Proton spin-lattice relaxation by paramagnetic centers may be dramatically enhanced if the paramagnetic center is rotationally immobilized in the magnetic field. The details of the relaxation mechanism are different from those appropriate to solutions of paramagnetic relaxation agents. We report here large enhancements in the proton spin-lattice relaxation rate constants associated with organic radicals when the radical system is rigidly connected with a rotationally immobilized macromolecular matrix such as a dry protein or a cross-linked protein gel. The paramagnetic contribution to the protein-proton population is direct and distributed internally among the protein protons by efficient spin diffusion. In the case of a cross-linked-protein gel, the paramagnetic effects are carried to the water spins indirectly by chemical exchange mechanisms involving water molecule exchange with rare long-lived water molecule binding sites on the immobilized protein and proton exchange. The dramatic increase in the efficiency of spin relaxation by organic radicals compared with metal systems at low magnetic field strengths results because the electron relaxation time of the radical is orders of magnitude larger than that for metal systems. This gain in relaxation efficiency provides completely new opportunities for the design of spin-lattice relaxation based contrast agents in magnetic imaging and also provides new ways to examine intramolecular protein dynamics.
Keywords paramagnetic relaxation; MRD; relaxation dispersion; relaxation agentMagnetic relaxation agents or contrast agents for clinical magnetic imaging applications are generally based on soluble chelate complexes of gadolinium(III) that provide one or more coordination positions for labile water molecules that carry the effects of the paramagnetic center to the total water population by chemical exchange of labile protons or water molecules [1][2][3]. In most cases, the relaxation efficiency of these compounds is limited by the short electron spin-lattice relaxation times in the range of tens to hundreds of picoseconds, which in turn defines the concentration range where these compounds may be used as practical contrast agents [4][5][6][7][8][9][10][11]. MRI contrast agents currently in use are generally soluble extracellular and blood-pool agents; however, targeting to provide specific anatomical or biochemical information will necessarily involve binding of an agent to a cell surface site or a specific macromolecular matrix [4,12,13]. For the relaxation agents presently used, the conjugation of