  Refractive index of tubulin is an important parameter underlying fundamental
electromagnetic and biophysical properties of microtubules - protein fibers
essential for several cell functions including cell division. Yet, the only
experimental data available in the current literature show values of tubulin
refractive index (n\,=\,2.36\,-\,2.90) which are much higher than established
theories predict based on the weighted contribution of the polarizability of
individual amino acids constituting the protein. To resolve this controversy,
we report here modeling and rigorous experimental analysis of refractive index
of purified tubulin dimer. Our experimental data revealed that the refractive
index of tubulin is n\,=\,1.64 at the wavelength 589\,nm and 25\,$^{\circ}{\rm
C}$, that is much closer to the values predicted by established theories than
the earlier experimental data provide.
