  It was once purported that biological systems were far too warm and wet to
support quantum phenomena mainly due to thermal effects disrupting quantum
coherence. However recent experimental results and theoretical analyses have
shown that thermal energy may assist, rather than disrupt, quantum coherence,
especially in the dry hydrophobic interiors of biomolecules. Specifically,
evidence has been accumulating for the necessary involvement of quantum
coherence and entanglement between uniquely arranged chromophores in light
harvesting photosynthetic complexes. Amazingly, the tubulin subunit proteins,
which comprise microtubules, also possess a distinct architecture of
chromophores, namely aromatic amino acids including tryptophan. The geometry
and dipolar properties of these aromatics are similar to those found in
photosynthetic units indicating that tubulin may support coherent energy
transfer. Tubulin aggregated into microtubule geometric lattices may support
such energy transfer, which could be of import for biological signaling and
communication essential to living processes. Here we perform a computational
investigation of energy transfer between chromophoric amino acids in tubulin
via dipole excitations coupled to the surrounding thermal environment. We
present the spatial structure and energetic properties of the tryptophan
residues in the microtubule constituent protein tubulin. Plausibility arguments
for the conditions favoring a quantum mechanism of signal propagation along a
microtubule are provided. Overall we find that coherent energy transfer in
tubulin and microtubules is biologically feasible.
