NMR proton spin-lattice relaxation constitutes an important tool for the characterization of the molecular dynamics in liquid crystals (LC). For LCs, in general, the main difference between the proton relaxation dispersion in mesophases and in the isotropic phase appears in the low frequency regime: typically a plateau is observed in the isotropic phase and specific frequency dependences, due to collective movements, are found in the LC phases. In more complex systems, peculiar frequency dependencies, in the low frequency regime may be found in the isotropic phase. The results presented in this paper reveal the presence of particular slow motional mechanisms related to the specific characteristics of the investigated systems.