Competitive Gas Phase Reactions for the Production of Isomers C2O2H4. Spectroscopic Constants of Methyl Formate.

New routes for the chemical formation of the C2O2H4 isomers in the gas phase are explored searching for a justification of the prominent astrophysical abundance of methyl formate with respect to the most stable one acetic acid. Kinetic rate constants at low temperatures are provided for eight barrierless reaction pathways. In addition, the spectroscopic parameters are computed using highly correlated ab initio methods for the main isotopologue of methyl formate and for five monosubstituted isotopologues containing 13C, 18O, and deuterium. Accurate rotational constants are obtained at the CCSD(T)-F12 level of theory. The dipole moments components are provided. Centrifugal distortion constants, rovibrational parameters, and Fermi displacements are predicted using second order perturbation theory. A variational procedure of reduced dimensionality is applied to determine band center positions for transitions corresponding to the large amplitude motions. The COC bending mode is considered explicitly as an independent coordinate to evaluate its resonances with the torsional energies. The effect of resonances is proven as negligible.

Site reactivity in the free radicals induced damage to leucine residues: a theoretical study.

Several recent computational studies have tried to explain the observed selectivity in radical damage to proteins. In this work we use Density Functional Theory and Transition State Theory including tunnelling corrections, reaction path degeneracy, the effect of diffusion, and the role of free radicals to get further insights into this important topic. The reaction between a leucine derivative and free radicals of biological significance, in aqueous and lipid media, has been investigated. Both thermochemical and kinetic analyses, in both hydrophilic and hydrophobic environments, have been carried out. DPPH, ˙OOH, ˙OOCH3, ˙OOCH2Cl, ˙OOCHCl2 and ˙OOCHCH2 radicals do not react with the target molecule. The reactions are proposed to be kinetically controlled. The leucine gamma site was the most reactive for the reactions with ˙N3, ˙OOCCl3, ˙OCH3, ˙OCH2Cl, and ˙OCHCl2 radicals, with rate constants equal to 1.97 × 10(5), 3.24 × 10(4), 6.68 × 10(5), 5.98 × 10(6) and 8.87 × 10(8) M(-1) s(-1), respectively, in aqueous solution. The ˙Cl, ˙OH and ˙OCCl3 radicals react with leucine at the beta, gamma, and delta positions at rates close to the diffusion limit with the alpha position which is the slowest path and the most thermodynamically favored. The presented results confirm that the Bell-Evans-Polanyi principle does not apply for the reactions between amino acid residues and free radicals. Regarding the influence of the environment on the reactivity of the studied series of free radicals towards leucine residues, it is concluded that hydrophilic media slightly lower the reactivity of the studied radicals, compared to hydrophobic ones, albeit the trends in reactivity are very similar.

Cyclic-3-hydroxymelatonin (C3HOM), a potent antioxidant, scavenges free radicals and suppresses oxidative reactions.

Cyclic 3-hydroxymelatonin (C3HOM) is an immediate product of melatonin's interaction with reactive oxygen species. Its presence has been detected in mice, rats and humans. In the current study, the antioxidant capacity and reducing power of this molecule have been systematically studied. C3HOM is found to be a more potent antioxidant than melatonin or vitamin C in terms of its ability to scavenge the hydroxyl radical (HO.) and to recover oxidized horseradish peroxidase to its ground state. The antioxidative mechanism of C3HOM is similar to that of the classic antioxidant, vitamin C, rather than to its precursor melatonin. C3HOM effectively prevents the oxidative degradation of cytochrome C induced by hydrogen peroxide (H2O2). It is speculated that some antioxidative activities of melatonin may be mediated by its metabolite, C3HOM. C3HOM prevents mitochondrial cytochrome C injury and, thus, it is likely to inhibit cellular apoptosis induced by the release of oxidized cytochrome C from mitochondria.

Rate constant dependence on the size of aldehydes in the NO(3) + aldehydes reaction. An explanation via quantum chemical calculations and CTST.

The reactions of NO(3) with formaldehyde, acetaldehyde, propanal, n-butanal, and isobutanal have been modeled using accurate ab initio and hybrid DFT methods with large basis sets. The results clearly indicate that the reaction is a simple aldehydic H atom abstraction; no adduct was found to support the idea of a complex mechanism. Alternative hydrogen abstractions were modeled for the alpha carbon hydrogen atoms and for the Cbeta of n-butanal; the differences in activation energies ruled out the possibility that competitive abstraction could be responsible for the anomalous increase of the rate constants with the size of aldehydes. The anomalous behavior was found to be a consequence of the preexponential factor increase, due to the enlargement of the internal rotation partition functions with the size of the aldehydes. The reaction rate constants, calculated using the conventional transition-state theory as applied to a proposed simple mechanism, reproduce remarkably well the reported experimental results. Consideration of the internal rotation partition functions is shown to be essential for the determination of the preexponential parameters and thus for the correct calculation of the rate constants. The tunneling correction was found negligible due to the features of the transition vector.

[Progress in the early diagnosis of cancer of the colon and rectum]. / Progresos en el diagnóstico precoz del cáncer de colon y recto.

Our experience with the air contrast examination in the cancer of colon and rectum diagnoses is showed. The colaboration among radiologist, endoscopist, pathologist and surgeon is important. In the large bowel tumors diagnosis, the radiologic and endoscopic prodedures should be evaluated together. The double colonic contrast has showed in our experience, better results than with the barium enema. Over 31 patients with both studies, we obtained 13 false negatives (with barium enema, doing then the double colonic contrast became positive 12 (92.4%).