycotoxin produced in planta by the endophytic fungus Neotyphodium lolii. The method is based on the large-scale isolation of the toxin by countercurrent chromatography (CCC). The lolitrem B content in endophyted ryegrass seed, 11 microg/g or 11 ppm, is extracted by stirring finely ground seeds with ethanol for 3 h at room temperature. The concentrated crude extract contains about 0.6 mg/g or 600 ppm of lolitrem B. It is then submitted to CCC purification with a biphasic four-solvent liquid system. A 160-fold enrichment was obtained in one step producing a raffinate containing 10% or 100 mg/g of the toxin. Further purifications were then performed by thin layer and low pressure liquid chromatography. Twenty-eight micrograms of lolitrem B with a 96% purity grade were obtained from 8 kg of seeds (yield 32%).     

Formation et réactivité de carbanions α phosphoramides. Mise en évidence d'une nouvelle réaction d'élimination

The reactivity of (diethoxy)-N-methyl-N-benzyl phosphoramide (I) and bis(dinethylamino)-N-methyl-N-benzylphosphoramide (II) when treated with strong bases (RLi, R₂NLi) is very different: (I) gives a novel elimination, whereas (II) under the same experimental conditions gives a stable carbanion whose reactivity has been investigated.     

The [CH2CHOH.+, CH3CHO] solvated enol radical cation

The bimolecular reaction of the CH₂CHOH^.+ enol ion (m/z 44) with acetaldehyde gives a strongly dominant product,m/z 45, formed mainly by proton transfer from the ion to the molecule. The abundance of the product coming from a H^. abstraction reaction from the neutral, albeit more exothermic, is negligible. In order to explain this result, the long lived [CH₂CHOH^.+, CH₃CHO] solvated ion was generated by reaction of the CH₂CHOH^.+ enol ion with (CH₃CHO) _n in the cell of a Fourier transform ion cyclotron resonance mass spectrometer. The structure of this solvated ion was clearly established. Labeling indicates that [CH₂CHOH^.+, CH₃CHO], upon low energy collisions, reacts by H^. abstraction more rapidly than by H⁺ transfer to the neutral moiety. This shows that the entropic factors are determinant when the enol ion reacts directly with acetaldehyde.     

General access to iminosugar C-glycoside building blocks by means of cross-metathesis: a gateway to glycoconjugate mimetics

[reaction: see text] Cross-metathesis reactions of alpha-1-C-allyl-1-deoxynojirimycin derivatives 7a,b and various functionalized alkenes mediated by Grubbs's catalyst 3 are reported. The reactions showed reasonable to very good yields and excellent E/Z selectivity. This methodology allows the efficient and convergent synthesis of iminosugar C-glycosides with a great degree of structural diversity in the aglycone, opening the way to a variety of new glycoconjugate mimetics of biological interest.     

Probing electrochemical properties of pi-conjugated thienylenevinylenes/fullerene C(60) adducts by ESI/MS: evidence for dimerized cation-radicals

Oligothienylenevinylenes/C(60) dyads n-C and triads n-C(2) are studied by electrospray mass spectrometry. A clear correlation is observed between the nature of the charged species detected by mass spectrometry, i.e. protonated molecule [M + H], (+) cation radical M(+.) and dication M(++), and the oxidation potentials of the molecules. Moreover, under defined solubility conditions, mass spectrometry provides conclusive evidences for the reversible dimerization of cation-radicals of n-C(2) compounds.     

Dialkyl formyl-1 methylphosphonates α-fonctionnels—II : Preparation par voie thermique et transformation en heterocycles α-phosphoniques

The thermal condensation of functional phosphonates bearing strongly withdrawing groups (RO)₂P(O)CH₂Z1 with dimethylformamide dimethyl acetal gives corresponding β-functional, β-phosphonic enamines (RO)₂P(O)C(Z)=CHNMe₂2. Acid or basic hydrolysis of the enamines frequently gives the free aldehyde (RO)₂P(O)CH(Z)—CHO 3. We show that the enamines can be used with success for the synthesis of heterocycles like, pyrazoles 4, pyrimidines 5, benzodiazepine 6 or indole 7, all of them substituted with a phosphonate group.     

All electron calculations of open-shell polyatomic molecules. III. Perturbation treatment of the spin polarization in vinyl and methyl radicals

A first order perturbation treatment starting with SCF-MO 'S in canonical or equivalent quasi-localized form is presented for the hyperfine coupling constants of vinyl and methyl radicals. The spin-polarisation contribution to hyperfine splittings is found to be large, negative for the proton of the radical center in both radicals and positive for the β protons of vinyl.     

Mechanisms and kinetics of polymerization of thermoplastic polyimides. II. Study of “bridged” dianhydride/aromatic amine systems

The aim of the present work was to identify and follow the main and side reactions involved in the ring dehydration of amic acid prepared from “bridged” dianhydrides whose central substituent is an electron acceptor or donor, and an aromatic diamine. Several isomeric structures may appear as a result of the opening reactivity and selectivity of anhydride groups towards the aromatic amine. Reaction mechanisms and kinetics were thus studied in solvent phase with HPLC and ¹³C-NMR and in solid molten phase by FTIR and solid ¹³C-NMR. The experimental conditions (liquid and solid) and the structure of the products (type of central substituent) affecting the mechanisms and kinetics of the reactions were noted. © 1993 John Wiley & Sons, Inc.