  In the standard DNA brick set-up, distinct 32-nucleotide strands of
single-stranded DNA are each designed to bind specifically to four other such
molecules. Experimentally, it has been demonstrated that the overall yield is
increased if certain bricks which occur on the outer faces of target structures
are merged with adjacent bricks. However, it is not well understood by what
mechanism such `boundary bricks' increase the yield, as they likely influence
both the nucleation process and the final stability of the target structure.
Here, we use Monte Carlo simulations with a patchy particle model of DNA bricks
to investigate the role of boundary bricks in the self-assembly of complex
multicomponent target structures. We demonstrate that boundary bricks lower the
free-energy barrier to nucleation and that boundary bricks on edges stabilize
the final structure. However, boundary bricks are also more prone to
aggregation, as they can stabilize partially assembled intermediates. We
explore some design strategies that permit us to benefit from the stabilizing
role of boundary bricks whilst minimizing their ability to hinder assembly; in
particular, we show that maximizing the total number of boundary bricks is not
an optimal strategy.
