T-Taxol and the electron crystallographic density in beta-tubulin

[chemical structure: see text]. T-Taxol has been proposed as the bioactive conformation on beta-tubulin and subsequently utilized in the design of a series of highly active bridged taxane analogues. A modified T-form with a reversed C-13 side chain orientation has recently been proposed as an equally plausible bioactive shape. A comparison of the two spatial alternatives within the tubulin binding site electron crystallographic density suggests strongly that T-Taxol is the bioactive conformation.