Single First-order Reaction Research Articles

Aerobic degradation of tetramethyl bisphenol F (TMBPF) with activated sludge: Kinetics and biotransformation products

This study investigated the bio-degradation kinetics of tetramethyl bisphenol F (TMBPF), a non-estrogenic alternative to bisphenol A (BPA). Batch biotransformation experiments were performed whereby samples were inoculated with activated sludge and analysed using liquid chromatography-Orbitrap-tandem mass spectrometry (LC-Orbitrap-MS) utilising two non-targeted workflows (commercial and freely available online) for biotransformation products (BTP) identification. The degradation of TMBPF followed single first-order reaction kinetics and depended on the initial concentration (ci) with faster degradation -kt = 0.16, (half-life = 4.4 days) at lower concentrations ci = 0.1 mg L−1, compared with -kt = 0.02 (half-live = 36.4 days) at ci = 10.0 mg L−1. After 18 days, only 8% of the original TMBPF remained at the lowest tested concentration (0.1 mg L−1). Twelve BTPs were identified, three of which were workflow and one condition-specific. The highest relative quantities of BTPs were observed in nutrient-mineral and mineral media after ten days, while after 14 days, 36 and 31% of TMBPF (ci = 1 mg L−1) remained in the nutrient-mineral and mineral media, respectively. Also, the kinetics of TMBPF and its BTPs were the same with and without an additional carbon source. A newly proposed biodegradation pathway for TMBPF involves cleavage of the methylene bridge, hydroxylation with further oxidation, sulphation, nitrification, nitro reduction with further oxidation, acetylation, and glycine conjugation, providing a deeper insight into the fate of TMBPF during biological wastewater treatment.

A semi-global reaction mechanism for the thermal decomposition of low-density polyethylene blended with ammonium polyphosphate and pentaerythritol

A semi-global reaction mechanism has been developed to capture the decomposition of a polymer, i.e. low-density polyethylene (LDPE) that contains a condensed-phase-active intumescent flame retardant, i.e. ammonium polyphosphate (APP) mixed with pentaerythritol (PER) with ratio 3:1 (wt/wt). The methodology combines thermogravimetric analysis (TGA) in anaerobic environment over a wide range of heating rates and inverse modeling of these experiments based on the Shuffled Complex Evolution (SCE) optimization algorithm. The decomposition of neat LDPE can be described by a single first-order reaction. In contrast, the decomposition of the APP/PER additive is characterized by strong interactions between the reactants and a 5-step reaction mechanism was designed driven by the mechanisms of the intumescence process. The direct coupling of the 1-step reaction mechanism for LDPE and the 5-step reaction mechanism for APP/PER was found to provide decent predictions for the decomposition of the 90%LDPE/10% APP/PER blend. The slight interactions between the LDPE matrix and the APP/PER additive were taken into account by updating the kinetic parameters of two reactions, leading to a final robust and accurate 6-step reaction model.

Flash Pyrolysis Kinetics of Extracted Lignocellulosic Biomass Components

Biomass is a complex material mainly composed of the three lignocellulosic components: cellulose, hemicellulose and lignin. The different molecular structures of the individual components result in various decomposition mechanisms during the pyrolysis process. To understand the underlying reactions in more detail, the individual components can be extracted from the biomass and can then be investigated separately. In this work, the pyrolysis kinetics of extracted and purified cellulose, hemicellulose and lignin are examined experimentally in a small-scale fluidized bed reactor (FBR) under N2 pyrolysis conditions. The FBR provides high particle heating rates (approx. 104 K/s) at medium temperatures (573–973 K) with unlimited reaction time and thus complements typically used thermogravimetric analyzers (TGA, low heating rate) and drop tube reactors (high temperature and heating rate). Based on the time-dependent gas concentrations of 22 species, the release rates of these species as well as the overall rate of volatiles released are calculated. A single first-order (SFOR) reaction model and a 2-step model combined with Arrhenius kinetics are calibrated for all three components individually. Considering FBR and additional TGA experiments, different reaction regimes with different activation energies could be identified. By using dimensionless pyrolysis numbers, limits due to reaction kinetics and heat transfer could be determined. The evaluation of the overall model performance revealed model predictions within the ±2σ standard deviation band for cellulose and hemicellulose. For lignin, only the 2-step model gave satisfying results. Modifications to the SFOR model (yield restriction to primary pyrolysis peak or the assumption of distributed reactivity) were found to be promising approaches for the description of flash pyrolysis behavior, which will be further investigated in the future.

Thermal behavior, kinetics and gas evolution characteristics for the co-pyrolysis of real-world plastic and tyre wastes

Investigations into the co-pyrolysis of four kinds of real-world plastics (high-density polyethylene (HDPE), low-density polyethylene (LDPE), polypropylene (PP), and polystyrene (PS)) and tyre were carried out in a TG-FTIR instrument. The thermal decomposition behavior, conversion kinetics and gas releasing properties during co-pyrolysis process were investigated. It was found that co-pyrolysis of plastics and tyre delayed the pyrolysis process with a wider decomposition temperature range, when compared with the pyrolysis of individual plastic. The kinetic analysis results indicated that the co-pyrolysis process could be divided into three stages by a single first-order reaction model with the activation energies of 65–87, 34–92, and 138–208 kJ/mol, respectively. A positive synergistic interaction was found between PS and tyre, while the co-pyrolysis of PP/HDPE/LDPE and tyre was negatively influenced. During co-pyrolysis of tyre with four kinds of plastics, CH4 was the majority product for HDPE, LDPE and PP, whereas for PS, the aromatics and C–H containing compounds were the main products. The results regarding co-pyrolysis of plastics and tyres provide an insight into an environmentally friendly recycling technology for polymeric wastes.

A Large Eddy Simulation Study on the Effect of Devolatilization Modelling and Char Combustion Mode Modelling on the Structure of a Large-Scale, Biomass and Coal Co-Fired Flame

This work focuses on the impact of the devolatilization and char combustion mode modelling on the structure of a large-scale, biomass and coal co-fired flame using large eddy simulations. The coal modelling framework previously developed for the simulation of combustion in large-scale facilities is extended for biomass capabilities. An iterative procedure is used to obtain devolatilization kinetics of coal and biomass for the test-case specific fuels and heating conditions. This is achieved by calibrating the model constants of two empirical models: the single first-order model and the distributed activation energy model. The reference data for calibration are devolatilization yields obtained with predictive coal and biomass multistep kinetic mechanisms. The variation of both particle density and diameter during char combustion is governed by the conversion mode, which is modelled using two approaches: the power law using a constant parameter that assumes a constant mode during char combustion and a constant-free model that considers a variable mode during combustion. Three numerical cases are considered: single first-order reaction with constant char combustion mode, distributed activation energy with constant char combustion mode, and single first-order reaction with variable char combustion mode. The numerical predictions from the large eddy simulations are compared with experimental results of a high co-firing rate large-scale laboratory flame of coal and biomass. Furthermore, results from single particle conversion under idealised conditions, isolating the effects of turbulence, are presented to assist the interpretation of the predictions obtained with large eddy simulations. The effects of the devolatilization and conversion mode modelling on the flame lift-off, flame length, and spatial distribution and radial profiles of O2 and CO2 are presented and discussed. Both the devolatilization and conversion mode modelling have a significant effect on the conversion of particles under idealised conditions. The large eddy simulations results show that the devolatilization model has a strong impact on the flame structure, but not on the flame lift-off. On the other hand, for the tested numerical conditions, the char combustion mode model has a marginal impact on the predicted results.

Removal of antibiotics during the anaerobic digestion of pig manure

Antibiotics are frequently used in animals to treat sickness and prevent infection especially in industrial meat production. Some of the antibiotics cannot be completely metabolized and, as an unavoidable result, are excreted and thus end up in manure which is then spread in the environment. Currently increasing amounts of manure is used in biogas production before spreading the residuals on agricultural fields. In this study, the removal patterns of sulfonamides (sulfadiazine, sulfamethizole, sulfamethoxazole) and macrolides (clarithromycin, erythromycin), as well as trimethoprim, were investigated during the anaerobic digestion of pig manure. Batch kinetic tests were conducted both at thermophilic and psychrophilic condition for 40 days. Some of the antibiotics (clarithromycin, sulfadiazine, sulfamethizole) were persistent in all experiments. Thus, no biodegradation was found for sulfadiazine and sulfamethizole in this study. From the studied compounds, only erythromycin was clearly removed and probably degraded during anaerobic digestion with 99% and 20% removal under thermophilic and psychrophilic condition. The removal of erythromycin was fitted to a single first-order kinetic reaction function, giving reaction rate constant of 0.29day−1 and 0.005day−1, respectively.

CFD based reactivity parameter determination for biomass particles of multiple size ranges in high heating rate devolatilization

This work presents a methodology that combines experimental measurements and Computational Fluid Dynamics (CFD) modeling to determine the global reaction kinetics of high heating rate biomass devolatilization. Three particle size ranges of woody biomass are analyzed: small (SF), medium (MF) and large (LF) fractions. Devolatilization mass loss is measured for each fraction in a laminar Drop-Tube Reactor (DTR) in nitrogen atmosphere, using two nominal reactor temperatures of 873 and 1173 K. Single First Order Reaction (SFOR) kinetics are determined by coupling an optimization routine with CFD models of the DTR. The global pre-exponential factors and activation energies for the SF, MF and LF particles are 5880 1/s and 42.7 kJ/mol, 48.1 1/s and 20.2 kJ/mol, and 102 1/s and 24.8 kJ/mol, respectively. These parameters are optimized for the isothermal heat transfer model available in CFD programs, and it is recommended that the specific heat capacity that was used in the optimization (1500 J/kgK) is used together with the parameters. Using the SF kinetics for small wood particles and either of the MF or LF kinetics for large particles, it is expected that more accurate devolatilization predictions can be obtained for the whole fuel size distribution in large scale CFD simulations.

Online-CPD-Coupled Large-Eddy Simulation of Pulverized-Coal Pyrolysis in a Hot Turbulent Nitrogen Jet

ABSTRACTThe pyrolysis characteristics of pulverized-coal particles in a hot turbulent nitrogen jet were investigated using large-eddy simulation (LES). In the present study an advanced pyrolysis model, the chemical percolation devolatilization (CPD) model, has been incorporated into LES in real time. The simulation results of the developed online-CPD-coupled LES were used to calibrate the kinetic parameters of the conventional single first-order reaction model (SFOM). Through the comparison between the CPD-coupled LES and the LES using the SFOM model, the CPD-coupled LES approach is found to be able to give a better prediction on particle pyrolysis in the high-temperature turbulent flow. Finally, the effects of important parameters, including the particle diameter, coal type, coal-feeding rate, carrier-phase velocity, and pyrolysis temperature, on the pulverized-coal pyrolysis process were investigated through parametric studies using the online-CPD-coupled LES method.

Pyrolysis characteristics and kinetics of low rank coals by TG-FTIR method

The paper studied the pyrolysis characteristics and kinetics of four typical low rank coals selected from different mines of China using thermogravimetry coupled with Fourier transform infrared spectrometry (TG-FTIR). The chemical structures of the low rank coals were investigated by the FTIR measurements. The FTIR spectra of the low rank coals were divided into four zones and functional groups of the chemical structures were investigated by curve-fitted method. The thermogravimetric (TG) and differential thermogravimetric (DTG) curves of low rank coals indicated that the overall pyrolysis processes could be divided into five stages based on characteristic temperatures and each stage had its unique characteristics. The pyrolysis reactions mainly occurred in the fast pyrolysis stage. The activation energy E and frequency factor A were calculated by a single first-order reaction model using Doyle integral method. The results showed that the E and A of the fast pyrolysis stage were the largest, while the obtained E of the slow pyrolysis stage was close to that of the fast polycondensation stage. The emissions of CO2 and CH4 were detected during pyrolysis process. The results revealed that the evolutions of CO2 and CH4 were affected mainly by content of carboxyl groups and content of aliphatic chains in the low rank coals, respectively. The pyrolysis characteristics and kinetics of low rank coals were closely associated with their complex chemical structures.

Furniture wood waste as a potential renewable energy source

In this study, the combustion behavior of raw waste wood from furniture and samples torrefied at temperatures of 473, 513, 553 and 593 K was investigated. TG-DTG analysis showed that the mass loss in the first stage of the process decreased with the temperature of torrefaction, whereas the temperature in the second stage increased. The influence of torrefaction and combustion parameters on greenhouse gas emissions were investigated by the FTIR technique. The characteristic combustion parameters were also determined. The ignition temperatures for the furniture wood waste and samples torrefied at 473, 513 and 553 K from 549 to 559 K, whereas that of the sample torrefied at 593 K was significantly higher (600 K). All samples were completely burnt at 813–843 K, after 29–35 min, depending on the torrefaction temperature. Kinetic parameters are determined using a two-step first-order reaction. The activation energy value for the first stage increased with the increasing temperature of torrefaction, from 68 to 125 kJ mol−1, whereas the temperature in the second stage decreased from 108 to 47 kJ mol−1. A similar correlation was observed for the pre-exponential value A. In the case of the torrefied furniture wood waste at 593 K, the combustion process runs as a single first-order reaction. The calculated data were fitted to the experimental data very accurately (R 2 > 0.9992 and standard deviation <6.7 %), and the kinetic model was correctly founded. The linear relationship between logA and E a provides a way to predict the kinetic parameters of the combustion process.

Unified Nusselt- and Sherwood-number correlations in axisymmetric finite-gap stagnation and rotating-disk flows

This paper develops a unified analysis of stagnation flow heat and mass transport, considering both semi-infinite domains and finite gaps, with and without rotation of the stagnation surface. An important objective is to derive Nusselt- and Sherwood-number correlations that represent heat and mass transport at the stagnation surface. The approach is based on computationally solving the governing conservation equations in similarity form as a boundary-value problem. The formulation considers ideal gases and incompressible fluids. The correlated results depend on fluid properties in terms of Prandtl, Schmidt, and Damköhler numbers. Heterogeneous chemistry at the stagnation surface is represented as a single first-order reaction. A composite Reynolds number represents the combination of stagnation flows with and without stagnation-surface rotation.

Pyrolysis of coal and woody biomass under N2 and CO2 atmospheres using a drop tube furnace - experimental study and kinetic modeling

Pyrolysis is the first chemical step in the overall solid fuel thermochemical process and has a significant influence on the subsequent stages. An accurate kinetic modeling of this process is thus required for a better understanding of the solid fuel conversion and for boiler CFD. This paper presents experimental investigations on the fast pyrolysis of pulverized coal and woody biomass in a drop tube furnace recently built at the GRE research lab. Experiments are carried out at several temperatures (600°C–1400°C) and under two atmospheres (N2 and CO2). The drop tube furnace is accurately equipped to get gas and wall temperature profiles. Char may be collected at different drop distances and the fuel volatile fraction is obtained by the ash-tracer method. Slow pyrolysis experiments using thermogravimetric analysis are also considered for comparison. A thermochemical conversion model of the solid fuels in the drop tube furnace is finally proposed based on non-isothermal techniques, as the particle time–temperature history in the furnace is known. The kinetic parameters for coal pyrolysis are extracted using the Kobayashi scheme. For the woody biomass, a single first-order reaction model is successfully used. The results show that the maximum yields of volatile matter is clearly affected by the temperature set point for coal, while no significant influence is observed for woody biomass. Finally, coal gasification under CO2 atmosphere is observed only for high temperatures and long residence times, thus indicating a change of the reaction scheme.

Modeling reaction–diffusion processes within catalyst washcoats: II. Macroscale processes informed by microscale simulations

This paper first develops a two-dimensional Thiele-type cylindrical-pore model that predicts catalytic washcoat performance, albeit for idealized cylindrical pores. The primary purpose for the cylindrical-pore model is to serve as a basis of comparison with three-dimensional models of catalytic performance in actual geometrically complex washcoat pores that are tomographically reconstructed from focused-ion-beam–scanning-electron-microscopy (FIB–SEM) measurements. In both models, the reaction–diffusion processes are characterized by a Damköhler number that is based on a pore diffusion coefficient and a single first-order reaction rate. Performance metrics include effective product flux from the pores, pore effectiveness, and reaction depth within the pore. In all cases, the models are generalized by casting the conservation equations and performance metrics as dimensionless variables. The paper then derives expressions to upscale individual pore performance to full-scale washcoats. The new understanding that emerges from these studies provides qualitative and quantitative insight that can assist the design and fabrication of improved washcoat microstructures.

Pyrolysis of solid fuels: Thermochemical behaviour, kinetics and compensation effect

The paper presents the pyrolysis behaviour of eleven different rank solid fuels, carried out using thermogravimetry (TG). Obtained experimental curves were compared with profiles determined using the Single First Order Reaction (SFOR) model and three different approaches of Distributed Activation Energy Model (DAEM). The aim of this work was to develop different models dedicated to pyrolysis, in particular further investigate the interrelation between the activation energy and the pre-exponential factor, its impact on the results quality and attribution with the fuel properties and pyrolysis mechanism. Obtained results showed that the compensation effect, often noted for SFOR, has also occurred for DAEM. Furthermore, analysis indicated an existence of a relationship between the characteristic temperature Tiso (and ln(kiso)) and fuel properties. Observed interrelation may be attributed to the change in reaction mechanism.

In situ observation of reduction reaction of calcium ferrites by XRD and XAFS

The lime-fluxed iron ore sinter is the major iron source in the steelmaking process. Its macroscopic properties, (e.g. strength, reduction-ability and reduction-disintegration) should strongly depend on characteristics, quantity and morphology of the bonding phase. It contains several calcium ferrite phases (CFs) including solid solutions of gang materials. Most of previous studies focused on the relation of micro-texture with properties of the sinter. However, properties of individual CFs, especially, of multi-component CFs have not been clarified yet. In this study, reduction process and rates of various CFs have been investigated by in situ observation using both X-Ray Absorption Fine Structure (XAFS) and X-Ray Diffraction (XRD) methods to clarify influence of CF species on the properties of the sinter. High temperature XAFS measurements were carried out using a synchrotron radiation source at the 9A beam line in Photon Factory (KEK-PF), Japan. Powdered single phase CFs, such as CaFe2O4, Ca2(Fe,Ca)6(Fe,Al,Si)6O20 (SFCA) were diluted with BN. Fe K and Ca K-edge XAFS spectra at 9000C and 7500C in He-H2 gas atmosphere were collected repeatedly by the quick-XAFS method. Absorption variations in normalized XANES spectra were used to calculate the reduction rate constants[1]. XRD patterns of various CFs at 9000C and 7500C were corrected repeatedly at 40-second intervals in N2-H2 gas atmosphere during reduction reaction progressed. Reduction rate of CaFe2O4, determined by XANES analysis was slower than that of Fe2O3. The reduction of Fe in CaFe2O4 was a single first-order reaction, although an induction period was clearly observed at the beginning of the reduction process. In situ XRD observations showed that CaFe2O4 was reduced into Fe and CaO via Fe2+ containing CFs such as CaFe3O5 and CaFe5O7 at 9000C. These may correspond to the induction period observed in the absorption variation. Analysis results for SFCA will be discussed in the presentation.

Evaluation of Thermal Evolution Profiles and Estimation of Kinetic Parameters for Pyrolysis of Coal/Corn Stover Blends Using Thermogravimetric Analysis

The thermal evolution profiles and kinetic parameters for the pyrolysis of two Montana coals (DECS-38 subbituminous coal and DECS-25 lignite coal), one biomass sample (corn stover), and their blends (10%, 20%, and 30% by weight of corn stover) have been investigated at a heating rate of 5°C/min in an inert nitrogen atmosphere, using thermogravimetric analysis. The thermal evolution profiles of subbituminous coal and lignite coal display only one major peak over a wide temperature distribution, ~152–814°C and ~175–818°C, respectively, whereas the thermal decomposition profile for corn stover falls in a much narrower band than that of the coals, ~226–608°C. The nonlinearity in the evolution of volatile matter with increasing percentage of corn stover in the blends verifies the possibility of synergistic behavior in the blends with subbituminous coal where deviations from the predicted yield ranging between 2% and 7% were observed whereas very little deviations (1%–3%) from predicted yield were observed in blends with lignite indicating no significant interactions with corn stover. In addition, a single first-order reaction model using the Coats-Redfern approximation was utilized to predict the kinetic parameters of the pyrolysis reaction. The kinetic analysis indicated that each thermal evolution profile may be represented as a single first-order reaction. Three temperature regimes were identified for each of the coals while corn stover and the blends were analyzed using two and four temperature regimes, respectively.

In situ QXAFS observation of the reduction of Fe2O3 and CaFe2O4

In situ QXAFS studies of the reduction of α-Fe2O3 and CaFe2O4 were conducted to determine their reduction kinetics and mechanisms. The reduction of α-Fe2O3 involved two steps, the first being a very fast process in which Fe3+ was reduced to Fe2+ and the second being the reduction of Fe2+ to Fe metal over a longer period. In contrast, the reduction of Fe in CaFe2O4 was a single first-order reaction, although an induction period was clearly observed at the beginning of the reduction process. The reduction processes were successfully studied using a combination of in situ QXAFS spectra at the Ca and Fe K-edges.

Thermal decomposition of lignin structural modification in termite digested softwood (II)

In light of increasing demand for renewable fuels and chemicals derived from nonfood biomass (lignocellulose), deciphering the mechanism of wood-cellulose utilization by wood-feeding termites became more imperative. To further explore lignin unlocking process of termite demonstrated in our previous investigations, study on thermal processing of termite digested softwood was continued using thermogravimetric analysis (TGA) at different heating rates from 2 to 50°Cmin−1, with the aid of the Friedman method. The differential thermogravimetry (DTG) curve was also deconvoluted using the Gauss and Lorentz model, into three fractions (lumps), each of which was represented by a single first-order reaction. The results showed that termite feces exhibited alterations in both the required activation energies at each thermal decomposition stage and the conversion rates of the three components of cellulose, hemicelluloses, and lignin, respectively. The activation energy for lignin decomposition decreased at its earlier decomposition stage, but increased when conversion reached 80%. The impact of structural modifications upon changes in kinetic parameters in different decomposition stages was then discussed. The fractional cellulose decomposition was decelerated but required higher temperature for the same thermal conversion extent as speculated to be a result of decrease in amorphous cellulose in termite feces. Moreover, termite digestion resulted in a more thermo-reactive hemicellulose, which possibly resulted from the bond cleavage within hemicellulose fraction and between hemicelluloses and lignin. It was proposed based on the active thermal behavior of the lignin fraction that preferential cleavage of bonds with higher dissociation energy as lignin structure unlocking process occurred within termite guts.

UV Photooxidation and Photopatterning of Alkanethiolate Self-Assembled Monolayers (SAMs) on GaAs (001)

We have investigated the photooxidation of alkanethiolate self-assembled monoalyers (SAMs) adsorbed on GaAs (001) using time-of-flight secondary ion mass spectrometry. Both -CH3- and -COOH-terminated SAMs undergo photoreaction to form sulfonated species upon exposure to UV light from a 500 W Hg arc lamp (lambda = 280-440 nm) in the presence of oxygen. In contrast to SAMs adsorbed on metals, the photooxidation of octadecanethiol adsorbed on GaAs can be fit to two first-order reactions: a fast initial reaction followed by a second slower reaction ( approximately 6 times slower). For SAMs with shorter alkyl chain lengths, the photooxidation process is can be fit to a single first-order reaction. Using the optimal photooxidation time, we also demonstrate that SAMs can be successfully UV photopatterned on GaAs substrates producing sharp, well-defined patterns.

Comparative Kinetics of Cofactor Association and Dissociation for the Human and Trypanosomal S-Adenosylhomocysteine Hydrolases. 1. Basic Features of the Association and Dissociation Processes

The S-adenosyl-l-homocysteine (AdoHcy) hydrolases catalyze the reversible conversion of AdoHcy to adenosine and homocysteine, making use of a catalytic cycle in which a tightly bound NAD+ oxidizes the 3-hydroxyl group of the substrate at the beginning of the cycle, activating the 4-CH bond for elimination of homocysteine, followed by Michael addition of water to the resulting intermediate and a final reduction by the tightly bound NADH to give adenosine. The equilibrium and kinetic properties of the association and dissociation of the cofactor NAD+ from the enzymes of Homo sapiens (Hs-SAHH) and Trypanosoma cruzi (Tc-SAHH) are qualitatively similar but quantitatively distinct. Both enzymes bind NAD+ in a complex scheme. The four active sites of the homotetrameric apoenzyme appear to divide into two numerically equal classes of active sites. One class of sites binds cofactor weakly and generates full activity very rapidly (in less than 1 min). The other class binds cofactor more strongly but generates activity only slowly (>30 min). In the case of Tc-SAHH, the final affinity for NAD+ is roughly micromolar and this affinity persists as the equilibrium affinity. In the case of Hs-SAHH, the slow-binding phase terminates in micromolar affinity also, but over a period of hours, the dissociation rate constant decreases until the final equilibrium affinity is in the nanomolar range. The slow binding of NAD+ by both enzymes exhibits saturation kinetics with respect to the cofactor concentration; however, binding to Hs-SAHH has a maximum rate constant around 0.06 s-1, while the rate constant for binding to Tc-SAHH levels out at 0.006 s-1. In contrast to the complex kinetics of association, both enzymes undergo dissociation of NAD+ from all four sites in a single first-order reaction. The equilibrium affinities of both Hs-SAHH and Tc-SAHH for NADH are in the nanomolar range. The dissociation rate constants and the slow-binding association rate constants for NAD+ show a complex temperature dependence with both enzymes; however, the cofactor always dissociates more rapidly from Tc-SAHH than from Hs-SAHH, the ratio being around 80-fold at 37 degrees C, and the cofactor binds more rapidly to Hs-SAHH than to Tc-SAHH above approximately 16 degrees C. These features present an opening for selective inhibition of Tc-SAHH over Hs-SAHH, demonstrated with the thioamide analogues of NAD+ and NADH. Both analogues bind to Hs-SAHH with approximately 40 nM affinities but much more weakly to Tc-SAHH (0.6-15 microM). Nevertheless, both analogues inactivated Tc-SAHH 60% (NAD+ analogue) or 100% (NADH analogue) within 30 min, while the degree of inhibition of Hs-SAHH approached 30% only after 12 h. The rate of loss of activity is equal to the rate of dissociation of the cofactor and thus 80-fold faster at 37 degrees C for Tc-SAHH.