Structural heterogeneity is also relevant to de novo protein design, and has been successfully used to create cyclic chameleon peptides that switch between exposed hydrophobic and hydrophilic surfaces to provide membrane permeability , two-state hinge proteins , and fold-switching metamorphic proteins . Membrane protein transporters have also been the subject of multiple-state de novo design efforts, such as the Zn 2+ -transporting four-helix bundle transmembrane protein Rocker .