title: Nuclear magnetic spin-lattice relaxation of water protons caused by metal cage compounds.
abstract: The nuclear magnetic spin-lattice relaxation rates of water protons are reported for solutions of manganese(II), copper(II), and chromium(III) cage complexes of the sarcophagine type. As simple aqueous solutions, the complexes are only modest magnetic relaxation agents, presumably because they lack protons on atoms in the first-coordination-sphere protons that are sufficiently labile to mix the large relaxation rate at the metal complex with that of the bulk solvent. The relaxation is approximately modeled using spectral density functions derived for translational diffusion of the interacting dipole moments with the modification that the electron spin relaxation rate is directly included as a contribution to the correlation time. In all cases studied, the electron spin relaxation rate is sufficiently large that it contributes directly to the water-proton spin relaxation process. The poor relaxation efficiency of the cage compound may, however, be improved dramatically by binding the complex to a protein. The efficiency is improved even further if the rotational motion of the protein is reduced drastically by an intermolecular cross-linking reaction. The relaxation efficiency of the cross-linked protein-cage complexes rivals that of the best first-coordination-sphere relaxation agents like [Gd(DTPA)(H2O)]2- and [Gd(DOTA)(H2O)]-.
authors.affiliation: Department of Biophysics, University of Rochester Medical Center, New York 14642.
authors.forename: L S
authors.initials: LS
authors.lastname: Szczepaniak
authors.forename: A
authors.initials: A
authors.lastname: Sargeson
authors.forename: I I
authors.initials: II
authors.lastname: Creasei
authors.forename: R J
authors.initials: RJ
authors.lastname: Geue
authors.forename: M
authors.initials: M
authors.lastname: Tweedle
authors.forename: R G
authors.initials: RG
authors.lastname: Bryant
mesh_terms: D001372:Aza Compounds / Q000737:chemistry*; D001665:Binding Sites; D001643:Bridged Bicyclo Compounds / Q000737:chemistry*; D002857:Chromium / Q000737:chemistry*; D003300:Copper / Q000737:chemistry*; D003432:Cross-Linking Reagents; D009682:Magnetic Resonance Spectroscopy; D008345:Manganese / Q000737:chemistry*; D011522:Protons; D014867:Water / Q000737:chemistry*
chemical_list: D001372:Aza Compounds; D001643:Bridged Bicyclo Compounds; D003432:Cross-Linking Reagents; D011522:Protons; D014867:Water; D002857:Chromium; D008345:Manganese; D003300:Copper