The importance of the phenyl-tropolone 'aS' configuration in colchicine's binding to tubulin.

Measuring ellipticities of (+/-)-colchicine and (+/-)-deacetamidocolchicine in the presence of tubulin afforded net positive CD bands with maxima at 340 nm resulting from reduction of the negative ellipticities upon binding of (-) enantiomers to the protein. Results of optical studies together with earlier NMR conformational analysis of these molecules substantiate the hypothesis that colchicinoids bind to tubulin with the phenyl-tropolone moiety in the 'aS' configuration. Natural colchicine which binds to tubulin, therefore, should be referred to as (-)-(aS,7S)-colchicine.