  In order to understand the physical processes of nanopore experiments at the
molecular level, microscopic information from molecular dynamics is greatly
needed. Coarse-grained models are a good alternative to classical all-atom
models since they allow longer simulations and application of lower electric
potentials, closer to the experimental ones. We performed coarse-grained
molecular dynamics of the ionic transport through the $\alpha$-hemolysin
protein nanopore, inserted into a lipid bilayer surrounded by solvent and ions.
For this purpose, we used the MARTINI coarse-grained force field and its
polarizable water solvent (PW). Moreover, the electric potential difference
applied experimentally was mimicked by the application of an electric field to
the system. We present, in this study, the results of 1.5 microsecond
long-molecular dynamics simulations of twelve different systems for which
different charged amino acids were neutralized, each of them in the presence of
nine different electric fields ranging between +/- 0.04 V/nm (a total of around
100 simulations). We were able to observe several specific features of this
pore, current asymmetry and anion selectivity, in agreement with previous
studies and experiments, and identified the charged amino acids responsible for
these current behaviors, therefore validating our coarse-grain approach to
study ionic transport through nanopores. We also propose a microscopic
explanation of these ionic current features using ionic density maps.
