The feasibility of coherent energy transfer in microtubules
Travis Craddock et al, Nova Southeastern University
Summary and review of the above paper
INTRODUCTION: Craddock’s paper emphasises the strong similarities between tryptophan complexes in microtubules and quantum coherent energy transfer in photosynthesis, a connection which is dumbed out in most discussions of quantum biology. The microtubules are themselves closely connected to mitochondria; these organelles are evolutionarily related to bacteria and plant chloroplasts that utilise quantum coherence for efficient energy transfer.
Recent experiments have shown that thermal energy may boost rather than disrupt quantum coherence, particularly in the ‘dry’ hydrophobic interiors of biomolecules. Quantum coherence is involved in the activity of chromophores in the light-harvesting complexes of photosynthesis. The tubulin proteins that form microtubules are made up of the same type of aromatic amino acids, such as tryptophan, that comprise the chromophores in photosynthetic light-harvesting systems. The geometry and dipolar properties of tubulins are similar to those found in the photosynthetic light-harvesting systems.
Over the last decade, research has demonstrated the efficacy of quantum features in biological systems. Quantum coherence in photosynthesis, magnoreception in birds, and quantum features in olfaction and vision have all been identified. However, the greatest amount of research attention has been concentrated on quantum coherence in photosynthesis, with quantum features found to function at room temperatures and in plants as well as bacteria and algae.
The light-absorbing nature of chlorophyl molecules allows light (photon) energy to be taken from the environment into the photosynthetic reaction centre. This involves photons passing from chromophore to chromophore. The first excited chromophore can donate its electronic energy to an acceptor chromophore as a result of electrical attraction between dipoles. The system depends on the optimal packing of chromphores and dipoles.
Microtubules as quantum candidates
Tubulin, the protein basis of microtubules, has a network of chromophoric tryptophan. Tests of the fluorescence quantum yield of tubulin are comparable to those for bacteriochlorophll involved in photosynthesis. The ‘red edge effect’ observed in vegetation is also observed in tubulin indicating energy transfer between tryptophan molecules. The spatial distribution of tryptophan in tubulin is seen to be similar to the distance between chromophores in some algae that have been seen to support quantum coherent transfer of electronic excitation. Excitation could travel the length of the tubulin dimer, and it is considered feasible that close and uninterrupted stacking of tubulins would allow the excitation to travel along the microtubule. The cylindrical lattice structure of the microtubule with its helical pathways could enhance energy transfer.
Microtubules appear to be more favourable to quantum energy transfer than other parts of the cytoskeleton because they have more than three times the amount of tryptophan molecules found in other parts of the cytoskeleton; further to that the distances between tryptophan clusters in other areas of the cytoskeleton appear too large. The author is therefore inclined to focus attention on the tryptophan network within tubulin dimers where dipolar coupling could support quantum coherence similar to that discovered in the FMO photosynthetic complex.