The carboxy-terminal segments of the R/‚-tubulins are flexible regions rich in acidic amino acid residues. It is generally believed that these regions play crucial roles in tubulin polymerization and interaction with many ligands, including colchicine. Exactly how these effects are exerted are not known at present. One such interesting aspect is the pH dependence of colchicine-tubulin interaction and the influence of the R-tail on the binding interaction. We have investigated the location of the colchicine- binding site on tubulin by docking. It has been located on the R/‚ interface on the N-terminal side, which is also supported by much of the solution data. This location is too far from the tail regions, suggesting that influence of the tail region is transmitted by a pH-dependent conformational change. Two-dimensional NMR studies indicate that at pH 7 a 13-residue peptide corresponding to R-tubulin tail shows little NOE constraints, suggesting extended conformation. On the contrary, at pH 5, a relatively compact structure was deduced from the interproton NOE constraints. Pulsed field gradient measurement of diffusion constant indicates that the peptide at pH 5 is substantially faster diffusing than at pH 7. The Perrin factors calculated from diffusion data indicates that the peptide structure at pH is significantly more compact than at pH 7. Temperature coefficients of several amide protons at pH 5 fall below 5 ppb/ o K, indicating a degree of protection. A difference is also seen in the CD spectra obtained at different pHs, consistent with the NMR data. We have investigated the probable spatial organization of the tail of the R-subunit of tubulin, in the high pH extended form and the low pH compact form. On the basis of correlation of pH dependence of many properties of tubulin and the conformation of the R-tail peptide, we propose that the intrinsic conformational preference of the tail-region modulate the tail -body interaction, which in turn has important bearing on colchicine binding properties.