NUCLEAR MAGNETIC-RELAXATION DISPERSION STUDIES OF WATER-SOLUBLE GADOLINIUM(III)-TEXAPHYRIN COMPLEXES

Water proton 1/T-1 nuclear magnetic relaxation dispersion (NMRD) profi les were measured for a water-soluble gadolinium(III) texaphyrin (Gd-t ex) complex as a function of temperature and in the presence and absen ce of 5% human serum albumin (HSA). Upon dissolving the complex in wat er (0.259 mM), the water relaxivity values decreased with time but rem ained higher than those of free Gd-(aq(3+)) at all fields. Concurrent measurements of free Gd3+ using metallochromic dyes indicated that dem etallation of the texaphyrin did not occur over a period of several da ys at 37 degrees C. The high relaxivity values and shape of the NMRD p rofile of this complex may be ascribed to a combination of large water coordination number (q estimated at 3.5) and long tau(R). Upon mixing an aqueous solution of the complex with 5% HSA, the low-field water r elaxivity slightly decreased whereas the high-field relaxivity increas ed relative to the free complex in water, and the relaxivities became nearly independent of temperature. These observations indicate that wa ter exchange between the inner coordination sphere of Gd-tex and bulk water becomes limiting in the presence of HSA.