Albumin-bound MRI contrast agents: the dilemma of the rotational correlation time.

Human serum albumin (HSA) binds numerous molecules, among which are suitably designed MRI contrast agents. The rotational tumbling of the protein is thus one of the parameters likely to affect the in vivo relaxivity of these agents. Literature unveils discrepancies about the value of the rotational correlation time (tau(R)) of HSA. In the present work, the tau(R) of this protein has been determined by studying the deuterium relaxation rate of small molecules known for their strong binding to HSA (warfarin and 4-hydroxycoumarin). Values of approx. 20-22 ns are obtained at 310 K in a 4% HSA solution and are in good agreement with the theoretical predictions.