Engineering oriented heme protein maquette monolayers through surface residue charge distribution patterns

We have designed and synthesized four-alpha-helix-bundle proteins that accommodate heme groups to act as molecular "maquettes" of more complex natural electron-transfer proteins. These bundles can be oriented at an air-water interface and transferred onto solid surfaces to facilitate the exploration of the factors that govern biological electron transfer. We find that the orientation of these maquettes on an air-water interface can be controlled by choosing the distribution of charged amino acids along the sides of the helices exposed to water. The four alpha-helices were assembled either as two subunits, where each subunit consists of two alpha-helices linked by a terminal cysteine disulfide bond, or as a single, four-helix covalent unit consisting of two helix-loop-helix molecules linked by a terminal cysteine. In either case, when each alpha-helix contains both positively charged lysines and negatively charged glutamates, addition of the heme binding bundles to an air-water interface causes them to open up and lie on the surface with alpha-helical axes oriented parallel to the interface. In contrast, when the positive and negative charges are segregated on different helices (two negative, two positive) of the single covalent four-alpha-helix-bundle unit, the bundle preserved its integrity on transfer to the air-water interface. Moreover, the presence of heme dictates the orientation of the alpha-helical axes of the bundle with respect to the surface plane. The alpha-helices adopt a parallel orientation in the absence of heme and a perpendicular orientation in the presence of heme. Circular dichroism (CD) and ultraviolet-visible (UV-vis) spectroscopy supported by linear dichroism demonstrate that these molecular orientations are preserved in Langmuir-Blodgett monolayer films on solid substrate surfaces.