Length-Independent Transport Rates in Biomolecules by Quantum Mechanical Unfurling

Ariel D. Levine, Chemistry, Technion, Haifa, Israel
Michael Iv, Chemistry, Technion, Haifa, Israel
Uri Peskin, Chemistry, Technion, Haifa, Israel

Natural processes, at their molecular level, require highly efficient charge transport through biomolecules. Experiments on DNA molecules have revealed charge transport rates which are independent of molecular length (within experimental error). However, the physical origin of this intriguing observation is still unclear. Classical transport theories predict weak length-dependence, but only a non-local theory (quantum mechanics) can possibly explain length-independent transport rates. In this work we show that length-independent charge transport rates through poly-A DNA are consistent with unfurling of initially localized charges into delocalized quantum states. Such states are ubiquitous also in long, ordered, biomolecules. The unfurling is triggered by fluctuations of the molecule in its ambient conditions. We also show that stochastic fluctuations in the DNA model parameters maintain length-independence charge transport rates for long DNA bridges (poly-A), as long as fluctuations in the local ionization energies and in the intra-strand couplings elements do not exceed ΔE~0.36 [eV] and ΔE~0.05 [eV] respectively. This is attributed to the existence of a significant energy gap between the donor and the band-like structure of the highly ordered poly-A DNA-bridge. Deviations from the length-independent rate are demonstrated when artificial local fluctuations are introduced, due to localization of the bridge orbitals and reduction in the number of delocalized states which contribute to charge transport by the unfurling mechanism. This new interpretation of charge transport in DNA is in line with recent work which relates efficient charge and energy transfer processes in biomolecules at ambient conditions to quantum mechanical delocalization.