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

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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, 7 S)-colchicine.