SELF-ASSEMBLED MONOLAYERS OF SYNTHETIC HEMOPROTEINS ON SILANIZED QUARTZ

To better understand the relation between structure and function of co mplex natural heme proteins, we have designed and synthesized de novo simplified versions called maquettes. Furthermore, we have assembled o rganized monolayers of these heme protein maquettes onto silanized qua rtz to mon define their structural and functional properties and to ta ke the first step in constructing robust devices that exploit biologic al chemistry. First, two di-alpha-helical peptides, alpha(2), were des igned and synthesized to self-assemble into a four-helix bundle [alpha (2)](2). The assembly has a well-developed hydrophobic interior and co ordinates iron protoporphyrin IX (heme) by bis-histidine ligation. The chemisorption of these synthetic hemoproteins onto silanized quartz w as achieved by reacting disulfide bridges in the loop region of each c l with thiols immobilized at the surface, Self-assembled monolayers of synthetic hemoproteins were characterized by UV/vis spectroscopy. UV absorption of the tryptophan reveals the presence of peptides on the s ubstrate, and circular dichroism (CD) seems to indicate that the axes of the alpha-helices are oriented at a angle less than 45 degrees rela tive to the substrate. The construction of monolayers of hemoproteins was successfully achieved in two different ways: (1) Hemes were incorp orated after the apoproteins were self-assembled onto the silanized qu artz substrate. This process led to bis-histidyl ligated hemes and to physisorbed porphyrins inside or at the surface of the monolayer, Phys isorbed hemes were removed by immersion of the film in NaCl and imidaz ole solutions, (2) Heme-containing holoproteins were prepared in solut ions before self-assembly onto silanized quartz. Linear dichroism of t he heme bis-ligated to the histidines showed an average tilt angle of the porphyrin plane of 40 degrees relative to the surface, consistent with an inclination of the whole assembly on the substrate suggested b y CD measurements. Monolayers of hemoproteins after reduction. bind CO by displacing one of the histidines. Their absorption spectra are rem arkably similar to the ones reported for the cytochrome c(3).