Control of the Orientational Order and Nonlinear Optical Response of the “Push-Pull” Chromophore RuPZn via Specific Incorporation into Densely Packed Monolayer Ensembles of an Amphiphilic Four-Helix Bundle Peptide: Characterization of the Peptide−Chromophore Complexes

“Push-pull” chromophores based on extended π-electron systems have been designed to exhibit exceptionally large molecular hyperpolarizabilities. We have engineered an amphiphilic four-helix bundle peptide to vectorially incorporate such hyperpolarizable chromophores having a metalloporphyrin moiety, with high specificity into the interior core of the bundle. The amphiphilic exterior of the bundle facilitates the formation of densely packed monolayer ensembles of the vectorially oriented peptide−chromophore complexes at the liquid−gas interface. Chemical specificity designed into the ends of the bundle facilitates the subsequent covalent attachment of these monolayer ensembles onto the surface of an inorganic substrate. In this article, we describe the structural characterization of these monolayer ensembles at each stage of their fabrication for one such peptide−chromophore complex designated as AP0-RuPZn. In the accompanying article, we describe the characterization of their macroscopic nonlinear optical properties

Control of the Orientational Order and Nonlinear Optical Response of the “Push−Pull” Chromophore RuPZn via Specific Incorporation into Densely Packed Monolayer Ensembles of an Amphiphilic 4-Helix Bundle Peptide: Second Harmonic Generation at High Chromophore Densities

The macroscopic nonlinear optical response of the “push−pull” chromophore RuPZn incorporated into a single monolayer of the amphiphilic 4-helix bundle peptide (AP0) covalently attached to a solid substrate at high in-plane density has been measured. The second-order susceptibility, χzzz, was found to be in the range of ∼15 × 10−9 esu, consistent with a coherent sum of the nonlinear contributions from the individual chromophores (β⃡) as previously measured in isotropic solution through hyper-Rayleigh scattering as well as estimated from theoretical calculations. The microscopic hyperpolarizability of the RuPZn chromophore is preserved upon incorporation into the peptide monolayer, suggesting that the chromophore−chromophore interactions in the densely packed ensemble do not substantially affect the first-order molecular hyperpolarizability. The polarization angle dependence of the second harmonic signal reveals that the chromophore is vectorially oriented in the two-dimensional ensemble. Analysis of the order parameter together with information obtained from grazing incidence X-ray diffraction help in determining the chromophore orientation within the AP0−RuPZn monolayer. Taking into account an average pitch angle of ∼20° characterizing the coiled-coil structure of the peptide bundle, the width of the bundle’s tilt angle distribution should be σ ≤ 20°, resulting in a mean value of the tilt angle 23° ≤ θ0 ≤ 37°

Structural Studies of Amphiphilic 4-Helix Bundle Peptides Incorporating Designed Extended Chromophores for Nonlinear Optical Biomolecular Materials

Extended conjugated chromophores containing (porphinato)zinc components that exhibit large optical polarizabilities and hyperpolarizabiliites are incorporated into amphiphilic 4-helix bundle peptides via specific axial histidyl ligation of the metal. The bundle's designed amphiphilicity enables vectorial orientation of the chromophore/peptide complex in macroscopic monolayer ensembles. The 4-helix bundle structure is maintained upon incorporation of two different chromophores at stoichiometries of 1−2 per bundle. The axial ligation site appears to effectively control the position of the chromophore along the length of the bundle

Acentric 2-D Ensembles of D-br-A Electron-Transfer Chromophores via Vectorial Orientation within Amphiphilic <i>n</i>-Helix Bundle Peptides for Photovoltaic Device Applications

We show that simply designed amphiphilic 4-helix bundle peptides can be utilized to vectorially orient a linearly extended donor–bridge–acceptor (D-br-A) electron transfer (ET) chromophore within its core. The bundle’s interior is shown to provide a unique solvation environment for the D-br-A assembly not accessible in conventional solvents and thereby control the magnitudes of both light-induced ET and thermal charge recombination rate constants. The amphiphilicity of the bundle’s exterior was employed to vectorially orient the peptide–chromophore complex at a liquid–gas interface, and its ends were tailored for subsequent covalent attachment to an inorganic surface, via a “directed assembly” approach. Structural data, combined with evaluation of the excited state dynamics exhibited by these peptide–chromophore complexes, demonstrate that densely packed, acentrically ordered 2-D monolayer ensembles of such complexes at high in-plane chromophore densities approaching 1/200 Ų offer unique potential as active layers in binary heterojunction photovoltaic devices

Computational de Novo Design and Characterization of a Protein That Selectively Binds a Highly Hyperpolarizable Abiological Chromophore

This work reports the first example of a single-chain protein computationally designed to contain four α-helical segments and fold to form a four-helix bundle encapsulating a supramolecular abiological chromophore that possesses exceptional nonlinear optical properties. The 109-residue protein, designated <i><b>SCRPZ-1</b></i>, binds and disperses an insoluble hyperpolarizable chromophore, ruthenium­(II) [5-(4′-ethynyl-(2,2′;6′,2″-terpyridinyl))-10,20-bis­(phenyl)­porphinato]­zinc­(II)-(2,2′;6′,2″-terpyridine)²⁺ (<i><b>RuPZn</b></i>) in aqueous buffer solution at a 1:1 stoichiometry. A 1:1 binding stoichiometry of the holoprotein is supported by electronic absorption and circular dichroism spectra, as well as equilibrium analytical ultracentrifugation and size exclusion chromatography. <i><b>SCRPZ-1</b></i> readily dimerizes at micromolar concentrations, and an empirical redesign of the protein exterior produced a stable monomeric protein, <i><b>SCRPZ-2</b></i>, that also displayed a 1:1 protein:cofactor stoichiometry. For both proteins in aqueous buffer, the encapsulated cofactor displays photophysical properties resembling those exhibited by the dilute <i><b>RuPZn</b></i> cofactor in organic solvent: femtosecond, nanosecond, and microsecond time scale pump–probe transient absorption spectroscopic data evince intensely absorbing holoprotein excited states having large spectral bandwidth that penetrate deep in the near-infrared energy regime; the holoprotein electronically excited triplet state exhibits a microsecond time scale lifetime characteristic of the <i><b>RuPZn</b></i> chromophore. Hyper-Rayleigh light scattering measurements carried out at an incident irradiation wavelength of 1340 nm for these holoproteins demonstrate an exceptional dynamic hyperpolarizabilty (β₁₃₄₀ = 3100 × 10^–30 esu). X-ray reflectivity measurements establish that this de novo-designed hyperpolarizable protein can be covalently attached with high surface density to a silicon surface without loss of the cofactor, indicating that these assemblies provide a new approach to bioinspired materials that have unique electro-optic functionality