L Moisés Canle

Myeloperoxidase-catalyzed chlorination: the quest for the active species.

Myeloperoxidase (MPO) is a dominating enzyme of circulating polymorphonuclear neutrophils that catalyzes the two-electron oxidation of chloride, thereby producing the strong halogenating agent hypochlorous acid (ClO(-)/HOCl). In absence of MPO the tripeptide Pro-Gly-Gly reacts with HOCl faster than the amino acid taurine (2-aminoethanesulfonic acid, Tau), while the MPO-mediated chlorination shows reverse order. A comparative study of the enzymatic oxidation of both substrates at pH 4.0-6.0, varying H2O2 concentration is presented. Initial and equilibrium rates studies have been carried on, reaction rates in the latter being slower due to the chemical equilibrium between MPO-I and MPO-II-HO2. A maximum of chlorination rate is observed for Pro-Gly-Gly and Tau when [H2O2] approximately 0.3-0.7 mM and pH approximately 4.5-5.0. Several mechanistic possibilities are considered, the proposed one implies that chlorination takes place via two pathways. One, for bulkier substrates, involves chlorination by free HOCl outside the heme cavity; ClO(-) is released from the active center, diffuses away the heme cavity, and undergoes protonation to HOCl. The other implies the existence of compound I-Cl(-) complex (MPO-I-Cl), capable of chlorinating smaller substrates in the heme pocket. Electronic structure calculations show the size of Pro-Gly-Gly comparable to the available gap in the substrate channel, this tripeptide being unable to reach the active site, and its chlorination is only possible by free HOCl outside the enzyme.

Combined Theoretical and Experimental Study of the Photophysics of Asulam

The photophysics of the neutral molecular form of the herbicide asulam has been described in a joint experimental and theoretical, at the CASPT2 level, study. The unique π → π* aromatic electronic transition (f, ca. 0.5) shows a weak red-shift as the polarity of the solvent is increased, whereas the fluorescence band undergoes larger red-shifts. Solvatochromic data point to higher dipole moment in the excited state than in the ground state (μ(g) < μ(e)). The observed increase in pKa in the excited state (pKa* - pKa, ca. 3) is consistent with the results of the Kamlet-Abboud-Taft and Catalán et al. multiparametric approaches. Fluorescence quantum yield varies with the solvent, higher in water (ϕ(f) = 0.16) and lower in methanol and 1-propanol (approx. 0.02). Room temperature fluorescence lifetime in aqueous solution is (1.0 ± 0.2) ns, whereas the phosphorescence lifetime in glassy EtOH at 77 K and the corresponding quantum yield are (1.1 ± 0.1) s and 0.36, respectively. The lack of mirror image symmetry between modified absorption and fluorescence spectra reflects different nuclear configurations in the absorbing and emitting states. The low value measured for the fluorescence quantum yield is justified by an efficient nonradiative decay channel, related with the presence of an easily accessible conical intersection between the initially populated singlet bright (1)(L(a) ππ*) state and the ground state (gs/ππ*)(CI). Along the main decay path of the (1)(L(a) ππ*) state the system undergoes an internal conversion process that switches part of the population from the bright (1)(L(a) ππ*) to the dark (1)(L(b) ππ*) state, which is responsible for the fluorescence. Additionally, singlet-triplet crossing regions have been found, a fact that can explain the phosphorescent emission detected. An intersystem crossing region between the phosphorescent state (3)(L(a) ππ*) and the ground state has been characterized, which contributes to the nonradiative deactivation of the excitation energy.

Understanding the mechanism of base-assisted decomposition of (N-halo),N-alkylalcoholamines.

The base-assisted decomposition of (N-X),N-methylethanolamine (X = Cl, Br) takes place mainly through two concurrent processes: a fragmentation and an intramolecular elimination. The global process follows second order kinetics, first order relative to both (N-X),N-methylethanolamine and base. Interaction of the base with the ionizable hydroxylic hydrogen triggers the reaction. The intramolecular elimination pathway leads to formaldehyde and 2-aminoethanol as reaction products via base-assisted proton transfer from the methyl to the partially unprotonated hydroxylic oxygen, with loss of halide. Meanwhile, the fragmentation pathway leads to methylamine and two equivalents of formaldehyde via bimolecular base-promoted concerted breakage of the molecule into formaldehyde, halide ion and N-methylmethanimine. Kinetic evidences allow a crude estimation of the concertedness and characterization of the transition structure for both processes, which are slightly asynchronous, the proton transfer to the base taking place ahead of the rest of the molecular events. The degree of asynchroneity increases as the bases become weaker. Electronic structure calculations, at the B3LYP/6-31++G** level, on the fragmentation pathway support the proposed mechanism.

A theoretical study on the mechanism of the base-promoted decomposition of N-chloro,N-methylethanolamine

The first step of the base-promoted decomposition of N-chloro,N-methylethanolamine in aqueous solution (CH3N(Cl)CH2CH2OH + HO- --> imine + Cl- + H2O (+ CH2O) --> amine + aldehyde) is investigated at the MP2/6-31++G(d,p) computing level. Solvation is included by using both a microsolvated model, in which two explicit water molecules simulate the specific solvent effects, and a hybrid cluster-continuum model, by applying a polarized continuum on the previous results, to account for the bulk effect of the solvent. Four alternative pathways (bimolecular fragmentation, Hofmann, Zaitsev and intramolecular eliminations) are possible for the rate-limiting step of this base-promoted decomposition. These reactive processes are bimolecular asynchronous concerted reactions. The common feature of the four pathways is the proton transfer to HO- being more advanced than all other molecular events, whereas imine formation is delayed. Non-reactive cyclic arrangements involving one of the explicit water molecules are found at transition structures of Hofmann and Zaitsev eliminations, such water molecule acting both as H+ donor and acceptor. Although MP2 calculations misjudge the absolute activation Gibbs free energy values, this computational level adequately predicts the enhancement in the decomposition rate due to the presence of the -OH group.