Abstract
Recently it was demonstrated that long-lived quantum coherence exists during excitation energy transport in photosynthesis. It is a valid question up to which length, time and mass scales quantum coherence may extend, how one may detect this coherence and what, if any, role it plays in the dynamics of the system. Here we suggest that the selectivity filter of ion channels may exhibit quantum coherence, which might be relevant for the process of ion selectivity and conduction. We show that quantum resonances could provide an alternative approach to ultrafast two-dimensional (2D) spectroscopy to probe these quantum coherences. We demonstrate that the emergence of resonances in the conduction of ion channels that are modulated periodically by time-dependent external electric fields can serve as signatures of quantum coherence in such a system. Assessments of experimental feasibility and specific paths towards the experimental realization of such experiments are presented.
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GENERAL SCIENTIFIC SUMMARY Introduction and background. Ion channels are transmembrane protein complexes that regulate the flux of ions across the cell membrane. Central to this function is the selectivity filter that combines a high ion throughput rate (~108 s−1) with a high discrimination rate for a specific ion type (~1:104). Understanding the underlying mechanism for the efficiency of this process is the subject of active research. Traditionally, the high throughput rate has been explained via a mechanism involving ion–ion channel attraction and ion–ion repulsion, while the high selectivity has been thought to rely on the relative dehydration energies for different ions. We suggest in this paper that quantum coherence might be present in the selectivity filter and may play a role in the ion selectivity and transport process.
Main results. The chains of ions and water molecules in the selectivity filter, each coordinated by eight oxygen atoms of carbonyl groups, were treated as one-dimensional coupled quantum harmonic oscillators connected to a source and a sink and subject to dephasing type noise and thermal excitations. We demonstrate that such a system, if driven by an external varying potential, exhibits quantum resonances that are absent in classical rate equation type models. We show that such resonances can be used to quantify the amount of quantum coherence in a system. Specific suggestions for experimental realization are presented.
Wider implications. If such coherences could indeed be detected, the question of their possible biological relevance would be of great importance. As they would be short lived, it might lead to a new understanding of bio-molecular functions relying on the interplay between quantum coherence and the environmentally induced decoherence.
Figure. Induced quantum resonances in the selectivity filter of the potassium channel in the presence of an external driving field with (dashed lines) and without (solid line) decoherence.