Test of Electron Delocalization Effects on Water-Proton Spin-Lattice Relaxation by Bromination of [Tetrakis(4-sulfonatopheny)porphine]manganese
详细信息 查看全文
- 作者：L. Henry Bryant ; Jr. ; Melinda Whaley Hodges ; and Robert G. Bryant
- 刊名：Inorganic Chemistry
- 出版年：1999
- 出版时间：March 8, 1999
- 年：1999
- 卷：38
- 期：5
- 页码：1002 - 1005
- 全文大小：50K
- 年卷期：v.38,no.5(March 8, 1999)
- ISSN：1520-510X
文摘
The potential value of electron spin delocalization as a means for substantially increasing the ability of aparamagnetic metal complex to induce nuclear spin relaxation of water protons has been examined by covalentattachment of bromine atoms in the 
-pyrrole positions of the [5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine]manganese complexes[MnIIITPPS]3- and [MnIITPPS].4- The water-proton spin-lattice relaxivities arereported as a function of magnetic field strength for the brominated and nonbrominated metalloporphyrins overthe range of magnetic field strengths corresponding to proton Larmor frequencies between 0.01 and 30 MHz.The brominated metalloporphyrins increase the water-proton relaxativities compared to the nonbrominatedmetalloporphrins, and, at low magnetic field strengths, the brominated [MnIITPPS]4- complex rivals the efficiencyof the hexaaquomanganese(II) ion. Attempts to fit the experimental data to theories for paramagnetic relaxation,which are based on the point-dipole approximation, result in distances between the paramagnetic center and thewater proton that are unreasonably short based on published structural data. The excess relaxivity implies that thepoint-dipole approximation may be inappropriate for these porphyrin systems and electron spin delocalizationmay provide a significant contribution to nuclear spin relaxation that may be fruitfully exploited in constructionof contrast agents for magnetic resonance imaging.
-pyrrole positions of the [5,10,15,20-tetrakis(4-sulfonatophenyl)-21H,23H-porphine]manganese complexes[MnIIITPPS]3- and [MnIITPPS].4- The water-proton spin-lattice relaxivities arereported as a function of magnetic field strength for the brominated and nonbrominated metalloporphyrins overthe range of magnetic field strengths corresponding to proton Larmor frequencies between 0.01 and 30 MHz.The brominated metalloporphyrins increase the water-proton relaxativities compared to the nonbrominatedmetalloporphrins, and, at low magnetic field strengths, the brominated [MnIITPPS]4- complex rivals the efficiencyof the hexaaquomanganese(II) ion. Attempts to fit the experimental data to theories for paramagnetic relaxation,which are based on the point-dipole approximation, result in distances between the paramagnetic center and thewater proton that are unreasonably short based on published structural data. The excess relaxivity implies that thepoint-dipole approximation may be inappropriate for these porphyrin systems and electron spin delocalizationmay provide a significant contribution to nuclear spin relaxation that may be fruitfully exploited in constructionof contrast agents for magnetic resonance imaging.