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.

This work reports the first example of a single-chain protein computationally designed to contain four α-helical segments and fold to form a 4-helix bundle encapsulating a supramolecular abiological chromophore that possesses exceptional nonlinear optical properties. The 109-residue protein designated SCRPZ-1, 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+ (RuPZn) in aqueous buffer solution at a 1:1 stoichiometry. A 1:1 binding stoichiometry of the holoprotein is supported by electronic absorption and CD spectra, as well as equilibrium analytical ultracentrifugation and size exclusion chromatography. SCRPZ-1 readily dimerizes at μM concentrations, and an empirical redesign of the protein exterior produced a stable monomeric protein, SCRPZ-2, 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 RuPZn cofactor in organic solvent: femtosecond-, nanosecond-, and microsecond-timescale pump-probe transient absorption spectroscopic data evince intensely absorbing holoprotein excited states having large spectral bandwidth that penetrate deep in the near-infrared (NIR) energy regime; the holoprotein electronically excited triplet state exhibits a microsecond timescale lifetime characteristic of the RuPZn chromophore. Hyper-Rayleigh light scattering (HRS) measurements carried out at an incident irradiation wavelength (λinc) 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 bio-inspired materials that have unique electro-optic functionality.

We demonstrate that cyano-phenylalanine (Phe(CN)) can be utilized to probe the binding of the inhalational anesthetic halothane to an anesthetic-binding, model ion channel protein hbAP-Phe(CN). The Trp to Phe(CN) mutation alters neither the alpha-helical conformation nor the 4-helix bundle structure. The halothane binding properties of this Phe(CN) mutant hbAP-Phe(CN), based on fluorescence quenching, are consistent with those of the prototype, hbAP1. The dependence of fluorescence lifetime as a function of halothane concentration implies that the diffusion of halothane in the nonpolar core of the protein bundle is one-dimensional. As a consequence, at low halothane concentrations, the quenching of the fluorescence is dynamic, whereas at high concentrations the quenching becomes static. The 4-helix bundle structure present in aqueous detergent solution and at the air-water interface, is preserved in multilayer films of hbAP-Phe(CN), enabling vibrational spectroscopy of both the protein and its nitrile label (-CN). The nitrile groups' stretching vibration band shifts to higher frequency in the presence of halothane, and this blue-shift is largely reversible. Due to the complexity of this amphiphilic 4-helix bundle model membrane protein, where four Phe(CN) probes are present adjacent to the designed cavity forming the binding site within each bundle, all contributing to the infrared absorption, molecular dynamics (MD) simulation is required to interpret the infrared results. The MD simulations indicate that the blue-shift of -CN stretching vibration induced by halothane arises from an indirect effect, namely an induced change in the electrostatic protein environment averaged over the four probe oscillators, rather than a direct interaction with the oscillators. hbAP-Phe(CN) therefore provides a successful template for extending these investigations of the interactions of halothane with the model membrane protein via vibrational spectroscopy, using cyano-alanine residues to form the anesthetic binding cavity.

Molecular exchange kinetics between a monolayer of antibody molecules formed on the air−water interface and the protein solution was studied by means of fluorescent labeling. It was shown that there is no inclusion of dissolved molecules in the previously formed monolayer during even 6 h of exposure regardless of monolayer surface density. The surface activity of IgG and horseradish peroxidase molecules was studied by means of surface compression isotherms, and the specific biological activity of the monolayers formed from these proteins was measured by enzyme and immunoassay techniques. It was shown that the surface activity of the proteins increases while specific biological activity decreases with exposure of the molecules on the water surface. Since the same effects were caused by denaturing agents, we propose that the surface activity of the proteins and the absence of surface−volume exchange are due to partial unfolding of the molecules which takes place on the water surface. Two models of the partial unfolding are discussed:  complete denaturation of some part of the molecules and partial unfolding of each molecule. The process of surface denaturation was shown to be slow and controllable. One can achieve a pronounced increase of protein surface activity with low degradation of the specific biological activity of the monolayer; thus, it can be used in the practice of protein Langmuir film deposition.