  We estimate the form factors that parametrise the hadronic matrix elements of
proton-to-pion transitions with the help of light-cone sum rules. These form
factors are relevant for semi-leptonic proton decay channels induced by
baryon-number violating dimension-six operators, as typically studied in the
context of grand unified theories. We calculate the form factors in a
kinematical regime where the momentum transfer from the proton to the pion is
space-like and extrapolate our final results to the regime that is relevant for
proton decay. In this way, we obtain estimates for the form factors that show
agreement with the state-of-the-art calculations in lattice QCD, if systematic
uncertainties are taken into account. Our work is a first step towards
calculating more involved proton decay channels where lattice QCD results are
not available at present.
