Engineering lanthanide complexes with controlled water exchange rates for magnetic resonance imaging
The control of the rate of water exchange in lanthanide complexes is critical to their future development as contrast agents in magnetic resonance imaging. In traditional T/sub 1/ shortening agents an optimal water residence lifetime (/spl tau//sub M/) of around 25 ns is required if the highest relaxivities are to be achieved. In contrast the newer PARACEST agents require that water exchange is much slower with /spl tau//sub M/ on the order of microseconds. It is now possible to rationally design complexes that exhibit water exchange rates that are ideal for their intended use. The selection of appropriate ligating groups allow a broad control mechanism. Anionic ligating groups such as carboxylates favour faster exchange whereas neutral ligands such as amides favour slower exchange. By altering the steric hindrance around the water coordination site water exchange rates can be fine tuned. The steric hindrance can be altered by controlling the coordination environment of the lanthanide ion and optimal water exchanges rates so achieved.
Solid state and solution dynamics of pyridine based tetraaza-macrocyclic lanthanide chelates possessing phosphonate ligating functionality (Ln-PCTMB): effect on relaxometry and optical properties.
The macrocyclic ligand 3,6,9-tris(methylenebutyl phosphonic acid)-3,6,9-15-tetraazabicyclo [9.3.1]pentadeca-1(15),11,13-triene (PCTMB) was synthesized and complexes of Eu(3+), Tb(3+), and Gd(3+) studied by X-ray crystallography, luminescence, and relaxometry. In the crystal these complexes are dimeric and possess 8-coordinate Ln(3+) centers that are linked by bridging phosphonates. The rigidity introduced by the pyridyl nucleus forces the EuPCTMB and TbPCTMB to adopt a twisted snub disphenoid (TSD) coordination geometry. Examination of the (5)D(0) --> (7)F(0) luminescent transition of EuPCTMB in the solid state confirmed the existence of a single distinct Eu(3+) coordination environment, whereas two Eu(3+) coordination environments were observed in aqueous solution. Lifetime analysis of aqueous TbPCTMB solutions determined that q = 0.1 and q = 1.0 for the two coordination environments and Stern-Volmer quenching constants (K(SV)(tau) = 1101 M(-1), K(SV)(Phi) = 40780 M(-1)) support the presence of a complicated monomer/dimer equilibrium. Relaxivity studies of GdPCTMB in H(2)O/CH(3)OH exhibited a concentration dependency (0.02 mM-10.00 mM) ranging from r(1) = 7.0 mM(-1) s(-1) to 4.0 mM(-1) s(-1) consistent with the trend observed by luminescence.
Organic constituents in process water from the in-situ retorting of oil from oil-shale kerogen
Capillary-column gas-chromatography/mass-spectrometry (GC/MS) was performed on the acid, base, and neutral fractions o'f liquid- and particulate-phase methylene chloride extracts of a composite sample of raw process water collected from separator Tank 6 by the Laramie Energy Technology Center. Of. the 160 extractable and chromatographable organic comP.ounds tentatively :identified, the following compou~d classes were found· (listed in decreasing order of. abundance):