  Microtubules are biological protein polymers with critical and diverse
functions. Their structures share some similarities with photosynthetic antenna
complexes, particularly in the ordered arrangement of photoactive molecules
with large transition dipole moments. As the role of photoexcitations in
microtubules remains an open question, here we analyze tryptophan molecules,
the amino acid building block of microtubules with the largest transition
dipole strength. By taking their positions and dipole orientations from
realistic models capable of reproducing tubulin experimental spectra, and using
a Hamiltonian widely employed in quantum optics to describe light-matter
interactions, we show that such molecules arranged in their native microtubule
configuration exhibit a superradiant ground state, which represents an
excitation fully extended on the chromophore lattice. We also show that such a
superradiant ground state emerges due to supertransfer coupling between the
ground states of smaller blocks of the microtubule. In the dynamics we find
that the spreading of excitation is ballistic in the absence of external
sources of disorder and strongly dependent on initial conditions. The velocity
of photoexcitation spreading is shown to be enhanced by the supertransfer
effect with respect to the velocity one would expect from the strength of the
nearest-neighbor coupling between tryptophan molecules in the microtubule.
Finally, such structures are shown to have an enhanced robustness to static
disorder when compared to geometries that include only short-range
interactions. These cooperative effects (superradiance and supertransfer) may
induce ultra-efficient photoexcitation absorption and could enhance excitonic
energy transfer in microtubules over long distances under physiological
conditions.
