  Dynamic Monte Carlo simulations are used to study coupled transport
(co-transport) through sub-nanometer-diameter pores. In this classic
Hodgkin-Keynes mechanism, an ion species uses the large flux of an abundant ion
species to move against its concentration gradient. The efficiency of
co-transport is examined for various pore parameters so that synthetic
nanopores can be engineered to maximize this effect. In general, the pore must
be narrow enough that ions cannot pass each other and the charge of the pore
large enough to attract many ions so that they exchange momentum. Co-transport
efficiency increases as pore length increases, but even very short pores
exhibit co-transport, in contradiction to the usual perception that long pores
are necessary. The parameter ranges where co-transport occurs is consistent
with current and near-future synthetic nanopore geometry parameters, suggesting
that co-transport of ions may be a new application of nanopores.
