NMR STRUCTURAL STUDIES

Trihalosulfonium Ions. Of the parent trihalosulfonium ions, X3S+ (X = F, Cl, Br), F3S+ and C13S+ were reported first. The reaction product of SF4 and BF3 was described by Bartlett as the SF4BF3 adduct,199 but it was subsequently proved by structural studies (NMR, Raman, IR)200 201 to be the F3S 1 BF4 salt. According to crystal structure analysis,202 the ion F3S+ is of C3v symmetry and has very short S—F bonds (1.495 and 1.499 A) indicative of a substantial positive charge on sulfur. Other F3S+MF6 (M = As, P, Sb) salts were studied by Gillespie and co-workers.200... 

Theoretical methods ranging from the now obsolete HMO studies to ab initio calculations have been used extensively on pyrazoles. Although not emphasized in earlier reviews (66AHC(6)347,67HC(22)l), the most recent publications (B-76MI40402,79RCR289) contain several references to theoretical studies. Some publications related to structural studies are to be found in the following sections, especially in connection with NMR spectroscopy (Section 4.04.1.3.4), UV spectroscopy (Section 4.04.1.3.6), PE spectroscopy (Section 4.04.1.3.9) and tautomerism (Section 4.04.1.5). 

Although experimental studies of DNA and RNA structure have revealed the significant structural diversity of oligonucleotides, there are limitations to these approaches. X-ray crystallographic structures are limited to relatively small DNA duplexes, and the crystal lattice can impact the three-dimensional conformation [4]. NMR-based structural studies allow for the determination of structures in solution however, the limited amount of nuclear overhauser effect (NOE) data between nonadjacent stacked basepairs makes the determination of the overall structure of DNA difficult [5]. In addition, nanotechnology-based experiments, such as the use of optical tweezers and atomic force microscopy [6], have revealed that the forces required to distort DNA are relatively small, consistent with the structural heterogeneity observed in both DNA and RNA. 

Recent extensive structural studies by X-ray crystallography and by P nmr spectroscopy have revealed an astonishing variety of con/imcto-polyphosphides with quasi-ionic... 

The structures of these two isomers of mono-N-acetyl derivative of 4 were decided on the basis of their NMR spectra as discussed in the part of structural studies. 

Structure determination of luciferin. Once a luciferin is obtained in a sufficient purity, the determination of luciferin structure should be attempted most of the important properties of luciferin are usually already obtained during the course of purification as a necessity. The structural study is considerably more straightforward than the extraction and purification, due to the availability of advanced methods, such as high-resolution mass spectrometry and various NMR techniques. If help or collaboration is needed in structure determination, the attractiveness of a luciferin will make it easy to find a good collaborator. However, the purified luciferin is usually an extremely precious material considering the effort spent in preparing it. To avoid accidental loss of the purified material, the chosen collaborator must have solid knowledge and experience in structure determination a criterion to be considered is that the person has successfully done the structure determination of at least one new natural product. 

Report 125 Structural Studies of Polymers by Solution NMR, H.N. Cheng, Hercules Incorporated. 

Structural Studies of Peptoids with Aliphatic Side Chains by CD, NMR, and X-ray Crystallography... 

Dickerson, Rich and Wemmer revealed by X-ray and NMR structural studies that the crescent-shaped molecule could bind A,T tracts in both 1 1 and 2 1 li-... 

Nieto, JL Baselga, J Hernandez-Fuentes, I Llorente, MA Pierola, IF, Polyacrylamide Networks. Kinetic and Structural Studies by High Field Hl-NMR with Polymerization in Situ, European Journal of Polymers 23, 551, 1987. 

The first step in determination of a structure by NMR spectroscopy involves assignment of individual proton resonances. Development of high-field spectrometers and the use of a second dimension (2D-NMR) along with isotopic substitution (11) and sophisticated pulse sequences (12) make it possible to almost completely assign the proton spectrum of proteins of about 15 kD molecular weight (13—17). Some 2D-pulse sequences commonly used in the study of macromolecules are shown in Figure 1. 

The proton spectra of thietane oxide (5a) and thietane dioxide (5b) have been studied in order to evaluate whether the oxidation at the sulfur atom changes the established 35° puckering of the ring , and whether a correlation is possible between structure and NMR parameters . 

A structural study on lipid A and the O-specific polysaccharide of the lipopoly-saccharide from a clinical isolate of Bacteroides vulgatus from a patient with Crohn s disease was conducted by Hashimoto and coworkers [39]. They separated two potent virulence factors, capsular polysaccharide (CPS) and lipopolysaccharide (LPS), from a clinical isolate of B. vulgatus and characterized the structure of CPS. Next, they elucidated the strucmres of O-antigen polysaccharide (OPS) and lipid A in the LPS. LPS was subjected to weak acid hydrolysis to produce the lipid A fraction and polysaccharide fraction. Lipid A was isolated by PLC, and its structure was determined by MS and NMR. 

A related study with a similar ruthenium catalyst led to the structural and NMR characterization of an intermediate that has the crucial Ru—C bond in place and also shares other features with the BEMAP-ruthenium diacetate mechanism.33 This mechanism, as summarized in Figure 5.4, shows the formation of a metal hydride prior to the complexation of the reactant. In contrast to the mechanism for acrylic acids shown on p. 378, the creation of the new stereocenter occurs at the stage of the addition of the second hydrogen. 

An NMR and structural study characterized the intermediates generated from diimine catalysts on reaction with diazodiphenylmethane.193 The dominant species in solution is dinuclear, but a monomeric metallocarbene species can be detected. 

There are several ways to deal with this problem. The simplest in concept is to use liquid imbibed particles that are uniform in structure, such as sugar cubes [12, 13] as already mentioned above. To my knowledge, this has never been done for structure studies. The second is to use NMR-invisible particles, for example, glass, and fill the spaces between the particles with a liquid such as water that can be imaged. The third is to use the usual radially heterogeneous particles and then use software to calculate the parameters. One first determines the centers of each particle and then decides whether the particles are in contact with each other or not by the distances between the centers [14]. 

NMR spectroscopy is a convenient method for structural study of the equilibrium between the colored and the colorless form of spirobenzopyran. In the H-NMR spectra, the chemical shifts of gem methyl groups in 3 -position, (V-methyl group,2,11 and methine protons in 3- and 4-position are important to distinguish between the colored and colorless forms. 

Comparative Evaluation of Related Compounds. Structural studies are incomplete, but NMR and gas chromatographic - mass spectroscopic analysis of a component (fraction 7) isolated by HPLC indicated biphenyl characteristics (30). In an attempt to evaluate the cytolytic effect of compounds with similar structure, 4-hydroxybiphenol and 2-hydroxybiphenol were tested for lytic effects on P. brevis. Bioassays of 2- and 4-hydroxybiphenol indicated that both compounds were cytolytic to P. brevis (31). 

The relatively large number and small amounts of these OFAs were surprising and made further structural studies difficult. Since fraction 4 6 had good activity and was well separated from other bands, it was used for further studies. Ozonolysls, low and high resolution MS, H-NMR and C-NMR data obtained on 4 6 allowed structure (I) (13-15) to be assigned. 

---