PAC studies of 111In binding to transferrin, tropolone and acetylacetone in aqueous solutions

Time integral and time-differential PAC measurements have been made over a wide temperature range in aqueous solutions of [111In]tropolonate and [111In]acetylacetonate. The quadrupole frequency in the latter is approximately 30% higher than that in the former and the molecular volumes derived from rotational correlation times show the expected differences. Apo-transferrin was separately added to the two 111In-chelates and the transfer of activity from chelate to transferrin followed as a function of relative molar concentrations. Very much larger molar ratios of transferrin to tropolone than of transferrin to acetylacetone were required before substantial transfer of 111In from chelate to transferrin took place. This difference in affinity for transferrin could be one significant factor in explaining the enhanced ability of [111In]tropolonate to label blood cells in the presence of plasma. The determination of PAC parameters in [111In]transferrin over a range of temperatures showed that the values of quadrupole frequency obtained depended on the number of binding sites assumed. For only one 111In site per molecule, the quadrupole frequency increases by over 50% as the temperature is reduced below the freezing point of the solution. If two 111In sites are assumed there appears to be a change in the percentage occupancy of the two sites on either side of the transition.