  We discuss IR limit of four-fermion scattering amplitudes in braneworld
models including intersecting-branes and SUSY SU(5) GUT version of it. With
certain compactification where instanton effect is negligible, grand
unification condition in D6-D6 intersecting-branes scenario subject to
experimental constraint on proton decay provides possibility for upper limit on
the string scale, $M_S$, through relationship between the string coupling,
$g_s$, and the string scale. We discuss how IR divergence is related to number
of twisted fields we have to introduce into intersection region and how it can
change IR behaviour of tree-level amplitudes in various intersecting-branes
models. Using number of twisted fields, we identify some intersecting-branes
models whose tree-level amplitudes are purely stringy in nature and
automatically proportional to $g_s/M^2_{S}$ at low energy. They are
consequently suppressed by the string scale. For comparison, we also derive
limit on the lower bound of the string scale from experimental constraint on
proton decay induced from purely stringy contribution in the coincident-branes
model, the limit is about $10^5$ TeV.
