Nuclear magnetic relaxation dispersion studies of water-soluble gadolinium(III)-texaphyrin complexes

Water proton 1/T-1 nuclear magnetic relaxation dispersion (NMRD) profiles were measured for a water-soluble gadolinium(III) texaphyrin (Gd-tex) complex as a function of temperature and in the presence and absence of 5% human serum albumin (HSA). Upon dissolving the complex in water (0.259 mM), the water relaxivity values decreased with time but remained higher than those of free Gd-(aq(3+)) at all fields. Concurrent measurements of free Gd3+ using metallochromic dyes indicated that demetallation of the texaphyrin did not occur over a period of several days at 37 degrees C. The high relaxivity values and shape of the NMRD profile 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 relaxivity slightly decreased whereas the high-field relaxivity increased relative to the free complex in water, and the relaxivities became nearly independent of temperature. These observations indicate that water exchange between the inner coordination sphere of Gd-tex and bulk water becomes limiting in the presence of HSA.