Applications of Dextran-magnetite as a sodium relaxation enhancer in biological systems

Abstract

Dextran-magnetite particles consist of macromolecular magnetite (Fe304) cores coated with the hydrophilic polymer dextran. These particles are completely soluble in aqueous solution and are similar to the iron-dextran complex which has been used to stimulate hematopoesis (I). However, unlike iron-dextran particles, dextran-magnetite particles are superparamaguetic and possess a large, inducible magnetic moment. In magnetic fields over a few thousand gauss, this magnetic moment approaches the saturation magnetization of 5660 G for ferromagnetic magnetite (2). This magnetic moment is two to three orders of magnitude larger than that for similarly sized paramagnetic materials at commonly used NMR field strengths (3). Almost a decade ago, dextran-magnetite was shown to be at least ten times more effective on a molar basis than the Mn2’ ion in reducing the T2 of water protons (4). Since this initial report, there has been relatively little use of this reagent in spectroscopic studies. Part of the reason for this may have been the fact that the reagent was not readily available. The particles used in the initial report were specially prepared and have not been commercially marketed. Recently, however, a simple synthetic procedure has been developed and published (5). Essentially, this protocol involves the precipitation of dextran-magnetite from a stoichiometric mixture of ferrous and ferric chlorides in a viscous dextran solution. The completely soluble particles possess a dense magnetite core ranging from 100-200 A in diameter and are approximately 30% dextran by weight. In addition to relaxing water protons, dextran-magnetite also effectively decreases the T2 of sodium in aqueous solution. In Fig. IA, the 23Na spectrum from a 140 nUI4 NaCl solution containing dextran-magnetite at an approximate molar particle concentration of 3.0 ti (14.5 g magnetite/liter) is displayed. This spectrum was recorded using a Bruker CXP-200 spectrometer operating at 52.9 MHz for sodium; the observed linewidth is slightly greater than 6900 Hz. In contrast, a well shimmed saline sample has a linewidth of only 20 Hz under the same conditions. Figure 1B also contains a spectrum obtained from fresh human erythrocytes which have been washed in 140 mM NaCl containing 3.0 N dextran-magnetite particles. Due to the relatively large size of the particles, they do not enter cells in solution. The resulting spectrum is readily seen to contain two components; a larger, broader component arising from the extracellular sodium and a smaller, narrower component arising from the intra-