Topological mirror images in protein structure computation: An underestimated problem

When calculating three-dimensional structures from NMR data, alternative solutions with very large RMS deviation can be obtained. Sometimes local or global inversions of the protein folding can be observed. We call these different solutions topological mirror images, as they keep the correct amino acid chirality. They are observed when the number of restraints is insufficient and represent different solutions from the same scalar information. Therefore they are common in small peptides where the NMR data are often limited and the secondary structure is not very well defined. They can also be observed in large molecules in regions of higher flexibility. In our experience the observation of topological mirror images is independent of the efficiency of sampling of the algorithm used. We present four examples of proteins with different size and folding. We also discuss ways to distinguish among the different solutions.