New Spectral Assignment Techniques for Paramagnetic Proteins

One important source of distance constraints for paramagnetic proteins are cross correlations between Curie spin relaxation and H-X dipolar relaxation (X = H, C, or N). These cross correlations can in principle be detected by the same pulse sequences as are commonly used in diamagnetic systems to detect cross correlation between chemical shift anisotropy and H- N dipolar relaxation. However, in paramagnetic systems, rapid transverse self-relaxation tends to quench the build-up of relaxation allowed coherence transfer. To overcome this problem, Kateb and Piccioli have proposed a modified HSQC experiment to observe and quantify relaxation-allowed coherence transfer before it is quenched by a short T2. 

Since paramagnetic broadening varies as the square of the nuclear gyromag-netic ratio, work has focused experiments involving low-y nuclei, particularly, and N. Bermel et describe the use of direct detection in 2D NMR experiments COSY, COCAMQ, NOESY) to detect 

In a review, Nocek et al discuss the use of proton pseudocontact chemical shift data to analyse structural perturbations in proteins that are too large for solution structure determination. The discussion focuses on oxygenated and deoxygenated states of hemoglobin. 

1 Determination of electron spin relaxation times. Electron spin relaxation times of paramagnetic metal centers in proteins can, at least in principle, be determined from NMR relaxation rates. Jensen and Led have discussed four different approaches to this problem based on from NMR Ti measurements of and N backbone nuclei. They conclude that only two of these methods are reliable when reorientation is slow. 

---