The interaction geometries of some AA (His, Phe, Tyr, Trp) on the flat surface of graphene are similar in the interplanar distances around 3.3 - 3.5 A, that recall the pi-pi interactions. In fact, the planar geometry of graphene enhance the pi-pi stacking occurring between the aromatic rings and the surface generating smaller interplanar distances when the aromatic AA adsorb. Consequently, these residue are more stable on planar graphene

All the biomolecules, studying different domains, tend to approach to the surface regardless of the type of the secondary structure o the starting orientations. And most of them reveals a partial loss of the secondary structure of the  protein section that is in contact with the surface. As reported in Figure 4, the worst interaction mode is the one with the long axes of the protein perpendicular to the surface.