The Diamagnetic Susceptibility of the Tubulin Dimer

An approximate value of the diamagnetic anisotropy of the tubulin 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 for an α-helical peptide bond given by Pauling (1979) and (b) using the previously determined anisotropy of fibrinogen as a calibration standard. The  JT−2 obtained from these measurements are similar to within 20%. Although Cotton-Mouton measurements alone cannot be used to estimate directly, the value we measured,  T−2cm2mg−1, is consistent with the above estimate for . The method utilized for the determination of the tubulin dimer diamagnetic susceptibility is applicable to other proteins and macromolecular assemblies as well.