The effects of temperature, viscosity, and molecular size on the aluminum-27 QCT NMR of transferrins.
A number of reports in recent years have demonstrated the feasibility of detecting quadrupolar metal ions bound tightly to rather large proteins via the quadrupolar central transition (QCT) NMR approach. In this article, an in-depth investigation of several interesting properties of transferrin-bound 27Al NMR signals, namely, their dependence on temperature, viscosity, and molecular size is presented. It is shown that (1) decreasing temperature and (2) increasing viscosity by adding reagents such as glycerol and ethylene glycol perturb only the linewidths of transferrin-bound 27Al signals, and, in fact, produce a decrease in signal linewidth. These effects are in accord with quadrupolar relaxation theory, which predicts that the linewidth of the central transition of a half-integer quadrupolar nucleus should decrease with increasing correlation time of the protein under nonextreme narrowing conditions. Furthermore, it is demonstrated that these trends, which are completely opposite to those generally observed in NMR spectroscopy, can be exploited to monitor ovotransferrin half-molecule reassociation reactions. In combination with the peculiar properties of transferrin-bound quadrupolar nuclei reported in the literature to date, the phenomena described here provide the basis for understanding the conditions and experimental parameters which may facilitate the application of the QCT NMR technique to the study of other quadrupolar nuclei and proteins.