  The repeating arrangement of tubulin dimers confers great mechanical strength
to microtubules, which are used as scaffolds for intracellular macromolecular
transport in cells and exploited in biohybrid devices. The crystalline order in
a microtubule, with lattice constants short enough to allow energy transfer
between amino acid chromophores, is similar to synthetic structures designed
for light harvesting. After photoexcitation, can these amino acid chromophores
transfer excitation energy along the microtubule like a natural or artificial
light-harvesting system? Here, we use tryptophan autofluorescence lifetimes to
probe inter-tryptophan energy hopping in tubulin and microtubules. By studying
how quencher concentration alters tryptophan autofluorescence lifetimes, we
demonstrate that electronic energy can diffuse over 6.6 nm in microtubules. We
discover that while diffusion lengths are influenced by tubulin polymerization
state (free tubulin versus tubulin in the microtubule lattice), they are not
significantly altered by the average number of protofilaments (13 versus 14).
We also demonstrate that the presence of the anesthetics etomidate and
isoflurane reduce exciton diffusion. Energy transport as explained by
conventional F\"orster theory (accommodating for interactions between
tryptophan and tyrosine residues) does not sufficiently explain our
observations. Our studies indicate that microtubules are, unexpectedly,
effective light harvesters.

Biological Physics;Mesoscale and Nanoscale Physics;Chemical Physics