  The exclusive photoproduction off nucleon of a large invariant mass lepton
pair in the backward region specified by the small Mandelstam variable $-u$ is
discussed in the framework of collinear QCD factorization. The amplitude is
factorized in terms of photon-to-nucleon Transition Distribution Amplitudes
(TDAs) which encode the photon content of the nucleon. Model estimates of these
new non-perturbative objects are described and the corresponding cross sections
calculated. The background due to the electromagnetic Bethe-Heitler process is
shown to be negligible in the kinematical regime of interest.
