MR-labeling of cells may be carried out by adding a Gd-based contrast agent to the incubation media. The amount of gadolinium internalized in HTC and C6 cells upon incubation with Gd-DTPA-BMA is circa one order of magnitude higher than those found with Gd-DTPA, Gd-DOTA and Gd-HPDO3A, respectively. The comparison of relaxometric and mass spectrometry determinations allows us to establish that only a minor fraction of intact Gd-DTPA-BMA is internalized into the cells. Moreover the binding/uptake behavior shown by Gd-DTPA-BMA resembles that found when GdCl 3 is added to the incubation medium. We suggest that the lower stability of Gd-DTPA-BMA is responsible for a shift in the dissociation equilibrium that results in the net transfer of Gd 3þ ions on the cell membrane followed by a slower internalization process. The transmetallation process is mediated by components of the incubation media, among which a dominant role is represented by phosphate anions. The uptake of Gd 3þ ions is clearly reflected in the drastic decrease of cell viability observed for cells labeled with Gd-DTPA-BMA.

The structural changes occurring in the clinopyroxenes with composition Di100, Di90En10 and Di80En20, due to the Ca-Mg substitution in the M2 site, have been studied. Evidence is given that with increasing Mg content a small percentage of the atoms converts from the M2 position to a new M2′ position which is solely occupied by Mg. The maximum conversion of M2 to M2′ found in this study is 7%. The closest parallel to the M2′ geometry is found in the ZnSiO3 pyroxene (C2/c). The presence of this new site causes significant changes in the tetrahedral configuration, because the M2′ atoms are not bonded to 03. The intermediate compositions, Di90En10 and Di80En20, may be thought of as the coexistence of two structural models: diopside and ZnSiO3 pyroxene (C2/c). © 1982 Springer-Verlag

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Under HR-MAS conditions, cells are subjected to high centrifugal forces that may cause irreversible cell damage. First, conditions have been defined to monitor and keep to a minimum unwanted effects in HR-MAS spectra arising from the loss of cell integrity. Then, the HR-MAS spectra of reasonably intact cells have been analyzed. Cell suspensions subjected to MAS rates as low as 1 kHz split into a two-compartment system that is composed of a cell-rich phase (H 2 O i ) and a cell-free phase (H 2 O o ). Each of these phases is characterized by its own water 1 H-NMR signal. Transport of water molecules between the cellrich and cell-free compartments is limited by the very low contact area between the two compartments, and water exchange dynamics consequently fall into the slow exchange limit on the NMR timescale. Since the exchange between the two water populations is "frozen,