J-coupling constants

Fig. 17.4 Common filter elements a X-half filter based on X pulse phase cycling [16, 17], b X-half filter with purge gradient [18], c X-half filter as in a, but with refocusing period for the hetero-nuclear antiphase magnetization [16, 17]. Sequences d [22], e [23] and f [18] show double filters based on single filter elements the delays r and r can be set to slightly different values to cover a broader range of ]J coupling constants (see text for a more detailed description).

The charge values have been used to correlate or calculate a variety of physical data including dipole moments (18), ESCA chemical shifts (16), H-NMR chemical shifts (19), and J. coupling constants (20), thereby relating these physical data to the fundamental values of IP and EA, in concert with a proper consideration of the network of bonds in molecules. 

All other factors are constants. The angle 9 is the projection angle between bond vectors and is the sole unknown in Eq. 1, and can be readily and precisely determined by measurement of the cross-correlated dipolar relaxation rate. It should be emphasized that 9 is measured directly, without the need of experimental calibration as in the Karplus curve for J-coupling constants for example. 

The teirperature dependencies of the chemical shift values for both Cl and C4 were determined in four different solvents (water, dimethyl sulfoxide, methanol and dioxane) and are shown in Figures 8 and 9. The resonance for Cl at 298 C varied from 101.6 ppm in D2O to 104.0 ppm in methanol. The resonance for C4 at the same temperature varied from 75.3 ppm in dimethyl sulfoxide to 78.3 ppm in methanol. The most pronounced tenperature dependence is observed in water and dioxane, where Cl and C4 signals varied from 101.4 ppm to 101.9 ppm (Cl, water, 278-358 K) and from 75.7 ppm to 76.5 ppm (C4, dioxane, 288-360 K), respectively. Thus, both tenperature and solvent dependence of C shifts indicate different conformational behavior of the molecule at various physico-chemico conditions. This feature is manifested even more clearly by the dependencies of the three-bond proton-carbon J and J coupling constants (< ) - Hl -Cl -04-C4 and f = H4-C4-04-C1 ) which are plotted against tenperature in Figures 10 and 11. 

Figure 10. A plot of temperature dependence of the j coupling constant of 2 for dioxane (curve labeled with stars), methanol (squares), dimethyl sulfoxide (rhombuses) and water (triangles).

Consequently, in PHIP spectra both doublets and triplets only exhibit two lines, since the central line of the triplet has an intensity of zero. This phenomenon has to be taken into account appropriately when analyzing PHIP spectra, since the size of the J-coupling constants might otherwise easily be misjudged by a factor of two ... 

JMOD (APT) is usually performed with broadband H decoupling and is relatively sensitive to the precise matching of the delay D2 to the J,., coupling constant, and so is less easier to use than the polarization techniques DEPT and INEPT. On the other hand, only one single experiment is necessary to measure the signals of all carbon multiplicities. 

Tables 6.1 and 6.2 summarize the NMR data obtained for the peracetylated glucose. Use this data as a reference in determining the structure of the unknown oligosaccharide. The data was obtained by direct analysis of the corresponding spectra and with the assistance of WIN-DAISY for the evaluation of H chemical shifts and H/ H J-coupling constants. The C T, values were evaluated using the T, fit routine in ID WIN-NMR.

Evaluated from GH OOI999.IR, GH 002999.1R with subsequent spectral analysis using WIN-DAISY the accuracy of the chemical shifts and of the J-coupling constants is 0.001 ppm and 0.1 Hz respectively... 

Proton signal assignments (Fig. 6.2), corresponding chemical shifts and estimated homonuclear J-coupling constants for all 3 carbohydrate monomers... 

The frequency difference (measured in Hz) between the two peaks of the doublet is called the coupling constant, denoted by J. Coupling constants are usually in the range of 0-18 Hz. and are independent of the strength of the applied magnetic field... 

Vicinal J-coupling constants provide valuable information for the determination of biomacromolecule conformation. The stmctural information is derived from Karplus equations (12) that provide empirical relationships between dihedral bond angles, and the J-coupling. Karplus equations obey the following general formula ... 

C. Torsion An e Restraints Derived From J-coupling Constants... 

The combination of MD and NMR is standard in stmctural studies of proteins and other biomolecular systems. In this method, called the restrained MD (rMD) [1,2], nuclear Over-hauser enhancement (NOE) data from NMR are used as limits for inter-proton distances, incorporated into the force field. Other NMR parameters, namely vicinal nuclear spin-spin, i.e. J coupling constants can also be used in the same way to restrict torsional angles to certain intervals (after being fitted to Karplus type of relationships [3]). In these particular investigations, MD simulations are used as an aid in structure refinement. This chapter will not deal with these aspects, however. 

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