Quantum Interference and Selectivity through Biological Ion Channels

The mechanism of selectivity in ion channels is still an open question in biology for more than half a century. Here, we suggest that quantum interference can be a solution to explain the selectivity mechanism in ion channels since interference happens between similar ions through the same size of ion channels. In this paper, we simulate two neighboring ion channels on a cell membrane with the famous double-slit experiment in physics to investigate whether there is any possibility of matter-wave interference of ions via movement through ion channels. Our obtained decoherence timescales indicate that the quantum states of ions can only survive for short times, i.e. ≈100 picoseconds in each channel and ≈17–53 picoseconds outside the channels, giving the result that the quantum interference of ions seems unlikely due to environmental decoherence. However, we discuss our results and raise few points, which increase the possibility of interference.

. Since, U 0 acts like the kinetic energy of quantum system, the unit of momentum could be expressed as P 0 = (U 0 M ) 1 2 . Subsequently, the conjugate variables of position q and momentum p are defined as q = R/R 0 and p = P/P 0 where R and P are the conventional position and momentum, respectively [2].
Moreover, the potential energyV and the HamiltonianĤ s operators of the system can be defined in this regime aŝ whereÛ (R) andĤ S are the potential energy and the Hamiltonian operators in the ordinary Schrodinger equation. Finally, the dimensionless Schrodinger equation can be written as Also, the canonical commutator in the dimensionless form is [q,p] = ih, whereh defined as As is clear in (2) and (3), a new dimensionless parameterh appears which show quantitavely the quantum behavior of the system. Strictly speaking, the situation in which one obtainsh 1, the system behaves quasi-classically. The values ofh between 0.01 to 0.1 are fair enough to show the macroscopic disposition of the proposed system [2]. Moreover, we can defineh in (3) ash whereλ 0 = λ 0 /2π. Here, λ 0 is the de Broglie wavelength of the system. For a macroscopic quantum system, λ 0 is too small compared to R 0 , which is nearly a fixed value for known models of potential. Thus,h < 0.1 seems appropriate condition for our future purposes. Smaller values ofh show more classical behavior of the macro-system. Moreover, according to the particle aspect of a quantum system, we define ω 0 = τ −1 0 = P 0 /R 0 M . On the other hand, for the wave aspect of a quantum system, we introduce another unit of momentum P 0 concluded from the phase velocity v 0 = ω 0 /k 0 (k 0 = 2π/λ 0 ), so that, P 0 = Mλ 0 ω 0 . Subsequently, regarding the relation (4), we conclude that Since for a macroscopic quantum system, the wave aspect is reduced, therefore P 0 P 0 . This is another reason of how the parameter ofh can explain the classical and quantum behavior of the macro-system.

Formulation of Double-Slit Interference Pattern of Potassium Ion via Movement through Ion Channels
So far, many studies have been done to investigate double-slit interference pattern of particles, atoms and molecules in experimental and theoretical contexts. In some of these works, the incoming state in double-slit experiment has been described by Gaussian wave packets [3,4]. The use of Gaussian wave packet is sufficiently general, because it includes the limit case of plane waves. On the other hand, due to the development of experimental techniques, possible deviations from the standard form of the interference pattern can be better explained by Gaussian states [5][6][7].
Our approach in this study will be based on Gaussian wave packet as a simulation for potassium ions which move through two neighbor ion channels in two dimensions. We suppose that the state of the ion behaves like a Gaussian state and after the slits, the ion can be behaved as a free particle in two directions x and y.
Moreover, we define the region W that is inaccessible to the ion, assumed to be a subset of the (x, y) plane where b is the slits width with depth 2a and d is the distance of the slits. The potential energy constituent of the Hamiltonian operator of the system at t ≥ 0 is an infinite step potential which does not permit a potassium ion of arbitrary energy to tunnel the region W . We also suppose that the wave function of the incoming ion to slits is factorized in its x and y components. It is assumed that x and y components of the wave function remain separated during and after passing the slits. We consider a Gaussian wave packet coming from the remote x region with probability distribution centered on a point moving with velocityhk 0x on the x-axis (y = 0) in dimensionless regime as [1] ψ(x, y, t) = µ(x, t)φ(y, t) where and φ(y, t) = [ β π We define the region in which the action of the Gaussian wave packet is investigated as After the slits, the wave function of the ion in the y-direction evolves as φ I (y, t) = 1 2h where the first integral is the Fourier transform of time evolution of the wave function in the momentum space and the last integral is the wave function of the ion in the momentum space at t = 0. One can obtain the above wave function by integrating over the variable p y as We suppose that the wave packet reaching the slits is very undetermined in the y-position probability distribution By setting β 2 ≈ 0 in (11) and neglecting the term iξ 2 /2ht against iyξ/2ht for large values of y (knows as far-field approximation) and after some calculations, the double-slit interference pattern of potassium ion can be obtained as The resulting interference patterns for potassium ions via movement through ion channels are drawn in different values of macroscopicity measureh in figure (1)-(4). As mentioned before, the macroscopicity measureh indicates the quantum character of the macroscopic system. Our results show that when the quantum nature of the potassium ion is evanesced, according to lower values ofh, the double-slit interference pattern becomes more classic. In other words, the values fromh = 0.1 toh = 0.01 are more quantum values which makes the interference pattern possible, while the values lower thanh = 0.01 are classical values that show the interference pattern cannot be formed.

Molecular dynamics simulation
Our MD simulations [8,9] are based on a model of the KcsA channel (Protein Data Bank, 1K4C.pdb), embedded in a palmitoyloleoyl phosphatidylcholine (POPC) lipid bilayer. The system was built from a cubic box of a 7.8 nm side that comprises KcsA (four subunits of 97 amino acids, 5292 atoms), water molecules (TIP3P model, 42296 atoms), 3K and 2K in the pore and 12 CL in the bulk (the entire system is electrically neutral). The AMBER 03 force field parameters [10] and GROMACS 4.5.3 software [11] was employed to perform the simulations with the time step of 1 fs. The protein was equilibrated during 10 ns in (N, V, T) then (N, P, T) ensembles. The temperature was kept at 300 K by Nose-Hoover coupling algorithm and the pressure was kept at 1 bar by Parrinello-Rahman coupling algorithm. The system is oriented along the z-axis. A cutoff was used for long-range interactions, namely: 1 nm for the van der Waals interaction and 1 nm for electrostatic interactions. Using the Particle-Mesh Ewald (PME) method, the electrostatic interactions are calculated. We have used the following abbreviations for amino acid identification: GLY1=GLY79, TYR=TYR78, GLY2=GLY77, VAL=VAL76, THR2=THR75, THR1=THR75. Each carbonyl group is a C=O compound in which the vibrations of C and O atoms are investigated separately. Taking typical values for membrane potentials in neurons, -70mV and +30mV, for resting and firing states are considered [12,13]. To be more general, we also considered other membrane potentials -100 mV and +100 mV [14] to obtain other velocities as well [8,9].