  We study how low-energy charge carriers scatter off periodic and linear
graphene grain boundaries oriented along the zigzag direction with a
periodicity three times greater than that of pristine graphene. These defects
map the two Dirac points into the same position, and thus allow for intervalley
scattering to occur. Starting from graphene's first-neighbor tight-binding
model we show how can we compute the boundary condition seen by graphene's
massless Dirac fermions at such grain boundaries. We illustrate this procedure
for the 3-periodic pentagon-only grain boundary, and then work out the
low-energy electronic scattering off this linear defect. We also compute the
effective generalized potential seen by the Dirac fermions at the grain
boundary region.
