Nuclear magnetic resonance, molecular rotation

The melting points, optical rotations, and uv spectral data for selected prostanoids are provided in Table 1. Additional physical properties for the primary PGs have been summarized in the Hterature and the physical methods have been reviewed (47). The molecular conformations of PGE2 and PGA have been determined in the soHd state by x-ray diffraction, and special H and nuclear magnetic resonance (nmr) spectral studies of several PGs have been reported (11,48—53). Mass spectral data have also been compiled (54) (see Mass spectrometry Spectroscopy). 

I. N. Levine (1975) Molecular Spectroscopy (John Wiley Sons, New York). A survey of the theory of rotational, vibrational, and electronic spectroscopy of diatomic and polyatomic molecules and of nuclear magnetic resonance spectroscopy. 

Nuclear magnetic resonance provides means to study molecular dynamics in every state of matter. When going from solid state over liquids to gases, besides mole- cular reorientations, translational diffusion occurs as well. CD4 molecule inserted into a zeolite supercage provides a new specific model system for studies of rotational and translational dynamics by deuteron NMR. 

Structural information at the molecular level can be extracted using a number of experimental techniques which include, but are not restricted to, optical rotation, infra-red and ultra-violet spectroscopy, nuclear magnetic resonance in the solid state and in solution, diffraction using electrons, neutrons or x-rays. Not all of them, however, are capable of yielding structural details to the same desirable extent. By far, experience shows that x-ray fiber diffraction (2), in conjunction with computer model building, is the most powerful tool which enables to establish the spatial arrangement of atoms in polymer molecules. 

Evidence of molecular rotation may be given by non-crystallographic evidence the transition from a rotating to a non-rotating state is accompanied by sudden changes in specific heat, in dielectric constant, and in width of nuclear magnetic resonance bands (see Chapter VIII). 

Initially, stereospecific analyses were done by Pitas et al. (1967) on whole milk fat and by Breckenridge and Kuksis (1968) on a molecular distillate of butter oil. They indicated that the short chain acids were selectively associated with the sn-3 position. In the butter oil distillate, over 90% of the TGs contained two long-chain and one short-chain fatty acids. This asymmetry has been confirmed by the observation of a small optical rotation of the TGs (Anderson et al. 1970), by proton magnetic spectroscopy (Bus et al. 1976), and by nuclear magnetic resonance spectroscopy (Pfeffer et al. 1977). Pfeffer et al. found 10.3 M% 4 0 (butyric) in the oil and determined that 97% of the acid was in the sn-3 position. It is worth noting that the analysis was done without alteration or fractionation of the oil. 

Methods used to obtain conformational information and establish secondary, tertiary, and quaternary structures involve electron microscopy, x-ray diffraction, refractive index, nuclear magnetic resonance, infrared radiation, optical rotation, and anisotropy, as well as a variety of rheological procedures and molecular weight measurements. Extrapolation of solid state conformations to likely solution conformations has also helped. The general principles of macromolecules in solution has been reviewed by Morawetz (17), and investigative methods are discussed by Bovey (18). Several workers have recently reexamined the conformations of the backbone chain of xylans (19, 20, 21). Evidence seems to favor a left-handed chain chirality with the chains entwined perhaps in a two fold screw axis. Solution conformations are more disordered than those in crystallites (22). However, even with the disorder-... 

Lastly, molecular motion affects solid-state spectra just as chemical exchange does in solution (Chapter 10 and Section 14.2). Nuclear magnetic resonance provided the proof that benzene molecules, structure 15-2, rotate in place about their sixfold axes (Example 4.3) in the crystal above 90 K (Kelvins absolute temperature 0°C = 273 K) ... 

Spectroscopic methods, such as FT-infrared (FTIR) and Raman spectroscopy detect changes in molecular vibrational characteristics in noncrystalline solid and supercooled liquid states. Various nuclear magnetic resonance (NMR) techniques and electron spin resonance (ESR) spectroscopy, however, are more commonly used, detecting transition-related changes in molecular rotation and diffusion (Champion et al. 2000). These methods have been used for studies of the amorphous state of a number of sugars in dehydrated and freeze-concentrated systems (Roudaut et al. 2004). 

Therefore, the determination of the mdecular conformation as a function of the internal rotation angle is most important. At present, nuclear magnetic resonance (NMR) spectroscopy is a most useful tool for the determination of the molecular conformation in solution. NMR investigations of the conformation of pdypep-tides, particulady cyclic peptides, have been reviewed recently (27,55). 

In the literature, one finds a bimodal distribution of parameter quality. On the one hand is the force field developer who makes monumental efforts to minimize the error between computed and experimental molecular properties. Parametarizations often involve fits to physical data such as molecular structure (bond lengths and bond angles), vibrational data, and heats of formation. Sometimes fittings also include molecular dipole moments, heats of sublimation, or rotational barriers from nuclear magnetic resonance or other spectroscopic measurements. Well-tested, high quality parameters are the result. Some of the better force fields were compared by Pettersson and Liljefors in Volume 9 of this series. ... 

Nuclear magnetic resonance (NMR) spectra can yield information on magnetic properties, rotational states and of the symmetry of both the molecules and then-environment. Mostly, is used as a probe, but in alkali salts, alkali atoms as Na or Li have also been applied. The effect of molecular dynamics, including pseudorotations, on the NMR line shape is thoroughly discussed in [20,28],... 

The first nuclear magnetic resonance (NMR) studies of T3 failed to isolate two distinct spectra for the distal and proximal conformers (8). In addition, molecular orbital (MO) calculations predicted a high barrier to internal rotation about the diphenyl ether bonds (j)). These data suggested that there was only one conformer present in solution, presumably distal, since this was shown to be the active form of the hormone. However, the first crystal structure of T3 showed the 3 -I conformation to be proximal (10), inconsistent with available binding and activity data. 

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