Single-step Mechanism Research Articles

K +–Na + exchange in phlogopite on the scale of a single layer

Hydrothermal atomic force microscopy (HAFM) was used to investigate K +–Na + ion exchange in phlogopite in-situ. The exchange of K + for Na + caused the interlayer distance to swell by approximately 5 Å. A distinct reaction front could be resolved between the K +-areas and the swollen (hydrated) Na +-areas, indicating a single reaction step mechanism. Although the fronts revealed kinematic variability due to inhomogeneities, the data indicate a diffusion mechanism within the interlayers. Diffusion coefficients ranged between 2 × 10 −8 and 35 × 10 −8 cm 2/s, depending on the depth of the interlayer, the solution composition, and temperature. An activation energy of 15 kJ/mol was calculated from the temperature dependence of the diffusion coefficients. In addition to the regular 5 Å swelling, bulge-shaped irregular swelling of up to 200 nm could be observed. This irregular swelling might be an initial stage of delamination. Reducing the Na +-concentration in the solution at a constant K +-concentration was found to reduce the exchange rate. The exchange ceases completely when the equilibrium ratio r(K +/Na +) of the solution is reached. The measured r(K +/Na +) of 0.013 indicates a lower K +-selectivity for interlayers that are closer to the surface. This lower selectivity is most likely related to a lower strain energy associated with the expansion of interlayers close to the surface. Reversing the exchange reaction caused the interlayers to shrink to their original height. The kinematics of the front of the reverse reaction were significantly enhanced. In parts, swollen Na +-areas were engulfed and trapped by the shrunken K +-areas. No morphological indications of remnant alterations other than these trapped islands and the irregular swelling were observed.

A DFT study of the possible role of vinylidene and carbene intermediates in the mechanism of the enzyme acetylene hydratase

The mode of action of the tungstoprotein, acetylene hydratase, is unresolved at present.We have carried out density functional theory (DFT) calculations to investigate two proposed mechanisms, and also propose a new mechanism based on our calculations. We find that the previously proposed single step mechanisms which involve the attack of a water molecule, catalysed by Asp13, have barriers greater than 40 kcal mol(-1). We propose a new mechanism which involves two tungsten complexes, a vinylidene and a carbene, as stable intermediates. We calculate the potential energy surfaces for their formation, and for the decomposition of the carbene complex to yield acetaldehyde. All the barriers along the pathway are less than 30 kcal mol(-1), except for the barrier to carbene formation which is 34 kcal mol(-1).

Modeling Instabilities in Lean Premixed Turbulent Combustors Using Detailed Chemical Kinetics

The development of non-conventional combustion technology with ultra-low emissions and the safe operation of combustion systems require a thorough understanding of the mechanisms of combustion instabilities. The objective of the present work is to investigate the role of unmixedness and chemical kinetics in driving combustion instabilities. The reaction-rate responses of different species to inlet flow variations have been studied using a perfectly stirred reactor model. Transient simulations of combustion of methane and propane with air, using both global single-step and detailed chemical kinetic mechanisms, have been conducted with imposed oscillations on inflow mass flow rate, temperature, and mixture equivalence ratio. The detailed mechanisms predicted fuel reaction-rate oscillations with amplitudes proportional to the imposed oscillations. However, increased amplitudes of the reaction rates of CO2 and OH were observed when the combustion became leaner, while the reaction-rate amplitudes of CO and H2 decreased. The single-step mechanisms predicted to some degree a similar reaction-rate behavior as the detailed mechanisms. However, near stoichiometric conditions, the fuel reaction rate of propane showed little influence by the imposed oscillations. When the mean equivalence ratio was lowered below a certain value, the fuel reaction-rate oscillations grew stronger and became larger than those seen with the detailed mechanism. This shows that simple mechanisms can by themselves introduce instabilities not seen with detailed mechanisms.

Urea-Induced Denaturation Process on Defatted Human Serum Albumin and in the Presence of Palmitic Acid

We report a study on the unfolding behavior of the most abundant protein contained in plasma, human serum albumin. The unfolding mechanisms in denaturing conditions induced by urea are studied for the defatted form (HSA) and for the palmitic acid:albumin (HSAPalm) complex. We employed the singular value decomposition method to determine the minimum number of structural states present in the unfolding processes. Low-resolution three-dimensional structures are reconstructed from the one-dimensional small-angle X-ray scattering patterns and are correlated with the parameters obtained from static and dynamic light scattering experiments. The unfolding process is pointed out by both ab initio and rigid body fitting methods that highlight a stepwise evolution of the protein structure toward open conformations. The superimpositions of the 3D structures provided independently by the two methods show very good agreements. The hydrodynamic radii estimated for the protein best fitting conformations are in satisfactory agreement with the experimental ones. The results show that the HSA unfolding process is consistent with previous spectroscopic studies that suggest a multistep unfolding pathway. In particular, a scheme in which domains I and II are opened in sequence and the presence of two intermediates are evidenced is presented. The opening sequence is different from that found using guanidine hydrochloride as denaturant agent. The stabilizing role of the fatty acids in the urea denaturation process is evident. The palmitic acid ligand strongly stabilizes the protein, which remains in the native form up to high denaturant concentrations. In this case, the unfolding process is characterized by a single-step mechanism.

Ancestral Origin of the ATTCT Repeat Expansion in Spinocerebellar Ataxia Type 10 (SCA10)

Spinocerebellar ataxia type 10 (SCA10) is an autosomal dominant neurodegenerative disease characterized by cerebellar ataxia and seizures. The disease is caused by a large ATTCT repeat expansion in the ATXN10 gene. The first families reported with SCA10 were of Mexican origin, but the disease was soon after described in Brazilian families of mixed Portuguese and Amerindian ancestry. The origin of the SCA10 expansion and a possible founder effect that would account for its geographical distribution have been the source of speculation over the last years. To unravel the mutational origin and spread of the SCA10 expansion, we performed an extensive haplotype study, using closely linked STR markers and intragenic SNPs, in families from Brazil and Mexico. Our results showed (1) a shared disease haplotype for all Brazilian and one of the Mexican families, and (2) closely-related haplotypes for the additional SCA10 Mexican families; (3) little or null genetic distance in small normal alleles of different repeat sizes, from the same SNP lineage, indicating that they are being originated by a single step mechanism; and (4) a shared haplotype for pure and interrupted expanded alleles, pointing to a gene conversion model for its generation. In conclusion, we show evidence for an ancestral common origin for SCA10 in Latin America, which might have arisen in an ancestral Amerindian population and later have been spread into the mixed populations of Mexico and Brazil.

Unsteady simulation of a CO/H2/N2/air turbulent non-premixed flame

The Sandia/ETH-Zurich CO/H2/N2 non-premixed unconfined turbulent jet flame (named ‘Flame A’) is numerically simulated by solving the unsteady compressible reactive Navier–Stokes equations in a three-dimensional axisymmetric formulation, hence, in a formally two-dimensional domain. The turbulent combustion closure model adopted is the Fractal Model, FM, developed as a subgrid scale model for Large Eddy Simulation. The fuel is injected from a straight circular tube and the corresponding Reynolds number is 16 700, while the air coflows. Since the thickness of the nozzle is 0.88 mm, and the injection velocity high, ∼ 104 m s−1, capturing the stabilization mechanism of the actual flame requires high spatial resolution close to the injector. Results are first obtained on a coarse grid assuming a fast-chemistry approach for hydrogen oxidation and a single step mechanism for carbon monoxide oxidation. With this approach the flame is inevitably anchored. Then, to understand the actual flame stabilization a more complex chemical mechanism, including main radical species, is adopted. Since using this chemistry and the coarse grid of previous simulation the flame blows off numerically, attention is focused on understanding the actual flame stabilization mechanism by simulating a small spatial region close to the injection with a very fine grid. Then, analysing these results, an artificial anchoring mechanism is developed to be used in simulations of the whole flame with a coarse grid. Unsteady characteristics are shown and some averaged radial profiles for temperature and species are compared with experimental data.

Synthesis of Shape Controlled Ferrite Nanoparticles by Sonochemical Technique

Synthesis of magnetic iron oxides/ferrites in the nano scale by sonochemical synthesis has become prominent recently. This technique facilitates the synthesis of magnetic particles in the nano scale attributed to the hotspot mechanism arising due to acoustic cavitation induced chemical reaction. Generally volatile organometallic precursor compounds favoring the formation of fully amorphous particles have been used to synthesize various nano magnetic materials. We report here the synthesis of ultrafine, < 10 nm magnetic iron oxide nanoparticles by sonochemical technique starting with a non-volatile precursor iron salt such as iron citrate which seems to favor the formation of semi crystalline/crystalline particles as the reaction takes place either in the interfacial region or in the bulk solution. Mono dispersed, ultra fine, approximately 4 nm spherical shaped magnetic maghemite particles having a saturation magnetization of 58.2 emu/g and coercivity of 118 Oe were obtained at low values of pH, 10 while higher pH, 11-13 favored the formation of elongated, cylindrical, acicular particles with a reduced magnetization. The coercivity was also found to decrease with increasing pH, with it being 118 Oe at pH 10 and 3 Oe at pH 13. When the ultrasound amplitude/intensity was low, 38% heat treatment of the samples at 300 degrees C (at pH 10) was required to make them crystalline, while application of high intensity ultrasound, 50% amplitude served as a single step mechanism for obtaining crystalline maghemite particles. The maghemite particles obtained at a pH of 10 could find applications in information storage media.

Density Functional Theory Investigation into Structure and Reactivity of Prenucleation Silica Species

We investigated the condensation and reverse hydrolysis reactions of several silica clusters ranging in size up to the octamer cage. Using density functional theory (DFT) at the B3LYP/6-31G(d,p) level of theory, we found that under neutral conditions the reactions proceeded through a single-step, SN2-type mechanism with formation of a pentacoordinated transition state. Under acidic conditions, the reactions of the smaller species followed a two-step mechanism with formation of a stable pentacoordinated intermediate, while the larger species proceeded through a single-step mechanism similar to the neutral route. In vacuo energy calculations showed a decreased activation barrier for formation of the larger ringed oligomers compared to the barriers of the smaller species. Calculation also showed an increasing kinetic and thermodynamic barrier for the silica clusters to break back into smaller less-condensed constituent species as they oligomerized to form larger species. To study the influence of solvation o...

A computational study and valence bond approach to the intramolecular electrophilic aromatic substitution mechanism of ortho-allyl- N-benzylanilines

Ab initio computational studies were performed for the intramolecular Friedel–Crafts alkylation of ortho-allyl- N-benzylanilines catalyzed by sulfuric acid, which results in the formation of both dihydrodibenz[ b, e]azepine and tetrahydrodibenz[ b, f]azocine isomers. This intramolecular reaction occurs through the rearrangement of the cationic species generated from the allyl moiety and falls into the type of electrophilic aromatic substitution rearrangements. Due to this fact, we discuss the reactivity trends of the intermediate species participated in this competitive 7- exo- trig versus 8- exo- trig cyclization process. The valence bond configuration mixing is used for this purpose and with this model the reduced potential energy surface and the reaction profile are constructed. The expected patterns such as single step or stepwise mechanism, and the trends in the reaction barriers for the electrophilic attack effects of the aryl substituents are discussed.

An investigation by means of correlation analysis into the mechanisms of oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane in various solvents

Relative rate constants have been measured for the oxidation of aryl methyl sulfides and sulfoxides by dimethyldioxirane in acetone, in mixtures of acetone with aprotic co-solvents of both higher and lower relative permittivity, and in aqueous acetone mixtures. Correlation analyses of the effects of substituents in the different solvents show that, with one exception, reactions take place via a single step mechanism in which the formation of the new SO bond and the elimination of acetone occur concertedly. The exception was oxidation of the sulfides in aqueous acetone containing the highest proportion of water of those studied (20% v/v). Here, the behaviour of the reaction is consistent with a two-step mechanism in which the oxidant reversibly attacks the sulfide to form an open-chain sulfonium betaine that subsequently fragments to sulfoxide and acetone. There is no evidence for the participation of an intermediate dioxathietane as has been found in the case of sulfide oxidations by (trifluoromethyl)methyldioxirane in CH(2)Cl(2) and similar aprotic solvents. It is not justified to generalise a mechanism involving a betaine, with or without a derived dioxathietane, to the reactions of dimethyldioxirane in acetone.

The Role of Decomposition Kinetics in Pyrolysis Modeling - Application to a Fire Retardant Polyester Composite

This work assesses the effect of decomposition kinetics on overall pyrolysis behavior using experimental data from thermogravimetric analysis (TGA) and Fire Propagation Apparatus (FPA) experiments. TGA data are presented for an unsaturated brominated polyester resin (reinforcement free), and the FPA is used to investigate the pyrolysis behavior of a fiber reinforced polymer (FRP) composite slab with matrix comprised of the same resin tested via TGA. Three different kinetic models are fit to the TGA data: singlestep n th order, 3-step n th order, and 3-step n th order with one autocatalytic step. These kinetics models are then used to simulate the pyrolysis of a composite slab in the FPA, with thermophysical properties estimated by genetic algorithm optimization. It is shown that the two 3-step mechanisms provide nearly identical calculations of total mass loss rate (MLR) in the FPA, while the single-step mechanism provides similar, but quantitatively different, MLR predictions. Although no broad conclusions regarding the importance of multi-step thermal decomposition kinetics can be drawn on the basis of a single study, detailed reaction mechanisms may be superfluous unless TGA curves show multiple distinct reaction peaks and/or all thermophysical properties/model input parameters are precisely known.

Autoignition in nonhomogeneous mixtures: Conditional statistics and implications for modeling

Conditional statistics associated with the problem of nonhomogeneous autoignition are investigated using direct numerical simulations (DNS). The chemical model is based on a single-step, second-order, and irreversible reaction mechanism with reaction rate expressed by the Arrhenius law. The mixture is initialized as a random distribution with variable mixture strength in decaying isotropic turbulence. Both low- and high-turbulence conditions are studied and three Lewis number cases are examined for the high-turbulence conditions. Simulation results show that under conditions of nonhomogeneous mixture and preheated air, autoignition is initiated in a fuel-lean mixture and evolves by propagation to richer mixtures. The propagation elements of the autoignition process are found in statistics of mean quantities for reactive scalars as well as covariances and variances of these scalars with the rate of dissipation. The addition of a second conditioning variable based on a reduced temperature is investigated. Results show that the addition of a second conditioning variable that measures the extent of completion of combustion may be a reasonable choice for nonhomogeneous autoignition modeling. However, additional nontrivial closure models are required for both reactive scalars' phase space equations and the transport equations for the second conditioning variable.

Kinetics of the Inhibition of Fusarium Serine Proteinases by Barley (Hordeum vulgare L.) Inhibitors

Fungal infections of barley and wheat cause devastating losses of these food crops. The endogenous proteinase inhibitors produced by plant seeds probably defend the plants from pathogens by inhibiting the degradation of their proteins by the pathogen proteases. We have studied the interactions of barley grain inhibitors with the subtilisin-like and trypsinlike proteinases of Fusarium culmorum. The inhibition kinetics of three inhibitor proteins, chymotrypsin/subtilisin inhibitor 2 (CI-2), barley alpha-amylase/subtilisin inhibitor (BASI), and Bowman-Birk trypsin inhibitor (BBBI), have been studied in detail for the first time using fungal enzymes. The kinetic studies were performed at physiological pH values to mimic in vivo conditions. Numerical approaches to kinetic analyses were used to calculate the inhibition constants, because the data analyses were complicated by some inhibitor turnover and the instability of enzymes and substrates. All were slow, tight-binding inhibitors that followed either a two-step mechanism (CI-2 and BASI) or a single-step mechanism (BBBI) under the conditions investigated. The overall Ki values derived were approximately 50 pM, 1 nM, and 0.1 nM for CI-2, BASI, and BBBI, respectively. The main difference between the CI-2 and the BASI inhibitions was accounted for by the stabilities of their final complexes and the rate constants for their second dissociation steps (9 x 10(-6)/s and 3 x 10(-4)/s, respectively). Understanding the inhibition mechanisms will be valuable in designing improved strategies for increasing the resistance of the grains to fungal infections.

Role of Zinc Content on the Catalytic Efficiency of B1 Metallo β-Lactamases

Metallo beta-lactamases (MbetaL) are enzymes naturally evolved by bacterial strains under the evolutionary pressure of beta-lactam antibiotic clinical use. They have a broad substrate spectrum and are resistant to all the clinically useful inhibitors, representing a potential risk of infection if massively disseminated. The MbetaL scaffold is designed to accommodate one or two zinc ions able to activate a nucleophilic hydroxide for the hydrolysis of the beta-lactam ring. The role of zinc content on the binding and reactive mechanism of action has been the subject of debate and still remains an open issue despite the large amount of data acquired. We report herein a study of the reaction pathway for binuclear CcrA from Bacteroides fragilis using density functional theory based quantum mechanics-molecular mechanics dynamical modeling. CcrA is the prototypical binuclear enzyme belonging to the B1 MbetaL family, which includes several harmful chromosomally encoded and transferable enzymes. The involvement of a second zinc ion in the catalytic mechanism lowers the energetic barrier for beta-lactam hydrolysis, preserving the essential binding features found in mononuclear B1 enzymes (BcII from Bacillus cereus) while providing a more efficient single-step mechanism. Overall, this study suggests that uptake of a second equivalent zinc ion is evolutionary favored.

Electrochemical behaviour of gadolinium ion in molten LiCl–KCl eutectic

This work presents the electrochemical study of GdCl3 in the molten LiCl–KCl eutectic in the temperature range 723–823K. Transient electrochemical techniques such as cyclic voltammetry and chronopotentiometry, on an inert metallic tungsten working electrode, have been used in order to investigate the reduction mechanism and transport parameters. This study shows that Gd3+ ions are reduced to Gd metal by a single step mechanism with exchange of three electrons. Diffusion coefficient of GdCl3 ions was determined at various temperatures, at 723K the value is D=0.8810−5cm2s−1. Apparent standard reduction potential of the redox couple Gd3+/Gd has been determined by the open-circuit chronopotentiometry technique at several temperatures. Also the Gibbs free energy of GdCl3 formation was determined and compared with thermodynamic data for pure compounds in the supercooled state in order to estimate the activity coefficient of Gd3+ in the molten LiCl–KCl eutectic.

Role of Zeolitic Oxygens During the Decomposition of 15N218O Over Fe-ferrierite

Isotopic species of dioxygen released during the decomposition of 15N 2 18 O over Fe-ferrierite show that the zeolite oxygens participate in the reaction. While Fe-ferrierite alone does not exchange its oxygens with 18O2 below 400 °C, this exchange is very rapid in the mixture of 18O2+N2O. The amount of participating zeolite oxygen (ca. 1–6 per iron atom) is practically the same in the latter case as in the decomposition of 15N 2 18 O. The time dependence of individual dioxygen isotope species released during the 15N 2 18 O decomposition points to the primary release of 18O2 which is very rapidly exchanged for the zeolite oxygen by a single-step mechanism.

Analysis of the Functional Modules of the tRNA 3′ Endonuclease (tRNase Z)

tRNA 3' processing is one of the essential steps during tRNA maturation. The tRNA 3'-processing endonuclease tRNase Z was only recently isolated, and its functional domains have not been identified so far. We performed an extensive mutational study to identify amino acids and regions involved in dimerization, tRNA binding, and catalytic activity. 29 deletion and point variants of the tRNase Z enzyme were generated. According to the results obtained, variants can be sorted into five different classes. The first class still had wild type activity in all three respects. Members of the second and third class still formed dimers and bound tRNAs but had reduced catalytic activity (class two) or no catalytic activity (class three). The fourth class still formed dimers but did not bind the tRNA and did not process precursors. Since this class still formed dimers, it seems that the amino acids mutated in these variants are important for RNA binding. The fifth class did not have any activity anymore. Several conserved amino acids could be mutated without or with little loss of activity.

{\rtf1\ansi\ansicpg1250\deff0\deflang1038\deflangfe1038\deftab708{\fonttbl{\f0\froman\fprq2\fcharset238{\*\fname Times New Roman;}Times New Roman CE;}} \viewkind4\uc1\pard\f0\fs20 A pseudo-isothermal kinetic analysis of the recrystallisation of nickel sulphide measured by non-isothermal DSC \par }

A pseudo-isothermal method for the kinetic analysis was applied to the recrystallisation of α to β nickel sulphide based on the Ozawa model for non-isothermal crystallisation in the presence of pre-existing nuclei. The aim of the analysis was to determine the number of steps involved in the recrystallisation and, hence, determine the applicability of the kinetic equation based on a single step mechanism, as is commonly applied in the kinetic analysis of solid-state processes using thermal methods. The kinetic analysis yielded evidence of at least two processes indicating that physical and mechanistic significance should not be drawn from the use of a single step approximation of the rate equation.

Evidence for a Single Step Mechanism in the Origin of Hyperdiploid Childhood Acute Lymphoblastic Leukemia.

High hyperdiploidy (>50 chromosomes) in childhood acute lymphoblastic leukemia (ALL) is characterized by non-random multiple trisomies and tetrasomies, involving in particular chromosomes X, 4, 6, 8, 10, 14, 17, 18, and 21. It is the most common cytogenetic subgroup in pediatric ALL, but in spite of this, the mechanisms behind its formation remain elusive. Four different pathways are possible: (1) initial near-haploidy followed by doubling of the chromosomes, (2) prior polyploidization with subsequent losses of chromosomes, (3) sequential gains of chromosomes in consecutive cell divisions, and (4) a simultaneous gain of all additional chromosomes in a single abnormal mitosis. Although these alternatives are difficult to distinguish experimentally, investigations of possible uniparental disomies (UPDs) and of the allelic ratios of loci located on tetrasomic chromosomes may provide some clues. In a previous study of 10 cases of hyperdiploid childhood ALL, we could exclude the first pathway as a general mechanism. Furthermore, the results did not favor the second alternative. Finally, our findings of equal allele dosage for chromosome 21 loci in cases with tetrasomy 21 suggested that the hyperdiploidy arose by a simultaneous gain of chromosomes. However, because all disomies and tetrasomies were not investigated, formation via a polyploid state or by a sequential gain could not be definitely excluded. In the present study, we have addressed this issue further by investigating 15 new cases of hyperdiploid childhood ALL using a total of 57 polymorphic microsatellite markers mapped to 23 of the 24 human chromosomes. Ten of the cases were analyzed with multicolor fluorescence in situ hybridization (M-FISH) and the remaining five with interphase FISH using probes for X, 4, 6, 8, 10, 14, 17, 18, and 21. Markers localized to all disomic and all tetrasomic chromosomes were then applied. One case displayed multiple UPDs and a non-typical pattern with only tetrasomies; this ALL probably arose via a near-haploid pathway. Two other cases had UPDs for 4/9 and 2/14 disomic chromosomes, respectively, and may have originated via a polyploid state. However, in the vast majority of the cases (12/15) there was no evidence for any UPDs, excluding that the hyperdiploidy originated by the near-haploid or polyploid mechanisms. Investigating the tetrasomies, 25 of a total of 27 tetrasomies present among the 15 cases displayed equal allele dosages, indicating a simultaneous gain of chromosomes as opposed to sequential gains. The two tetrasomies with unequal allele dosages were present in the same case, suggesting that this ALL constituted an exception. In conclusion, the results of the present study strongly suggest that hyperdiploidy in childhood ALL generally arises by a simultaneous gain of all additional chromosomes in a single abnormal mitosis.

Artificial Photosynthetic Reaction Centers with Porphyrins as Primary Electron Acceptors

A triad consisting of a carotenoid (C), a dimesitylporphyrin (P), and a tris(heptafluoropropyl)porphyrin (PF), C−P−PF, has been synthesized and found to undergo rapid singlet−singlet energy transfer between the porphyrin moieties so that their excited states are in equilibrium. Photoinduced electron transfer from the first excited singlet state of P, or hole transfer from the first excited singlet state of PF, yields C−P•+−PF•-. Electron transfer from C then yields the final charge-separated state C•+−P−PF•- with a quantum yield of 0.73 and a lifetime of 500 ns in tetrahydrofuran solution at ambient temperature. The final charge-separated state decays to form primarily a triplet excited state localized on the carotenoid, 3C−P−PF, rather than the ground state. A second triad in which P is metalated (PZn) has also been synthesized. In this system, the excited singlet states of the porphyrins are no longer in equilibrium; fast electron transfer from excited PZn to PF to form C−PZn•+−PF•- and also fast energy transfer from PZn to PF with subsequent hole transfer from PF to PZn converge to give the same C−PZn•+−PF•- species, which evolves to C•+−PZn−PF•- with a quantum yield of 0.14. This state decays to 3C−PZn−PF with a quantum yield of 0.06 in tetrahydrofuran at room temperature. The charge recombination reaction follows a single-step mechanism for C•+−P−PF•- from room temperature to 77 K and for C•+−PZn−PF•- below 250 K. Above 250 K, a two-step pathway is accessed for the recombination reaction of C•+−PZn−PF•- in addition to the direct recombination mechanism. This new pathway involves an endergonic step to populate C−PZn•+−PF•- having an Ea of ∼0.23 eV. Certain photophysical characteristics of these triads, in particular the recombination to the triplet state, are reminiscent of those of artificial reaction centers with C60 as the primary electron acceptor.