Conformation of thiocolchicine and two B-ring-modified analogues bound to tubulin studied with optical spectroscopy

The interaction of tubulin with thiocolchicine and two thiocolchicine analogues, one lacking the B ring and one with a six-membered B ring, has been studied by using near-UV and CD spectroscopies. Rapid, reversible binding of the latter analogue to tubulin demonstrates the ability of the colchicine binding site to accommodate the phenyltropone system with a more coplanar conformation than is present in free colchicine. There is no evidence, however, that bound thiocolchicine should have a much less twisted conformation than free thiocolchicine. Thiocolchicine and the bicyclic analogue appear to have approximately the same conformation of the phenyltropone system, in both the free and the bound states, suggesting that this conformation has an optimal arrangement of the phenyl and tropone rings for binding to tubulin. In contrast to colchicine and related derivatives, the three thiocolchicine analogues show pronounced near-UV CD bands upon association to tubulin. No simple relation could be found between the sign pattern of the CD components in the near-UV band of the thiocolchicinoid chromophore and its axial chirality.