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T. David Westmoreland






The purpose of this study was to elucidate the solution structure of three Mn(II)-based complexes, [Mn(DOTAM)]2+, [Mn(HxDOTA)]x-2, and [Mn(HxDOTP)]x-6, using temperature and pH-dependent 17O transverse relaxivity measurements. pH-dependent 17O relaxivities were found to generally correlate with the concentration of [Mn(H2O)6]2+ in solution, thereby giving a qualitative indication of stability for each complex over a wide pH range. Based on the stability for each complex, solution structure was probed for the number of bound water molecules, q, at high and low pH. The effective q value of [Mn(DOTAM)]2+, [Mn(HxDOTA)]x-2, and [Mn(HxDOTP)]x-6, at high pH was determined to be 0, indicating that the stable solution structures of these complexes do not have exchangeable water molecules. It was determined that there were approximately six bound water molecules in the solution structures of [Mn(HxDOTA)]x-2 and [Mn(HxDOTP)]x-6 at low pH, which indicates the destabilization of the complex and the presence of [Mn(H2O)6]2+. The temperature dependence of the 17O transverse relaxivity of [Mn(H2O)6]2+ was also measured and modeled using nonlinear least squares fitting procedures, and the results were compared to similar analyses for the temperature dependent 17O relaxivities of [Mn(HxDOTA)]x-2 and [Mn(HxDOTP)]x-6, further confirming the presence of [Mn(H2O)6]2+ at low pH.

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