Several Bi(III) complexes are used in medicine as drugs. Bi(DO3A-Bu) has recently been proposed as a nonionic contrast agent in X-ray imaging (H(3)DO3A-Bu = 10-[2,3-dihydroxy-(1-hydroxymethyl)propyl]-1,4,7,10-tetraazacyclododecane-1,4,7,-triacetic acid). The solution equilibria and NMR structure and dynamics of Bi(DO3A-Bu) and of the similar Bi(DOTA)(-) have been investigated (H(4)DOTA = 1,4,7,10-tetraazacyclododecane-1,4,7,10-tetraacetic acid). The stability constants were determined with the study of the competition equilibria between Br(-) ions and the ligands DOTA or DO3A-Bu for the Bi(III) by spectrophotometry. The stability constants, obtained for Bi(DOTA)(-) and Bi(DO3A-Bu), are very high, log K = 30.3 and 26.8, respectively. Potentiometric titrations indicated the dissociation of one of the protons among the three alcoholic OH groups in Bi(DO3A-Bu). The dissociation constant is log K = 7.53 (0.09) indicating that at physiological pH about 50% of the species possess -1 charge. It was shown by (1)H and (13)C NMR spectroscopy that the OH group attached to the middle carbon atom of the "butriol" side chain is coordinated to the Bi(III) and starts to deprotonate at pH > 5.5. The crystal structure of NaBi(DOTA).H(2)O shows an octacoordinated arrangement of the donor atoms around the Bi(III), with no water in the inner sphere. The crystals belong to the centrosymmetric space group C2/c. The temperature dependent (1)H and (13)C NMR spectra indicate that both Bi(DOTA)(-) and Bi(DO3A-Bu)(-) complexes are fluxional. For Bi(DOTA), the Delta(deltadeltadeltadelta) right harpoon over left harpoon Lambda(lambdalambdalambdalambda) fluxionality was identified, and on the basis of the activation parameters, a synchronous motion was suggested for the fluxional motion resulting in the change of ring conformation and of the helicity of the complex. The transition state is supposed to be more symmetrical than the initial state. The deprotonated Bi(DO3A-Bu) has a highly asymmetric NMR structure in solution, and its fluxional motion is slower than that of Bi(DOTA)(-).

The effects of dealumination, pore size, and calcination on the efficiency (as expressed in the relaxivity) of Gd3+-loaded zeolites for potential application as magnetic resonance imaging (MRI) contrast agents were studied. Partial dealumination of zeolites NaY or NaA by treatment with (NH4)2SiF6 or diluted HCl resulted in materials that, upon loading with Gd3+, had a much higher relaxivity than the corresponding non-dealuminated materials. Analysis of the 1H NMR dispersion profiles of the various zeolites showed that this can be mainly ascribed to an increase of the amount of water inside the zeolite cavities as a result of the destruction of walls between cavities. However, the average residence time of water inside the Gd3+-loaded cavities did not change significantly, which suggests that the windows of the Gd3+-loaded cavities are not affected by the dealumination. Upon calcination, the Gd3+ ions moved to the small sodalite cavities and became less accessible for water, resulting in a decrease in relaxivity. The important role of diffusion for the relaxivity was demonstrated by a comparison of the relaxivity of Gd3+-loaded zeolite NaY and NaA samples. NaA had much lower relaxivities due to the smaller pore sizes. The transversal relaxivities of the Gd3+-doped zeolites are comparable in magnitude to the longitudinal ones at low magnetic fields (<60 MHz). However at higher fields, the transversal relaxivities steeply increased, whereas the longitudinal relaxivities decreased as field strength increased. Therefore, these materials have potential as T1 MRI contrast agents at low field, and as T2 agents at higher fields.

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The (1)H NMR spectra of [H(x)DOTA]((4-x)-) species are reported as a function of pH and temperature in aqueous solution. The spectra show line broadening both in ligand proton signals and also in the water proton signal by titration with KOH solution. The formation of different [K(H(x)DOTA)]((3-x)-) complexes is found to be responsible for this behaviour. At high pH the usual fluxional motions, i.e. the ring inversion and the change in the acetate arms' helicity, which are also characteristic for other but inert metal-DOTA complexes, have been detected. However, because of the kinetic lability of K(+)-O and K(+)-N coordinative bonds a new type of rearrangement appears. This new motion requires breaking of coordinative bonds in the complex and can be described as a certain type of "ring slewing" around the ring C-C bonds. At low temperature (about 270 K) the ring slewing slows down and becomes negligible compared with the ring inversion and the change in the arms' helicity. These two latter processes have the same rate. When the temperature is higher (about 320 K) the ring slewing accelerates and its rate exceeds the rate of ring inversion. At this temperature the change in the acetate arms' helicity has the same rate as the ring slewing. Additionally, in the pH range 4-5 a slow intermolecular proton exchange process has been observed between the water and the dissociable protons of [K(H(x)DOTA)]((3-x)-). A water-assisted proton exchange mechanism is proposed on the basis of the activation parameters. This finding supports the previously suggested slow proton motion hypothesis for the formation of DOTA complexes.