Molecular dynamics simulations of MRI-relevant GdIII chelates: direct access to outer-sphere relaxivity

The structure and dynamics of the surrounding water were studied through molecular dynamics (MD) simulations for several GdIII polyaminocarboxylate and polyaminophosphonate complexes in aqueous solution. The radial distribution functions (rdf) show that a few water molecules are bonded to the ligand through hydrogen bonds to hydrophilic groups such as carboxylates and phosphonates. Residence times are of the order of 20-25 ps for the polyaminocarboxylate and 56ps for the polyaminophosphonate chelates. No preferred orientation or bonding of water molecules is observed in the hydrophobic region of the anisotropic macrocyclic complexes. Our rdf allow calculation of the outer-sphere contribution to the nuclear magnetic resonance dispersion (NMRD) profiles using Freed's finite differences method, including electronic relaxation. The results show that the commonly used analytical force-free model is only an empirical relationship. When experimental outer-sphere NMRD profiles are available ([Gd(teta)]- and [Gd(dotp)]5-(teta=N,N',N",N"'-tetracarboxymethyl-1,4,8,11- tetraazacyclotetradecane; dotp = N,N',N",N"'-tetraphosphonatomethyl-1,4,7,10-tetraazacyclododecane) the calculated curves are in good agreement. In the case of [Gd(teta)]-, the comparison with the experimental NMRD profile has led us to predict a very fast electronic relaxation, which has been confirmed by the EPR spectrum.