Accelerating slow excited state proton transfer

Visible light excitation of the ligand-bridged assembly [(bpy)₂Ru_a^II(L)Ru_b^II(bpy)(OH₂)⁴⁺] (bpy is 2,2′-bipyridine; L is the bridging ligand, 4-phen-tpy) results in emission from the lowest energy, bridge-based metal-to-ligand charge transfer excited state (L^−•)Ru_b^III-OH₂ with an excited-state lifetime of 13 ± 1 ns. Near–diffusion-controlled quenching of the emission occurs with added HPO₄²⁻ and partial quenching by added acetate anion (OAc⁻) in buffered solutions with pH control. A Stern–Volmer analysis of quenching by OAc⁻ gave a quenching rate constant of k_q = 4.1 × 10⁸ M⁻¹⋅s⁻¹ and an estimated pK_a* value of ∼5 ± 1 for the [(bpy)₂Ru_a^II(L^•−)Ru_b^III(bpy)(OH₂)⁴⁺]* excited state. Following proton loss and rapid excited-state decay to give [(bpy)₂Ru_a^II(L)Ru_b^II(bpy)(OH)³⁺] in a H₂PO₄⁻/HPO₄²⁻ buffer, back proton transfer occurs from H₂PO₄⁻ to give [(bpy)₂Ru_a^II(L)Ru_b(bpy)(OH₂)⁴⁺] with k_PT,2 = 4.4 × 10⁸ M⁻¹⋅s⁻¹. From the intercept of a plot of k_obs vs. [H₂PO₄⁻], k = 2.1 × 10⁶ s⁻¹ for reprotonation by water providing a dramatic illustration of kinetically limiting, slow proton transfer for acids and bases with pK_a values intermediate between pK_a(H₃O⁺) = −1.74 and pK_a(H₂O) = 15.7.     

Mechanism of and exquisite selectivity for O-O bond formation by the heme-dependent chlorite dismutase

Chlorite dismutase (Cld) is a heme b-dependent, O–O bond forming enzyme that transforms toxic chlorite (ClO₂⁻) into innocuous chloride and molecular oxygen. The mechanism and specificity of the reaction with chlorite and alternate oxidants were investigated. Chlorite is the sole source of dioxygen as determined by oxygen-18 labeling studies. Based on ion chromatography and mass spectrometry results, Cld is highly specific for the dismutation of chlorite to chloride and dioxygen with no other side products. Cld does not use chlorite as an oxidant for oxygen atom transfer and halogenation reactions (using cosubstrates guaiacol, thioanisole, and monochlorodimedone, respectively). When peracetic acid or H₂O₂ was used as an alternative oxidant, oxidation and oxygen atom transfer but not halogenation reactions occurred. Monitoring the reaction of Cld with peracetic acid by rapid-mixing UV-visible spectroscopy, the formation of the high valent compound I intermediate, [(Por^•+)Fe^IV = O], was observed [k₁ = (1.28 ± 0.04) × 10⁶ M⁻¹ s⁻¹]. Compound I readily decayed to form compound II in a manner that is independent of peracetic acid concentration (k₂ = 170 ± 20 s⁻¹). Both compound I and a compound II-associated tryptophanyl radical that resembles cytochrome c peroxidase (Ccp) compound I were observed by EPR under freeze-quench conditions. The data collectively suggest an O–O bond-forming mechanism involving generation of a compound I intermediate via oxygen atom transfer from chlorite, and subsequent recombination of the resulting hypochlorite and compound I.     

Energy transfer to a proton-transfer fluorescence probe: Tryptophan to a flavonol in human serum albumin

A protein fluorescence probe system, coupling excited-state intermolecular Förster energy transfer and intramolecular proton transfer (PT), is presented. As an energy donor for this system, we used tryptophan, which transfers its excitation energy to 3-hydroxyflavone (3-HF) as a flavonol prototype, an acceptor exhibiting excited-state intramolecular PT. We demonstrate such a coupling in human serum albumin–3-HF complexes, excited via the single intrinsic tryptophan (Trp-214). Besides the PT tautomer fluorescence (λ_max = 526 nm), these protein–probe complexes exhibit a 3-HF anion emission (λ_max = 500 nm). Analysis of spectroscopic data leads to the conclusion that two binding sites are involved in the human serum albumin–3-HF interaction. The 3-HF molecule bound in the higher affinity binding site, located in the IIIA subdomain, has the association constant (k₁) of 7.2 × 10⁵ M⁻¹ and predominantly exists as an anion. The lower affinity site (k₂ = 2.5 × 10⁵ M⁻¹), situated in the IIA subdomain, is occupied by the neutral form of 3-HF (normal tautomer). Since Trp-214 is situated in the immediate vicinity of the 3-HF normal tautomer bound in the IIA subdomain, the intermolecular energy transfer for this donor/acceptor pair has a 100% efficiency and is followed by the PT tautomer fluorescence. Intermolecular energy transfer from the Trp-214 to the 3-HF anion bound in the IIIA subdomain is less efficient and has the rate of 1.61 × 10⁸ s⁻¹, thus giving for the donor/acceptor distance a value of 25.5 Å.     

Fast events in protein folding: Relaxation dynamics of secondary and tertiary structure in native apomyoglobin

We report the fast relaxation dynamics of “native” apomyoglobin (pH 5.3) following a 10-ns, laser-induced temperature jump. The structural dynamics are probed using time-resolved infrared spectroscopy. The infrared kinetics monitored within the amide I absorbance of the polypeptide backbone exhibit two distinct relaxation phases which have different spectral signatures and occur on very different time scales (ν = 1633 cm⁻¹, τ = 48 ns; ν = 1650 cm⁻¹, τ = 132 μs). We assign these two spectral components to discrete substructures in the protein: helical structure that is solvated (1633 cm⁻¹) and native helix that is protected from solvation by interhelix tertiary interactions (1650 cm⁻¹). Folding rate coefficients inferred from the observed relaxations at 60°C are k_f(solvated) = (7 to 20) × 10⁶ s⁻¹ and k_f(native) = 3.6 × 10³ s⁻¹, respectively. The faster rate is interpreted as the intrinsic rate of solvated helix formation, whereas the slower rate is interpreted as the rate of formation of tertiary contacts that determine a native helix. Thus, at 60°C helix formation precedes the formation of tertiary structure by over three orders of magnitude in this protein. Furthermore, the distinct thermodynamics and kinetics observed for the apomyoglobin substructures suggest that they fold independently, or quasi-independently. The observation of inhomogeneous folding for apomyoglobin is remarkable, given the relatively small size and structural simplicity of this protein.     

Reactive and nonreactive quenching of O(1D) by the potent greenhouse gases SO2F2, NF3, and SF5CF3

A laser flash photolysis–resonance fluorescence technique has been employed to measure rate coefficients and physical vs. reactive quenching branching ratios for O(¹D) deactivation by three potent greenhouse gases, SO₂F₂(k₁), NF₃(k₂), and SF₅CF₃(k₃). In excellent agreement with one published study, we find that k₁(T) = 9.0 × 10^-11 exp(+98/T) cm³ molecule^-1 s^-1 and that the reactive quenching rate coefficient is k_1b = (5.8 ± 2.3) × 10^-11 cm³ molecule^-1 s^-1 independent of temperature. We find that k₂(T) = 2.0 × 10^-11 exp(+52/T) cm³ molecule^-1 s^-1 with reaction proceeding almost entirely (∼99%) by reactive quenching. Reactive quenching of O(¹D) by NF₃ is more than a factor of two faster than reported in one published study, a result that will significantly lower the model-derived atmospheric lifetime and global warming potential of NF₃. Deactivation of O(¹D) by SF₅CF₃ is slow enough (k₃ < 2.0 × 10^-13 cm³ molecule^-1 s^-1 at 298 K) that reaction with O(¹D) is unimportant as an atmospheric removal mechanism for SF₅CF₃. The kinetics of O(¹D) reactions with SO₂ (k₄) and CS₂ (k₅) have also been investigated at 298 K. We find that k₄ = (2.2 ± 0.3) × 10^-10 and k₅ = (4.6 ± 0.6) × 10^-10 cm³ molecule^-1 s^-1; branching ratios for reactive quenching are 0.76 ± 0.12 and 0.94 ± 0.06 for the SO₂ and CS₂ reactions, respectively. All uncertainties reported above are estimates of accuracy (2σ) and rate coefficients k_i(T) (i = 1,2) calculated from the above Arrhenius expressions have estimated accuracies of ± 15% (2σ).     

Giant circular dichroism of high molecular weight chlorophyllide-apomyoglobin complexes

Chlorophyllide a and the apoprotein of myoglobin (Mb) spontaneously form three types of complex. The M (M_r ≈ 3 × 10⁵) and H (M_r ≥ 4 × 10⁶) complexes, but not the L (M_r ≈ 1.7 × 10⁴), display a circular dichroism (CD) spectrum that is highly red-shifted, nonconservative, and very intense—characteristics shared by the CD spectra of reaction center complexes from purple photosynthetic bacteria. At its 710-nm peak, the H complex CD spectrum has a larger magnitude, 0.06 differential absorbance per unit total absorbance, than has been reported for chlorophyll in any medium.     

Unliganded gating of acetylcholine receptor channels

We estimated the unliganded opening and closing rate constants of neuromuscular acetylcholine receptor-channels (AChRs) having mutations that increased the gating equilibrium constant. For some mutant combinations, spontaneous openings occurred in clusters. For 25 different constructs, the unliganded gating equilibrium constant (E₀) was correlated with the product of the predicted fold-increase in the diliganded gating equilibrium constant caused by each mutation alone. We estimate that (i) E₀ for mouse, wild-type α₂βδε AChRs is ≈1.15 × 10⁻⁷; (ii) unliganded AChRs open for ≈80 μs, once every ≈15 min; (iii) the affinity for ACh of the O(pen) conformation is ≈10 nM, or ≈15,600 times greater than for the C(losed) conformation; (iv) the ACh-monoliganded gating equilibrium constant is ≈1.7 × 10⁻³; (v) the C→O isomerization reduces substantially ACh dissociation, but only slightly increases association; and (vi) ACh provides only ≈0.9 k_BT more binding energy per site than carbamylcholine but ≈3.1 k_BT more than choline, mainly because of a low O conformation affinity. Most mutations of binding site residue αW149 increase E₀. We estimate that the mutation αW149F reduces the ACh affinity of C only by 13-fold, but of O by 190-fold. Rate–equilibrium free-energy relationships for different regions of the protein show similar slopes (Φ values) for un- vs. diliganded gating, which suggests that the conformational pathway of the gating structural change is fundamentally the same with and without agonists. Agonist binding is a perturbation that (like most mutations) changes the energy, but not the mechanism, of the gating conformational change.     

Importance of trivalency and the eg1 configuration in the photocatalytic oxidation of water by Mn and Co oxides

Prompted by the early results on the catalytic activity of LiMn₂O₄ and related oxides in the photochemical oxidation of water, our detailed study of several manganese oxides has shown that trivalency of Mn is an important factor in determining the catalytic activity. Thus, Mn₂O₃, LaMnO₃, and MgMn₂O₄ are found to be very good catalysts with turnover frequencies of 5 × 10⁻⁴ s⁻¹, 4.8 × 10⁻⁴ s⁻¹, and 0.8 ×10⁻⁴ s⁻¹, respectively. Among the cobalt oxides, Li₂Co₂O₄ and LaCoO₃—especially the latter—exhibit excellent catalytic activity, with the turnover frequencies being 9 × 10⁻⁴ s⁻¹ and 1.4 × 10⁻³ s⁻¹, respectively. The common feature among the catalytic Mn and Co oxides is not only that Mn and Co are in the trivalent state, but Co³⁺ in the Co oxides is in the intermediate t_2g⁵e_g¹ state whereas Mn³⁺ is in the t_2g³e_g¹ state. The presence of the e_g¹ electron in these Mn and Co oxides is considered to play a crucial role in the photocatalytic properties of the oxides.     

A streptavidin mutant with altered ligand-binding specificity

The biotin-binding site of streptavidin was modified to alter its ligand-binding specificity. In natural streptavidin, the side chains of N23 and S27 make two of the three hydrogen bonds with the ureido oxygen of biotin. These two residues were mutated to severely weaken biotin binding while attempting to maintain the affinity for two biotin analogs, 2-iminobiotin and diaminobiotin. Redesigning of the biotin-binding site used the difference in local electrostatic charge distribution between biotin and these biotin analogs. Free energy calculations predicted that the introduction of a negative charge at the position of S27 plus the mutation N23A should disrupt two of the three hydrogen bonds between natural streptavidin and the ureido oxygen of biotin. In contrast, the imino hydrogen of 2-iminobiotin should form a hydrogen bond with the side chain of an acidic amino acid at position 27. This should reduce the biotin-binding affinity by approximately eight orders of magnitude, while leaving the affinities for these biotin analogs virtually unaffected. In good agreement with these predictions, a streptavidin mutant with the N23A and S27D substitutions binds 2-iminobiotin with an affinity (K_a) of 1 × 10⁶ M⁻¹, two orders of magnitude higher than that for biotin (1 × 10⁴ M⁻¹). In contrast, the binding affinity of this streptavidin mutant for diaminobiotin (2.7 × 10⁴ M⁻¹) was lower than predicted (2.9 × 10⁵ M⁻¹), suggesting the position of the diaminobiotin in the biotin-binding site was not accurately determined by modeling.     

Fickian-Based Empirical Approach for Diffusivity Determination in Hollow Alginate-Based Microfibers Using 2D Fluorescence Microscopy and Comparison with Theoretical Predictions

Hollow alginate microfibers (od = 1.3 mm, id = 0.9 mm, th = 400 µm, L = 3.5 cm) comprised of 2% (w/v) medium molecular weight alginate cross-linked with 0.9 M CaCl₂ were fabricated to model outward diffusion capture by 2D fluorescent microscopy. A two-fold comparison of diffusivity determination based on real-time diffusion of Fluorescein isothiocyanate molecular weight (FITC MW) markers was conducted using a proposed Fickian-based approach in conjunction with a previously established numerical model developed based on spectrophotometric data. Computed empirical/numerical (D_empiricial/D_numerical) diffusivities characterized by small standard deviations for the 4-, 70- and 500-kDa markers expressed in m²/s are (1.06 × 10⁻⁹ ± 1.96 × 10⁻¹⁰)/(2.03 × 10⁻¹¹), (5.89 × 10⁻¹¹ ± 2.83 × 10⁻¹²)/(4.6 × 10⁻¹²) and (4.89 × 10⁻¹² ± 3.94 × 10⁻¹³)/(1.27 × 10⁻¹²), respectively, with the discrimination between the computation techniques narrowing down as a function of MW. The use of the numerical approach is recommended for fluorescence-based measurements as the standard computational method for effective diffusivity determination until capture rates (minimum 12 fps for the 4-kDa marker) and the use of linear instead of polynomial interpolating functions to model temporal intensity gradients have been proven to minimize the extent of systematic errors associated with the proposed empirical method.     

Electrochemically Pretreated Sensor Based on Screen-Printed Carbon Modified with Pb Nanoparticles for Determination of Testosterone

Testosterone (TST), despite its good properties, may be harmful to the human organism and the environment. Therefore, monitoring biological fluids and environmental samples is important. An electrochemically pretreated screen-printed carbon sensor modified with Pb nanoparticles (pSPCE/PbNPs) was successfully prepared and used for the determination of TST. The surface morphology and electrochemical properties of unmodified and modified sensors were characterized by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), scanning and transmission electron microscopy (SEM and TEM), and energy-dispersive X-ray spectroscopy (EDS). Selective determinations of TST at the pSPCE/PbNPs were carried out by differential pulse adsorptive stripping voltammetry (DPAdSV, E_{Pb dep.and TST acc.} of −1.1 V, t _{Pb dep.and TST acc.} of 120 s, ΔE_A of 50 mV, ν of 175 mV s⁻¹, and t_m of 5 ms) in a solution containing 0.075 mol L⁻¹ acetate buffer of pH = 4.6 ± 0.1, and 7.5 × 10⁻⁵ mol L⁻¹ Pb(NO₃)₂. The analytical signal obtained at the potential around −1.42 V (vs. silver pseudo-reference electrode) is related to the reduction process of TST adsorbed onto the electrode surface. The use of pSPCE/PbNPs allows obtaining a very low limit of TST detection (2.2 × 10⁻¹² mol L⁻¹) and wide linear ranges of the calibration graph (1.0 × 10⁻¹¹–1.0 × 10⁻¹⁰, 1.0 × 10⁻¹⁰–2.0 × 10⁻⁹, and 2.0 × 10⁻⁹–2.0 × 10⁻⁸ mol L⁻¹). The pSPCE/PbNPs were successfully applied for the determination of TST in reference material of human urine and wastewater purified in a sewage treatment plant without preliminary preparation.     

Electrochemically Activated Screen-Printed Carbon Sensor Modified with Anionic Surfactant (aSPCE/SDS) for Simultaneous Determination of Paracetamol,Diclofenac and Tramadol

In this work, an electrochemically activated screen-printed carbon electrode modified with sodium dodecyl sulfate (aSPCE/SDS) was proposed for the simultaneous determination of paracetamol (PA), diclofenac (DF), and tramadol (TR). Changes of surface morphology and electrochemical behaviour of the electrode after the electrochemical activation with H₂O₂ and SDS surface modification were studied by scanning electron microscopy (SEM), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The influence of various parameters on the responses of the aSPCE/SDS such as pH and concentration of the buffer, SDS concentration, and techniques parameters were investigated. Using optimised conditions (E_acc. of −0.4 V, t_acc. of 120 s, ΔE_A of 150 mV, ν of 250 mV s⁻¹, and t_m of 10 ms), the aSPCE/SDS showed a good linear response in the concentration ranges of 5.0 × 10⁻⁸–2.0 × 10⁻⁵ for PA, 1.0 × 10⁻⁹–2.0 × 10⁻⁷ for DF, and 1.0 × 10⁻⁸–2.0 × 10⁻⁷ and 2.0 × 10⁻⁷–2.0 × 10⁻⁶ mol L⁻¹ for TR. The limits of detection obtained during the simultaneous determination of PA, DF, and TR are 1.49 × 10⁻⁸ mol L⁻¹, 2.10 × 10⁻¹⁰ mol L⁻¹, and 1.71 × 10⁻⁹ mol L⁻¹, respectively. The selectivity of the aSPCE/SDS was evaluated by examination of the impact of some inorganic and organic substances that are commonly present in environmental and biological samples on the responses of PA, DF, and TR. Finally, the differential pulse adsorptive stripping voltammetric (DPAdSV) procedure using the aSPCE/SDS was successfully applied for the determination of PA, DF, and TR in river water and serum samples as well as pharmaceuticals.     

Microstructure and Wear Properties of Laser Cladding WC/Ni-Based Composite Layer on Al–Si Alloy

The microstructural and wear properties of laser-cladding WC/Ni-based layer on Al–Si alloy were investigated by scanning electron microscope (SEM), X-ray diffraction (XRD), energy dispersive spectrometer (EDS) and wear-testing. The results show that, compared with the original specimen, the microhardness and wear resistance of the cladding layer on an Al–Si alloy were remarkably improved, wherein the microhardness of the layer achieved 1100 HV and the average friction coefficient of the layer was barely 0.14. The mainly contributor to such significant improvement was the generation of a WC/Ni-composite layer of Al–Si alloy during laser cladding. Two types of carbides, identified as M₇C₃ and M₂₃C₆, were found in the layer. The wear rate of the layer first increased and then decreased with the increase in load; when the load was 20 N, 60 N and 80 N, the wear rate of layer was1.89 × 10⁻³ mm³·m⁻¹, 3.73 × 10⁻³ mm³·m⁻¹ and 2.63 × 10⁻³ mm³·m⁻¹, respectively, and the average friction coefficient (0.14) was the smallest when the load was 60 N.     

Antibody catalysis of peptidyl-prolyl cis-trans isomerization in the folding of RNase T1

An antibody generated to an α-keto amide containing hapten 1 catalyzes the cis-trans isomerization of peptidyl-prolyl amide bonds in peptides and in the protein RNase T1. The antibody-catalyzed peptide isomerization reaction showed saturation kinetics for the cis-substrate, Suc-Ala-Ala-Pro-Phe-pNA, with a k_cat/K_m value of 883 s⁻¹M⁻¹; the reaction was inhibited by the hapten analog 13 (K_i = 3.0 ± 0.4 μM). Refolding of denatured RNase T1 to its native conformation also was catalyzed by the antibody, with the antibody-catalyzed folding reaction inhibitable both by the hapten 1 and hapten analog 13. These results demonstrate that antibodies can catalyze conformational changes in protein structure, a transformation involved in many cellular processes.     

Engineering a model protein cavity to catalyze the Kemp elimination

Synthetic cavitands and protein cavities have been widely studied as models for ligand recognition. Here we investigate the Met102 → His substitution in the artificial L99A cavity in T4 lysozyme as a Kemp eliminase. The resulting enzyme had k_cat/K_M = 0.43 M^-1 s^-1 and a (k_cat/K_M)/k_uncat = 10⁷ at pH 5.0. The crystal structure of this enzyme was determined at 1.30 Å, as were the structures of four complexes of substrate and product analogs. The absence of ordered waters or hydrogen bonding interactions, and the presence of a common catalytic base (His102) in an otherwise hydrophobic, buried cavity, facilitated detailed analysis of the reaction mechanism and its optimization. Subsequent substitutions increased eliminase activity by an additional four-fold. As activity-enhancing substitutions were engineered into the cavity, protein stability decreased, consistent with the stability-function trade-off hypothesis. This and related model cavities may provide templates for studying protein design principles in radically simplified environments.     

Triple Perovskite Nd1.5Ba1.5CoFeMnO9−δ-Sm0.2Ce0.8O1.9 Composite as Cathodes for the Intermediate Temperature Solid Oxide Fuel Cells

Triple perovskite has been recently developed for the intermediate temperature solid oxide fuel cell (IT-SOFC). The performance of Nd_1.5Ba_1.5CoFeMnO_9−δ (NBCFM) cathodes for IT-SOFC is investigated in this work. The interfacial polarization resistance (R_P) of NBCFM is 1.1273 Ω cm²~0.1587 Ω cm² in the range of 700–800 °C, showing good electrochemical performance. The linear thermal expansion coefficient of NBCFM is 17.40 × 10⁻⁶ K⁻¹ at 40–800 °C, which is significantly higher than that of the electrolyte. In order to further improve the electrochemical performance and reduce the thermal expansion coefficient (TEC) of NBCFM, Ce_0.8Sm_0.2O_2−δ (SDC) is mixed with NBCFM to prepare an NBCFM-xSDC composite cathode (x = 0, 10, 20, 30, 40 wt.%). The thermal expansion coefficient decreases monotonically from 17.40 × 10⁻⁶ K⁻¹ to 15.25 × 10⁻⁶ K⁻¹. The surface oxygen exchange coefficient of NBCFM-xSDC at a given temperature increases from 10⁻⁴ to 10⁻³ cm s⁻¹ over the po2 range from 0.01 to 0.09 atm, exhibiting fast surface exchange kinetics. The area specific resistance (ASR) of NBCFM-30%SDC is 0.06575 Ω cm² at 800 °C, which is only 41% of the ASR value of NBCFM (0.15872 Ω cm²). The outstanding performance indicates the feasibility of NBCFM-30% SDC as an IT-SOFC cathode material. This study provides a promising strategy for designing high-performance composite cathodes for SOFCs based on triple perovskite structures.     

Rapid Detection of Listeria by Bacteriophage Amplification and SERS-Lateral Flow Immunochromatography

A rapid Listeria detection method was developed utilizing A511 bacteriophage amplification combined with surface-enhanced Raman spectroscopy (SERS) and lateral flow immunochromatography (LFI). Anti-A511 antibodies were covalently linked to SERS nanoparticles and printed onto nitrocellulose membranes. Antibody-conjugated SERS nanoparticles were used as quantifiable reporters. In the presence of A511, phage-SERS nanoparticle complexes were arrested and concentrated as a visible test line, which was interrogated quantitatively by Raman spectroscopy. An increase in SERS intensity correlated to an increase in captured phage-reporter complexes. SERS limit of detection was 6 × 10⁶ pfu·mL⁻¹, offering detection below that obtainable by the naked eye (LOD 6 × 10⁷ pfu·mL⁻¹). Phage amplification experiments were carried out at a multiplicity of infection (MOI) of 0.1 with 4 different starting phage concentrations monitored over time using SERS-LFI and validated by spot titer assay. Detection of L. monocytogenes concentrations of 1 × 10⁷ colony forming units (cfu)·mL⁻¹, 5 × 10⁶ cfu·mL⁻¹, 5 × 10⁵ cfu·mL⁻¹ and 5 × 10⁴ cfu·mL⁻¹ was achieved in 2, 2, 6, and 8 h, respectively. Similar experiments were conducted at a constant starting phage concentration (5 × 10⁵ pfu·mL⁻¹) with MOIs of 1, 2.5, and 5 and were detected in 2, 4, and 5 h, respectively.     

Ambipolar-transporting coaxial nanotubes with a tailored molecular graphene–fullerene heterojunction

Despite a large steric bulk of C₆₀, a molecular graphene with a covalently linked C₆₀ pendant [hexabenzocoronene (HBC)–C₆₀; 1] self-assembles into a coaxial nanotube whose wall consists of a graphite-like π-stacked HBC array, whereas the nanotube surface is fully covered by a molecular layer of clustering C₆₀. Because of this explicit coaxial configuration, the nanotube exhibits an ambipolar character in the field-effect transistor output [hole mobility (μ_h) = 9.7 × 10⁻⁷ cm² V⁻¹ s⁻¹; electron mobility (μ_e) = 1.1 × 10⁻⁵ cm² V⁻¹ s⁻¹] and displays a photovoltaic response upon light illumination. Successful coassembly of 1 and an HBC derivative without C₆₀ (2) allows for tailoring the p/n heterojunction in the nanotube, so that its ambipolar carrier transport property can be optimized for enhancing the open-circuit voltage in the photovoltaic output. As evaluated by an electrodeless method called flash-photolysis time-resolved microwave conductivity technique, the intratubular hole mobility (2.0 cm² V⁻¹ s⁻¹) of a coassembled nanotube containing 10 mol % of HBC–C₆₀ (1) is as large as the intersheet mobility in graphite. The homotropic nanotube of 2 blended with a soluble C₆₀ derivative [(6,6)-phenyl C₆₁ butyric acid methyl ester] displayed a photovoltaic response with a much different composition dependency, where the largest open-circuit voltage attained was obviously lower than that realized by the coassembly of 1 and 2.