Chemical structure elucidation

Ion trap MS is particularly suited for chemical structure elucidation, as it allows for simultaneous ion storage, ion activation and fragmentation, and product ion analysis. The fragmentation pathway of selected ions and the fragmentation products provide information on the molecular structure. Compared with triple-quadrupole and especially with sector instruments, the ion trap instrument provides more efficient conversion of precursor ion into product ions. However, the CID process via resonance excitation, although quite efficient in terms of conversion yield, generally results in only one (major) product ion in the product-ion mass spectrum. MS/MS with a quadrupole ion trap offers a number of advantages ... 

In the last twenty years, many of the developed and validated high performance liquid chromatography methods with conventional diode array or fluorescence detectors (DAD, FLD) were improved and substituted by new hyphenation with mass spectrometric instrumentation and/or NMR, especially for the analyses of raw materials derived from Natural sources. The main goal of this coupling is achieved by improvement of selectivity and sensitivity of new instrumental configurations [7], Furthermore, with these configurations it is possible to obtain, in only one analysis, the complete chemical structure elucidation, identification and quantification of targeted compounds. 

Contemporary approaches to chemical stmcture elucidation are now heavily reliant on mass spectrometry and NMR spectroscopy. Since the advent of 2D NMR methods, in many laboratories vibrational data are either not acquired or not considered, which represents a paradigm shift from approaches to chemical structure elucidation as recently as 20 years ago when vibrational spectroscopic data were an integral part of the structure elucidation data assembled to characterize an unknown structure. In contrast, we have found it useful to continue to acquire and utilize vibrational data for the characterization of impurities and degradation products [64,65]. 

Recently two different disciplines, chemical structural elucidation and transmission electron microscopy, were utilized in the study of pectin, with particular emphasis on tobacco pectin. The goal was to help bridge the gap between knowledge of their chemical structures to understanding the complex physical structures revealed by microscopy. To provide background on chemical structure, a study established that tobacco pectin was present as a series of related rhamnogalacturonans. 

In conclusion, we have demonstrated that high resolution TEM is a valuable complement to x-ray fiber diffraction analysis and chemical structural elucidation. Its application provided information about the organization of pectin in cell walls and in calcium-free gels. Using freeze-dried samples that were Pt/C replicated, we demonstrated tobacco pectin filaments in a gel to be of the same diameter as the filaments on the noncutinized lower epidermal surface of senescing Coker 319 tobacco leaves. These filaments were 7.1 3A and 4.6 4.8A, respectively, and roughly the same diameter, 7A, as fiber-diffraction modeled citrus pectin (32). Replicated... 

Chemical structure elucidation of the carbohydrate moiety of a glycoprotein is a difficult problem because there is often chain branching and because the individual carbohydrate chains are not always identical. Specific exoglycosidases which release monosaccharide units only from the terminal nonreducing end of the carbohydrate chain are most useful in structural analysis. Among the enzymes available for sequence analysis... 

The first member of the oligosaccharide antibiotics to have its complete chemical structure elucidated was eveminomicin D (5). The structure of eveminomicin D then represented a template for the assignment of the structures of flambamycin (23), avilamycins (14), curamycins (19), and, more recently, eveminomicin 13-384 Components 1 and 5 (13). 

Mass spectrometry is the most widely used analytical technique available to scientists for quantification, confirmation, identification, and chemical structural elucidation. It is based on the in vacuo separation of ions, in the gas phase, according to their mass-to-charge (m/z) ratio. Current LC-MS instruments allow for the determination of almost all antibiotics in food, which have molecular weights between 100 and 1200 Da, with the majority in the range of 200-500 Da. 

The simplest ID NMR experiments involve the apphcation of a pulse followed by observation of the resulting signal in the time domain, with subsequent Fourier transformation of the data to the frequency domain for presentation in a format that we, as chemists, can understand. Pulsed NMR methods had their inception in 1966 [33] and have almost completely supplanted earlier continuous wave (CW) methods. For reasons of sensitivity, only ID NMR spectra were typically acquired prior to the 1970s. The advent of pulsed Fourier transform NMR instruments made it possible to acquire natural abundance C NMR spectra on a routine basis in the early 1970s. With the routine availabihty of C NMR data came the compilation of chemical shift data bases and a very different way of approaching chemical structure elucidation. 

Ammonia as a reagent gas was found to yield very simple Cl mass spectra. Finally, a recent report analysed a number of additives (antioxidants and light stabilisers) in PP compomids. Three ionisation methods (El, Cl, FI) were used, and supplemental MS-MS and atomic composition (AC-MS) results were used for chemical structure elucidation/confirmation of various ingredients. 

The way the spins of the protons and neutrons in the nucleus combine can lead to nuclei with spin quantum numbers (/), varying from 0 to 6, and isotopes of the same element can have, and usually do have, different spin quantum numbers. Isotopic nuclear spin is important in NMR. For example, has I — 0 and has I = A, so that NMR spectroscopy is not possible with nuclei but is possible with Fortunately for chemists the natural carbon isotopes include 1 % and NMR is a very important tool in chemical structure elucidation [14]. Nuclear spin can also have subtle effects in other spectroscopies [15], and nuclear mass is important in molecular vibrational and rotational spectroscopies. 

G.E. Martin, AJ. Williams, Utilizing long-range 2-D NMR spectroscopy for chemical structure elucidation and confirmation, in R.K. Harris, R.E. WasyUdshen (Eds.), Encyclopedia of Magnetic Resonance, John Wiley Sons, Ltd., London, 2010, http //dx.doi.Org/10.1002/9780470034590.emrstml083. 

In summary, based on the above NMR evidence, I committed to structure 9 shown in Figure 6. It is at this point in the story that I draw the readers attention to the fact that it has become increasingly common that natural products that have their chemical structures elucidated by NMR spectroscopy have those structures later revised, usually as a result of a natural product total synthesis campaigns Sleepless night 1. 

Chemical structures elucidated, if necessary, by a variety of techniques including NMR spectroscopy and single-crystal X-ray structure analysis (Wiley). 

In this review, 22 different types of alkaloids could be identified in plants of the family Menispermaceae over the past 27 years. To be considered as a separate type, there must be at least 2 different alkaloids with the same basic skeleton. In the case where only one alkaloid has been identified, these were classified as "Others". Another problem encountered was the bibliographic citation of some alkaloids with no chemical structure elucidated. Those all oids were classified as CSND (chemical structure not defined). The other types of alkaloids were classified as indicated in Table m. 

Pandit S B and Nadkarni V M (1994) Sequential interconnected interpenetrating polymer networks of polyurethane and polystyrene. 1. S5mthesis and chemical structure elucidation, Macromolecules 27 4583-4594. 

---