Current Organic Chemistry - Volume 5, Issue 3, 2001

The structural elucidation of complex organic molecules relies heavily on the application of proton detected heteronuclear NMR. Among these techniques, the HMBC NMR experiment is probably the most useful 2D NMR method The HMBC (C-H) experiment allows the assignment of structural fragments through correlations between protons and carbons separated by more than one bond, usually two or three bonds ( 2 J CH and 3 J CH ) via 1 H,13 C-coupling constants. It is also possible to obtain valuable information through longer correlations, n J CH n 3, performing several HMBC experiments with different long-range delays and using a deeper threshold in the contour plot. There have been several attempts to improve the results of the HMBC experiment, mainly focused on the question of optimization of the long-range delay, Delta 2 . The D-HMBC, 3D-HMBC, CT-HMBC, ACCORD-HMBC, IMPEACH-MBC and CIGAR-HMBC experiments which provide much better experimental access to sample long-range couplings are briefly discussed. These long-range correlations have proven to be crucial in the structure elucidation of molecules with proton deficient skeleton.

The siderophores produced by the so-called fluorescent members of the bacterial genus Pseudomonas called pyoverdins are chromopeptides with complex structures. Their peptidic part - linear or partially cyclic - comprises unusual and partially modified amino acids. So far about fifty different pyoverdines have been encountered. A fast establishing at least of their gross structure is of importance for the identification of strains in medicine, agriculture and in waste degradation. Mass spectrometric techniques have become a potent tool in this respect. Results which have been obtained by the various techniques will be reviewed.

Proton transfer processes play a key role in many reactions of organic compounds and in many biochemical mechanisms. In particular, proton transfer in hydrogen bond complexes in solutions has attracted considerable attention. NMR spectroscopy is known to be a powerful tool in studying hydrogen bond and the proton transfer equilibrium. This review focuses on one specific application of this method, based isotope effect are shortly reviewed. The main subject of discussion is a relationship between the primary p DeltaH(D) on measurement of the deuterium isotope effect on chemical shifts. The factors affecting the magnitude of this and the secondary n delta13C(D),n delta15 N(D), n delta17 O(D) deuterium isotope effects and the hydrogen bond strength and symmetry, proton dynamic and position of the proton transfer equilibrium. The possibility of using the deuterium isotope effect to study proton transfer equilibrium is suggested and illustrated with representative examples of tautomeric equilibrium in beta-diketones, beta-thioketones, Schiff and Mannich bases, and proton transfer equilibrium in complexes of organic and inorganic acids with pyridine. Some experimental details of the method are given and its advantages are indicated.

Saponins and triterpenoid glycosides are polar constituents of plants many of which exhibit a broad spectrum of biological activities. They often occur as intricate mixtures difficult to separate by conventional procedures. Improvements in liquid chromatographic techniques and availability of new matrices have improved the possibility of purifying even the isomeric saponins. The application of newer isolation and purification processes are illustrated in the present review with examples both from the literature and from the reviewers laboratories. Advances in mass spectrometry, for example tandem mass spectrometry, offer considerable advantages in structure determination of complex saponins, and their use has been illustrated. The major structural information however comes from NMR spectra. Due to the complex structures, interpretation of NMR spectra recorded even with high field instruments is not easy. The present review summarizes the available 2D NMR techniques particularly HMBC, NOESY and HOHAHA for simplifying spectral interpretation with suitable examples. In special cases, use of molecular mechanics and molecular dynamics studies of structure leads to valuable information on conformation of molecules. Their application was found to be of considerable advantage for saponins possessing polysubstituted monosaccharides.

In the last two decades sensitive physico-chemical methods enabling analyses of natural products mixtures (secondary metabolites and biopolymers) have been developed. Hyphenation of chromatograhic techniques with different types of mass spectrometers is one of the most effective analytical methods for identification and structural studies of compounds demonstrating different physico-chemical properties and wide range of molecular masses. Various types of chromatography for separation, as well as mass spectrometers equipped with different analyzers and ionization sources are in common use. Chromatographic techniques suitable for separation of unpolar or highly polar compounds are combined with mass spectrometers, the achievable instrumentation assures high sensitivity of analyses, in some circumstances, even on attomole level. The possibilities for investigations on different classes of natural products, present in complicated mixtures obtained from biological sources, so called metabolic profiling, may be considered as complementary studies in many fields of biochemical and medical sciences. This information complement data obtained during research conducted on animal and (slash) or plant genome and proteome projects.

In complex organic molecules, macromolecules and biological samples, 1 H NMR spectra can be very complex and assignments of resonances can be difficult. Thus NMR pulse sequence methods to edit and simplify spectra have been introduced. During the last few years, there have been a number of reports on the use of heteronuclear multiple-quantum filtration to simplify 1 H NMR spectra. The method is known as inverse-detected or 1 H-detected NMR spectroscopy. These pulse sequences start and end with 1 H single-quantum coherence, and heteronuclear single- or multiple-quantum coherence is generated and allowed to evolve in the middle stage of the pulse sequence. In addition, we have developed a family of NMR pulse sequences, known generically as MAXY, for the provision of separate one-, two- and three-dimensional 1 H NMR spectra of CH, CH 2 and CH 3 groups. These methods utilize the natural abundance of 13 C spins and are based upon the selection of the different possible maximum multiple-quantum coherences of the various groups. These approaches have been applied to the measurement of chemical shifts, coupling constants, relaxation rates and molecular diffusion coefficients. These recent results are summarized in this article. Possibilities for the future are also outlined.

The regularities of the solvent effect manifested in the absorption spectra of certain types of Schiff bases were investigated. In solvents capable of hydrogen-bonding, this solvent effect is observed for those Schiff bases in which there is an OH group in an ortho or para position on the aldehyde ring, and the electron density of the azomethine nitrogen is appropriately high. Studies in solvent mixtures demonstrated that the phenomenon can be interpreted in terms of an enol keto tautomer equilibrium. It was found that, in response to salts (particularly CaCl 2 ) dissolved in ethanol, the equilibrium is shifted extensively in the direction of the keto form, as a function of the salt concentration. This experience was utilized to determine the molar extinction of the keto form at the maximum of the band at around 400 nm. In the knowledge of this, the equilibrium constant and the concentration of the keto form in various solvents were calculated. Calculation methods were described for determination of the full UV and visible absorption spectra of the keto and the enol forms. A correlation was found between the magnitude of the solvent effect and the acidity and basicity parameters of the solvents used. It was confirmed that both the acidity and the basicity play roles in the solvent effect. The results of the investigation further permit the assumption that the keto form develops via an enol zwitterions equilibrium.