Multiconformational analysis of solution NOE data for the Ac-(L)proline-(D)alanine-NHMe dipeptide in a nonprotic solvent.

Solution NOE data for the dipeptide, Ac-(L)proline-(D)alanine-NHMe (1), have been obtained in the viscous solvent, 75:25 polychlorotrifluoroethylene:chloroform-d1. In this solvent at a 500 MHz 1H NMR spectrometer frequency, 1 exhibits large, negative NOE enhancements. The quantified 2D-NOESY time courses for 1 are analyzed with the multiconformational analysis technique, conformer population analysis (CPA). Thus, experimental NOE data for a molecule that is known to adopt at least three conformational motifs are used to explore the factors influencing the structural characterization of conformationally supple molecules by NOE. Both the quantitative and qualitative interpretations of solution NOE data for the simple molecule, 1, are demonstrated to critically depend on the extent to which one relies on empirical force-field energetics to determine which structures are energetically viable and on the methods by which trial structures are generated. Other influences include the relative weighting of contributions to the fitting-error function by large vs small NOE cross peaks and inclusion vs exclusion of null data, i.e., whether the absence of NOE cross peaks is included in the fitting procedure. Factors that do not appear to exert significant influence on the interpretation of these NOE data include (1) whether conformational interconversion is assumed to be slow or fast with respect to T1, (2) the inclusion of diagonal cross-peak data in the fitting procedure, and (3) what structures are assumed in the calibration of rotational correlation times from the observed data.