Structure determination of di-O-benzylidene derivatives of L-iditol by 1H and 13C NMR spectroscopy

The configuration and stable conformation of two dibenzylidene-L -iditol isomers formed upon benzalation of L -iditol, as well as that of a third isomer obtained by partial hydrolysis of the tribenzylidene derivative—the main product of the benzalation reaction—were determined by ¹H and ¹³C NMR spectroscopy.     

Direct determination of tautomerism in purine derivatives by low-temperature NMR spectroscopy

An investigation of the tautomerism of the purine derivatives N,N-dimethyl-N′-(7(9)-H-purin-6-yl)-formamidine 1, 6-chloropurine 3 and 6-methoxy purine 5 at low temperatures by NMR spectroscopy has been carried out. Knowledge of tautomeric equilibria is important for predicting N-alkylation positions, hydrogen-bonding patterns, and interactions with biological targets. In the NMR spectra of 1 and 5 at 213 K we observed two sets of signals, whereas at laboratory temperature there was only a single set of signals, reflecting the time-averaged contribution of both components. Based on characteristic values of ¹³C and ¹⁵N chemical shifts and of vicinal ¹H-¹³C scalar coupling constants, the two components of 1 were determined to be the N7-H (71%) and N9-H (29%) tautomers and those of 5 as the N7-H (18%) and N9-H (82%) tautomers. The investigation of 3 revealed a substantial predominance of the N9-H tautomer without any separation of NMR signals at 213 K.     

Structure determination of aligned samples of membrane proteins by NMR spectroscopy

The paper briefly reviews the process of determining the structures of membrane proteins by NMR spectroscopy of aligned samples, describes the integration of recent developments in the interpretation of spectra of aligned proteins and illustrates the application of these methods to the trans-membrane helical domain of a protein. The emerging methods of interpreting the spectral parameters from aligned samples of isotopically labeled proteins provide opportunities for simultaneously assigning the spectra and determining the structures of the proteins, and also for comparing the results from solid-state NMR experiments on completely aligned samples with those of solution NMR experiments on weakly aligned samples.     

Structure determination of ramosine, a guaianolide, by NMR spectroscopy

Ramosine, a new sesquiterpene lactone, was isolated from the chloroform fraction of Amberboa ramosa and the structure was assigned as 4beta-(hydroxymethyl)-3beta,4alpha-dihydroxy-8alpha-[(S)-3-hydroxy-2-ethylenepropionyloxy]-1alphaH, 5alphaH,6betaH,7alphaH,11betaH,11alpha-methylguaia-10(14)-en-6, 12-olide by extensive NMR studies.     

Structure determination of selaginellins G and H from Selaginella pulvinata by NMR spectroscopy

Selaginellins G ( 1 ) and H ( 2 ), two new selaginellin derivatives, were isolated from the whole plant of Selaginella pulvinata. Their structures were elucidated, and complete assignments of the ¹H and ¹³C NMR spectroscopic data were achieved by 1D and 2D NMR experiments (HSQC, HMBC, COSY and ROESY). Compound 1 displayed good antifungal activity against Candida albicans with an IC₅₀ value of 5.3 µg/ml. Copyright © 2010 John Wiley & Sons, Ltd.     

Geometry determination of tetrasubstituted stilbenes by proton NMR spectroscopy

A simple spectroscopic method was applied to determine the geometry of tetrasubstituted alkenes. The observation of the ⁵J-coupling constants in proton NMR spectra on the ¹³C satellite signals could confirm the previous misassignment of 2,3-diphenylbutene. Hence, the (E)-isomer showed a 1.5 Hz coupling constant, whereas the (Z)-isomer showed a 1.1 Hz coupling constant. Based on this new assignment and a stereospecific preparation, we also propose a revision concerning the NMR data of 2,3-diphenyl-2,3-butanediol.     

Differentiation of diastereomeric bornane derivatives by 13C NMR spectroscopy

Three diastereomeric bornane derivatives have been assigned configurational structures by ¹³C NMR measurements in the presence of Yb(dpm)₃.     

13C NMR spectroscopy of coumarin derivatives

The ¹³C chemical shifts of 209 naturally occurring and synthetic coumarin derivatives are listed and a number of methods for signal assignments are explained. Substituent effects on ¹³C chemical shifts (SCS) in monosubstituted coumarins and non-additivities of SCS in coumarins with more than one substituent are discussed in detail.     

13C NMR spectroscopy of oleanane derivatives

The chemical shifts of the carbon atoms in the¹³C NMR spectrum of 188 natural and synthetic oleanane derivatives published up to 1988 are given. The possibilities of¹³C NMR spectroscopy for the identification and study of the structures of new derivatives are discussed.Pacific Ocean Institute of Bioorganic Chemistry, Far Eastern Branch, Russian Academy of Sciences, Vladivostok. Translated from Khimiya Prirodnykh Soedinenii, No. 1, pp. 3–31, January–February, 1992.     

Structure determination of salvadorin, a novel dimeric dihydroisocoumarin from Salvadora oleoides, by NMR spectroscopy

Salvadorin, a new dimeric dihydroisocoumarin (1), was isolated from the chloroform fraction of Salvadora oleoides. Its chemical structure was established as 8-benzyl-6-[6-(6-ethyl-7-methyl-5,8-dihydro-2-naphthalenyl)-1-oxo-3,4-dihydro-1H-isochromen-8yl]-3, 4-dihyro-1H-isochromen-1-one, through spectroscopic techniques and chemical analysis.     

Structure determination of ursene-type triterpenes by NMR techniques

Two new ursene-type triterpenes, nudicauline A and nudicauline B, have been isolated from Launaea nudicaulis. Their structures have been assigned as 3beta-hydroxy-urs-11-ene (1) and 3beta-acetyl-urs-11-ene (2), respectively, by extensive NMR studies. In addition, olean-11,13(18)-diene (3), 3beta-hydroxy-13(28)-epoxy-urs-11-ene (4) and 3-keto-13(28)-epoxy-urs-11-ene (5) are also reported for the first time from this species.     

Structural determination of diterpenes from Daphne genkwa by NMR spectroscopy

Five daphnane type diterpenes have been isolated from the chloroform soluble fraction of Daphne genkwa. The structure of the new compound (1) was assigned as 5beta-hydroxyresiniferonol-6alpha,7alpha-epoxy-12beta-acetoxy-9,13,14-ortho-2E-decenoate by extensive NMR studies.     

Comparative conformational studies of cyclic derivatives by 13C NMR spectroscopy

The ¹³C chemical shift of the substituted or functionalized carbon of various medium and large rings is plotted against ring size (6–15 carbons). The curves thus obtained allow a conformational analysis of the corresponding derivatives.     

Structure determination of a membrane protein with two trans-membrane helices in aligned phospholipid bicelles by solid-state NMR spectroscopy

The structure of the membrane protein MerFt was determined in magnetically aligned phospholipid bicelles by solid-state NMR spectroscopy. With two trans-membrane helices and a 10-residue inter-helical loop, this truncated construct of the mercury transport membrane protein MerF has sufficient structural complexity to demonstrate the feasibility of determining the structures of polytopic membrane proteins in their native phospholipid bilayer environment under physiological conditions. PISEMA, SAMMY, and other double-resonance experiments were applied to uniformly and selectively (15)N-labeled samples to resolve and assign the backbone amide resonances and to measure the associated (15)N chemical shift and (1)H-(15)N heteronuclear dipolar coupling frequencies as orientation constraints for structure calculations. (1)H/(13)C/(15)N triple-resonance experiments were applied to selectively (13)C'- and (15)N-labeled samples to complete the resonance assignments, especially for residues in the nonhelical regions of the protein. A single resonance is observed for each labeled site in one- and two-dimensional spectra. Therefore, each residue has a unique conformation, and all protein molecules in the sample have the same three-dimensional structure and are oriented identically in planar phospholipid bilayers. Combined with the absence of significant intensity near the isotropic resonance frequency, this demonstrates that the entire protein, including the loop and terminal regions, has a well-defined, stable structure in phospholipid bilayers.