Paramagnetic Constraints in Structure Determination

Introduction

The collection of paramagnetic constraint data and the use of those data as a part of a structure determination is fairly straight forward.  RDCs can be collected using either the J-modulation or HSQC-TROSY methods described in the RDC section.  Paramagnetic relaxation enhancements (PREs) are collected based on attenuation of signal intensity in HSQC or TROSY spectra.  When Curie contributions from paramagnetic metals dominate it will be convenient to make measurements at multiple field strengths as the effects are field squared dependent and field variation provides a useful way to probe different distance ranges.  We have access to spectrometers operating from 600 to 900 MHz.  Pseudocontact shifts (PCSs) are measured by comparison of HSQC (or TROSY) cross-peak positions in diamagnetic (La³⁺) and paramagnetic complexes (Dy³⁺ or Tb³⁺).  Pairing of shifted and non-shifted peaks is facilitated by the fact that shifts in in both ¹H and ¹⁵N dimensions are nearly equal on the ppm scale and are therefore connected by diagonal lines.  Paramagnetic relaxation interferences (PRIs) produce differential effects on the α and β cross peaks of coupled HSQC spectra and the cross-correlation effects can be measured using experiments that we have developed for the measurement of correlation times from CSA/DD interference (Ref. 1).  Integration of these data into structure characterization protocols is accomplished using programs such as XPLOR-NIH (Ref. 2) or REDCAT (Ref. 3)  Routines for integration of PREs, RDCs, PCSs and PRI all exist, and use as a part of a structure determination protocol are relatively straightforward (Ref. 4).

References

1.   Liu, Y.Z., and Prestegard, J.H. (2008) Direct measurement of dipole-dipole/CSA cross-correlated relaxation by a constant-time experiment. J. Magn. Reson. 193, 23-31.

2.   Schwieters, C.D., Kuszewski, J.J., and Clore, G.M. (2006). Using Xplor-NIH for NMR molecular structure determination. Prog. NMR Spect. 48, 47-62.

3.   Valafar, H., and Prestegard, J.H. (2004) REDCAT: a residual dipolar coupling analysis tool. J. Magn. Reson. 167, 228-241

4.   Banci, L., Bertini, I., Cavallaro, G., Giachetti, A., Luchinat, C., and Parigi, G. (2004) Paramagnetism-based restraints for Xplor-NIH. J. Biomol. NMR 28, 249-261.