Reactive intermediates relevant to the carbonylation of CH(3)Mn(CO)(5). Activation parameters for key dynamic processes

Reported is a time-resolved infrared and optical kinetics investigation of the transient species CH(3)C(O)Mn(CO)(4) (I(Mn)) generated by flash photolysis of the acetyl manganese pentacarbonyl complex CH(3)C(O)Mn(CO)(5) (A(Mn)) in cyclohexane and in tetrahydrofuran. Activation parameters were determined for CO trapping of I(Mn) to regenerate A(Mn) (rate = k(CO) [CO][I(Mn)]) as well as the methyl migration pathway to form methylmanganese pentacarbonyl CH(3)Mn(CO)(5) (M(Mn)) (rate = k(M)[I(Mn)]). These values were Delta H(++)(CO) = 31 +/- 1 kJ mol(-1), Delta S(++)(CO) = -64 +/- 3 J mol(-1) K(-1), Delta H(++)(M) = 35 +/- 1 kJ mol(-1), and Delta S(++)(M) = -111 +/- 3 J mol(-1) K(-1). Substantially different activation parameters were found for the methyl migration kinetics of I(Mn) in THF solutions where Delta H(++)(M) = 68 +/- 4 kJ mol(-1) and Delta S(++)(M) = 10 +/- 10 J mol(-1) K(-1), consistent with the earlier conclusion (Boese, W. T.; Ford, P. C. J. Am. Chem. Soc. 1995, 117, 8381-8391) that the composition of I(Mn) is different in these two media. The possible isotope effect on k(M) was also evaluated by studying the intermediates generated from flash photolysis of CD(3)C(O)Mn(CO)(5) in cyclohexane, but this was found to be nearly negligible (k(M)(h)/k(M)(d) (298 K) = 0.97 +/- 0.05, Delta H(++)(M)(d) = 37 +/- 4 kJ mol(-1), and Delta S(++)(M)(d) = -104 +/- 12 J mol(-1) K(-1)). The relevance to the migratory insertion mechanism of CH(3)Mn(CO)(5), a model for catalytic carbonylations, is discussed.     

Oxygen Atom Transfer, Coupled Electron-Proton Transfer, and Correlated Electron-Nucleophile Transfer Reactions of Oxomolybdenum(IV) and Dioxomolybdenum(VI) Complexes

The oxo-Mo(IV) complexes LMoO(S(2)PR(2)-S,S') [L = hydrotris(3,5-dimethylpyrazol-1-yl)borate; R = Me, Et, Pr(i)(), Ph] were prepared by reacting MoO(S(2)PR(2))(2) and KL in refluxing toluene. The dioxo-Mo(VI) complexes cis-LMoO(2)(S(2)PR(2)-S) (R = Pr(i)(), Ph) were prepared by oxidation of the oxo-Mo(IV) complexes or by reaction of LMoO(2)Cl with NaS(2)PR(2). Oxygen atom transfers from Me(2)SO to LMoO(S(2)PR(2)) were first-order with respect to Me(2)SO and complex; the overall second-order rate constants at 40 degrees C range from 9.0(1) x 10(-)(5) M(-)(1).s(-)(1) for LMoO(S(2)PMe(2)) to 2.08(5) x 10(-)(4) M(-)(1).s(-)(1) for LMoO(S(2)PPr(2)); activation parameters were in the ranges DeltaH() = 63(1) to 73(1) kJ.mol(-)(1), DeltaS() = -88(1) to -111(1) J.K(-)(1).mol(-)(1), and DeltaG() = 100(2) kJ.mol(-)(1) for LMoO(S(2)PMe(2)) to 98(2) kJ.mol(-)(1) for LMoO(S(2)PPr(2)). Oxygen atom transfer from pyridine N-oxide to LMoO(S(2)PPr(2)) was also second-order with a rate constant of 1.54(5) x 10(-)(3) M(-)(1).s(-)(1) at 40 degrees C, DeltaH() = 62(1) kJ.mol(-)(1), DeltaS() = -90(1) J.K(-)(1).mol(-)(1), and DeltaG() = 90(1) kJ.mol(-)(1). The second-order rate laws and large negative entropies of activation are consistent with associative mechanisms for the above reactions. Oxygen atom transfer from LMoO(2)(S(2)PPr(2)) to PPh(3) was first-order with respect to reactants, with an overall second-order rate constant of 2.5(3) x 10(-)(4) M(-)(1).s(-)(1) at 30 degrees C. In toluene at 40 degrees C, all the above complexes catalyzed the oxidation of PPh(3) by Me(2)SO, with turnover rates of ca. 0.9 mol of PPh(3)/(mol of catalyst/h). Reduction of LMoO(2)(S(2)PR(2)) by SH(-) led to the generation of the dioxo-Mo(V) anions [LMoO(2)(S(2)PR(2)-S)](-), which were slowly converted to the analogous oxothio-Mo(V) complexes [LMoOS(S(2)PR(2)-S)](-). Dioxygen reacted with [LMoOS(S(2)PPr(2))](-) to produce the oxothio-Mo(VI) complex LMoOS(S(2)PPr(2)-S). The (hydroxo)oxo-Mo(V) complexes LMoO(OH)(S(2)PR(2)-S) were formed upon reduction of LMoO(2)(S(2)PR(2)) with PPh(3) in wet (3-5 M H(2)O) tetrahydrofuran or upon ferrocenium oxidation of LMoO(S(2)PR(2)) in wet tetrahydrofuran. In dry solvents, LMoO(S(2)PR(2)) were oxidized to the corresponding cations, [LMoO(S(2)PR(2)-S,S')](+), which reacted with water to form LMoO(OH)(S(2)PR(2)). The Mo(V) complexes have been characterized by EPR spectroscopy.     

Spectroscopic studies on the interaction of bovine (BSA) and human (HSA) serum albumins with ionic surfactants

Bovine (BSA) and human (HSA) serum albumins are frequently used in biophysical and biochemical studies since they have a similar folding, a well known primary structure, and they have been associated with the binding of many different categories of small molecules. One important difference of BSA and HSA is the fact that bovine albumin has two tryptophan residues while human albumin has a unique tryptophan. In this work results are presented for the interaction of BSA and HSA with several ionic surfactants, namely, anionic sodium dodecyl sulfate (SDS), cationic cethyltrimethylammonium chloride (CTAC) and zwitterionic N-hexadecyl-N,N-dimethyl-3-ammonium-1-propanesulfonate (HPS), as monitored by fluorescence spectroscopy of intrinsic tryptophans and circular dichroism spectroscopy. On the interaction of all three surfactants with BSA, at low concentrations, a quenching of fluorescence takes place and Stern-Volmer analysis allowed to estimate their 'effective' association constants to the protein: for SDS, CTAC and HPS at pH 7.0 these constants are, respectively, (1.4+/-0.1) x 10(5) M(-1), (8.9+/-0.1) x 10(3) M(-1) and (1.4+/-0.1) x 10(4) M(-1). A blue shift of maximum emission is observed from 345 to 330 nm upon surfactant binding. Analysis of fluorescence emission spectra allowed to separate three species in solution which were associated to native protein, a surfactant protein complex and partially denatured protein. The binding at low surfactant concentrations follows a Hill plot model displaying positive cooperativity and a number of surfactant binding sites very close to the number of cationic or anionic residues present in the protein. Circular dichroism data corroborated the partial loss of secondary structure upon surfactant addition showing the high stability of serum albumin. The interaction of the surfactants with HSA showed an enhancement of fluorescence at low concentrations, opposite to the effect on BSA, consistent with the existence of a unique buried tryptophan residue in this protein with considerable static quenching in the native state. The effects of surfactants at low concentrations were very similar to those of myristic acid suggesting a non specific binding through hydrophobic interaction modulated by eletrostatic interactions. The changes in the vicinity of the tryptophan residues are discussed based on the recently published crystallographic structure of HSA myristate complex (S. Curry et al., Nat. Struct. Biol. 5 (1998) 827).     

Chemical ionization mass spectra of acetals of beta-D-glycopyranosylnitromethanes

O-Isopropylidene and O-benzylidene acetals of common 2, 6-anhydro-1-deoxy-1-nitroalditols (beta-D-glycopyranosylnitromethanes) derived from D-glucose, D-galactose and D-mannose were studied by chemical ionization mass spectrometry (CIMS) using methane, isobutane, ammonia or pyridine as reaction gas. Production of [M+H](+) adduct ions dominates in the case of methane or isobutane possessing proton affinity values PA = 552 or 683 kJ mol(-1), respectively. The collision-induced dissociation time-of-flight product ion spectra of [M+H](+) ions differ characteristically according the stereochemical arrangement of the pyranoid ring. These differences can be helpful when assigning stereochemical arrangements for the pyranoid ring. The dominant process in ammonia (PA = 853 kJ mol(-1)) CIMS for most of the compounds studied is the production of the cluster ions [M+NH(4)](+). The cluster [M+pyridineH](+) ions are observable only for substances possessing the O-benzylidene group (PA of pyridine = 924 kJ mol(-1)). Copyright 2000 John Wiley & Sons, Ltd.     

Protonation thermochemistry of aminoacetonitrile

The gas-phase basicity (GB) of aminoacetonitrile (NH2CH2CN, 1) has been determined from measurement of proton transfer equilibrium constants in an ion cyclotron resonance mass spectrometer (GB(1) = 789.3 +/- 1.0 kJ x mol(-1)). Molecular orbital calculations up to the G2 level demonstrate that protonation occurs preferentially on the nitrogen atom of the NH2 group, and provide a theoretical proton affinity (PA(1)) of 824.0 kJ x mol(-1). Exact calculation of the entropy associated with hindered rotations and consideration of Boltzman distribution of conformers allow a theoretical estimate of the molar protonation entropy S degrees (1H+) - S degrees (1) = 8.6 J x mol(-1) x K(-1). Combining this value with experimental GB(1) leads to an 'experimental' proton affinity of 819.2 kJ x mol(-1), in close agreement with the G2 expectation.     

A restricted-open-shell complete-basis-set model chemistry

A restricted-open-shell model chemistry based on the complete basis set-quadratic Becke3 (CBS-QB3) model is formulated and denoted ROCBS-QB3. As the name implies, this method uses spin-restricted wave functions, both for the direct calculations of the various components of the electronic energy and for extrapolating the correlation energy to the complete-basis-set limit. These modifications eliminate the need for empirical corrections that are incorporated in standard CBS-QB3 to compensate for spin contamination when spin-unrestricted wave functions are used. We employ an initial test set of 19 severely spin-contaminated species including doublet radicals and both singlet and triplet biradicals. The mean absolute deviation (MAD) from experiment for the new ROCBS-QB3 model (3.6+/-1.5 kJ mol(-1)) is slightly smaller than that of the standard unrestricted CBS-QB3 version (4.8+/-1.5 kJ mol(-1)) and substantially smaller than the MAD for the unrestricted CBS-QB3 before inclusion of the spin correction (16.1+/-1.5 kJ mol(-1)). However, when applied to calculate the heats of formation at 298 K for the moderately spin-contaminated radicals in the G2/97 test set, ROCBS-QB3 does not perform quite as well as the standard unrestricted CBS-QB3, with a MAD from experiment of 3.8+/-1.6 kJ mol(-1) (compared with 2.9+/-1.6 kJ mol(-1) for standard CBS-QB3). ROCBS-QB3 performs marginally better than standard CBS-QB3 for the G2/97 set of ionization energies with a MAD of 4.1+/-0.1 kJ mol(-1) (compared with 4.4+/-0.1 kJ mol(-1)) and electron affinities with a MAD of 3.9+/-0.2 kJ mol(-1) (compared with 4.3+/-0.2 kJ mol(-1)), but the differences in MAD values are comparable to the experimental uncertainties. Our overall conclusion is that ROCBS-QB3 eliminates the spin correction in standard CBS-QB3 with no loss in accuracy.     

A close look at fluorescence quenching of organic dyes by tryptophan.

Understanding fluorescence quenching processes of organic dyes by biomolecular compounds is of fundamental importance for in-vitro and in-vivo fluorescence studies. It has been reported that the excited singlet state of some oxazine and rhodamine derivatives is efficiently and almost exclusively quenched by the amino acid tryptophan (Trp) and the DNA base guanine via photoinduced electron transfer (PET). We present a detailed analysis of the quenching interactions between the oxazine dye MR121 and Trp in aqueous buffer. Steady-state and time-resolved fluorescence spectroscopy, together with fluorescence correlation spectroscopy (FCS), reveal three contributing quenching mechanisms: 1) diffusion-limited dynamic quenching with a bimolecular quenching rate constant k(d) of 4.0 x 10(9) s(-1) M(-1), 2) static quenching with a bimolecular association constant K(s) of 61 M(-1), and 3) a sphere-of-action contribution to static quenching described by an exponential factor with a quenching constant lambda of 22 M(-1). The latter two are characterized as nonfluorescent complexes, formed with approximately 30 % efficiency upon encounter, that are stable for tens of nanoseconds. The measured binding energy of 20-30 kJ mol(-1) is consistent with previous estimates from molecular dynamics simulations that proposed stacked complexes due to hydrophobic forces. We further evaluate the influence of glycerol and denaturant (guanidine hydrochloride) on the formation and stability of quenched complexes. Comparative measurements performed with two other dyes, ATTO 655 and Rhodamine 6G show similar results and thus demonstrate the general applicability of utilizing PET between organic dyes and Trp for the study of conformational dynamics of biopolymers on sub-nanometer length and nanosecond time-scales.     

Solvation properties of N-substituted cis and trans amides are not identical: significant enthalpy and entropy changes are revealed by the use of variable temperature 1H NMR in aqueous and chloroform solutions and ab initio calculations

The cis/trans conformational equilibrium of N-methyl formamide (NMF) and the sterically hindered tert-butylformamide (TBF) was investigated by the use of variable temperature gradient 1H NMR in aqueous solution and in the low dielectric constant and solvation ability solvent CDCl3 and various levels of first principles calculations. The trans isomer of NMF in aqueous solution is enthalpically favored relative to the cis (deltaH(o) = -5.79 +/- 0.18 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = -0.23 +/- 0.17 kJ mol(-1)) playing a minor role. The experimental value of the enthalpy difference strongly decreases (deltaH(o) = -1.72 +/- 0.06 kJ mol(-1)), and the contribution of entropy at 298 K (298 x deltaS(o) = -1.87 +/- 0.06 kJ mol(-1)) increases in the case of the sterically hindered tert-butylformamide. The trans isomer of NMF in CDCl3 solution is enthalpically favored relative to the cis (deltaH(o) = -3.71 +/- 0.17 kJ mol(-1)) with entropy differences at 298 K (298 x deltaS(o) = 1.02 +/- 0.19 kJ mol(-1)) playing a minor role. In the sterically hindered tert-butylformamide, the trans isomer is enthalpically disfavored (deltaH(o) = 1.60 +/- 0.09 kJ mol(-1)) but is entropically favored (298 x deltaS(o) = 1.71 +/- 0.10 kJ mol(-1)). The results are compared with literature data of model peptides. It is concluded that, in amide bonds at 298 K and in the absence of strongly stabilizing sequence-specific inter-residue interactions involving side chains, the free energy difference of the cis/trans isomers and both the enthalpy and entropy contributions are strongly dependent on the N-alkyl substitution and the solvent. The significant decreasing enthalpic benefit of the trans isomer in CDCl3 compared to that in H2O, in the case of NMF and TBF, is partially offset by an adverse entropy contribution. This is in agreement with the general phenomenon of enthalpy versus entropy compensation. B3LY/6-311++G** and MP2/6-311++G** quantum chemical calculations confirm the stability orders of isomers and the deltaG decrease in going from water to CHCl3 as solvent. However, the absolute calculated values, especially for TBF, deviate significantly from the experimental values. Consideration of the solvent effects via the PCM approach on NMF x H2O and TBF x H2O supermolecules improves the agreement with the experimental results for TBF isomers, but not for NMF.     

Dynamic 1H NMR study around the carbon-carbon single bond and partial carbon-carbon double bond in the two particular phosphorus ylides and in an enaminoester

Herein, a series of separate dynamic (1)H NMR effects are reported at different temperatures within a particular enaminoester involving a phenanthridine. These effects are attributed to restricted rotation around the two single bonds such as carbon-carbon (H(a)-C-C-H(b)) and nitrogen-carbon (NCCOOCH(3)). Activation energies (E(a)) for these interconversion processes in their rotational isomers are equal to 20 and 20 ± 1 kJ mol(-1), respectively. In addition, three dynamic (1)H NMR effects are investigated at different temperatures for a particular phosphorus ylide involving a 2-indolinone around the carbon-carbon single bond (H-C-C-PPh(3)) within the two Z- and E-rotational isomers and partial carbon-carbon double bond (OCH(3)-C=C-PPh(3)). Activation energies (E(a)) for these interconversion processes in rotational isomers are equal to 53, 63 and 73 ± 1 kJ mol(-1) , respectively. This behavior was also observed for other phosphorus ylide containing 2-mercaptobenzoxazole around the carbon-carbon single bond and partial carbon-carbon double bond with their relevant activation energies containing 13, 10 and 75 ± 1 kJ mol(-1), respectively.     

Kinetics of Water Exchange on the Dihydroxo-Bridged Rhodium(III) Hydrolytic Dimer

()()Conventional (18)O isotopic labeling techniques have been used to measure the water exchange rates on the Rh(III) hydrolytic dimer [(H(2)O)(4)Rh(&mgr;-OH)(2)Rh(H(2)O)(4)](4+) at I = 1.0 M for 0.08 < [H(+)] < 0.8 M and temperatures between 308.1 and 323.1 K. Two distinct pathways of water exchange into the bulk solvent were observed (k(fast) and k(slow)) which are proposed to correspond to exchange of coordinated water at positions cis and trans to bridging hydroxide groups. This proposal is supported by (17)O NMR measurements which clearly showed that the two types of water ligands exchange at different rates and that the rates of exchange matched those from the (18)O labeling data. No evidence was found for the exchange of label in the bridging OH groups in either experiment. This contrasts with findings for the Cr(III) dimer. The dependence of both k(fast) and k(slow) on [H(+)] satisfied the expression k(obs) = (k(O)[H(+)](tot) +k(OH)K(a1))/([H(+)](tot) + K(a1)) which allows for the involvement of fully protonated and monodeprotonated Rh(III) dimer. The following rates and activation parameters were determined at 298 K. (i) For fully protonated dimer: k(fast) = 1.26 x 10(-)(6) s(-)(1) (DeltaH() = 119 +/- 4 kJ mol(-)(1) and DeltaS() = 41 +/- 12 J K(-)(1) mol(-)(1)) and k(slow) = 4.86 x 10(-)(7) s(-)(1) (DeltaH() = 64 +/- 9 kJ mol(-)(1) and DeltaS() = -150 +/- 30 J K(-)(1) mol(-)(1)). (ii) For monodeprotonated dimer: k(fast) = 3.44 x 10(-)(6) s(-)(1) (DeltaH() = 146 +/- 4 kJ mol(-)(1) and DeltaS() = 140 +/- 11 J K(-)(1) mol(-)(1)) and k(slow) = 2.68 x 10(-)(6) s(-)(1) (DeltaH() = 102 +/- 3 kJ mol(-)(1) and DeltaS() = -9 +/- 11 J K(-)(1) mol(-)(1)). Deprotonation of the Rh(III) dimer was found to labilize the primary coordination sphere of the metal ions and thus increase the rate of water exchange at positions cis and trans to bridging hydroxides but not to the same extent as for the Cr(III) dimer. Activation parameters and mechanisms for ligand substitution processes on the Rh(III) dimer are discussed and compared to those for other trivalent metal ions and in particular the Cr(III) dimer.     

Modified Gaussian-2 level investigation of the identity ion-pair SN2 reactions of lithium halide and methyl halide with inversion and retention mechanisms

Identity ion-pair S(N)2 reactions LiX + CH(3)X --> XCH(3) + LiX (X = F, Cl, Br, and I) have been investigated in the gas phase and in solution at the level of the modified Gaussian-2 theory. Two possible reaction mechanisms, inversion and retention, are discussed. The reaction barriers relative to the complexes for the inversion mechanism [DeltaH(cent) ( not equal )(inv)] are found to be much higher than the corresponding values for the gas phase anionic S(N)2 reactions, decreasing in the following order: F (263.6 kJ mol(-1)) > Cl (203.3 kJ mol(-1)) > Br (174.7 kJ mol(-1)) > I (150.7 kJ mol(-1)). The barrier gaps between the two mechanisms [DeltaH(cent) ( not equal ) (ret) - DeltaH(cent) ( not equal ) (inv)] increase in the order F (-62.7 kJ mol(-1)) < Cl (4.4 kJ mol(-1)) < Br (24.9 kJ mol(-1)) < I (45.1 kJ mol(-1)). Thus, the retention mechanism is energetically favorable for fluorine and the inversion mechanism is favored for other halogens, in contrast to the anionic S(N)2 reactions at carbon where the inversion reaction channel is much more favorable for all of the halogens. The stabilization energies for the dipole-dipole complexes CH(3)X. LiX (DeltaH(comp)) are found to be similar for the entire set of systems with X = F, Cl, Br, and I, ranging from 53.4 kJ mol(-1) for I up to 58.9 kJ mol(-1) for F. The polarizable continuum model (PCM) has been used to evaluate the direct solvent effects on the energetics of the anionic and ion-pair S(N)2 reactions. The energetic profiles are found to be still double-well shaped for most of the ion-pair S(N)2 reactions in the solution, but the potential profile for reaction LiI + CH(3)I is predicted to be unimodal in the protic solvent. Good correlations between central barriers [DeltaH(cent) ( not equal ) (inv)] with the geometric looseness of the inversion transition state %C-X( not equal ), the dissociation energies of the C-X bond (D(C-X)) and Li-X bond (D(Li-X)) are observed, respectively.     

Facile ketene-ketene and ketene-ketenimine rearrangements: a study of the 1,3-migration of alpha-substituents interconverting alpha-imidoylketenes and alpha-oxoketenimines, a pseudopericyclic reaction

Theoretical calculations (B3LYP/6-311+G(3df,2p)//B3LYP/6-31G) of the 1,3 migration of NR(2) transforming alpha-oxoketenimines 1 to alpha-imidoylketenes 3 and vice versa indicate that this process is a pseudo-pericyclic reaction with a low activation energy (NH(2) 97 kJ mol(-1), N(CH3)(2) 62 kJ mol(-1)). The oxoketenimines were found to be more stable (by 18-35 kJ mol(-1)) which is in line with experimental observations. The hindered amine rotation in the amide and amidine moieties adjacent to the cumulenes are important in the migration of the NR(2) group, as one of the rotation transition states is close to the 1,3 migration pathway. This gives an interesting potential energy surface with a valley-ridge inflection (VRI) between the orthogonal hindered amine rotation and 1,3 migration transition states. The imidoylketene may also undergo ring closure to an azetinone 5; however, this is metastable, and under the conditions that allow the 1,3-migration, the oxoketenimine 1 will be favored. The imine NH E/Z-interconversion of the ketenimine group takes place by inversion and has a low activation barrier ( approximately 40 kJ mol(-1)). In all the amidines examined the E/Z-interconversion of the imine function was predicted to be by rotation with a high barrier (>80 kJ mol(-1)), in contrast to all other reported imine E/Z-interconversions which are by inversion.     

The rate of O2 and CO binding to a copper complex, determined by a "flash-and-trap" technique, exceeds that for hemes

The observation and fast time-scale kinetic determination of a primary dioxygen-copper interaction have been studied. The ability to photorelease carbon monoxide from [Cu(I)(tmpa)(CO)](+) in mixtures of CO and O(2) in tetrahydrofuran (THF) between 188 and 218 K results in the observable formation of a copper-superoxide species, [Cu(II)(tmpa)(O(2)(-))](+) lambda(max) = 425 nm. Via this "flash-and-trap" technique, temperature-dependent kinetic studies on the forward reaction between dioxygen and [Cu(I)(tmpa)(thf)](+) afford activation parameters DeltaH = 7.62 kJ/mol and DeltaS = -45.1 J/mol K. The corresponding reverse reaction proceeds with DeltaH = 58.0 kJ/mol and DeltaS = 105 J/mol K. Overall thermodynamic parameters are DeltaH degrees = -48.5 kJ/mol and DeltaS degrees = -140 J/mol K. The temperature-dependent data allowed us to determine the room-temperature second-order rate constant, k(O2) = 1.3 x 10(9) M(-1) s(-1). Comparisons to copper and heme proteins and synthetic complexes are discussed.     

Chlorine dioxide reduction by aqueous iron(II) through outer-sphere and inner-sphere electron-transfer pathways

The reduction of ClO(2) to ClO(2)(-) by aqueous iron(II) in 0.5 M HClO(4) proceeds by both outer-sphere (86%) and inner-sphere (14%) electron-transfer pathways. The second-order rate constant for the outer-sphere reaction is 1.3 x 10(6) M(-1) s(-1). The inner-sphere electron-transfer reaction takes place via the formation of FeClO(2)(2+) that is observed as an intermediate. The rate constant for the inner-sphere path (2.0 x 10(5) M(-1) s(-1)) is controlled by ClO(2) substitution of a coordinated water to give an inner-sphere complex between ClO(2) and Fe(II) that very rapidly transfers an electron to give (Fe(III)(ClO(2)(-))(H(2)O)(5)(2+))(IS). The composite activation parameters for the ClO(2)/Fe(aq)(2+) reaction (inner-sphere + outer-sphere) are the following: DeltaH(r)++ = 40 kJ mol(-1); DeltaS(r)++ = 1.7 J mol(-1) K(-1). The Fe(III)ClO(2)(2+) inner-sphere complex dissociates to give Fe(aq)(3+) and ClO(2)(-) (39.3 s(-1)). The activation parameters for the dissociation of this complex are the following: DeltaH(d)++= 76 kJ mol(-1); DeltaS(d)++= 32 J K(-1) mol(-1). The reaction of Fe(aq)(2+) with ClO(2)(-) is first order in each species with a second-order rate constant of k(ClO2)- = 2.0 x 10(3) M(-1) s(-1) that is five times larger than the rate constant for the Fe(aq)(2+) reaction with HClO(2) in H(2)SO(4) medium ([H(+)] = 0.01-0.13 M). The composite activation parameters for the Fe(aq)(2+)/Cl(III) reaction in H(2)SO(4) are DeltaH(Cl(III))++ = 41 kJ mol(-1) and DeltaS(Cl(III))++ = 48 J mol(-1) K(-1).     

Steady-state and time-resolved fluorescence studies on Trichosanthes cucumerina seed lectin

Steady-state and time-resolved fluorescence spectroscopic studies have been carried out on Trichosanthes cucumerina seed lectin (TCSL). The fluorescence emission maximum of TCSL in the native state as well as in the presence of 0.1 M lactose is centered around 331 nm, which shifts to 347 nm upon denaturation with 8 M urea, indicating that all the tryptophan residues of this protein in the native state are in a predominantly hydrophobic environment. The exposure and accessibility of the tryptophan residues of TCSL and the effect of ligand binding on them were probed by quenching studies employing two neutral quenchers (acrylamide and succinimide), an anionic quencher (I(-)) and a cationic quencher (Cs(+)). Quenching was highest with acrylamide and succinimide with the latter, which is bulkier, yielding slightly lower quenching values, whereas the extent of quenching obtained with the ionic quenchers, I(-) and Cs(+) was significantly lower. The presence of 0.1 M lactose led to a slight increase in the quenching with acrylamide and iodide, whereas quenching with succinimide and cesium ion was not significantly affected. When TCSL was denatured with 8 M urea, both acrylamide and succinimide yielded upward-curving Stern-Volmer plots, indicating that the quenching mechanism involves both dynamic and static components. Quenching data obtained with I(-) and Cs(+) on the urea-denatured protein suggest that charged residues could be present in close proximity to some of the Trp residues. The Stern-Volmer plots with Cs(+) yielded biphasic quenching profiles, indicating that the Trp residues in TCSL fall into at least two groups that differ considerably in their accessibility and/or environment. In time-resolved fluorescence experiments, the decay curves could be best fit to biexponential patterns, with lifetimes of 1.78 and 4.75 ns for the native protein and 2.15 and 5.14 ns in the presence of 0.1 M lactose.     

Thermodynamic and kinetic studies on the reaction between the vitamin B12 derivative beta-(N-methylimidazolyl)cobalamin and N-methylimidazole: ligand displacement at the alpha axial site of cobalamins

The equilibria and kinetics of substitution of the 5,6-dimethylbenzimidazole at the alpha site of beta-(N-methylimidazolyl)cobalamin by N-methylimidazole have been investigated, and the product, bis(N-methylimidazolyl)cobalamin, has been characterized by visible and 1H NMR spectroscopies. The equilibrium constant for (N-MeIm)Cbl+ + N-MeIm right harpoon over left harpoon (N-MeIm)2Cbl+ was determined by 1H NMR spectroscopy (9.6 +/- 0.1 M(-1), 25.0 degrees C, I = 1.5 M (NaClO4)). The observed rate constant for this reaction exhibits an unusual inverse dependence on N-methylimidazole concentration, and it is proposed that substitution occurs via a base-off solvent-bound intermediate. Activation parameters typical for a dissociative ligand substitution mechanism are reported at two different N-MeImT concentrations, 5.00 x 10(-3) M (DeltaH++ = 99 +/- 2 kJ x mol(-1), DeltaS++ = 39 +/- 5 J x mol(-1) x K(-1), DeltaV++ = 15.0 +/- 0.7 cm3 x mol(-1), and 1.00 M (DeltaH++ = 109.4 +/- 0.8 kJ x mol(-1), DeltaS++ = 70 +/- 3 J x mol(-1) x K(-1), DeltaV++ = 16.8 +/- 1.1 cm3 x mol(-1)). According to the proposed mechanism, these parameters correspond to the equation of (N-MeIm)2Cbl+ and the ring-opening reaction of the alpha-DMBI of (N-MeIm)Cbl+ to give the solvent-bound intermediate in both cases, respectively.     

Spectroscopic studies on binding of 1-phenyl-3-(coumarin-6-yl)sulfonylurea to bovine serum albumin

The interaction of 1-phenyl-3-(coumarin-6-yl)sulfonylurea (SU22) with bovine serum albumin (BSA) has been investigated by fluorescence quenching spectroscopy combined with UV-absorption, circular dichroism (CD), Fourier transform infrared (FT-IR) spectroscopy techniques under simulative physiological conditions for the first time. Fluorescence data and UV-absorption spectra revealed that the quenching mechanism of fluorescence of BSA by SU22 was a static quenching process and the number of binding sites was about 0.8858; the thermodynamic parameters (DeltaG=-29.23 kJ mol(-1), DeltaH=-47.48 kJ mol(-1), and DeltaS=-61.24 J mol(-1)K(-1)) explained that hydrogen bond and Van der Waals interaction were the main binding force stabilizing the complex. The binding average distance between SU22 and BSA was obtained (3.20 nm) on the basis of the F?rster's theory. In addition, The CD spectra and FT-IR spectra have proved that BSA secondary structure changed in the presence of SU22 in aqueous solution.     

Metal dilution effects on the spin-crossover properties of the three-dimensional coordination polymer Fe(pyrazine)[Pt(CN)4

The 1A1 left arrow over right arrow 5T2 spin transition has been investigated in the solid solutions of Fe(x)M(1-x)(pyrazine)[Pt(CN)4] (M = Ni or Co, 0 < or = x < or = 1) having a three-dimensional polynuclear structure. Both Ni and Co dilutions tend to decrease the hysteresis width and smooth the transition curves. The enthalpy (entropy) change associated with the spin transition was found to decrease from 26 kJ mol(-1) (84 J K(-1) mol(-1)) for x = 1 to 12 kJ mol(-1) (47 J K(-1) mol(-1)) for 47% Co dilution and to 15 kJ mol(-1) (54 J K(-1) mol(-1)) for 59% Ni dilution. Raman spectroscopy revealed a mixed one- and two-mode behavior in the solid solutions. For the first time, a correlation between vibrational frequencies exhibiting one-mode behavior and the entropy change, which drives the spin crossover, is established.     

Interaction of monosulfonate tetraphenyl porphyrin (H2TPPS1) with plant-esterase: determination of the binding mechanism by spectroscopic methods

The interaction of monosulfonate tetraphenyl porphyrin (H(2)TPPS(1)) with plant-esterase was investigated using fluorescence and UV-vis absorption spectroscopy. Fluorescence quenching, from which the binding parameters were evaluated, revealed that the quenching of the esterase by H(2)TPPS(1) resulted from the formation of a dye-esterase complex. According to the modified Stern-Volmer equation, the effective quenching constants (K(a)) between H(2)TPPS(1) and plant-esterase at four different temperatures (297 K, 300 K, 303 K, and 306 K) were obtained to be 14.132×10(5), 5.734×10(5), 2.907×10(5), and 2.291×10(5) M(-1), respectively. The thermodynamic parameters, enthalpy change (ΔH) and entropy change (ΔS) for the reaction were calculated to be -181.67 kJ M(-1) and -0.49 kJ M(-1)K(-1), indicating that van der Waals force and hydrogen bonds were the dominant intermolecular force in stabilizing the complex. Site marker competitive experiments showed that the binding of H(2)TPPS(1) to plant-esterase primarily took place in the active site. The binding distance (r) was obtained to be 5.99 nm according to F?rster theory of non-radioactive energy transfer. The conformation of plant-esterase was investigated by synchronous fluorescence and UV-vis absorption spectroscopy, and the results confirmed some micro-environmental and conformational changes of plant-esterase molecules.     

Direct measurements of the rate constants of the reactions NCN + NO and NCN + NO2 behind shock waves

The high-temperature rate constants of the reactions NCN + NO and NCN + NO(2) have been directly measured behind shock waves under pseudo-first-order conditions. NCN has been generated by the pyrolysis of cyanogen azide (NCN(3)) and quantitatively detected by sensitive difference amplification laser absorption spectroscopy at a wavelength of 329.1302 nm. The NCN(3) decomposition initially yields electronically excited (1)NCN radicals, which are subsequently transformed to the triplet ground state by collision-induced intersystem crossing (CIISC). CIISC efficiencies were found to increase in the order of Ar < NO(2) < NO as the collision gases. The rate constants of the NCN + NO/NO(2) reactions can be expressed as k(NCN+NO)/(cm(3) mol(-1)s(-1)) = 1.9 × 10(12) exp[-26.3 (kJ/mol)/RT] (±7%,ΔE(a) = ± 1.6 kJ/mol, 764 K < T < 1944 K) and k(NCN+NO(2))/(cm(3) mol(-1)s(-1)) = 4.7 × 10(12) exp[-38.0(kJ/mol)/RT] (±19%,ΔE(a) = ± 3.8 kJ/mol, 704 K < T < 1659 K). In striking contrast to reported low-temperature measurements, which are dominated by recombination processes, both reaction rates show a positive temperature dependence and are independent of the total density (1.7 × 10(-6) mol/cm(3) < ρ < 7.6 × 10(-6) mol/cm(3)). For both reactions, the minima of the total rate constants occur at temperatures below 700 K, showing that, at combustion-relevant temperatures, the overall reactions are dominated by direct or indirect abstraction pathways according to NCN + NO → CN + N(2)O and NCN + NO(2) → NCNO + NO.