THE EFFECTS OF TEMPERATURE, VISCOSITY, AND MOLECULAR-SIZE ON THE AL-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 prot eins via the quadrupolar central transition (QCT) MMR approach, In thi s article, an in-depth investigation of several interesting properties of transferrin-bound Al-27 NMR signals, namely, their dependence on t emperature, viscosity, and molecular size is presented, It is shown th at (1) decreasing temperature and (2) increasing viscosity by adding r eagents such as glycerol and ethylene glycol perturb only the linewidt hs of transferrin-bound Al-27 signals, and, in fact, produce a decreas e in signal linewidth. These effects are in accord with quadrupolar re laxation theory, which predicts that the linewidth of the central tran sition of a half-integer quadrupolar nucleus should decrease with incr easing correlation time of the protein under nonextreme narrowing cond itions, Furthermore, it is demonstrated that these trends, which are c ompletely opposite to those generally observed in NMR spectroscopy, ca n be exploited to monitor ovotransferrin half-molecule reassociation r eactions, In combination with the peculiar properties of transferrin-b ound quadrupolar nuclei reported in the literature to date, the phenom ena described here provide the basis for understanding the conditions and experimental parameters which may facilitate the application of th e QCT NMR technique to the study of other quadrupolar nuclei and prote ins. (C) 1996 Academic Press, Inc.