A lattice-gated exciton pair in α-tubulin with femtosecond electronic coherence at physiological temperature

Version 1.0 (November 2026). First public version (preprint). Revised after two rounds of simulated peer review in Synthesis Lab. Code and data: https://doi.org/10.5281/zenodo.23858682 (version 1.0.0). Abstract Electronic coherence in proteins is widely expected to vanish at physiological temperature: the environment is too warm, too wet and too noisy. We test that expectation on one aromatic pair, phenylalanines F169 and F202 of α-tubulin, whose rings sit 4.76 Å apart in the microtubule lattice, inside an aromatic relay that couples GTP hydrolysis to lattice compaction (Zehr et al., 2026). Combining fragment and supermolecular time-dependent density functional theory with molecular dynamics of the α monomer, of partial lattices and of closed, nucleotide-bound microtubule rings, we find that the lattice decides how often the rings touch: a closed expanded lattice holds the contact about a fifth to a quarter of the time in two force fields, a compacted lattice about an eighth, and isolated pieces of the lattice let it open. At the contact the coupling is large but fluctuates by more than its own mean and changes sign, driven by in-plane slip of the rings. Cys200 decides whether the pair mixes: with the cysteine treated quantum mechanically the native pair is detuned and barely mixes, while the C200S mutation brings it to resonance. With one bath convention for every environment, and the same open-system machinery and detector applied to the FMO light-harvesting complex, a strongly mixed pair in the closed, nucleotide-bound lattice keeps its eigenstate coherence for 39–83 fs at 300 K, against 80 fs for FMO's BChl 1–2 pair with the same machinery and detector: the same order in absolute time, although the pair's slower beat means this is a fraction of one oscillation. That lifetime is a dephasing time set by the coupling noise and barely depends on the mixing; the mixing decides how much population crosses between the rings before it, about a third for the strongly mixed pair against an eighth to a fifth for the detuned native one. Visible population beats in the lattice are not a robust measure: they change with the basis set, the detection threshold and the depth of the hierarchy, so we draw no conclusion from them. We compute how each condition moves the coupling noise, and with it the coherence, from the free monomer to the closed lattice, and give experimental tests for each part with the result that would refute it: lattice state, the C200S and C200A mutants, mutant-difference circular dichroism and absorption, and two-dimensional electronic spectroscopy.