  The motion of polymers with inhomogeneous structure through nanopores is
discussed theoretically. Specifically, we consider the translocation dynamics
of polymers consisting of double-stranded and single-stranded blocks. Since
only the single-stranded chain can go through the nanopore the double-stranded
segment has to unzip before the translocation. Utilizing a simple analytical
model, translocation times are calculated explicitly for different polymer
orientations, i.e., when the single-stranded block enters the pore first and
when the double-stranded segment is a leading one. The dependence of the
translocation dynamics on external fields, energy of interaction in the
double-stranded segment, size of the polymer and the fraction of
double-stranded monomers is analyzed. It is found that the order of entrance
into the pore has a significant effect on the translocation dynamics. The
theoretical results are discussed using free-energy landscape arguments.
