ISSN:
1434-1948
Keywords:
MRI Contrast agents
;
GdIII complexes
;
Relaxometry
;
Water exchange
;
Prototropic exchange
;
Chemistry
;
General Chemistry
Source:
Wiley InterScience Backfile Collection 1832-2000
Topics:
Chemistry and Pharmacology
Notes:
A thorough investigation of the proton and oxygen-17 relaxation rates of water nuclei has been carried out for a solution containing an amphiphilic, paramagnetic GdIII chelate of potential interest as a contrast agent for MRI. It has been found that at pH = 7, 298 K and 20 MHz (proton observation frequency), the contribution to the overall relaxation enhancement from the water molecule in the inner coordination sphere is dominated by the relaxation time (T1MH) of its protons. However, upon formation of a slowly tumbling adduct with β-cyclodextrin, the observed relaxation enhancement is also affected by the exchange lifetime (T1MH) of the coordinated water and by the transfer rate of its protons. This situation has been exploited to assess the prototropic exchange rate from the coordinated water to the bulk, which is base-catalyzed. In fact, under these experimental conditions, at basic pH, the proton transfer is faster than the exchange of the whole water molecule, and it becomes the factor determining the observed relaxation enhancement. The effect is further enhanced at low temperature as a consequence of the concomitant lengthening of either τR (which causes a reduction of T1MH) and/or the exchange lifetime of the coordinated water molecule.
Type of Medium:
Electronic Resource
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