All-atom molecular dynamics simulations of an artificial sodium channel in a lipid bilayer: the effect of water solvation/desolvation of the sodium ion

All-atom molecular dynamics is used to investigate the transport of Na⁺ across a 1,2-dioleoyl-sn-glycero-3-phosphocholine lipid bilayer facilitated by a diazacrown hydraphile. Specifically, the free energy of Na⁺ passing through the bilayer is calculated using the adaptive biasing force method to study the free energy associated with the increase in Na⁺ transport in the presence of the hydraphile molecule. The results show that water interaction greatly influences Na⁺ transport through the lipid bilayer as water is pulled through the bilayer with Na⁺ forming a water channel. The hydraphile causes a reduction in the free energy barrier for the transport of Na⁺ through the head group part of the lipid bilayer since it complexes the Na⁺ reducing the necessity for water to be complexed and, therefore, dragged through with Na⁺, an energetically unfavorable process. The free energy associated with Na⁺ being desolvated within the bilayer is significantly decreased in the presence of the hydraphile molecule; the hydraphile increases the number of solvation states of Na⁺ that can be adopted, and this increase in the number of available configurations provides an entropic explanation for the success of the hydraphile.