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Determination of the Prototropic Exchange Rate at the Water Molecule Coordinated to an Anionic Paramagnetic Gd III Chelate
Author(s) -
Aime Silvio,
Crich Simonetta Geninatti,
Gianolio Eliana,
Terreno Enzo,
Beltrami Andrea,
Uggeri Fulvio
Publication year - 1998
Publication title -
european journal of inorganic chemistry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.667
H-Index - 136
eISSN - 1099-0682
pISSN - 1434-1948
DOI - 10.1002/(sici)1099-0682(199809)1998:9<1283::aid-ejic1283>3.0.co;2-b
Subject(s) - chemistry , paramagnetism , proton , molecule , relaxation (psychology) , chelation , adduct , coordination sphere , inorganic chemistry , organic chemistry , physics , quantum mechanics , psychology , social psychology
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 Gd III 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 (T 1 M H ) 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 (T 1 M H ) 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 τ R (which causes a reduction of T 1 M H ) and/or the exchange lifetime of the coordinated water molecule.