The wa ter 1 H and 17 O NMR re lax ation prop er ties of aque ous so lu tions con tain ing Gd(III) che lates of the DTPA-BIPA, DTPA-BAA, DTPA-BDEA and DTPA-BMA lig ands were in ves ti gated, and the re sults were com pared with those pre vi ously re ported for the Gd(III) com plex with DTPA ligand. The sig nif i cant de vi ation from the monoexponential be hav ior at the es ti ma tion of tem per a ture de pend ence of 1 H lon gi tu di nal relax ation rate for Gd(III) com plexes with DTPA-BIPA, DTPA-BAA and DTPA-BDEA lig ands is char ac ter is tic of the slow chem i cal ex change. The mea sure ments of 17 O NMR trans verse re lax ation rates and EPR transverse elec tronic re lax ation rates in the range 276-343 K, al lowed the as sess ment of the wa ter-exchange lifetime be tween the co or di na tion site and the bulk sol vent. With this ap proach, we ob tained the wa ter-exchange life time ( M ) of 1515, 1429, 1538 and 2128 ns for [Gd(DTPA-BIPA)(, re spec tively. Compared with the M value of 303 ns for [Gd(DTPA)(H2O)] 2-, we are able to con clude that the wa ter res i dence life time in creases when the two termi nal carboxylate groups of DTPA were sub sti tuted by two am ide groups. IN TRO DUC TIONSta bile com plexes of the gad o lin ium(III) ion in aqueous so lu tion are gen er at ing much in ter est as ac tual and po tential con trast agents in bio med i cal mag netic res o nance im aging (MRI).1 Para mag netic metal com plexes are used to increase the im age con trast for MRI. 1,2 The first clin i cally utilized con trast en hance ment agent was [Gd(DTPA)(H 2 O)] 2-, which dis trib utes in the extracellular space and sig nif i cantly in creases pro ton re lax ation rates. Para mag netic metal ions func tion as con trast agents due to the in ter ac tions be tween the elec tron spins of the paramag netic cen ter and the pro ton nu clei, con se quently re sult ing in the in crease of the re lax ation rates for the ob served wa ter pro tons near the ions. For small para mag netic com plexes, wa ter co or di na tion to the metal ion di rectly (in ner sphere) and dif fu sion in the outer sphere en vi ron ment are the two main con tri bu tors to re lax ation en hance ment given by equation (1).(1) R is 1 and R os 1 re fer to the in ner sphere and outer sphere con tri bu tion to the lon gi tu di nal relaxivity, re spec tively. The in ner sphere con tri bu tion for R1 is given by equa tion (2). (2)where M is the con cen tra tion of the metal com plex, q is the num ber of co or di nated in ner sphere wa ter mol e cules, T 1M is the lon gi tu di nal re lax ation time and M is the wa ter res i dence life time on the metal ion. T 1M is a func tion of the in ter ac tion strength and it can be ob tained us ing the Sol o mon-BloembergenMor gan (SBM) the ory, a sim pli fied ver sion is shown in equations (3-4). 1,3 (3) (4) where r is the ef fec tive dis tance be tween the Gd(III) elec tron spin and the wa ter pro tons, H is the pro ton magnetogyric ratio, g is the Landè fac tor, B is the Bohr magneton, S is the spin quan tum ...
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