Intrinsic Chemical Reactivity Research Articles

Gaseous ions and chemical mass spectrometry

The intrinsic chemical reactivities of ions can be monitored in the gas phase using mass spectrometers that are coupled to appropriate ion sources and reaction cells. Here the author surveys his own experiences over the past 40 years as an ion chemist. He used flow-tube techniques for room temperature measurements of the intrinsic chemical reactivities of a large variety of positive and negative ions, including bare atomic ions, ions found in solution, biological ions, carbonaceous ions, and interstellar, cometary, and ionospheric ions. Progress in the measurement of chemical reactions of these ions with flow-tube mass spectrometry in the author’s laboratory was been driven largely by developments in techniques of ion injection into the flow tube and of ion production (e.g., by electron impact, plasma ionization, and electrospray ionization). Chemical topics that are covered include: acid-base and nucleophilic displacement reactions that have bridged the gap between the gas phase and solution; interstellar ions and their role in molecular synthesis such as the synthesis of amino acids; the chemistry of fullerene cations as a function of charge state; fundamentals and applications of the chemistry of atomic cations with an emphasis on transition metal and lanthanide cations; atomic metal-ion catalysis; and chemical reactions of singly and multiply charged biological anions and cations in the gas phase.Key words: ions, mass spectrometry, kinetics, ion chemistry.

Evaluation of the chemical reactivity in lignin precursors using the Fukui function

The hydroxycinnamyl alcohols: p-coumarol, coniferol and sinapol are considered the basic units and precursors of lignins models. In this work, the specific reactivity of these molecules was studied. We investigate their intrinsic chemical reactivity in terms of the Fukui function, applying the principle of hard and soft acids and bases (HSAB) in the framework of the density functional theory (DFT). Comparisons of their nucleophilic, electrophilic and free radical reactivity show their most probably sites to form linkages among them. It is found that the most reactive sites, for reactions involving free radicals, are the carbons at the beta-position in the p-coumarol and sinapol molecules, whilst the regions around the carbon-oxygen bond of the phenoxyl group are the most reactive in coniferol.

Shewanella putrefaciens produces an Fe(III)-solubilizing organic ligand during anaerobic respiration on insoluble Fe(III) oxides

The mechanism of Fe(III) reduction was investigated using voltammetric techniques in anaerobic incubations of Shewanella putrefaciens strain 200 supplemented with Fe(III) citrate or a suite of Fe(III) oxides as terminal electron acceptor. Results indicate that organic complexes of Fe(III) are produced during the reduction of Fe(III) at rates that correlate with the reactivity of the Fe(III) phase and bacterial cell density. Anaerobic Fe(III) solubilization activity is detected with either Fe(III) oxides or Fe(III) citrate, suggesting that the organic ligand produced is strong enough to destabilize Fe(III) from soluble or solid Fe(III) substrates. Results also demonstrate that Fe(III) oxide dissolution is not controlled by the intrinsic chemical reactivity of the Fe(III) oxides. Instead, the chemical reaction between the endogenous organic ligand is only affected by the number of reactive surface sites available to S. putrefaciens. This report describes the first application of voltammetric techniques to demonstrate production of soluble organic-Fe(III) complexes by any Fe(III)-reducing microorganism and is the first report of a Fe(III)-solubilizing ligand generated by a metal-reducing member of the genus Shewanella.

Chemical reactivity and ion beam irradiation behaviour of perovskite- and zirconolite-nuclear ceramics type

Oxide ceramics of two neighboring families: perovskite A(II)B(IV)O3 and zirconolite A(II)B(IV)C(IV)2O7 have been synthesized by a classical solid route. Substitution of divalent cation (Ca) by trivalent cation (Nd) was tested on zirconolite compositions. Then, the ceramic pellets were submitted to aqueous leaching tests at 90°C in deionized water. Some of them were previously ion irradiated with 150keV Xe+ within a fluence range 5×1013–1×1015cm−2 in order to study the effect of ion damaging on their intrinsic chemical reactivity. X-ray diffraction (XRD), electron microprobe analysis (EMA), scanning electron microscopy (SEM) and ion beam analysis (IBA) methods were used to characterize the evolution of the crystallinity level and the surface chemical composition of the ceramics after each step (synthesis, irradiation, leaching). The alteration mechanism of unirradiated titanate ceramics appears to be not uniform at the sample surface. Chemical durability of zirconolite is shown to be dependent both on the pH of the aqueous solution and the ceramic composition. Surface hydration only concerns a very thin layer, typically 200nm and the hydrogen content does not go beyond 1–2at.%. No differences have been detected in the leaching behaviour of unirradiated or irradiated perovskite samples.

Frontier orbital and density functional study of the variation of the hard–soft behavior of monoborane (BH3) and boron trifluoride (BF3) as a function of angles of reorganization from planar (D3h) to pyramidal (C3v) shape

AbstractIn chemical response the BH3 and BF3 molecules undergo the physical process of planar (D3h) to pyramidal (C3v) reorganization in shape as the condition precedent to the event of chemical reaction under the requirement of symmetry. A frontier orbital and density functional study of the variation of the stability of electronic structures and chemical reactivity of associated with the physical process of D3h to C3v geometry reorganization has been performed. The theoretical parameters viz. eigenvalues of HOMO and LUMO, the HOMO and LUMO energy gap, the global hardness and global softness, the chemical potential, the condensed Fukui function, and local softness of B atom, the reaction site, have been computed over a wide range of ∠XBX angles. The nature of variation in the intrinsic chemical reactivity, global and local, of the molecules associated with their geometry reorganization during the chemical event of charge transfer interaction involving their frontier molecular orbitals has been quantitatively explored. The hardness profiles as a function of reaction coordinates are consistent with the principle of maximum hardness (PMH). Results demonstrate that the hardness and softness are not a static and invariable property of molecules but a dynamic and variable function of molecular structure. The hardness parameters and the HOMO–LUMO gap of the molecules are so modified with the distortion of molecular geometry that, after a certain stage of molecular deformation, the profiles of such parameters of the molecules intersect and cross each other, signifying that the relative order of the intrinsic hardness of their equilibrium geometry is reversed. The intrinsically hard molecule BF3 becomes softer than the intrinsically soft molecule BH3 as a consequence of structural distortion. The increase in chemical reactivity computed in terms of density functional parameters are transparent and justified in terms of the profiles of the eigenvalues of the frontier orbitals. The profiles of chemical potential reveal the inherent difference in the tendency of backdonation from two molecules. The computed values of Fukui functions and local softness parameters of the B atom site demonstrate that the concept of local softness can be exploited for a theoretical analysis and understanding of the characteristic chemical events of the molecules under consideration. The profiles of the Fukui functions and local softness parameters of the two molecules seem to reflect and reveal their intrinsic difference in the tendency of receiving donation in the LUMO (electrophilicity) and that of backdonation from the HOMO (nucleophilicity) and the inherent difference of overall reactivity of the two molecules by a simultaneous operation of two opposing processes of charge transfer. © 2003 Wiley Periodicals, Inc. Int J Quantum Chem, 2003

Factors governing intrinsic chemical reactivity differences between clavulanic and penicillanic acids.

To help elucidate why penicillin-G is inactivated by certain bacterial beta-lactamase enzymes, whereas clavulanic acid (Clav, which is similar to penicillin-G except at positions 1, 2, and 6) inhibits beta-lactamase, the intrinsic chemical reactivities of these two antibiotics were assessed in this work. Ab initio and continuum dielectric methods were used to map out the gas-phase and solution-phase free-energy profiles for the alkaline hydrolyses of Clav and penicillanic acid (Peni, which is similar to penicillin-G except at position 6) as well as of a fictitious hybrid compound, Peni-db, which is similar to Clav and Peni except at positions 1 and 2, respectively. Furthermore, the ring strain energies of various lactam rings and the five-membered rings of Peni and Clav as well as their respective rate-limiting transition states were computed to assess the contribution of four- and five-membered ring strains to the antibiotic's activity. The predicted product distribution, rate-limiting step, and relative reaction rates for the alkaline hydrolysis of Peni and Clav are in accord with the experimental findings. The rate-limiting step in the alkaline hydrolysis of Peni, Clav, or Peni-db is the approach of the negatively charged hydroxide ion toward the anionic reactant to form a tetrahedral intermediate. The alkaline hydrolysis of Clav generates more stable products than that of Peni mainly because the O1 atom and the hydroxyethylidene group in Clav facilitate the opening of the five-membered ring; furthermore, the O1 atom can abstract a proton easier than the less polar S1 in Peni. Clav undergoes basic hydrolysis faster than Peni mainly because its hydroxyethylidene group leads to an increase in the positive charge on the carbonyl C7 atom, therefore enhancing favorable electrostatic interactions with the incoming hydroxide anion. To a lesser extent, the oxygen at position 1 in Clav also contributes to the rate acceleration because of the greater solvent stabilization of the oxygen-containing transition state as compared to the respective ground state. The inherent strain of the four-membered beta-lactam ring or five-membered ring does not enhance the alkaline hydrolyses of beta-lactam molecules such as Peni or Clav, consistent with the observation that the rate-limiting step does not involve a breakdown of the four-membered beta-lactam ring or five-membered thiazolidine/oxazolidine rings.

Inhibition of serine proteases by functionalized sulfonamides coupled to the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold

A challenge associated with drug design is the development of selective inhibitors of proteases (serine or cysteine) that exhibit the same primary substrate specificity, that is, show a preference for the same P 1 residue. While these proteases have similar active sites, nevertheless there are subtle differences in their S and S′ subsites which can be exploited. We describe herein for the first time the use of functionalized sulfonamides as a design and diversity element which, when coupled to the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold yields potent, time-dependent inhibitors of the serine proteases human leukocyte elastase (HLE), proteinase 3 (PR 3) and cathepsin G(Cat G). Our preliminary findings suggest that (a) appending to the 1,2,5-thiadiazolidin-3-one 1,1 dioxide scaffold recognition and diversity elements that interact with both the S and S′ subsites of a target protease may result in optimal enzyme selectivity and potency and, (b) functionalized sulfonamides constitute a powerful design and diversity element with low intrinsic chemical reactivity and potentially wide applicability.

Glutathione depletion in rat hepatocytes: a mixture toxicity study with α,β-unsaturated esters

1. Glutathione (GSH) depletion is often reported as an early cytotoxic effect, caused by many reactive organic chemicals. In the present study, GSH depletion in primary rat hepatocytes was used as an in vitro effect-equivalent to measure the toxic potency of α,β-unsaturated esters (acrylates and methacrylates). 2. When these compounds were administered as a mixture, GSH depletion was dose additive. The result of the mixture study shows that GSH depletion may be a useful effect-equivalent for the risk assessment of mixtures of α,β-unsaturated esters. 3. To get more insight in the underlying mechanisms of GSH depletion, the metabolism of two esters was investigated in greater detail. One of them, allyl methacrylate, was metabolized to acrolein. This metabolic pathway can explain the high potency of allyl methacrylate to deplete GSH despite its low intrinsic chemical reactivity.

Alpha-ketoacids are potent slow binding inhibitors of the hepatitis C virus NS3 protease.

The replication of the hepatitis C virus (HCV), an important human pathogen, crucially depends on the proteolytic maturation of a large viral polyprotein precursor. The viral nonstructural protein 3 (NS3) harbors a serine protease domain that plays a pivotal role in this process, being responsible for four out of the five cleavage events that occur in the nonstructural region of the HCV polyprotein. We here show that hexapeptide, tetrapeptide, and tripeptide alpha-ketoacids are potent, slow binding inhibitors of this enzyme. Their mechanism of inhibition involves the rapid formation of a noncovalent collision complex in a diffusion-limited, electrostatically driven association reaction followed by a slow isomerization step resulting in a very tight complex. pH dependence experiments point to the protonated catalytic His 57 as an important determinant for formation of the collision complex. K(i) values of the collision complexes vary between 3 nM and 18.5 microM and largely depend on contacts made by the peptide moiety of the inhibitors. Site-directed mutagenesis indicates that Lys 136 selectively participates in stabilization of the tight complex but not of the collision complex. A significant solvent isotope effect on the isomerization rate constant is suggestive of a chemical step being rate limiting for tight complex formation. The potency of these compounds is dominated by their slow dissociation rate constants, leading to complex half-lives of 11-48 h and overall K(i) values between 10 pM and 67 nM. The rate constants describing the formation and the dissociation of the tight complex are relatively independent of the peptide moiety and appear to predominantly reflect the intrinsic chemical reactivity of the ketoacid function.

Electron beam sensitivity of butyl acrylate copolymers: effects of composition on reactivity

Partial transesterification of poly(butyl acrylate) (PBuA) was developed and used as a novel method for obtaining acrylate copolymers possessing the same distribution of the degree of polymerization (DP) as that of the parent PBuA and bearing various organofunctional side groups with olefinic unsaturations or heteroatoms. The series of copolymers exposed to accelerated electron beams exhibited radiation sensitivity expectedly dependent on chemical composition. The copolymer reactivity was examined and quantified by applying the Charlesby-Pinner formalism to the gel fraction content as a function of the absorbed dose. The modifications of reactivity for small variations of the copolymer composition were determined by comparing the values of radiochemical yields for scission G(S) and for crosslinking G(X) of the slightly modified polymers with that of the parent homopolymer. The resulting differential contributions of infinitesimal amounts of functional comonomer units are discussed in terms of intrinsic chemical reactivity. Additional physical effects are also invoked for higher contents in the comonomer that induce variations of polymer segments mobility in the bulk state.

Molecular basis of halothane hepatitis.

The formation of covalent adducts to cellular macromolecules, including proteins, phospholipids, and DNA or RNA, is associated with the exposure of humans and animals (most probably to any given living organism) to a large number of xenobiotics, including drugs as well as occupational and environmental pollutants (Hinson and Roberts, 1992; Park and Kitteringham, 1990; Nelson and Pearson, 1990). Only few parent compounds form, based on their intrinsic chemical reactivity, such adducts spontaneously; the majority of compounds do form adducts to cellular macromolecules only after metabolic activation to reactive intermediates (Park and Kitteringham, 1990; vanWelie et al., 1992). From an immunological point of view, adducted cellular target molecules constitute “modified self” or even “non-self” structures, which might provoke immune responses against themselves (Allison 1989; deWeck 1983). In general, low molecular mass organic compounds (<l000 Da), such as most drugs, are thought to be non-immunogenic per se. However, many of these compounds (coined haptens) may become immunogenic, when covalently linked to a macromolecular carrier such as a protein. Once formed, drug-carrier conjugates might act as immunogens and elicit immune responses at the humoral level, at the cellular level, or at both levels (Allison 1989; deWeck 1983). These immune responses might be directed against at least three different types of antigenic determinants. First, haptenic epitopes may include the derivative of the xenobiotic (i.e. hapten) bound to the carrier molecule. Second, new antigenic determinants (NAD) may comprise newly created linear or conformational structures on the carrier molecule elicited upon binding of the hapten to the carrier molecule. Last, cryptic autoantigenic determinants of the carrier molecule, which normally are seen as self or are ignored, could bypass the mechanisms of immunological self-tolerance as a consequence of hapten binding (Allison, 1989; Blooksma and Schuurman, 1989; Roitt et al., 1985). An immunological basis for the development, in susceptible individuals, of a number of severe, sometimes life-threatening adverse effects after therapeuticKeywordsLipoic AcidProtein Disulfide IsomeraseMolecular MimicryOctanoic AcidPyruvate Dehydrogenase ComplexThese keywords were added by machine and not by the authors. This process is experimental and the keywords may be updated as the learning algorithm improves.

Ethylene biosynthesis: processing of a substrate analog supports a radical mechanism for the ethylene-forming enzyme

Background: The chemical mechanism of the final step of ethylene biosynthesis (the conversion of 1-aminocyclopropanecarboxylic acid, ACC 1 to ethylene by ACC oxidase, the ethylene-forming enzyme, EFE) is poorly understood. Two possibilities have been suggested: a radical mechanism and an N-hydroxylation mechanism. We investigated reaction pathways available to radical intermediates in this reaction using an ACC analog, 1-aminocyclobutanecarboxylic acid (ACBC) as a substrate. Results: ACBC was converted to dehydroproline (A 1-pyrroline-2-carboxylic acid) by the EFE via a ring expansion process. The possibility that an N-hydroxy-aminoacid (produced during two-electron oxidation) acts as an intermediate in this process was eliminated by control experiments. Chemical model reactions involving two-electron oxidants, such as a positive halogen (X +) which presumably generate N-halo derivatives, produce only decarboxylation products. Radical-based oxidants, in contrast, generate dehydroproline. Model reactions involving sequential single-electron transfer mechanisms also produce dehydroproline; thus our results support the proposal that the EFE-catalyzed step of ethylene biosynthesis proceeds using a radical-based mechanism. Conclusions: Our results provide support for a radical mechanism in the final step of ethylene biosynthesis and refute an alternative N-hydroxylation mechanism. This work extends the idea that the intrinsic chemical reactivity of a high energy iron—oxo intermediate can account for the observed products in ethylene biosynthesis.

Influence of the Halogen-Substituent Pattern of Fluoronitrobenzenes on Their Biotransformation and Capacity to Induce Methemoglobinemia

In the present study both the biotransformation patterns and the capacity to induce methemoglobinemia of a series of fluoronitrobenzenes were investigated. This was done to investigate to what extent variation in the number and position of the halogen substituents influence the metabolic fate of the fluoronitrobenzenes, thereby influencing their capacity to induce methemoglobinemia. The results obtained were compared to the effect of the fluorine substituent patterns on the calculated electronic characteristics and, thus, on the chemical reactivity of the fluoronitrobenzenes. Analysis of thein vivometabolic profiles demonstrates a dependence of the extent of nitroreduction, of glutathione conjugation, and of aromatic hydroxylation with the pattern of halogen substitution. With an increasing number of fluorine substituents at electrophilic carbon centers, 24-hr urine recovery values decreased and fluoride anion elimination increased, due to increased reactivity of the fluoronitrobenzenes with cellular nucleophiles.In vitrostudies even demonstrated a clear correlation between calculated parameters for the electrophilicity of the fluoronitrobenzenes and the natural logarithm of their rate of reaction with glutathione or with bovine serum albumin, taken as a model for cellular nucleophiles (r= 0.97 andr= 0.98, respectively). Increased possibilities for the conjugation of the fluoronitrobenzenes to cellular nucleophiles were accompanied by decreased contributions of nitroreduction and aromatic hydroxylation to the overallin vivometabolite patterns, as well as by a decreased capacity of the fluoronitrobenzenes to induce methemoglobinemia.In vitrostudies on the rates of nitroreduction of the various fluoronitrobenzenes by cecal microflora and rat liver microsomes revealed that the changes in the capacity of the fluoronitrobenzenes to induce methemoglobinemia were not due to differences in their intrinsic reactivity in the pathway of nitroreduction, leading to methemoglobinemia-inducing metabolites. Thus, the results of the present study clearly demonstrate that the number and position of fluorine substituents in the fluoronitrobenzenes influence the capacity of the fluoronitrobenzenes to induce methemoglobinemia, not because their intrinsic chemical reactivity for entering the nitroreduction pathway is influenced. The different methemoglobinemic capacity must rather result from differences in the inherent direct methemoglobinemic capacity and/or reactivity of the various toxic metabolites and/or from the fact that the halogen substituent pattern influences the electrophilic reactivity, thereby changing the possibilities for reactions of the nitrobenzenes with glutathione and, especially, other cellular nucleophiles. When the number of fluorine substituents increases, the electrophilicity of the fluoronitrobenzenes can become so high that glutathione conjugation is no longer able to compete efficiently with covalent binding of the fluoronitrobenzenes to cellular macromolecules. As a consequence, it can be suggested that with an increasing number of fluorine substituents at electrophilic carbon centers in a nitrobenzene derivative, a toxic end point of the nitrobenzene other than formation of methemoglobinemia can be foreseen.

Rapid formation of the native 14-38 disulfide bond in the early stages of BPTI folding.

Using recombinant variants of BPTI, we have determined the rate constants corresponding to formation of each of the fifteen possible disulfide bonds in BPTI, starting from the reduced, unfolded protein. The 14-38 disulfide forms faster than any of the other 14 possible disulfides. This faster rate results from significantly higher intrinsic chemical reactivities of Cys-14 and Cys-38, in addition to local structure in the reduced protein that facilitates formation of the 14-38 disulfide bond. This disulfide bond is found in native BPTI. Our results suggest that a significant flux of folding BPTI molecules proceed through the one-disulfide intermediate with the 14-38 disulfide bond, denoted [14-38], that has recently been detected on the BPTI folding pathway. In addition to providing a detailed picture of the early events in the folding of BPTI, our results address quantitatively the effect of local structure in the unfolded state on folding kinetics.

Effects of ligand homologation and ligand reactivity on the apparent kinetic specificity of papain

Papain, a prototype cysteine proteinase, shows a pronounced kinetic preference for substrates and inhibitors based on the Ac- l-Phe-Gly-structural motif. Replacing the l-Phe at position P 2 with D-Phe, or with a less hydrophobic residue such as Leu or Met, results in decreases of substrate or inhibitory activity of up to 400-fold. In this study we examined the effect of homologating the P 1 glycine moiety to ß-alanine in the context of specific ester and amide substrates, peptidyl nitrile and -aldehyde transition state analog inhibitors, and peptidyl Michael acceptors as irreversible affinity labels. Papain discriminates extremely strongly (i.e., from 1000-fold to ⩾ 29000-fold) against the ‘homologs’ based on ß-alanine at P 1 compared to ‘analogs’ based on glycine at P 1. However, with highly reactive ligands such as p-nitrophenyl esters, homolog/analog discrimination is greatly reduced (i.e., ⩽ 10-fold). These observations are interpreted in terms of (1) cooperativity between several non-covalent enzyme-ligand interactions and the covalent interaction of the ligand P 1 moiety with Cys-25 of papain, (2) the deceased ability to homologs to utilize these cooperative interactions optimally because of their extended size, and (3) a decrease in the importance of the cooperative interactions as the intrinsic chemical reactivity of the ligand increases. Some implications of this analog vs. homolog discrimination for peptidyl disulfide and peptidyl chloromethane probes of protease specificity and mechanism are discussed.

Identification and Characterization of O-Biotinylated Hydroxy Amino Acid Residues in Peptides

Recent findings have indicated that certain hydroxyl groups of peptides have enhanced intrinsic chemical reactivity and can be O-acylated by N-hydroxysuccinimide esters of biotin. Analytical procedures are described for the identification of O-acylated derivatives of seryl, threonyl, and tyrosyl residues after reaction with the N-hydroxysuccinimide ester of biotin containing the extended spacer arm 6-aminohexanoic acid. The identification and chemical characterization of O-biotinylated residues included amino acid analysis, peptide sequence determination, and mass spectrometric analysis. The application of these analytical procedures provided unequivocal identification of O-biotinylated residues for a number of peptides investigated. In a separate study, we also show that O-biotinylated amino-terminal seryl residues occurring after proteolysis of peptides or proteins can rapidly undergo an O → N acyl shift resulting in blockage of sequence analysis by the Edman degradation procedure.

Crystal structure of selenosubtilisin at 2.0-.ANG. resolution

The three-dimensional structure of selenosubtilisin, an artificial selenoenzyme, has been solved at 2.0-A resolution by the method of molecular replacement. Selenosubtilisin is a chemical derivative of the bacterial serine protease subtilisin in which the catalytically essential serine residue has been replaced with a selenocysteine. Its unique hydrolytic and redox properties reflect the intrinsic chemical reactivity of the selenium prosthetic group. Structural analysis of the modified protein reveals that the selenium moiety is selectively incorporated into the side chain of residue 221 and confirms the seleninic acid oxidation state expected from treatment of the enzyme with hydrogen peroxide prior to crystallization. Although the seleninic acid replaces the essential nucleophile in the enzyme's catalytic triad and introduces a negative charge into the active site, the interaction between His64 and Asp32 is not altered by the modification. Hydrogen bonds from the oxygen atoms of the seleninic acid to His64 and to Asn155 in the oxyanion hole confine the prosthetic group to a single well-defined conformation within the active site. These interactions thus provide a structural basis for understanding the seleninic acid's unusually low pKa, the enzyme's relatively sluggish rate of reaction with thiols, and its much more efficient peroxidase activity. Aside from the active site region, the structure of the protein is essentially the same as that previously reported for native subtilisin Carlsberg, indicating the viability of chemical modification strategies for incorporating site-specific changes into the protein backbone. Comparison of the three-dimensional structures of selenosubtilisin and glutathione peroxidase, an important naturally occurring selenoenzyme, provides the means to evaluate how the function of the selenium prosthetic group varies with molecular context.

Mechanistic relationship among mutagenicity, skin sensitization, and skin carcinogenicity.

Twenty organic Salmonella mutagens, seven of which (including benzo[a]pyrene) are established skin carcinogens, and one of which (2-chloroethanol) is a well-defined noncarcinogen to skin, have been evaluated for skin-sensitizing activity using the local lymph node assay. The relative mutagenicity of the agents to Salmonella was also established. Fourteen of the chemicals were positive in the local lymph node assay, including the seven skin carcinogens. 2-Chloroethanol was inactive as a sensitizing agent. We suggest that a variety of factors contributes to the lack of sensitizing activity of the remaining six bacterial mutagens: extremes of intrinsic chemical reactivity, high water solubility reducing dermal translocation, and inappropriate dermal metabolism. Two reference skin-sensitizing agents (an oxazolinone and fluorescein isothiocyanate) were established as in vitro clastogens after their recognition as nonmutagens to Salmonella. These data imply that mutagenicity, rather than simply activity in the Salmonella assay, is a primary stimulus for electrophilic sensitization and carcinogenic initiation in the skin. We conclude that genotoxicity data for an agent can provide indications of the agent's potential to induce skin sensitization and that genotoxins which are skin-sensitizing agents have an enhanced potential to initiate skin carcinogenesis. We suggest that common, albeit individually distinct, structure-activity relationships underpin genotoxicity, skin sensitization, and the initiation of skin carcinogenesis. These relationships should simplify the hazard evaluation of chemicals and contribute to a reduction in animal usage. Several predictions of skin carcinogenicity are made based on the data presented.

Structure-activity relationships for inhibition of papain by peptide Michael acceptors.

Two series of peptidyl Michael acceptors, N-Ac-L-Phe-NHCH2CH = CH-E with different electron withdrawing groups (E = CO2CH3, 1a; SO2CH3, 1b; CO2H, 1c; CN, 1d; CONH2, 1e; and C6H4-p-NO2, 1f) and R-NHCH2CH = CHCOOCH3 with different recognition and binding groups (R = N-Ac-D-Phe, 2a; N-Ac-L-Leu, 3a; N-Ac-L-Met, 4a; PhCH2CH2CO, 5a; PhCO, 6a), were synthesized and evaluated as inactivators against papain. It was found that the inhibition of papain by peptidyl Michael acceptors is a general phenomenon and that the intrinsic chemical reactivity of the E group in the Michael acceptors has a direct effect on the kinetics of the inactivation process as reflected in k2/Ki. At pH 6.2, the reactivity of papain toward the Michael acceptors is about 283,000-fold higher than the reactivity of the model thiol 3-mercaptopropionate. This large increase in reactivity is attributable to at least 2 factors; one is the low apparent pKa of Cys-25 of papain, and the other is the recruitment of catalytic power by specific enzyme-substrate interactions. The unexpectedly high reactivity of 1c (E = COOH) was rationalized by proposing a direct interaction of the acid group with His-159 in the active site of papain. The unexpected inactivity of 1f (E = C6H4-p-NO2) as a Michael acceptor and its very powerful competitive inhibition of papain were rationalized by molecular graphics which showed the nitrophenyl moiety rotated out of conjugation with the olefin and interacting instead with the hydrophobic S1' region of papain. A plot of log (k2/Ki) for 1a-6a vs log (kcat/Km) for analogous R-Gly-p-NA substrates was linear (r = 0.98) with slope of 0.83, suggesting that binding energy from specific enzyme-ligand interactions can be used to drive the self-inactivation reaction to almost the same extent as it is used to drive catalysis.

Heat and mass transfer limitations in gasification of carbon by carbon dioxide

Rates of gasification of carbon by carbon dioxide were measured in the laboratory using a thermogravimetry apparatus. The experiments were conducted at various temperatures and sample bed depths using powders of graphite and coconut char as sources of carbon, under flowing gas of CO2. Various CO–CO2 mixtures were also employed for the study of coconut char. The measured rate data were subjected to a heat and mass transfer analysis to find out the values of isothermal and non‐isothermal effectiveness factors. These values were utilized to predict the intrinsic chemical reactivities of graphite and coconut char samples, free from heat and mass‐transfer limitations. Extrapolation of experimental data to hypothetical zero bed depth showed good agreements with the above at various temperatures, thus providing valuable cross‐checks. The activation energies were found to be 260 and 250 kJ/mol for graphite and coconut char, respectively, under CO2. Carbon monoxide was found to decrease the rate of gasification but not as drastically as some literature reports predict.