Triplet Energy\nMigration in Cytoskeletal Polymers

Dexter\nenergy transfer (DET) of triplet electronic states\nis used\nto direct energy in photovoltaics, quench reactive singlet oxygen\nspecies in biological systems, and generate them in photodynamic therapy.\nHowever, the extent to which repeated DET between aromatic residues\ncan lead to triplet energy migration in proteins has not been investigated.\nHere, we computationally describe DET rates in microtubules, actin\nfilaments and the intermediate filament, vimentin. We discover instances\nwhere interaromatic residue Dexter couplings within individual protein\nsubunits of these polymers are similar those of small molecules used\nfor organic electronics. However, interaromatic residue coupling is\nmostly weak (<10<sup>–3</sup> eV), limiting triplet energy\ndiffusion lengths to 6.1, 0.5 and 1.0 Å in microtubules, actin\nfilaments and vimentin, respectively. On the other hand, repeated\nförster resonance energy transfer (FRET) between aromatic residues\nleads to singlet energy diffusion lengths of 12.4 Å for actin\nfilaments and about 8.6 Å for both microtubules and vimentin\nfilaments. Our work shows that singlet energy migration dominates\nover triplet energy migration in cytoskeletal polymers.