The observation of proton to nitrogen-15 heteronuclear Overhauser effects in the microcrystalline protein Crh is used to confirm that the principal mechanism of relaxation of amide nitrogens is due to the fluctuation of the N-H dipolar couplings caused by N-H bond dynamics. Our observations reveal the central role of water as the main source of proton magnetization, and we provide an analysis of the different pathways that could lead to the observed results.