Water‐Exchange Rate and Rotational Dynamics of [Gd(TTDA‐PY)(H 2 O)] − and [Gd((S)‐4‐Bz‐TTDA)(H 2 O)] 2− in Aqueous Solution: A Variable Temperature 17 O NMR Study

• Published in: Journal of the Chinese Chemical Society (Taipei) 2004-10, Vol.51 (5B), p.1147-1152
• Main Authors: Ou, Ming‐Hung, Liu, Gin‐Chung, Wang, Yun‐Ming
• Format: Article
• Language: English
• ISSN: 0009-4536; 2192-6549
• DOI: 10.1002/jccs.200400170

17 O NMR longitudinal and transverse relaxation rates and chemical shifts were measured at variable temperatures at 9.4 T magnetic fields for aqueous solutions of two Gd(III) TTDA‐like complexes (TTDA = 3,6,10‐tri(carboxymethyl)‐3,6,10‐triazadodecanedioic acid), [Gd(TTDA‐PY)(H 2 O)] (TTDA‐PY = 3,10‐di‐(carboxymethyl)‐6‐pyridylmethyl‐3,6,10‐triazadodecanedioic acid) and [Gd((S)‐4‐Bz‐TTDA)(H 2 O)] 2− ((S)‐4‐Bz‐TTDA = ((S)‐4‐benzyl‐3,6,10‐tri(carboxymethyl)‐3,6,10‐triazadodecanedioic acid). The results were analyzed in terms of the water‐exchange kinetics and the rotational motion of the complex. The water‐exchange kinetic ( k ex 298 ) parameters for [Gd(TTDA‐PY)(H 2 O)] ((30.3 1.8) 10 6 s −1 ) and [Gd((S)‐4‐Bz‐TTDA)(H 2 O)] 2− ((115 2.5) 10 6 s −1 ) are lower than that of [Gd(TTDA)(H 2 O)] 2− ( k ex 298 = (330 4.0) 10 6 s −1 ) and are significantly higher than that of [Gd(DTPA)(H 2 O)] 2− ( k ex 298 = (4.1 0.3) 10 6 s −1 )(DTPA = diethylenetriamine‐ N,N,N′,N″,N″ ‐pentaacetic acid). The higher k ex 298 values for [Gd((S)‐4‐Bz‐TTDA)(H 2 O)] 2− and [Gd(TTDA)(H 2 O)] 2− suggest that the longer backbone of the multidentate ligand may be pulled tightly into the first coordination sphere, resulting in high steric constraints at the water binding site. The rotational correlation time (τ R ) for [Gd(TTDA‐PY)(H 2 O)] (129 ps) and [Gd((S)‐4‐Bz‐TTDA)(H 2 O)] 2− (141 ps) are significantly higher than those of [Gd(DTPA)(H 2 O)] 2− (103 ps)and [Gd(TTDA)(H 2 O)] 2− (75 ps), and this is owing to that R is related to the magnitude of their molecular weight.