Intramolecular Exchange Reactions Research Articles

Engineering a Fluorescent Protein Color Switch Using Entropy-Driven β-Strand Exchange.

Protein conformational switches are widely used in biosensing. They are often composed of an input domain (which binds a target ligand) fused to an output domain (which generates an optical readout). A central challenge in designing such switches is to develop mechanisms for coupling the input and output signals via conformational changes. Here, we create a biosensor in which binding-induced folding of the input domain drives a conformational shift in the output domain that results in a sixfold green-to-yellow ratiometric fluorescence change in vitro and a 35-fold intensiometric fluorescence increase in cultured cells. The input domain consists of circularly permuted FK506 binding protein (cpFKBP) that folds upon binding its target ligand (FK506 or rapamycin). cpFKBP folding induces the output domain, an engineered green fluorescent protein (GFP) variant, to replace one of its β-strands (containing T203 and specifying green fluorescence) with a duplicate β-strand (containing Y203 and specifying yellow fluorescence) in an intramolecular exchange reaction. This mechanism employs the loop-closure entropy principle, embodied by the folding of the partially disordered cpFKBP domain, to couple ligand binding to the GFP color shift. This study highlights the high-energy barriers present in GFP folding which cause β-strand exchange to be slow and are also likely responsible for the shift from the β-strand exchange mechanism in vitro to ligand-induced chromophore maturation in cells. The proof-of-concept design has the advantages of full genetic encodability and potential for modularity. The latter attribute is enabled by the natural coupling of binding and folding and circular permutation of the input domain, which theoretically allows different binding domains to be compatible for insertion into the GFP surface loop.

Single-step fluorescent probes to detect decrotonylation activity of HDACs through intramolecular reactions.

Lysine crotonylation plays vital roles in gene transcription and cellular metabolism. Nevertheless, methods for dissecting the molecular mechanisms of decrotonyaltion remains limited. So far, there is no single-step fluorescent method developed for enzymatic decrotonylation activity detection. The major difficulty is that the aliphatic crotonylated lysine doesn't allow π-conjugation to a fluorophore and decrotonylation can not modulate the electronic state directly. Herein, we have designed and synthesized two activity-based single-step fluorogenic probes KTcr-I and KTcr-II for detecting enzymatic decrotonylation activity. These two probes can be recognized by histone deacetylases and undergo intramolecular nucleophilic exchange reaction to generate fluorescence signal. Notably, peptide sequence-dependent effect was observed. KTcr-I can be recognized by Sirt2 more effectively, while KTcr-II with LGKcr peptide sequence preferentially reacted with HDAC3. Compared to other methods of studying enzymatic decrotonylation activity, our single-step fluorescent method has a number of advantages, such as facileness, high sensitivity, cheap facility and little material consumed. We envision that the probes developed in this study will provide useful tools to screen inhibitors which suppress the decrotonylation activity of HDACs. Such probes will be useful for further delineating the roles of decrotonylation enzyme and aid in biomarker identification and drug discovery.

Theoretical study for conformational analysis and kinetics on the internal halogen exchange thermally induced of trichloro-(1,1-difluoroethyl)silane in the gas phase

Consecutive first-order, homogeneous intramolecular fluorine-chlorine exchange reactions of trichloro-(1,1-difluoroethyl)silane have been theoretically studied using ab initio post-Hartree-Fock G4MP2 and DFT (B3LYP, M06, M06-2X) levels of theory. The reactions were studied at 127 °C and 0.1 Torr of temperature and pressure respectively, yielding (1-choro-1-fluoroethyl)-dichlorofluorosilane, and chloro-difluoro(1,1-dichloroethyl)silane. A very good agreement has been found between B3LYP/6-311+G(2df,p) calculations and previously reported experimental results. Conformational mobility in the structure of the intermediate and final product was studied in order to assign the highest and lowest in energy conformers. Analysis of activation parameters, Natural Bond Order (NBO) and Atoms In Molecules (AIM) suggests that in the first exchange reaction, the fluorine atom is almost completely transferred to silicon prior to chlorine migration, producing a penta-coordinated Si center and a positive-charged carbon in the transition state (TS). The second fluorine-chlorine switch proceeds through a more synchronized three-membered cyclic transition state, as opposed to the results obtained in a previous work, which proposes that the cyclic TS take place in the first halogen interchange. The differences in synchronicity (Sy) parameters between the two reactions steps are attributed to the difference in the uniformity of the electron distribution. According to IRC calculations, transition states are early and both exchanges are exothermic.

Intramolecular Metal Exchange Reaction Promoted by Thiol Ligands.

The synthesis of an alloy nanocluster that is atomically precise is the key to understanding the metal synergy effect at the atomic level. Using the Ag2Au25(SR)18 nanocluster as a model, we reported a third approach for the metal exchange reaction, that is, intramolecular metal exchange. The surface adsorbed metal ions (i.e., Ag) can be exchanged with the kernel metal atoms (i.e., Au) that are promoted by thiol ligands. The exchanged gold atoms can be further stripped by the thiol ligands, and produce the AgxAu25−x(SR)18− nanocluster.

Intra- and inter-protein couplings of backbone motions underlie protein thiol-disulfide exchange cascade

The thioredoxin (Trx)-coupled arsenate reductase (ArsC) is a family of enzymes that catalyzes the reduction of arsenate to arsenite in the arsenic detoxification pathway. The catalytic cycle involves a series of relayed intramolecular and intermolecular thiol-disulfide exchange reactions. Structures at different reaction stages have been determined, suggesting significant conformational fluctuations along the reaction pathway. Herein, we use two state-of-the-art NMR methods, the chemical exchange saturation transfer (CEST) and the CPMG-based relaxation dispersion (CPMG RD) experiments, to probe the conformational dynamics of B. subtilis ArsC in all reaction stages, namely the enzymatic active reduced state, the intra-molecular C10–C82 disulfide-bonded intermediate state, the inactive oxidized state, and the inter-molecular disulfide-bonded protein complex with Trx. Our results reveal highly rugged energy landscapes in the active reduced state, and suggest global collective motions in both the C10–C82 disulfide-bonded intermediate and the mixed-disulfide Trx-ArsC complex.

Halogen effect on mixed-valence state of biosmocenium(II, IV) salts

The mixed-valence biosmocenium(II, IV)–Cl adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVCl]Y(1a:YPF6, 1b:YBF4) and biosmocenium-Br adduct [(C5H5)OsII(μ2-η5:η5-C10H8) (C5H5)OsIVBr]PF6(2) were prepared. The structures of 1b and 2 were determined by single crystal X-ray analysis, and it was found that Os-X bond was fixed on cis position to the other half of osmocenyl moiety. Although 1b and 2 had the trapped-valence state in solid state, valence-detrapping depending on temperature was observed in solution state by using variable-temperature 1H NMR spectroscopy. It was found that intramolecular electron exchange reaction OsII–OIV ⇄ OsIV–OsII occurred accompanied by halogen migration. The activation energy of the reaction was determined. The energy decreased in the order A > 2 > 1a depending on a kind of halogen ligand bonded to Os center, and the tendency can be explained by HSAB theory.

Investigations of the Degenerate Intramolecular Charge Exchange in Symmetric Organic Mixed Valence Compounds: Solvent Dynamics of Bis(triarylamine)paracyclophane Redox Systems

Triarylamines are important hole-transport components in optoelectronic devices. Understanding the factors controlling their intra- and intermolecular electron transfer properties is crucial to the application and optimization of organic hole conductors. Here, we report on the degenerate intramolecular electron exchange reactions of several purely organic mixed valence compounds based on the bis(triarylamine)paracyclophane structural unit, which are archetypical molecular wires. Different bridging moieties are compared, and the foremost impact of the solvent environment on the rate of electron transfer is demonstrated. Comparing the rate constants found for many different solvents, we find that surprisingly the electron transfer reaction is limited by the solvent dynamic effect and not strongly impacted by the peculiarities of the bridging moiety, a finding which was not anticipated for this type of long-range, thermally activated intramolecular charge transfer from previous studies. Rate constants are measured by dynamic electron paramagnetic resonance spectroscopy. Our insight was possible using various solvents spanning a wide range of longitudinal relaxation times (0.24 ps ≤ τL ≤ 516 ps) and Pekar factors (0.298 ≤ γ ≤ 0.526).

An Efficient Synthesis of Benzimidazoles via Palladium-Catalyzed Amine Exchange Reaction from Trialkylamines to o-Phenylenediamine in an Aqueous Medium

Transition metal-catalyzed alkyl group transfer between alkylamines has been known as amine exchange reaction (amine scrambling reaction) and used for the synthesis of unsymmetrical amines and N-heterocycles and the study of the metabolism of amines. During the course of our studies directed towards transition metal-catalyzed C-N bond activation of alkylamines, we developed an alkyl (or alkanol) group transfer from alkylamines (or alkanolamines) to Natom of anilines as well as α-carbon of ketones, which leads to a regioselective α-alkylation of ketones. The former transfer eventually leads to indoles and quinolines under the employed reaction conditions. However, except for our findings, there have been known only a few examples for the synthesis of N-heterocycles using such an amine exchange reaction. It is known that hydropyrimidines, imidazolidines and imidazoles could be formed by palladium-catalyzed intermolecular amine exchange reaction between diamines and alkylamines. Diamines were found to be cyclized to pyrrolidine, piperidine, and azepane via ruthenium-catalyzed intramolecular amine exchange reaction. On the other hand, in connection with this report, Murahashi et al. reported that N-methylbenzylamine reacts with o-phenylenediamine in the presence of Pd/C to give 2-phenylbenzimidazole and 1benzyl-2-phenylbenzimidazole in 37% and 25% yields, respectively. Under these circumstances, the present reaction was disclosed during the course of seeking for a more efficient catalytic system on an intrinsic amine exchange reaction. Herein this report describes an efficient synthesis of benzimidazoles via palladium-catalyzed amine exchange reaction from trialkylamines to o-phenylenediamine in an aqueous medium. To investigate the effect of reaction variants such as solvent, reaction temperature and time, o-phenylenediamine (1) and tributylamine (2a) were chosen as a model reaction. Treatment of equimolar amounts of 1 and 2a in toluene at 110 C for 20 h in the presence of a catalytic amount of 5% Pd/C afforded 2-propylbenzimidazole (3a) in 47% isolated yield with 67% conversion of 1 (run 1). Performing the reaction for a longer reaction time under two-fold molar ratio of 2a to 1 gave no improvement in the yield of 3a (run 2). Higher reaction temperature in toluene was needed for the effective formation of 3a (run 3). However, the reaction carried out under the further addition of H2O resulted in an increased yield of 3a (72%) along with concomitant formation of further N-alkylated benzimidazole 4 (2%) (run 4). In spite of further elaboration for the optimization of reaction conditions (runs 5-7), the best result in terms of the yield of 3a and the selectivity of 3a to 4 is best accomplished under the standard set of condition shown in run 4 of Table 1. Based on reaction conditions of Table 1, various trialkylamines 2 were subjected to the reaction with 1 in order to investigate the reaction scope, and several representative results are summarized in Table 2. An array of trialkylamines (2a-e) having straight alkyl chains reacted with 1 and the corresponding benzimidazoles (3a-e) were obtained in a range of 57-72% yields. Generally, the product yield gradually decreased as the alkyl chain length on 2a-e increases. Thus, in the reaction with 2d and 2e, a longer reaction time was needed for the allowable yield of products. Furthermore, in the case of 2e, three-fold molar ratio of 2e to 1 was necessary for the effective formation of 3e. When the reaction was carried out with two-fold molar ratio of 2e to 1 for 40 h under the employed conditions, 3e was obtained in 31% yield. In the reaction with trialkylamines (2f and 2g) having branched alkyl chains, similar reaction rate and yield were observed with triisoamylamine (2g), whereas higher reaction

Zirconium-catalyzed chemoselective methylalumination of ethers, amines, and sulfides bearing two terminal alkenyl groups

Chemoselectivity in the methylalumination reaction of unsymmetrical ethers, amines, and sulfides bearing two different terminal alkenyl groups, a 13-tetradecenyl group and an allyl, 4-pentenyl or 6-heptenyl group was examined. The methylalumination of the allyl derivatives proceeded with complete chemoselectivity to afford only the 13-tetradecenyl-monomethylated products. In the methylalumination reactions of the 4-pentenyl and the 6-heptenyl derivatives, in addition to the 13-tetradecenyl-monomethylated products, and dimethylated products were also obtained. However, as in the case of the allyl derivatives, monomethylation to the shorter 4-pentenyl or 6-heptenyl group was not observed, except in the case of 6-heptenyl 13-tetradecenyl amine. The unique selectivity was rationalized upon how readily the intramolecular ligand exchange reaction between intermediate zirconocenium–alkene and zirconocenium–heteroatom complexes could occur.

Structure and dynamics of binary and ternary lanthanide(iii) and actinide(iii) tris[4,4,4-trifluoro-1-(2-thienyl)-1,3-butanedione] (TTA) complexes. Part 2, the structure and dynamics of binary and ternary complexes in the Y(iii)/Eu(iii) –TTA – tributylphosphate (TBP) system in chloroform as studied by NMR spectroscopy

The stoichiometric reaction mechanisms, rate constants and activation parameters for inter- and intramolecular ligand exchange reactions in the binary Y/Eu(TTA)(3)(OH(2))(2)-HTTA and the ternary Y/Eu(TTA)(3)(OH(2))(2)-TBP systems have been studied in chloroform using (1)H and (31)P NMR methods. Most complexes contain coordinated water that is in very fast exchange with water in the chloroform solvent. The exchange reactions involving TTA/HTTA and TBP are also fast, but can be studied at lower temperature. The rate constant and activation parameters for the intramolecular exchange between two structure isomers in Y(TTA)(3)(OH(2))(2) and Y(TTA)(3)(TBP)(OH(2)) were determined from the line-broadening of the methine protons in coordinated TTA. The rate equations for the intermolecular exchange between coordinated TTA and free HTTA in both complexes are consistent with a two-step mechanism where the first step is a fast complex formation of HTTA, followed by a rate determining step involving proton transfer from coordinated HTTA to TTA. The rate constants for both the inter- and intramolecular exchange reactions are significantly smaller in the TBP system. The same is true for the activation parameters in the Y(TTA)(3)(OH(2))(2)-HTTA and the ternary Y/Eu(TTA)(3)(TBP)(OH(2))-HTTA systems, which are ΔH(≠) = 71.8 ± 2.8 kJ mol(-1), ΔS(≠) = 62.4 ± 10.3 J mol(-1) K(-1) and ΔH(≠) = 38.8 ± 0.6 kJ mol(-1), ΔS(≠) = -93.0 ± 3.3 J mol(-1) K(-1), respectively. The large difference in the activation parameters does not seem to be related to a difference in mechanism as judged by the rate equation; this point will be discussed in a following communication. The rate and mechanism for the exchange between free and coordinated TBP follows a two-step mechanism, involving the formation of Y(TTA)(3)(TBP)(2).

Radiolytic cyclization of stem-and-loop structured oligodeoxynucleotide with neighboring arrangement of α,ω-bis-disulfides

Upon X-ray irradiation of hypoxic aqueous solution, modified oligodeoxynucleotides (ODNs) bearing a pair of disulfides at both ends of the strand that forms a stem-and-loop structure with a neighboring arrangement of α,ω-bis-disulfides underwent efficient cyclization via an intramolecular exchange reaction at the disulfide moieties with a multiple turnover process. Mechanistic studies revealed that hydrogen atoms generated in the radiolysis of water are key active species initiating a chain reaction to produce cyclic ODN disulfides, in which addition of hydrogen atom results in dissociation of the original disulfide bond to generate a thiyl radical intermediate as the chain carrier for the succeeding disulfide exchange into cyclization. The properties were also assessed for the resultant cyclic ODN disulfide that has several favorable features for use in the transcriptional decoy strategy. The cyclic ODN disulfides produced by the present radiolytic method showed high thermal stability, resistance to nuclease, and high binding activity to a representative transcriptional factor of nuclear factor κB.

Assessing the magnitude of folding forces along the oxidative folding pathway of multi-disulfide-containing proteins.

Oxidative folding involves the acquisition of the native set of disulfide bonds and native structure of a protein from its reduced, unfolded form. Two major factors that influence the distribution of disulfide bonds (over their entropically favored probability of formation) during the regeneration, eventually favoring and sustaining the native set of disulfide bonds, are enthalpic interactions that lead to conformational ordering among intermediates and conformational folding, resulting in native (or nativelike) structure which protects the native disulfide bonds from reduction and intramolecular thiol-disulfide exchange reactions. Here, we report a novel technique to provide a quantitative description of the enthalpic and conformational folding forces present at any stage of the oxidative folding process of multi-disulfide-containing proteins which is made feasible by assessing their ability to form "structures" that are able to protect disulfide bonds from reduction.

Reversible Unfolding of Bovine β-Lactoglobulin Mutants without a Free Thiol Group

Bovine beta-lactoglobulin (beta-lg) has been used extensively as a model for studying protein folding. One of the problems preventing clarification of the folding mechanism is the incomplete reversibility from the unfolded state, probably caused by the thiol-disulfide exchange between a free thiol at Cys-121 and two disulfide bonds. We constructed and expressed three beta-lg subtype A mutants in which Cys-121 was replaced by Ala, Ser, or Val (i.e. C121A, C121S, and C121V). We studied the reversibilities of these mutants from urea denaturation using circular dichroism, tryptophan fluorescence, reversed-phase and gel-filtration high performance liquid chromatographies, and SDS-PAGE. The folded structure of each mutant was similar to that of wild-type beta-lg. Urea-induced unfolding at pH 7.0 and 3.0 showed that although the C121S mutation notably decreases the stability, the destabilizing effects of the C121A and C121V mutations are less severe. For all of the mutants, complete refolding from the unfolded state in 8 M urea at both pH 7.0 and 3.0 was observed. Kinetics of the formation of the irreversibly unfolded species of wild-type beta-lg in 8 M urea at pH 7.0 indicated that, first, an intramolecular thiol-disulfide exchange occurs to produce a mixture of species with non-native disulfide bonds followed by the intermolecular thiol-disulfide exchange producing the oligomers. These results indicate that intramolecular and intermolecular thiol-disulfide exchange reactions cause the low reversibility of wild-type beta-lg especially at neutral pH and that the mutation of Cys-121 improves the reversibility, enabling us to study the folding of beta-lg more exactly under various conditions.

Bis(2‐aminobutanol)dichloroplatinum(II) Complexes and Their Singly and Doubly Ring‐Closed Butanolato Species − Novel Prodrugs for Platinum‐Based Antitumour Chemotherapy?

AbstractCytotoxic hydroxyethyl‐substituted (amine)platinum(II) and ‐(IV) complexes have recently attracted attention because of the ability of their hydroxyethyl groups to coordinate through the oxygen atom to the platinum centre during oxidation with hydrogen peroxide or through intramolecular ligand exchange reactions in dichloroplatinum(II) complexes. The last point in particular is of great interest, because the intramolecular attack of the hydroxy group dramatically influences the mode of action of platinum(II) compounds. On the other hand, there is also the chance to use such reactions specifically for the synthesis of novel anticancer platinum‐based drugs for chemotherapy. We have therefore focused our chemistry program on the synthesis of dichloroplatinum(II) complexes that are in a position to form singly and, especially, doubly ring‐closed alcoholato species and on investigation of their structures by X‐ray crystallography. It was possible to determine the crystal structures of [Pt{(R)‐(−)‐HL}2Cl2], [Pt{(S)‐(+)‐HL}2Cl2], [Pt{(R)‐(−)‐HL}{(S)‐(+)‐HL}Cl2], [Pt{(S)‐(+)‐HL}{(S)‐(+)‐L}Cl], [Pt{(R)‐(−)‐L}2] and [Pt{(S)‐(+)‐L}2] (HL = 2‐aminobutanol‐κN, L = 2‐aminobutanolato‐κ2N,O). The results obtained may represent the first step towards novel prodrugs for platinum‐based antitumour chemotherapy. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2003)

Stereocontrolled total synthesis of (-)-ephedradine A (orantine).

The stereocontrolled total synthesis of (-)-ephedradine A has been accomplished. The synthesis features an asymmetric C-H insertion reaction, an intramolecular ester-amide exchange reaction, and a Sharpless asymmetric aminohydroxylation reaction. Construction of the complex macrocyclic ring was performed by Ns-strategy and an intramolecular aza-Wittig reaction.

The Intramolecular Ligand-Exchange Reaction of (SP-4-2)-Dichlorobis(2-hydroxyethylamine)platinum(II) and (OC-6-22)-Tetrachlorobis(2-hydroxyethylamine)platinum(IV), a1H and15N,1H-HMQC NMR Study

The bis(ethanolamine)platinum complexes (SP-4-2)-dichlorobis(2-hydroxyethylamine)platinum(II) (1) and (OC-6-22)-tetrachlororbis(2-hydroxyethylamine)platinum(IV) (2) have been synthesized and their chemistry in aqueous solution has been investigated due to the fact that 1 forms very stable monoadducts with 5′-GMP. In water 1 and 2 are converted into (SP-4-3)-chloro(2-ethanolatoamine-κ2N,O)(2-hydroxyethylamine)platinum(II) (3) and (OC-6-31)-trichloro(2-ethanolatoamine-κ2N,O)(2-hydroxyethylamine)platinum(IV) (4) with a chelating ethanolatoamine ligand by a intramolecular ligand exchange reaction, which was confirmed by 1H and 2D 15N,1H-HMQC NMR experiments and the crystal structure determination of 4.

Inter- and intramolecular ester exchange reactions in the ring-opening polymerization of (D,L)-lactide using lanthanide alkoxide initiators

Polylactide samples obtained by the ring-opening polymerization of (D,L)-lactide by using different lanthanide alkoxide initiators were examined by SEC, 13 C NMR and MALDI-TOF MS techniques. The evidence of substantial transesteriffcation was observed in the case of a polymerization initiated by lanthanum alkoxide and bimetallic aluminium-yttrium alkoxide initiators. In the case of Y and Sm alkoxide initiators, the MWD remains narrow up to high conversions for reasonable polymerization times, but the prolongation of the post-polymerization time increases the amount of ester-eschange reactions, The extent of the transesterification reactions was evaluated on the basis of the intensity of the stereosequence resonances for the methine carbon signal in the 13 C NMR spectra. MALDI-TOF MS analysis revealed the presence of add-numbered oligomers in all spectra. Thus, transesterification reactions occus from the beginning of the polymerization. Linear and cyclic oligomers were detected in some cases indicating the simultaneous occurrence of inter- and intramolecular exchange reactions. The microstructure analysis of the polymers by 13 C NMR spectroscopy indicates a preference for a syndiotactic addition during the polymerization process.

Thermal Decomposition Processes in Aromatic Polycarbonates Investigated by Mass Spectrometry

The thermal decomposition pathways leading to the formation of volatile compounds and to char residue in poly(bisphenol A carbonate) (PC), poly(resorcinol carbonate) (PRC), and poly(hydroquinone carbonate) (PHC) have been investigated by mass spectrometry. The structure of the volatile compounds obtained in the temperature range 300−700 °C, by direct pyrolysis mass spectrometry (DPMS), suggests that these polycarbonates undergo thermal decomposition by a number of different pyrolysis processes. In the initial stage of the thermal degradation are generated cyclic oligomers by an intramolecular exchange reaction, whereas the evolution of CO2 and H2O is spread over all the pyrolysis temperature range, being responsible for the formation of ether bridges (decarboxylation) and phenolic end groups (hydrolysis). A disproportionation reaction of the BPA isopropylidene bridges of PC itself takes place at higher temperature yielding phenyl and isopropylidene end groups, whereas pyrolysis products containing dibenzo...

Solvent effects on intramolecular electron exchange in the 1,4-dinitrobenzene radical anion

The rate constants of the intramolecular electron exchange reaction in the 1,4-dinitrobenzene radical anion in linear alcohols were determined from alternating line-broadening effects in EPR spectra. Rate constants at 293 K range from 2.9×109 s-1 in methanol to 2.7×108 s-1 in octan-1-ol. The rate constant was found to depend on the reverse of the longitudinal correlation time of the solvent, τL, as expected for a solvent-controlled adiabatic reaction. Applying Marcus theory and the ellipsoidal cavity model for evaluating λ0, the amount of transferred charge between nitro groups, z, and the outer-sphere reorganisation energy were determined. z changes from 0.56 at 273 K to 0.52 at 313 K. λ0 at 293 K changes from 61 kJ mol-1 in octan-1-ol to 80 kJ mol-1 in methanol. The activation energies were obtained from the temperature dependence of the rate constants, changing from 26 kJ mol-1 in methanol to 36 kJ mol-1 in octan-1-ol. The variation of this energy clearly reflects the increase in the exponential term of τL for the more viscous alcohols. The results for the 1,4-dinitrobenzene radical anion were compared with those previously reported for the radical anion of 1,3-dinitrobenzene. Rate constants are smaller in the former, by three orders of magnitude, due to conjugation of the nitro groups.

Mechanisms of Ligand Substitution Reactions in Ternary Dioxouranium(VI) Complexes

The activation parameters of various inter- and intramolecular ligand exchange reactions in ternary complexes of the type UO2LF3 (where L is one of the bidentate ligands picolinate, 4-nitropicolinate, 4-(3-pentyl)picolinate, oxalate, or carbonate) and UO2L2F (where L = picolinate or oxalate) have been determined by different NMR techniques. The activation entropies for the reactions have been used to discuss their intimate mechanism, particularly the solvent participation. Additional mechanistic information has been obtained from studies of H+/D+ catalysis of the various reactions. Through the use of the proton-catalyzed pathway, the rate of carbonate exchange in UO2CO3F33- was varied by a factor of ca. 10. The fact that the rate of fluoride exchange remained constant clearly indicates that the exchanges of carbonate and fluoride follow parallel pathways. The activation entropies of most of the fluoride exchange reactions in UO2LF3 have values close to 10 J K-1 mol-1, indicating dissociative (D) or dissoc...