Journal of Biomolecular NMR

, Volume 65, Issue 3–4, pp 143–156 | Cite as

Evaluating the influence of initial magnetization conditions on extracted exchange parameters in NMR relaxation experiments: applications to CPMG and CEST

Article

Abstract

Transient excursions of native protein states to functionally relevant higher energy conformations often occur on the μs–ms timescale. NMR spectroscopy has emerged as an important tool to probe such processes using techniques such as Carr–Purcell–Meiboom–Gill (CPMG) relaxation dispersion and Chemical Exchange Saturation Transfer (CEST). The extraction of kinetic and structural parameters from these measurements is predicated upon mathematical modeling of the resulting relaxation profiles, which in turn relies on knowledge of the initial magnetization conditions at the start of the CPMG/CEST relaxation elements in these experiments. Most fitting programs simply assume initial magnetization conditions that are given by equilibrium populations, which may be incorrect in certain implementations of experiments. In this study we have quantified the systematic errors in extracted parameters that are generated from analyses of CPMG and CEST experiments using incorrect initial boundary conditions. We find that the errors in exchange rates (kex) and populations (pE) are typically small (<10 %) and thus can be safely ignored in most cases. However, errors become larger and cannot be fully neglected (20–40 %) as kex falls near the lower limit of each method or when short CPMG/CEST relaxation elements are used in these experiments. The source of the errors can be rationalized and their magnitude given by a simple functional form. Despite the fact that errors tend to be small, it is recommended that the correct boundary conditions be implemented in fitting programs so as to obtain as robust estimates of exchange parameters as possible.

Keywords

Conformational exchange CPMG CEST Spin relaxation Boundary conditions 

Notes

Acknowledgments

We thank Enrico Rennella for helpful discussions and Rina Rosenzweig, Alaji Bah and Jacob Brady for providing protein samples. This project is supported by grants from the Canadian Institutes of Health Research and the Natural Sciences and Engineering Research Council of Canada. L.E.K. holds a Canada Research Chair in Biochemistry.

Supplementary material

10858_2016_45_MOESM1_ESM.pdf (380 kb)
Supplementary material 1 (PDF 380 kb)

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Copyright information

© Springer Science+Business Media Dordrecht 2016

Authors and Affiliations

  1. 1.Departments of Molecular Genetics, Biochemistry and ChemistryThe University of TorontoTorontoCanada
  2. 2.Program in Molecular Structure and FunctionHospital for Sick ChildrenTorontoCanada

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