SHG from a Single Monolayer of the RuPZn Cofactor within the 4-Helix Bundle AP0

Grazia Gonella, Venkata Krishnan, Joseph Strzalka, Andrey Tronin, Hai-Lung Dai, Michael J. Therien & J. Kent Blasie

Electron density profiles for the AP0 peptide monolayer vectorially oriented at the water-air interface in the presence and absence of the bound cofactor RuPZn. The difference profile (inset) reveals the location of the cofactor within the hydrophilic domain of the bundle.

Polarization dependent SHG from a single monolayer of the vectorially oriented RuPZn-AP0 complex on a fused silica substrate.

Extended chromophores can be designed to possess extraordinary non-linear optical properties. When incorporated into designed n-helix bundle peptides, the peptide can control the microscopic properties of the cofactor and allow for the vectorial-orientation of the peptide-cofactor complex at interfaces, including on surfaces. Together, they can provide for the translation of the cofactor’s microscopic property into a potential device application, either single-molecule based or ensemble-based. Here, we demonstrate that the design of an amphiphilic 4-helix bundle peptide AP0 can be used to vectorially incorporate the non-linear optical cofactor RuPZn, based on an ethyne-bridged zinc-porphyrin polypyridyl-ruthenium. The designed amphiphilicity of the peptide has allowed the covalent attachment of the peptide-cofactor complex to the surface of inorganic substrates, e.g., silicon. As a result, highly efficient second harmonic generation (SHG) from a monolayer ensemble of the complex has been observed, preserving the designed large hyperpolarizability of the cofactor as the incoherent sum, in the absence of undesirable interactions between cofactors.