What was immediately apparent from these studies was that for tubulin-active molecules, modelling gave binding constants within 5-fold of 3 (4, 5, and (aR)-6), while those that were mostly inactive differed by at least 100 fold (Figure ). Compounds 4 and 5 both bound to tubulin in silico with dihedral angle profiles that were comparable to 3, whereas (aR)-6 bound with much larger dihedral angles. When unbound, each of the compounds had comparable average dihedral angles that was near coplanar, but differed noticeably in their distribution. Specifically, compound 3 had dihedral angle distributions nearly uniformly from 50° to 130°, while (aR)-6 was mostly in the 80° to 100° range (Figure ). 4 profiled more similarly to (aR)-6, and 5 more similar to 3, consistent the energetic profiling results (see Scheme 1C). Thus, (aR)-6 has the smallest change in structure to binding, illustrative of its high conformational rigidity. Looking more closely at the structures generated in silico, two main hydrogen bond contacts are observed (Figure ). The most stable of these contacts is that between the oxygen of the tropone carbonyl and the hydrogen of the amide backbone of valine 181 (not shown). A second hydrogen bond is observed between the thiol hydrogen of a highly flexible cysteine 241 and the trimethoxybenzene-associated oxygen atoms, and more specifically those of the methoxy groups at the meta and/or para position (illustrated in Figure ). MTC (3) shows consistent hydrogen bonding between this thiol and at least one of these two trimethoxybenzene-associated methoxy groups, whereas it is less frequent in the case of (aR)-6. Given that the para methoxy groups are in line with the AC chiral axes, the methoxy at the para position is not influenced as much by the differences in dihedral angles as much as the meta methoxy groups would be. This could help explain the diminished potency of (aR)-6 as compared to 3 observed experimentally.