PARAMAGNETIC CHALLENGES IN NMR MEASUREMENTS OF FOODS

NMR spectroscopy is more and more used in the evaluation of food quality, although all its potentialities have not been yet exploited. Paramagnetic metal ions, either naturally present or added to foodstuff, provide an additional source of information that can help to characterize the food. The presence of “paramagnetic” signals in spectra of complicated systems is extremely helpful for their interpretation and provides information otherwise not obtainable. In complex mixtures, NMR spectra are diagnostic, for example, for the oxidation and spin states of iron porphyrins in myoglobin and hemoglobin, yielding information about the metal ion coordination sphere.1 In turn, the latter determines the extent to which paramagnetism affects the relaxation rates of those water molecules that are able to bind protein sites in proximity of the heme pocket; the relaxation enhancement is then transmitted to the bulk water molecule by chemical exchange2 Paramagnetic metal ions, like some lanthanides, can also be added to samples submitted to NMR investigation in order to simplify the spectra, and to easily identify species which are present in the sample but hidden from the overlapping signals of the same large molecule or belonging to other more abundant substances present in a mixture. For instance, Yb3+ replaces calcium ions in many biomolecules and its effects have been exploited to obtain distance information, increasing the quality of NMR structures3 Nuclear relaxation is deeply affected by coupling with unpaired electrons: the observed relaxation rates of water protons present in food could be highly increased by the presence of metal ions, provided that water molecules have access to them (inner solvation sphere) or to their proximity (outer solvation sphere). Keeping this in mind, all explanations for different relaxation times measured on food with LF-NMR should take into account possible paramagnetic effects, especially for those cases in which the concentration of the metal, as well as its spin stateHigh resolution NMR and low resolution NMRD techniques, that are consolidated tools to investigate metalloproteins in solution, are shown to hold great potential also in the characterization of foodstuff. The presence of paramagnetic metal ions such as iron(II) and iron(III) induces isotropic shifts and nuclear relaxation that can be exploited to obtain information, e.g., about food ageing. Paramagnetic relaxation effects are also measured on the water pool of foodstuff, and this should be taken as a caveat for the current interpretation of water proton relaxation changes only in terms of changes in hydration and cellular structure of tissues.