To further study these binding modes, we clustered the bound frames of 5b mapped on the protein surface with respect to their RMSD after superimposing Hsp90. Among the binding modes were several that form interactions to the C-terminal helix interface, with 5b positioned such that it mimics interactions formed by H5' in the dimer (Figure and). To corroborate that this binding mode is favorable, we computed the effective binding free energies by the MM-GB/SA approach for the trajectory that led to it. Already first transient interactions with the protein resulted in effective energies down to ~-30 kcal mol -1 . The effective energies decreased further to ~-45 kcal mol -1 once the ligand bound in the C-terminal interface, thereby forming interactions with Hsp90 that remained stable even when the trajectory was extended to 1 µs, indicating that such poses are particularly favorable (Figure ). As to the magnitude of the effective energies, note that configurational entropy contributions were not considered, since estimating such contributions by normal mode analysis may introduce additional uncertainties. Overall, the probability density of bound 5b poses, the proportion of replicas, and the results of the MM-GB/SA computations indicate that 5b preferentially binds to the C-terminal helix interface, where it can adopt poses that mimic H5'.