Solution Structures of α- and β-Amanitin via Distance Geometry Calculations and a Relaxation-Matrix Analysis of Nuclear Overhauser Effect Spectra

Abstract

The solution structures of α- and β-amanitin in dimethyl sulfoxide/chloroform (85/15) at 286 K have been examined through analysis of two-dimensional double-quantum-filtered correlated spectra and nuclear-Overhauser-effect spectra (NOESY). The NOESY data were analyzed by both a two-spin approximation and a relaxation-matrix analysis. Analysis of NOESY data acquired with a long mixing time (τ m > T 1) provides experimental verification that the two-spin approximation underestimates interproton distances greater than approximately 3.0 Å. The interproton distances obtained from relaxation-matrix analyses were used in combination with distance-geometry-based calculations to compute a set of 10 solution structures for each compound. The sets of solution structures compare favorably with reported crystal structures for these compounds. A simplified relaxation-matrix-analysis procedure is presented which can estimate interproton distances from a set of references distances and a single NOESY volume matrix. This simplified analysis was used together with a cross-validation procedure to investigate the accuracy of computed distances. In the present case, distances obtained with the relaxation analysis are slightly more accurate than distances obtained with a simplified two-spin analysis.