  Models with dark energy decaying into dark matter have been proposed to solve
the coincidence problem in cosmology. We study the effect of such coupling in
the matter power spectrum. Due to the interaction, the growth of matter density
perturbations during the radiation dominated regime is slower compared to
non-interacting models with the same ratio of dark matter to dark energy today.
This effect introduces a damping on the power spectrum at small scales
proportional to the strength of the interaction and similar to the effect
generated by ultrarelativistic neutrinos. The interaction also shifts
matter--radiation equality to larger scales. We compare the matter power
spectrum of interacting quintessence models with the measurments of 2dFGRS. We
particularize our study to models that during radiation domination have a
constant dark matter to dark energy ratio.
