Utilizing Chemical Raman Enhancement: A Route for Metal Oxide Support-Based Biodetection

Raman scattering enhancement was observed in systems where different metal oxide semiconductors (TiO2, Fe2O3, ZrO2, and CeO2) were modified with enediol ligands. The intensity of Raman scattering was dependent on laser frequency and correlated with the extinction coefficient of the CT complex of the enediol ligands and nanoparticles. The mechanism of Raman enhancement was studied by varying both the chemical composition of the enediol ligand and the chemical composition (and crystal structure) of the nanoparticles. We found that the intensity of the Raman signal depends on the number of surface binding sites, electron density of the ligands, and their dipole moment. Changes in chemical composition caused variations in the intensity, frequency, and number of Raman bands observed. We also showed that Raman scattering is observed for the bioconjugated system, where a peptide is linked to the surface of the particle through a catechol linker, and further investigated the potential for such a system in the development of Raman-based in vivo and in vitro biodetection, cell labeling and imaging, and nanotherapeutic strategies

Wavelength-Dependent Energy and Charge Transfer in MOF: A Step toward Artificial Porous Light-Harvesting System

Chromophore assemblies within well-defined porous coordination polymers, such as metal–organic frameworks (MOFs), can emulate the functionality of the antenna rings of chlorophylls in light-harvesting complexes (LHCs). The chemical, electronic, and structural diversities define MOFs as a promising platform where photogenerated excitons can be displaced to redox catalysts similar to the reaction center of the LHC. The precise positioning of the pigments and complementary redox units enables us to understand the charge/energy-transfer process within these crystalline solid compositions. In this study, we postsynthetically anchored tetra­phenyl­porphyrinato zinc­(II) (TPPZn)-derived complementary pigment within the 1D pores of 1,3,6,8-tetrakis­(p-benzoic­acid)­pyrene (H4TBAPy)-derived NU-1000 MOF to form a high-density donor–acceptor system. The ground- and excited-state redox potentials of the donor and acceptor were chosen to facilitate an energy transfer (EnT) from the excited MOF (i.e., NU-1000*) to TPPZn and a charge transfer (CT) from excited porphyrin (i.e., TPPZn*). Thus, the processes depend on the excitation wavelength. The energy transfer process was spectroscopically probed by excitation–emission mapping: MOF emission was completely quenched at 460 nm, where the pyrene-centered emission was expected. Instead, the excited MOF efficiently transfers the energy to manifest a TPPZn-centered emission at 670 nm (kEnT ≈ 4.7 × 1011 s–1). The excited TPPZn pigment, with a neighboring TBAPy linker, forms an artificial “special-pair”-like system driving the charge-separation process (kCT = 1.2 × 1010 s–1). The findings demonstrate a synthetic MOF-based artificial LHC system where their well-defined structure will open up new possibilities as the separated charge can hop along the 1D pore channel for further mechanistic understanding and future developments

Diverse Bilayer Morphologies Achieved via α‑Helix-to-β-Sheet Transitions in a Short Amphiphilic Peptide

Transmembrane proteins are functional macromolecules that direct the flow of small molecules and ions across a lipid bilayer. Here, we propose the development of helical peptide amphiphiles that will serve as both the bilayer and the functional unit of a self-assembled peptide bilayer membrane. The peptide, K3L12, was designed not only to possess dimensions similar to that of a lipid bilayer but also to yield a structurally robust, α-helical bilayer. The formation of α-helices is pH-dependent, and upon annealing the sample, a transition from α-helices to β-sheets can be controlled, as indicated by optical and vibrational spectroscopies. Imaging the materials confirms morphologies similar to that of a lipid bilayer but rich in α-helices. Annealing the samples yields a shift in the morphology from bilayers to curled disks, fibers, and sheets. The structural robustness of the material can facilitate the incorporation of many functions into the bilayer assembly

The Rate of O₂ and CO Binding to a Copper Complex, Determined by a “Flash-and-Trap” Technique, Exceeds that for Hemes

The observation and fast time-scale kinetic determination of a primary dioxygen−copper interaction have been studied. The ability to photorelease carbon monoxide from [CuI(tmpa)(CO)]+ in mixtures of CO and O2 in tetrahydrofuran (THF) between 188 and 218 K results in the observable formation of a copper-superoxide species, [CuII(tmpa)(O2-)]+ λmax = 425 nm. Via this “flash-and-trap” technique, temperature-dependent kinetic studies on the forward reaction between dioxygen and [CuI(tmpa)(thf)]+ afford activation parameters ΔH⧧ = 7.62 kJ/mol and ΔS⧧ = −45.1 J/mol K. The corresponding reverse reaction proceeds with ΔH⧧ = 58.0 kJ/mol and ΔS⧧ = 105 J/mol K. Overall thermodynamic parameters are ΔH° = −48.5 kJ/mol and ΔS° = −140 J/mol K. The temperature-dependent data allowed us to determine the room-temperature second-order rate constant, kO2 = 1.3 × 109 M-1 s-1. Comparisons to copper and heme proteins and synthetic complexes are discussed

Computational Design and Elaboration of a <i>de Novo</i> Heterotetrameric α-Helical Protein That Selectively Binds an Emissive Abiological (Porphinato)zinc Chromophore

The first example of a computationally de novo designed protein that binds an emissive abiological chromohore is presented, in which a sophisticated level of cofactor discrimination is pre-engineered. This heterotetrameric, C2-symmetric bundle, AHis:BThr, uniquely binds (5,15-di[(4-carboxymethyleneoxy)phenyl]porphinato)zinc [(DPP)Zn] via histidine coordination and complementary noncovalent interactions. The A2B2 heterotetrameric protein reflects ligand-directed elements of both positive and negative design, including hydrogen bonds to second-shell ligands. Experimental support for the appropriate formulation of [(DPP)Zn:AHis:BThr]2 is provided by UV/visible and circular dichroism spectroscopies, size exclusion chromatography, and analytical ultracentrifugation. Time-resolved transient absorption and fluorescence spectroscopic data reveal classic excited-state singlet and triplet PZn photophysics for the AHis:BThr:(DPP)Zn protein (kfluorescence = 4 × 108 s−1; τtriplet = 5 ms). The A2B2 apoprotein has immeasurably low binding affinities for related [porphinato]metal chromophores that include a (DPP)Fe(III) cofactor and the zinc metal ion hemin derivative [(PPIX)Zn], underscoring the exquisite active-site binding discrimination realized in this computationally designed protein. Importantly, elements of design in the AHis:BThr protein ensure that interactions within the tetra-α-helical bundle are such that only the heterotetramer is stable in solution; corresponding homomeric bundles present unfavorable ligand-binding environments and thus preclude protein structural rearrangements that could lead to binding of (porphinato)iron cofactors

Photochemical Organic Oxidations and Dechlorinations with a μ-Oxo Bridged Heme/Non-Heme Diiron Complex

Steady state and laser flash photolysis studies of the heme/non-heme μ-oxo diiron complex [(6L)FeIIIOFeIIICl]+ (1) have been undertaken. The anaerobic photolysis of benzene solutions of 1 did not result in the buildup of any photoproduct. However, the addition of excess triphenylphosphine resulted in the quantitative photoreduction of 1 to [(6L)FeII···FeIICl]+ (2), with concomitant production by oxo-transfer of 1 equiv of triphenylphosphine oxide. Under aerobic conditions and excess triphenylphosphine, the reaction produces multiple turnovers (∼28) before the diiron complex is degraded. The anaerobic photolysis of tetrahydrofuran (THF) or toluene solutions of 1 likewise results in the buildup of 2. The oxidation products from these reactions included γ-butyrolactone (∼15%) for the reaction in THF and benzaldehyde (∼23%) from the reaction in toluene. In either case, the O-atom which is incorporated into the carbonyl product is derived from dioxygen present under workup or under aerobic photolysis conditions. Transient absorption measurements of low-temperature THF solutions of 1 revealed the presence of an (P)FeII-like {P = tetraaryl porphyrinate dianion} species suggesting that the reactive species is a formal (heme)FeII/FeIVO(non-heme) pair. The non-heme FeIVO is thus most likely responsible for CH bond cleavage and subsequent radical chemistry. The photolysis of 1 in chlorobenzene or 1,2-dichlorobenzene resulted in C−Cl cleavage reactions and the formation of {[(6L)FeIIICl···FeIIICl]2O}2+ (3), with chloride ligands that are derived from solvent dehalogenation chemistry. The resulting organic products are biphenyl trichlorides or biphenyl monochlorides, derived from dichlorobenzene and chlorobenzene, respectively. Similarly, product 3 is obtained by the photolysis of benzene−benzyl chloride solutions of 1; the organic product is benzaldehyde (∼70%). A brief discussion of the dehalogenation chemistry, along with relevant environmental perspectives, is included

Efficient Photodissociation of O₂ from Synthetic Heme and Heme/M (M = Fe, Cu) Complexes

Single wavelength excitation (λex = 355 or 532 nm) of low-temperature stabilized (198 K) synthetic heme−dioxygen and heme−dioxygen/M complexes, where M = copper or iron in a non-heme environment, results in the dissociation of dioxygen as indicated by the generation of the ferrous heme (Soret band, 427 nm) and the bleaching of the ferric-superoxide (FeIII(O2-)) 410-nm Soret band in the transient absorption difference spectrum. Dioxygen rebinds to the four heme complexes studied with comparable rate constants (∼6−9 × 105 M-1 s-1). However, the quantum yield for complete dissociation of O2 from our simplest heme−O2 complex (F8)FeIII(O2-) (φ = 0.60) is higher than the other complexes measured (φ = ∼0.2−0.3) as well as that for oxy-myoglobin (φ = 0.3)

Carbon Monoxide Coordination and Reversible Photodissociation in Copper(I) Pyridylalkylamine Compounds

Systematic studies of CO coordination and photodissociation have been carried out for a series of copper(I) carbonyl compounds possessing tripodal tetradentate ligands, [CuI(L)(CO)]B(C6F5)4 (L = Me2N-TMPA (1Me2N), MeO-TMPA (1MeO), H-TMPA (1H), PMEA (2pmea), PMAP (2pmap), BQPA (3bqpa). Detailed structural, electrochemical, and infrared characterization has been accomplished. In addition, various experimental techniques were utilized to determine equilibrium binding constants (KCO), association (kCO), and dissociation (k-CO) rate constants, as well as the thermodynamic (ΔH°, ΔS°) and activation parameters (ΔH⧧, ΔS⧧) that regulate these processes. With increased ligand-electron-donating ability, greater π back-bonding results in stronger Cu−CO bonds, leading to KCO values on the order 1Me2N−CO > 1MeO−CO > 1H−CO. With systematic synthetic expansion of the five-membered chelate rings like 1R to six-membered chelate rings like 2R, the stability of the CO adduct decreases, 1H−CO > 2pmea−CO > 2pmap−CO. The CO-binding properties of 3bqpa did not follow trends observed for the other compounds, presumably because of its bulkier ligand framework. Through solid- and solution-state analyses, we concluded that the photolabile carbonyl species in solution possess a tridentate coordination mode, forming strictly five-membered chelate rings to the copper ion with one dangling arm of the tripodal ligand. Carbon monoxide reversibly photodissociated from complexes 1Me2N−CO, 1MeO−CO, 1H−CO, and 3bqpa−CO in coordinating (CH3CN) and weakly coordinating (THF) solvent but not from 2pmea−CO and 2pmap−CO. Comparisons to O2-binding data available for these copper complexes as well as to small molecule (O2, CO, NO) reactions with hemes and copper proteins are discussed

Carbon Monoxide Coordination and Reversible Photodissociation in Copper(I) Pyridylalkylamine Compounds

Systematic studies of CO coordination and photodissociation have been carried out for a series of copper(I) carbonyl compounds possessing tripodal tetradentate ligands, [CuI(L)(CO)]B(C6F5)4 (L = Me2N-TMPA (1Me2N), MeO-TMPA (1MeO), H-TMPA (1H), PMEA (2pmea), PMAP (2pmap), BQPA (3bqpa). Detailed structural, electrochemical, and infrared characterization has been accomplished. In addition, various experimental techniques were utilized to determine equilibrium binding constants (KCO), association (kCO), and dissociation (k-CO) rate constants, as well as the thermodynamic (ΔH°, ΔS°) and activation parameters (ΔH⧧, ΔS⧧) that regulate these processes. With increased ligand-electron-donating ability, greater π back-bonding results in stronger Cu−CO bonds, leading to KCO values on the order 1Me2N−CO > 1MeO−CO > 1H−CO. With systematic synthetic expansion of the five-membered chelate rings like 1R to six-membered chelate rings like 2R, the stability of the CO adduct decreases, 1H−CO > 2pmea−CO > 2pmap−CO. The CO-binding properties of 3bqpa did not follow trends observed for the other compounds, presumably because of its bulkier ligand framework. Through solid- and solution-state analyses, we concluded that the photolabile carbonyl species in solution possess a tridentate coordination mode, forming strictly five-membered chelate rings to the copper ion with one dangling arm of the tripodal ligand. Carbon monoxide reversibly photodissociated from complexes 1Me2N−CO, 1MeO−CO, 1H−CO, and 3bqpa−CO in coordinating (CH3CN) and weakly coordinating (THF) solvent but not from 2pmea−CO and 2pmap−CO. Comparisons to O2-binding data available for these copper complexes as well as to small molecule (O2, CO, NO) reactions with hemes and copper proteins are discussed

Carbon Monoxide Coordination and Reversible Photodissociation in Copper(I) Pyridylalkylamine Compounds

Systematic studies of CO coordination and photodissociation have been carried out for a series of copper(I) carbonyl compounds possessing tripodal tetradentate ligands, [CuI(L)(CO)]B(C6F5)4 (L = Me2N-TMPA (1Me2N), MeO-TMPA (1MeO), H-TMPA (1H), PMEA (2pmea), PMAP (2pmap), BQPA (3bqpa). Detailed structural, electrochemical, and infrared characterization has been accomplished. In addition, various experimental techniques were utilized to determine equilibrium binding constants (KCO), association (kCO), and dissociation (k-CO) rate constants, as well as the thermodynamic (ΔH°, ΔS°) and activation parameters (ΔH⧧, ΔS⧧) that regulate these processes. With increased ligand-electron-donating ability, greater π back-bonding results in stronger Cu−CO bonds, leading to KCO values on the order 1Me2N−CO > 1MeO−CO > 1H−CO. With systematic synthetic expansion of the five-membered chelate rings like 1R to six-membered chelate rings like 2R, the stability of the CO adduct decreases, 1H−CO > 2pmea−CO > 2pmap−CO. The CO-binding properties of 3bqpa did not follow trends observed for the other compounds, presumably because of its bulkier ligand framework. Through solid- and solution-state analyses, we concluded that the photolabile carbonyl species in solution possess a tridentate coordination mode, forming strictly five-membered chelate rings to the copper ion with one dangling arm of the tripodal ligand. Carbon monoxide reversibly photodissociated from complexes 1Me2N−CO, 1MeO−CO, 1H−CO, and 3bqpa−CO in coordinating (CH3CN) and weakly coordinating (THF) solvent but not from 2pmea−CO and 2pmap−CO. Comparisons to O2-binding data available for these copper complexes as well as to small molecule (O2, CO, NO) reactions with hemes and copper proteins are discussed