Mechanistic studies of a calcium-dependent MRI contrast agent.
Intracellular Ca(2+) plays an important role in signal transduction, and we are developing new MRI techniques to study its regulation in living animals. We have reported on an MRI contrast agent (DOPTA-Gd) where the relaxivity of the complex is controlled by the presence or absence of the divalent ion Ca(2+). By structurally modulating inner-sphere access of water to a chelated Gd(3+) ion, we observe a substantial and reversible change in T(1) upon the addition of Ca(2+) and not other divalent ions. Luminescence lifetime and NMRD measurements of the complex have been acquired, and several parameters contribute to the Ca(2+) dependent relaxivity change of DOPTA-Gd. The number of inner-sphere water molecules is more than doubled after the Ca(2+) concentration is increased. This finding strongly supports the proposed conformational change of DOPTA-Gd when Ca(2+) is bound. Relaxometric measurements confirm these results and provide an indication that second-sphere water molecules are probably responsible for paramagnetic relaxation enhancement in the absence of Ca(2+). After Ca(2+) is bound to DOPTA-Gd, the molecule undergoes a substantial conformational change that opens up the hydrophilic face of the tetraazacyclododecane macrocycle. This change dramatically increases the accessibility of chelated Gd(3+) ion to bulk solvent. The design of this class of calcium-activated MR contrast agent was based primarily on the assumption that the number of coordinated inner-sphere water molecules would be the dominating factor in observed relaxivity measurements. This result has been confirmed; however, careful mechanistic studies reveal that additional factors are involved in this process.
Li
Wen-Hong
WH
Division of Biology, Beckman Institute, California Institute of Technology, Pasadena, CA 91125, USA.
Parigi
Giacomo
G
Fragai
Marco
M
Luchinat
Claudio
C
Meade
Thomas J
TJ
D002118:Calcium / Q000737:chemistry; D004563:Electrochemistry; D005682:Gadolinium / Q000737:chemistry*; D008279:Magnetic Resonance Imaging; D009942:Organometallic Compounds / Q000737:chemistry*; D010084:Oxidation-Reduction; D014867:Water / Q000737:chemistry
D009942:Organometallic Compounds; D014867:Water; D005682:Gadolinium; D002118:Calcium