Ethyne-Bridged (Porphinato)Zinc(II)−(Porphinato)Iron(III) Complexes: Phenomenological Dependence of Excited-State Dynamics upon (Porphinato)Iron Electronic Structure

We report the synthesis, spectroscopy, potentiometric properties, and excited-state dynamical studies of 5-[(10,20-di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]zinc(II)−[5‘-[(10‘,20‘- di-((4-ethyl ester)methylene-oxy)phenyl)porphinato]iron(III)-chloride]ethyne (PZn−PFe-Cl), along with a series of related supermolecules ([PZn−PFe-(L)1,2]+ species) that possess a range of metal axial ligation environments (L = pyridine, 4-cyanopyridine, 2,4,6-trimethylpyridine (collidine), and 2,6-dimethylpyridine (2,6-lutidine)). Relevant monomeric [(porphinato)iron-(ligand)1,2]+ ([PFe(L)1,2]+) benchmarks have also been synthesized and fully characterized. Ultrafast pump−probe transient absorption spectroscopic experiments that interrogate the initially prepared electronically excited states of [PFe(L)1,2]+ species bearing nonhindered axial ligands demonstrated subpicosecond-to-picosecond relaxation dynamics to the ground electronic state. Comparative pump−probe transient absorption experiments that interrogate the initially prepared excited states of PZn−PFe-Cl, [PZn−PFe-(py)2]+, [PZn−PFe-(4-CN-py)2]+, [PZn−PFe-(collidine)]+, and [PZn−PFe-(2,6-lutidine)]+ demonstrate that the spectra of all these species are dominated by a broad, intense NIR S1 → Sn transient absorption manifold. While PZn−PFe-Cl*, [PZn−PFe-(py)2]+*, and [PZn−PFe-(4-CN-py)2]+* evince subpicosecond and picosecond time-scale relaxation of their respective initially prepared electronically excited states to the ground state, the excited-state dynamics observed for [PZn−PFe-(2,6-lutidine)]+* and [PZn−PFe-(collidine)]+* show fast relaxation to a [PZn+−PFe(II)] charge-separated state having a lifetime of nearly 1 ns. Potentiometric data indicate that while ΔGCS for [PZn−PFe-(L)1,2]+* species is strongly influenced by the PFe+ ligation state [ligand (ΔGCS):  4-cyanopyridine (−0.79 eV) < pyridine (−1.04 eV) 2Cl2], the pump−probe transient absorption dynamical data demonstrate that the nature of the dominant excited-state decay pathway is not correlated with the thermodynamic driving force for photoinduced charge separation, but depends on the ferric ion ligation mode. These data indicate that sterically bulky axial ligands that drive a pentacoordinate PFe center and a weak metal axial ligand interaction serve to sufficiently suppress the normally large magnitude nonradiative decay rate constants characteristic of (porphinato)iron(III) complexes, and thus make electron transfer a competitive excited-state deactivation pathway

Computational De Novo Design and Characterization of a Four-Helix Bundle Protein that Selectively Binds a Nonbiological Cofactor

We report the complete de novo design of a four-helix bundle protein that selectively binds the nonbiological DPP−Fe(III) metalloporphyrin cofactor (DPP−Fe(III) = 5, 15-Di[(4-carboxymethyleneoxy)phenyl]porphinato iron(III)). A tetrameric, D2-symmetric backbone scaffold was constructed to encapsulate two DPP−Fe(III) units through bis(His) coordination. The complete sequence was determined with the aid of the statistical computational design algorithm SCADS. The 34-residue peptide was chemically synthesized. UV−vis and CD spectroscopy, size-exclusion chromatography, and analytical ultracentrifugation indicated the peptide undergoes a transition from a predominantly random coil monomer to an α-helical tetramer upon binding DPP−Fe(III). EPR spectroscopy studies indicated the axial imidazole ligands were oriented in a perpendicular fashion, as defined by second-shell interactions that were included in the design. The 1-D 1H NMR spectrum of the assembled protein displayed features of a well-packed interior. The assembled protein possessed functional redox properties different from those of structurally similar systems containing the heme cofactor. The designed peptide demonstrated remarkable cofactor selectivity with a significantly weaker binding affinity for the natural heme cofactor. These findings open a path for the selective incorporation of more elaborate cofactors into designed scaffolds for constructing molecularly well-defined nanoscale materials

Computational De Novo Design and Characterization of a Four-Helix Bundle Protein that Selectively Binds a Nonbiological Cofactor [<i>J. </i><i>Am. Chem. Soc.</i> <b>2005</b>, <i>127</i>, 1346−1347].

Computational De Novo Design and Characterization of a Four-Helix Bundle Protein that Selectively Binds a Nonbiological Cofactor [J. Am. Chem. Soc. 2005, 127, 1346−1347]

Broad Spectral Domain Fluorescence Wavelength Modulation of Visible and Near-Infrared Emissive Polymersomes

Incorporation of an extended family of multi[(porphinato)zinc(II)] (PZn)-based supermolecular fluorophores into the lamellar membranes of polymersomes (50 nm to 50 μm diameter polymer vesicles) gives rise to electrooptically diverse nano-to-micron (meso) scale soft materials. Studies that examine homogeneous suspensions of 100 nm diameter emissive polymersomes demonstrate fluorescence energy modulation over a broad spectral domain of the visible and near-infrared (600−900 nm). These polymersomal structures highlight that the nature of intermembranous polymer-to-fluorophore contacts depends on the position and identity of the porphyrins' phenyl ring substituents. Emissive polymersomes are shown to possess reduced spectral heterogeneity with respect to the established optical signatures of these PZn-based supermolecular fluorophores in solution; additionally, selection of fluorophore ancillary substituents predictably controls the nature of polymer−emitter noncovalent interactions to provide an important additional mechanism to further modulate the fluorescence band maxima of these meso-scale emissive vesicles

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

Amphiphilic Four-Helix Bundle Peptides Designed for Light-Induced Electron Transfer Across a Soft Interface

A family of four-helix bundle peptides were designed to be amphiphilic, possessing distinct hydrophilic and hydrophobic domains along the length of the bundle's exterior. This facilitates their vectorial insertion across a soft interface between polar and nonpolar media. Their design also now provides for selective incorporation of electron donor and acceptor cofactors within each domain. This allows translation of the designed intramolecular electron transfer along the bundle axis into a macroscopic charge separation across the interface