NMR of paramagnetic proteins.

Abstract

AbstractIt was recognized early on in the development of the protein NMR field that paramagnetic proteins offered opportunities for useful structure-function studies by NMR spectroscopy that were not available with diamagnetic proteins. Thus, for example, in 1965 Kowalsky (1) reported 1H NMR spectra of ferricytochrome-c, and within a few years Wüthrich (2), McDonald and Phillips (3,4), and Redfield and Gupta (5) had obtained structural information concerning the cofactor site of cytochrome-c that only became available from X-ray diffraction studies later. Redfield and Gupta (5) also reported in their seminal paper the first measurements of nuclear Overhauser effects, saturation-transfer, and nuclear T1s in any protein, let alone a paramagnetic protein. Part of the success of this early work resulted from the electron relaxation times for cytochrome-c being relatively short, coupled with the considerable anisotropy in the distribution of its unpaired electron. Consequently, resonance linewidths were not broadened appreciably by the electron relaxation, and sizable pseudocontact shifts gave rise to a marked chemical shift dispersion. This example illustrates that the distinguishing features separating NMR spectroscopy of paramagnetic samples and related diamagnetic samples are all dependent on the electronic properties of the paramagnetic center. However, although the paramagnetism is often the source of valuable information, it can also present significant practical difficulties for the acquisition of good quality NMR spectra. In the present work, we describe these difficulties, and review experimental protocols employed to extract the maximum information possible from NMR spectra of paramagnetic samples. Throughout the work we give examples of the kind of information attainable for paramagnetic proteins by NMR, and, in Section 4., briefly review the progress made in the structure determination of paramagnetic proteins by NMR. Before turning to the practical aspects discussed in Section 3., we provide a brief introduction to relevant theoretical aspects of paramagnetic proteins.KeywordsParamagnetic CenterHeteronuclear CorrelationElectron Relaxation TimePseudocontact ShiftTOCSY ExperimentThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.