Properties of B-ring analogues of colchicine.

Absorption spectra of colchicine and its analogues are affected by the presence of the B-ring, although it is not part of the chromophore (C-ring). Thus, 2-methoxy-5-(2',3'4'-trimethoxyphenyl)tropone has absorption maxima at 341 nm, whereas that of desacetamidocolchicine is at 353 nm. A similar red shift in the lambda max of colchicine, desacetamidocolchicine and 2-methoxy-(2',3',4'-trimethoxyphenyl)tropone also occurs when they are immobilized in the binding site to tubulin or in pure glycerol. We also observed that the B-ring of colchicine alone or with substituent does not affect the UV-induced rearrangement of colchicine to lumicolchicine. However, in the absence of the B-ring, as in the case of 2-methoxy-5-(2',3'4'-trimethoxyphenyl)tropone, the rearrangement reaction of the C-ring slows down significantly.