| Abstract
| - We have designed and synthesized four-α-helix-bundle proteins that accommodate heme groups to act asmolecular “maquettes” of more complex natural electron-transfer proteins. These bundles can be oriented atan air−water interface and transferred onto solid surfaces to facilitate the exploration of the factors thatgovern biological electron transfer. We find that the orientation of these maquettes on an air−water interfacecan be controlled by choosing the distribution of charged amino acids along the sides of the helices exposedto water. The four α-helices were assembled either as two subunits, where each subunit consists of twoα-helices linked by a terminal cysteine disulfide bond, or as a single, four-helix covalent unit consisting oftwo helix−loop−helix molecules linked by a terminal cysteine. In either case, when each α-helix containsboth positively charged lysines and negatively charged glutamates, addition of the heme binding bundles toan air−water interface causes them to open up and lie on the surface with α-helical axes oriented parallel tothe interface. In contrast, when the positive and negative charges are segregated on different helices (twonegative, two positive) of the single covalent four-α-helix-bundle unit, the bundle preserved its integrity ontransfer to the air−water interface. Moreover, the presence of heme dictates the orientation of the α-helicalaxes of the bundle with respect to the surface plane. The α-helices adopt a parallel orientation in the absenceof 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 orientationsare preserved in Langmuir−Blodgett monolayer films on solid substrate surfaces.
|
