Atmospheric Spectroscopy and Photochemistry at Environmental Water Interfaces.

The air-water interface is ubiquitous in nature, as manifested in the form of the surfaces of oceans, lakes, and atmospheric aerosols. The aerosol interface, in particular, can play a crucial role in atmospheric chemistry. The adsorption of atmospheric species onto and into aerosols modifies their concentrations and chemistries. Moreover, the aerosol phase allows otherwise unlikely solution-phase chemistry to occur in the atmosphere. The effect of the air-water interface on these processes is not entirely known. This review summarizes recent theoretical investigations of the interactions of atmosphere species with the air-water interface, including reactant adsorption, photochemistry, and the spectroscopy of reactants at the water surface, with an emphasis on understanding differences between interfacial chemistries and the chemistries in both bulk solution and the gas phase. The results discussed here enable an understanding of fundamental concepts that lead to potential air-water interface effects, providing a framework to understand the effects of water surfaces on our atmosphere.

Theoretical kinetic study of the reactions of cycloalkylperoxy radicals.

Reactions of alkyl radicals with oxygen are key reactions in the low-temperature oxidation of hydrocarbons, but they have not been extensively studied yet in the case of cycloalkanes. Isomerizations of cycloalkylperoxy radicals and formation of cyclic ethers are especially important. In the present work, a theoretical study of the gas-phase reactions of cyclopentylperoxy and cyclohexylperoxy radicals has been carried out by means of quantum chemical calculations at the CBS-QB3 level. Computations on cyclopentylperoxy decomposition pathways are reported here for the first time. Thermochemical data have been obtained by means of isodesmic reactions, and the contribution of hindered rotors has been explicitly taken into account. Transition state theory has been used to calculate rate constants for all the elementary reactions. Three-parameter Arrhenius expressions have been derived in the temperature range 300-1000 K. Tunneling effects have been accounted for in the case of H-atom transfers. Our results compare well with experimental data and previous calculations available in the literature. In particular, the predicted rate constants for processes involving cyclohexylperoxy radicals, which have been introduced in a reaction mechanism scheme proposed before, exhibit excellent agreement with experiments at low and intermediate temperatures.

Theoretical kinetic study of thermal unimolecular decomposition of cyclic alkyl radicals.

Whereas many studies have been reported on the reactions of aliphatic hydrocarbons, the chemistry of cyclic hydrocarbons has not been explored extensively. In the present work, a theoretical study of the gas-phase unimolecular decomposition of cyclic alkyl radicals was performed by means of quantum chemical calculations at the CBS-QB3 level of theory. Energy barriers and high-pressure-limit rate constants were calculated systematically. Thermochemical data were obtained from isodesmic reactions, and the contribution of hindered rotors was taken into account. Classical transition state theory was used to calculate rate constants. The effect of tunneling was taken into account in the case of CH bond breaking. Three-parameter Arrhenius expressions were derived in the temperature range of 500-2000 K at atmospheric pressure, and the CC and CH bond breaking reactions were studied for cyclic alkyl radicals with a ring size ranging from three to seven carbon atoms, with and without a lateral alkyl chain. For the ring-opening reactions, the results clearly show an increase of the activation energy as the pi bond is being formed in the ring (endo ring opening) in contrast to the cases in which the pi bond is formed on the side chain (exo ring opening). These results are supported by analyses of the electronic charge density that were performed with Atoms in Molecules (AIM) theory. For all cycloalkyl radicals considered, CH bond breaking exhibits larger activation energies than CC bond breaking, except for cyclopentyl for which the ring-opening and H-loss reactions are competitive over the range of temperatures studied. The theoretical results compare rather well with the experimental data available in the literature. Evans-Polanyi correlations for CC and CH beta-scissions in alkyl and cycloalkyl free radicals were derived. The results highlight two different types of behavior depending on the strain energy in the reactant.

The structure of chlorophyll a-water complexes: insights from quantum chemistry calculations.

Computations show that chlorophyll a is able to coordinate a maximum of two water molecules in hydrophobic media that form a bridge between the Mg atom and the methyl ester carbonyl group.

Detailed kinetic study of the ring opening of cycloalkanes by CBS-QB3 calculations.

This work reports a theoretical study of the gas-phase unimolecular decomposition of cyclobutane, cyclopentane and cyclohexane by means of quantum chemical calculations. A biradical mechanism has been envisaged for each cycloalkane, and the main routes for the decomposition of the biradicals formed have been investigated at the CBS-QB3 level of theory. Thermochemical data(DeltaHf(o), S(o), Cp(o)) for all the involved species have been obtained by means of isodesmic reactions. The contribution of hindered rotors has also been included. Activation barriers of each reaction have been analyzed to assess the energetically most favorable pathways for the decomposition of biradicals. Rate constants have been derived for all elementary reactions using transition-state theory at 1 atm and temperatures ranging from 600 to 2000 K. Global rate constant for the decomposition of the cyclic alkanes in molecular products have been calculated. Comparison between calculated and experimental results allowed us to validate the theoretical approach. An important result is that the rotational barriers between the conformers, which are usually neglected, are of importance in decomposition rate of the largest biradicals. Ring strain energies (RSE) in transition states for ring opening have been estimated and show that the main part of RSE contained in the cyclic reactants is removed upon the activation process.

Theoretical study of the reduction mechanism of sulfoxides by thiols.

Theoretical computations have been carried out to investigate the reaction mechanism of the sulfoxide reduction by thiols in solution. This reaction is a suitable model for enzymatic processes involving methionine sulfoxide reductases (Msrs). Recent investigations on the Msr mechanism have clearly shown that a sulfenic acid intermediate is formed on the catalytic cysteine of the active site concomitantly to the methionine product. In contrast, experimental studies for the reaction of a number of thiols and sulfoxides in solution did not observe sulfenic acid formation. Only, a disulfide was identified as the final product of the process. The present study has been carried out at the MP2/6-311+G(3d2f,2df,2p)//B3LYP/6-311G(d,p) level of theory. The solvent effect in DMSO has been incorporated using a discrete-continuum model. The calculations provide a basic mechanistic framework that allows discussion on the apparent discrepancy existing between experimental data in solution and in the enzymes. They show that, in the early steps of the process in solution, a sulfurane intermediate is formed the rate of which is limiting. Then, a proton transfer from a second thiol molecule to the sulfurane leads to the formation of either a sulfenic acid or a disulfide though the latter is much more stable than the former. If a sulfenic acid is formed in solution, it should react with a thiol molecule making its experimental detection difficult or even unfeasible.

Comparison of three effective Hamiltonian models of increasing complexity: triazene in water as a test case.

A critical comparison of widely used solvation models is reported. It is illustrated by a study of the triazene molecule in liquid water. We consider the following approaches: (1) a continuum model based on multicentric multipole expansions of the charge distribution, (2) the averaged solvent electrostatic potential from molecular dynamics (ASEP/MD) method, and (3) molecular dynamics simulations using a combined quantum mechanics/molecular mechanics potential (QM/MM/MD). We find that the solvation induces appreciable changes in the geometry and charge distribution of triazene. These changes are only qualitatively reproduced by the dielectric continuum model, which clearly underestimates induced dipole moments and solute-solvent interaction energy. We also show that the use of effective point charges placed on solute nuclei during the classical simulations may cause significant errors in the description of the solvent structure. The addition of charges representing nitrogen atom lone pairs is compulsory to reproduce the QM/MM/MD simulation results. Moreover, our results validate the use of the mean field approximation in the study of solvent effects. A major conclusion of this study is that the ASEP/MD method constitutes a reliable alternative to the much more computationally demanding QM/MM/MD methods.

Simulation of liquid water using semiempirical Hamiltonians and the divide and conquer approach.

This work examines the ability of semiempirical methods to describe the structure of liquid water. Particularly, the standard AM1 and PM3 methods together with recently developed PM3-PIF and PM3-MAIS parametrizations have been considered. We perform molecular dynamics simulations for a system consisting of 64 or 216 water molecules in a periodic cubic box. The whole system is described quantum mechanically. Calculations with 64 molecules have been carried out using standard SCF techniques whereas calculations with 216 molecules have been done using the divide and conquer approach. This method has also been used in one simulation case with 64 molecules for test purposes. Within this scope, the molecular dynamics program ROAR have been coupled with a linear scaling semiempirical code (DivCon) implemented in a parallel MPI version. The predicted liquid water structure using either AM1 or PM3 is shown to be very poor due to well-known limitations of these methods describing hydrogen bonds. In contrast, PM3-PIF and PM3-MAIS calculations lead to results in reasonably good agreement with experimental data. The best results for the heat of vaporization are obtained with the PM3-PIF method. The average induced dipole moment of the water molecule in the liquid is underestimated by all semiempirical techniques, which seems to be related to the NDDO approximation and to the use of minimal basis sets. A brief discussion on charge-transfer effects in liquid water is also presented.

Theoretical study of the 1,3-hydrogen shift of triazene in water.

The 1,3-hydrogen shift of triazene in aqueous solution was studied with a combination of QM/MM methods. First, the different species involved were characterized and the activation free-energies calculated with ASEP/MD, a method that makes use of the mean field approximation. Then the reaction dynamics was simulated with a QM/MM/MD method. A very strong influence of the solvent was observed, both specific, with the participation of a water molecule, and from the rest of the solvent. The effect of solvation on the geometry and electron distribution of triazene is important: N-N bond lengths tend to be more similar and the molecule acquires a planar structure. For the transition state structure, a substantial degree of ionic nature was found. Dynamic solvent effects were also analyzed.

A model for double asymmetric induction in the stereocontrolled reduction of glycosyl alpha-ketoesters with oxazaborolidines.

Experimental diastereoselectivities for the stereocontrolled reduction of glycosyl alpha-ketoesters into the corresponding alpha-hydroxyesters have recently been reported with unexpected results. The process is catalyzed by a chiral oxazaborolidine derivative (the so-called CBS catalyst) and represents the key step in the synthesis of glycosyl alpha-amino acids synthons, a class of compounds that allow preparation of natural glycopeptides analogues exhibiting potential therapeutic relevance. Good to very good diastereomeric excesses have been obtained for a series of reactions with different glucidic derivatives, but surprisingly, the major product obtained does not correspond to that predicted by using Corey's model. In the present work, we carry out a theoretical investigation of these reactions at the density functional level. Separated effects from the catalyst and from the glucidic derivative have been computed to rationalize the observed diastereoselectivities and the double asymmetric induction.

Cinc y enfermedad pulmonar obstructiva crónica / Zinc and chronic pulmonary obstructive disease

Fundamento. Evaluar las concentraciones orgánicas de cinc en la enfermedad pulmonar obstructiva crónica (EPOC) y en controles sanos, así como evidenciar si existen diferencias entre ambos grupos. Determinar si las concentraciones de cinc plasmático se correlacionan con las halladas en hematíes y faneras. Pacientes y métodos. Se estudian 40 pacientes con EPOC, varones, edad media de 62 ñ 7 años e índice de masa corporal (IMC) de 27 ñ 4. El grupo control lo forman 40 individuos; edad media de 57 ñ 9 años e IMC de 24 ñ 5. Se excluyeron los pacientes con patologías concomitantes o en tratamientos que pudieran aumentar la excreción de cinc. A todos los pacientes se les ha realizado historia clínica y exploración física completas, analítica general con hemograma, bioquímica con parámetros hepáticos y lipídicos y determinación del cinc ungueal, plasmático, en cabellos y en hematíes, mediante espectrofotometría de absorción atómica. El método estadístico empleado ha sido el análisis de la varianza de una vía y el coeficiente de correlación lineal de Pearson para estudiar la asociación entre variables. Resultados. El cinc plasmático, intraeritrocitario y ungueal fue similar en los sujetos sanos y en los pacientes; en cambio, la concentración media en cabellos resultó significativamente menor en los pacientes (156 ñ 46) respecto al grupo control (185 ñ 64) con p < 0,05. No se han encontrado diferencias significativas entre fumadores y no fumadores, ni entre bebedores y no bebedores respecto a las concentraciones de cinc orgánico. Conclusión. Los pacientes con EPOC tienen unas concentraciones de cinc en pelo inferiores a la de los sujetos sanos, mientras que el cinc en hematíes, plasma y a nivel ungueal fue similar en ambos grupos. La determinación de cinc en cabellos puede ser un buen método para detectar deficiencias crónicas y asintomáticas de este metal en el organismo. No se ha encontrado correlación entre la determinación de cinc plasmática y las determinaciones en hematíes y faneras. (AU)

[Nocturnal arterial pressure and the cardiovascular risk factors in patients with arterial hypertension]. / Presión arterial nocturna y factores de riesgo cardiovascular en pacientes con hipertensión arterial.

BACKGROUND: Blood pressure (BP) has a circadian rhythm. Most of the people presents a BP fall between 10-20% during nighttime hours (dipper condition). Measurement of these circadian variations is performed by continuous blood pressure ambulatory monitoring. We have studied the possible relation between blood pressure nocturnal fall and cardiovascular risk factors in hypertensive patients. METHODS: We selected 100 hypertensive patients from the Hypertension and Lipids Unit of San Cecilio University Hospital of Granada, Spain. They were divided into two groups: dippers group and non-dippers, depending of whether or not the fall of nocturnal systolic and diastolic BP was > 10%. All patients included in the study underwent complete clinical history, exhaustive physical examination, complementary examinations, urine and hematology tests, and continuous blood pressure ambulatory monitoring by the Space Labs system. RESULTS: The heart rate was found to be significantly higher (p = 0.0253) in the hypertensive dippers group than in the non-dippers. The latter showed higher plasma creatinine values (p = 0.0343) and lower potassemia values (p = 0.0140) than the dippers group. The dippers group presented concentrations of HDL-cholesterol significantly higher (p = 0.008) and diurnal diastolic BP values (p = 0.0211) also higher than the other group. CONCLUSIONS: Non dippers hypertensive patients present a higher number of cardiovascular risk factors as well as a higher tendency to renal lesions, and worse prognosis than the dippers group.

[Zinc and chronic obstructive pulmonary disease]. / Cinc y enfermedad pulmonar obstructiva crónica.

BACKGROUND: To evaluate the zinc organic concentration in the chronic obstructive pulmonary disease (COPD) in relation to a control group. Also, to analyze if the serum zinc levels are related with zinc concentration in erythrocyte, nail and hair. PATIENTS AND METHODS: Forty patients of COPD have been studied, all males, average age 62 +/- 7 years and body mass index (BMI) of 27 +/- 4. Forty patients were included in the control group, with average age 57 +/- 9 years and BMI of 24 +/- 5. The patients with concomitant disease or booth treatment that could increase the zinc excretion were excluded. In all patients clinical history and examination, hematology and biochemistry tests, hepatic and lipid parameters, and nail, plasma, hair and erythrocyte zinc levels by atomic absorption spectrophotometry were done. Variance analysis and Pearson test were done. RESULTS: Blood, intraerythrocyte and nail zinc were similar in healthy subjects and patients; however, the median concentration of zinc in hair was significantly lower in patients (156 +/- 46 micrograms/g versus control group (185 +/- 64 micrograms/g) (p < 0.05). No differences were found between smokers and no smokers, and drinkers and no drinkers in relation with body concentration of zinc. CONCLUSION: The zinc concentration in hair can be a good method to evaluate the chronic deficiency in the human body. The COPD patients could be susceptible to develop zinc deficiency; the situation increases the possibility of infection diseases.