Thesis Title: Subtitle - NMR Spectroscopy Research Group

46 Chapter 2. Possum: paramagnetically orchestrated spectral solver of unassigned methyls.
d Fraction of correct assignments for the paramagnetic cz- 186/ /Yb 3+ and cz- 186/ /Dy 3+
complexes, as indicated. The number in the denominator is the number of methyl groups for which
cross-peaks were observed in the presence of Yb 3+ or Dy 3+ .
e Fraction of correct assignments for the diamagnetic cz- 186/ /La 3+ complex. The number in the
denominator is the number of methyl groups for which cross-peaks were observed in at least one of
the paramagnetic complexes.
2.4.6 PCS and flexibility
Structural differences between the crystal structure of 186 determined under cryogenic
conditions (Hamdan et al., 2002b) and the solution structure of the cz- 186/ complex become
apparent as differences between measured and predicted PCS. In a few cases, the structural
differences interfere with the resonance assignment. Figure 2.6a illustrates the situation for the cz-
186/ /Dy 3+ complex, where the measured PCS of Leu161 are smaller than predicted and would
more closely match the values predicted for Leu131. This can be explained by a small displacement
of the peptide segment comprising residues 151-161 that decreases the PCS of both methyls of
Leu161. Smaller PCS than expected were also observed for the backbone amides of this segment. 45
The correct assignment would be obtained by focusing on the difference in 13 C PCS between both
methyl groups rather than their magnitude (Figure 2.6a) or by using the data of the cz- 186/ /Yb 3+
complex which are less strongly distance dependent in the 11 Å distance range (Figure 2.6b).