The Diamagnetic Susceptibility of the Tubulin Dimer

An approximate value of the diamagnetic anisotropy of the tubulin dimer, Δ χ dimer , has been determined assuming axial symmetry and that only the α -helices and β -sheets contribute to the anisotropy. Two approaches have been utilized: (a) using the value for the Δ χ α for an α -helical peptide bond given by Pauling (1979) and (b) using the previously determined anisotropy of fibrinogen as a calibration standard. The Δ χ dimer ≈ 4 × 10 −27  JT −2 obtained from these measurements are similar to within 20%. Although Cotton-Mouton measurements alone cannot be used to estimate Δ χ directly, the value we measured, CM dimer = (1.41 ± 0.03) × 10 −8  T −2 cm 2 mg −1 , is consistent with the above estimate for Δ χ dimer . The method utilized for the determination of the tubulin dimer diamagnetic susceptibility is applicable to other proteins and macromolecular assemblies as well.