The nitridation of sapphire substrates was monitored in situ by reflection high-energy electron diffraction. The evolution of the lattice-mismatch evidences the formation of an AIN relaxed layer when exposing the sapphire surface heated at 850 degrees C to an ammonia flow. GaN thin films were grown by gas-source molecular beam epitaxy on variously nitridated sapphire substrates. High-resolution transmission electron microscopy study reveals the existence of two different epitaxial relationships between GaN and sapphire (0 0 0 1). The well-known orientation with the c-axis of the GaN crystal perpendicular to the Al2O3 surface is observed when starting the growth on a nitridated substrate. On the other hand, growing GaN directly on bare Al2O3 surfaces results in a different crystallographic orientation where the c-axis is tilted by about 19 degrees with respect to the sapphire basal plane. Photoluminescence measurements show that both the intensity of the yellow-band emission (similar to 2.2 eV) and the residual donor-acceptor pair recombinations are affected by the nitridation state of the starting surface. The dependence of the optical properties of GaN thin films versus the NH3 exposure time is then used to optimize the nitridation step.