Regular Article
A Relaxation-Matrix Analysis of Distance-Constraint Ranges for NOEs in Proteins at Long Mixing Times

https://doi.org/10.1006/jmrb.1995.1004Get rights and content

Abstract

Long-mixing-time data (τm > 200 ms) from NOE spectra have largely been ignored as a source of protein structural information due to the effects of spin diffusion on calculated interproton distances when using the two-spin approximation. An effective approach for incorporating spin-diffusion effects in an average way into refinements is to choose distance bounds based on distributions of distances observed in NOE back calculations on homologous proteins from a protein structure database. We have determined distributions of interproton distances characteristic of newly observed NOE cross peaks for the proteins crambin, PTI, and echistatin at long mixing times. A relaxation-matrix analysis was used to model the effects of spin diffusion. Constraint ranges were constructed from the interproton distance distributions which can be used in standard protein-refinement programs based on the two-spin approximation. Back calculations are also used to analyze constraint ranges typically used for protein structure determinations based on NOE spectra at shorter mixing times.

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