Determination of Stereochemistry

D2O has the further advantage that the solvent isotope effect on mutarotation is usually greater than that on glycosidase catalysis. The temperature coefficient of most enzymes, moreover, is less than that of mutarotation, so it is often advantageous to cool the sample. Finally, the more powerful the magnet, the fewer the FIDs and hence the shorter the time that is required to obtain a good spectrum. Tours de force of NMR-based glycosidase stereochemistry 

A particular problem is the demonstration of inversion with sialidases, since they act on ot-A-acetylneuraminides, but the mutarotational equilibrium is 10% ot 90% The chemical shifts of the two H3 protons have different chemical shifts in the ot- and p-forms of A-acetylneuraminic acid and its mutarotation is not inconveniently fast, so demonstrating retention is straightforward. However, to demonstrate inversion, it is necessary to match the rate of liberation of aglycone with that of a retaining sialidase and plot time courses for both enzymes, in order to distinguish inversion from uninformative conversion of substrate to the equilibrium mixture i.e. it has to be demonstrated that the chemical flux through the system is easily adequate to maintain non-equilibrium proportions of a- and p-A-acetylneuraminic acid. 

Whereas alteration of enzyme or substrate structure causes large perturbations to the potential energy functions describing enzyme-substrate interactions, a consequence of the Born-Oppenheimer approximation is that isotopic substitution causes no perturbation of potential energy functions at all. Isotope effects are therefore amongst the most powerful methods of determining enzyme mechanism. If they are measured by any sort of competition method (for example, isotope enrichment in the unreacted starting material or product) then, because one isotopomer acts as a competitive inhibitor of the 

The Northrop-Cleland nomenclature system for isotope effects greatly simplifies their discussion the non-abundant isotope and, in the case of secondary effects, site of substitution are written as superscripts to V or VjK in parentheses, so that °(F/A) refers to an a-deuterium kinetic isotope effect on kcatZ-Klm and (K) refers to an effect on kcat- The effects on individual rate and equilibrium constants are written as superscripts is the p-tritium effect on an equilibrium constant and k+2 is the effect on the k+2 step. Although in principle potentially ambiguous (e.g. could in principle refer to or the relatively short-lived F), in practice any ambiguity is resolved from the context. 

If Cr is unity, the expression simplifies to K, the isotope effect on the equilibrium constant. 

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M.)., Zartler, E. R. A Novel method for the determination of stereochemistry in six-membered chairlike rings using residual dipolar couplings. /. Org. Chem. 

Borowitz, I.J., Yee, K.C., and Crouch, R.K., Determination of stereochemistry in vinyl phosphorylated species by nuclear magnetic resonance shift reagents. Revised mechanistic pathways for the Perkow reaction, /. Org. Chem., 38,1713, 1973. 

Although structural elucidation of lignans is not a difficult task, the similarities between the structures can create problems. In particular, the determination of stereochemistry at the chiral center requires NOE/ NOESY NMR experiments and/or X-ray analyses. The enantiomeric excesses of the known lignans (+)-lariciresinol, (-)-secoisolariciresinol and (+)-taxiresinol, isolated from Japanese yew T. cuspidata roots, were determined by chiral high-performance liquid chromatographic analyses [78] except for (+)-pinoresinol (77% enantiomeric excess), they were found to be optically pure by Kawamura et al. In an earlier study, the presence of taxiresinol in Taxus species was reported by Mujumdar et al. [69] after they had isolated it from the heartwood of T. baccata, although they did not study its stereochemistry. 

In phosphine complexes, the v MP modes (around 400 cm-1)36,64,20s are often obscured by aryl-group vibrations. Main efforts for the determination of stereochemistry are concentrated on 31P nmr spectroscopy, especially in the case of platinum(II) and platinum(IV) complexes36,371 (195Pt nucleus ... 

A gentle introduction to the determination of stereochemistry with two dimensions only. Suggested solution... 

Slightly more difficult determination of stereochemistry moving from two dimensions to three Revision of the Karplus relationship and of conjugate addition. 

This result is of some mechanistic importance because it permits the determination of stereochemistry of Pd(II) oxymetallation by stereospecific replacement of Pd(II) by CO. This technique was used for determining the mode of methoxypalladation of the butenes (Section III, A, 3, b). 

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