Problems with NMR

PROBLEMS WITH OVER-EXTRAPOLATION OF NMR SPECTA

The pitfalls of making absolute structural assignments based upon NMR data alone are illustrated by similarities in the 13C-NMR spectral shifts in the metabolites of the Coenzyme A pathway. Differences in the shifts of NMR spectra between members of the vitaletheine modulator family of compounds are expected to be even more subtle than in this comparison in the literature of other structures containing the ß-alanine and cysteamine moieties. Hence, over-reliance upon theoretical NMR programs for assigning structures to the compounds in this series can create real problems, including the grave situation in which proliferative therapeutics are substituted for anti-proliferative ones. Although these programs provide some rather good spectra with regards to carbon NMR, they generally do not compensate for the effects of hydrogen bonding upon proton spectral shifts. (Note that the predicted spectra for ß-alanyl-taurine is remarkably similar to those for authentic vitaletheine V4 and its theoretical decomposition products, especially when decarboxylated).

This revelation about the limitations of theoretical NMR analyses is made with some regrets, since it would be convenient to be able to further structural agendas (other than the alleged sulfonate structures) with these estimating programs. For example, in some estimating programs, peaks in the 13C-NMR spectrum for the amide, carbonimidate, and imidate tautomers of vitalethine, and of authentic vitaletheine V4 and its decomposition products are virtually superimposible (even to as little as ±4 ppm from observed) and these arguments could be used to support the suspicion that some of the peaks in the spectra of these compounds overlap. These estimating programs also appear to support the solvated structure of the benzyl derivative (mass=560.69614). Loss of peaks in the 100-120 ppm range from authentic vitaletheine modulators are also predicted, since loss of ZnO from authentic vitaletheine V4, dehydration of the benzyl derivative, or tautomerization of vitaletheine should shift peaks in this region downfield to those coinciding with the carbonimidate tautomer (>170 ppm). Thus, while useful for predicting candidates for chemical assignment, the obvious problems with these programs in predicting proton NMR of hydrogen-bonded structures and an up to 40 ppm 95% confidence spreads in 13-C NMR theoretical spectra for these compounds indicate that mass spectrometry and other less-theoretical analyses are more reliable sources of definitive data than the estimating programs for now.

Proton NMR estimates in particular cannot be relied upon to predict the relative positions of methylenes in hydrogen-bonded structures. It is common knowledge that the magnitude of effects of substituents upon spectra shifts in proton NMR fall off dramatically beyond the alpha position, so much so that many estimating strategies do not even employ corrections for more distal (beta) contributions. The sulfenate groups, which can be oxidized to sulfinate and even sulfonate moieties under more oxidizing conditions than those exemplified, effectively separate the influence of one methylene-containing monomer from the other as far as proton NMR is concerned. Consequently, the proton NMR spectra in this series of compounds appear so closely related that they may be misinterpreted as being identical. While it is important to recognize the limitations of proton NMR in this series of compounds, changes in the spectral shifts for water and N-H peaks in proton NMR may eventually be useful in establishing structural assignments, including the degrees of oxidation and cyclization in the various products. These differences logically should be most obvious in the proton spectra of hydrogen-bonded moieties, such as hydrating water and when protonated, the alkoxy-like groups of carbamate, imidate, and carbonimidate. For the time being, however, over-reliance upon NMR theoretical spectra is unwarranted, and one ignores other pertinent scientific information such as mass spectrometry, elemental analysis, IR, melting points, and wet chemistry results at one's own hazard.

GO TO:

Home	Overview	People	Journal	Nutrition
Environment	Treatments	WWW Links	Outline	e-mail us