Isotope effects should be negligible in docking simulations, although they are sometimes taken into account in molecular dynamics simulations by adding the parameters for the heavy atom to the force field being used. However, for Nitrogen, the change in mass between Nitrogen-14 and Nitrogen-15 is less than 10% and could probably be ignored.

I agree with Rudy, it is unlikely that the heavier mass of the N-15 will affect parameters of the energy functions used in docking and effect would be negligible. In your case, docking of peptide poises more important challenges of how to deal with the ligand and receptor flexibility to effectively search the conformational space, and is more likely to affect ability to reach conclusive, good quality, reproducible results than subtle parameter differences.

thank you very much