Diffusion of molecules in the bulk of a low density amorphous ice from molecular dynamics simulations.

The diffusion of molecules in interstellar ice is a fundamental phenomenon to take into account while studying the formation of complex molecules in this ice. This work presents a theoretical study on the diffusion of H2O, NH3, CO2, CO, and H2CO in the bulk of a low density amorphous (LDA) ice, while taking into account the physical conditions prevailing in space, i.e. temperatures below 150 K and extremely low pressure. This study was undertaken by means of molecular dynamics simulations. For CO2 for which no experimental data were available we conducted our own experiments. From our calculations we show that, at low temperatures, the diffusion of molecules in the bulk of a LDA ice is driven by the self-diffusion of water molecules in the ice. With this study we demonstrate that molecular dynamics allows the calculation of diffusion coefficients for small molecules in LDA ice that are convincingly comparable to experimentally measured diffusion coefficients. We also provide diffusion coefficients for a series of molecules of astrochemical interest.

Kinetics of the NH3 and CO2 solid-state reaction at low temperature.

Ammonia and carbon dioxide play an important role in both atmospheric and interstellar ice chemistries. This work presents a theoretical and experimental study of the kinetics of the low-temperature NH3 and CO2 solid-state reaction in ice films, the product of which is ammonium carbamate (NH4(+)NH2COO(-)). It is a first-order reaction with respect to CO2, with a temperature-dependent rate constant fitted to the Arrhenius law in the temperature range 70 K to 90 K, with an activation energy of 5.1 ± 1.6 kJ mol(-1) and a pre-exponential factor of 0.09-0.08(+1.1) s(-1). This work helps to determine the rate of removal of CO2 and NH3, via their conversion into ammonium carbamate, from atmospheric and interstellar ices. We also measure first-order desorption energies of 69.0 ± 0.2 kJ mol(-1) and 76.1 ± 0.1 kJ mol(-1), assuming a pre-exponential factor of 10(13) s(-1), for ammonium carbamate and carbamic acid, respectively.

Contribution of high-energy conformations to NMR chemical shifts, a DFT-BOMD study.

This paper highlights the relevance of including the high-energy conformational states sampled by Born-Oppenheimer molecular dynamics (BOMD) in the calculation of time-averaged NMR chemical shifts. Our case study is the very flexible glycerol molecule that undergoes interconversion between conformers in a nonrandom way. Along the sequence of structures from one backbone conformer to another, transition states have been identified. The three (13)C NMR chemical shifts of the molecule were estimated by averaging their calculated values over a large set of BOMD snapshots. The simulation time needed to obtain a good agreement with the two signals present in the experimental spectrum is shown to be dependent on the atomic orbital basis set used for the dynamics, with a necessary longer trajectory for the most extended basis sets. The large structural deformations with respect to the optimized conformer geometries that occur along the dynamics are related to a kinetically driven conformer distribution. Calculated conformer type populations are in good agreement with experimental gas phase microwave results.

Structure, dynamics, and energetics of lysobisphosphatidic acid (LBPA) isomers.

Lysobisphosphatidic acid (LBPA), or bis(monoacylglycerol)phosphate, is a very interesting lipid, that is mainly found in late endosomes. It has several intriguing characteristics, which differ from those of other animal glycerophospholipids, that may be related to its specific functions, particularly in the metabolism of cholesterol. Its phosphodiester group is bonded at the sn-1 (sn-1') positions of the glycerols rather than at sn-3 (sn-3'); the position of the two fatty acid chains is still under debate but, increasingly, arguments favor the sn-2, sn-2' position in the native molecule, whereas isolation procedures or acidic conditions lead to the thermodynamically more stable sn-3, sn-3' structure. Because of these peculiar features, it can be expected that LBPA shape and interactions with membrane lipids and proteins are related to its structure at the molecular level. We applied quantum mechanical methods to study the structures and stabilities of the 2,2' and 3,3' LBPA isomers, using a step-by-step procedure from glycerol to precursors (in vitro syntheses) and to the final isoforms. The structures of the two positional LBPA isomers are substantially different, showing that the binding positions of the fatty acid chains on the glycerol backbone determine the shape of the LBPA molecule and thus, possibly, its functions. The 3,3' LBPA structures obtained are more stable with respect to the 2,2' form, as expected from experiment. If one argues that the in vivo synthesis starts from the present glycerol conformers and considering the most stable bis(glycero)phosphate structures, the 2,2' isoform should be the most probable isomer.

Density functional theory-based conformational analysis of a phospholipid molecule (dimyristoyl phosphatidylcholine).

The conformational space of the dimyristoyl phosphatidylcholine (DMPC) molecule has been studied using density functional theory (DFT), augmented with a damped empirical dispersion energy term (DFT-D). Fourteen ground-state isomers have been found with total energies within less than 1 kcal/mol. Despite differences in combinations of their torsion angles, all these conformers share a common geometric profile, which includes a balance of attractive, repulsive, and constraint forces between and within specific groups of atoms. The definition of this profile fits with most of the structural characteristics deduced from measured NMR properties of DMPC solutions. The calculated vibrational spectrum of the molecule is in good agreement with experimental data obtained for DMPC bilayers. These results support the idea that DMPC molecules preserve their individual molecular structures in the various assemblies.

Two bonding configurations of acetylene on Si(001)-(2 x 1): a combined high-resolution electron energy loss spectroscopy and density functional theory study.

Two coexisting adsorption states of molecularly adsorbed acetylene on the Si(001)-(2 x 1) surface have been identified by a combined study based on the high-resolution electron energy loss spectroscopy and density functional computations. Seven possible adsorbate-substrate structures are considered theoretically including their full vibrational analysis. Based on a significantly enhanced experimental resolution, the assignment of 15 C2H2- and C2D2-derived vibrational modes identifies a dominant di-sigma bonded molecule adsorbed on top of a single Si-Si dimer. Additionally there is clear evidence for a second minority species which is di-sigma bonded between two Si-Si dimers within the same dimer row (end-bridge geometry). The possible symmetries of the adsorbate complexes are discussed based on the specular and off-specular vibrational measurements. They suggest lower than ideal C(2v) and C(s) symmetries for on-top and end-bridge species, respectively. At low coverages the symmetry reductions might be lifted.

Selectivity descriptors for the Michael addition reaction as obtained from density functional based approaches.

Density functional (DF) based numerical approaches for computing orbital and atomic reactivity indices were employed in the study of selectivity descriptors for the 1,4 Michael addition reaction. To this aim, atomic and orbital Fukui indices and atomic softnesses for 2-arylmethylene-1,4-butanolides and N,N-disubstituted phenylacetamides were computed. Further on, these local selectivity descriptors have been rationalized in terms of the Pearson's hard-soft-acid-base principle to explain the observed regioselectivity. It is shown that the methods employed for local (atomic and orbital) reactivity index computations are useful and reliable for prediction of the regioselectivity upon conjugate addition of ambident nucleophiles to 2,3-unsaturated carboxylic esters. All the results reveal similar degree of localization/hardness of the 1,4-butanolides C4 and active alpha-carbon belonging to the N,N-dimethyl-phenylacetamide, while the soft alpha-carbon in LiCH2CN reacts with the soft C2 1,4-butanolide center.

On the hardness evaluation in solvent for neutral and charged systems.

The influence of water on the hardness values of a series of neutral and charged molecules has been studied in the framework of density functional theory using the polarizable continuum model to take into account solvent effects. Three working formulas already widely tested in gas-phase have been used and the results compared. Two of the methods employed going from gas phase to solvent phase give values that do not change, while the results of the third method show remarkable changes. To check the reliability of the hardness behavior found, a test based on the hard-soft/acid-base principle and the calculation of the free energy of reaction has been applied using the adopted procedures.

[The impact of the hostel life style on the health status of students]. / Otrazhenie na obshtezhitiinata forma na zhivot vurkhu zdravnoto sustoianie na studentite.

The work aims at studying the specificity of the social living conditions, organization, regimen, psychosocial and other factors of life and their effect on the health status of students from the hostels of the students' town in Sofia. An inquiry is used, including 69 questions, connected with the organization and the conditions of living and education in the hostels. A statistical method for examining the chronic somatic morbidity is applied. By the inquiry method are investigated 1034 young people (boys and girls), living in the hostels of the Student's town. The statistical method assists in studying the chronic morbidity of 10.051 students, living in the hostels and of 9.562 students living in other places. The results show that in the hostel form of living there are infringements of the conditions and organization of the living standard, daily regimen, nutrition etc. The registered negative deviations correlate with the presence of higher chronic somatic morbidity, higher prevalence in spreading of socially important diseases-cardio-vascular, gastro-intestinal, renal. A hygiene-preventive program for improving the living standard, extending the dispensarization, carrying of prophylactic and treatment undertakings, etc. was recommended.

[Problems of the health status of the youth from a student town]. / Vurkhu niakoi problemi na zdravnoto sustoianie na mladezhta ot Studenskiia grad.

Studies are carried out on the health status and morbidity of students from the territory of the student town and health-hygienic undertakings are recommended for improving the health of the students and formation of health habits. For 8000 students were traced the following data: prophylactic examinations, acute morbidity, morbidity according to medical registrations studied from the statistical cards, the follow-up care of patients with chronic diseases. A scientific statistical method for studying the medical documentation is used for examining the acute and chronic morbidity. The basic moments in the structure of the students pathology are described, stimulated by the new moments of life in the conditions of the students town and the peculiarities in the health status of the students as special age group of the population. Conclusions are made concerning the carrying out of directed hygiene-preventive undertakings.

[Health and hygiene research on constructive toys intended for children over 7 under natural conditions]. / Zdravnokhigienni izsledvaniia na konstruktivni igarchki za detsa nad 7 godini v estestveni usloviia.

Physiological and questionnaire investigations are carried out on 10 constructive toys, with the participation of 38 children 7 to 12 years old. A testimonial of the toys is made according to mass, dimensions, technological treatment, packing etc. A number of discrepancies with the hygienic requirements were established: 1) imperfection of the technological treatment (sharp edges, tops and holes, difficult assembling and disassembling of details, not fitting some details at construction, presence of odour, dust in the packings); 2) discrepancy with the announced age destination; 3) faults (incongruity of colours, small dimensions of models and not clear and contrast models, absence of age destination); 4) unphysiological strain and fatigue during play with part of the toys. On the basis of the results are developed hygiene recommendations to the manufacturing of constructive toys for children over 7 years.

[Health status of schoolchildren under five-day-week school program]. / Zdravno sustoianie uchenitsite, obuchavani pri petdnevna uchebna sedmitsa.

The effect of a five-day educative programme is studied with respect to the health status and morbidity of schoolchildren--from entering VIIth class in the Unified secondary professional school to completion Xth class. The data received are compared with the results from health exam on children, trained on six-day week. The results from the investigation show more favorable dynamics of the chronic, acute and neuro-psychic morbidity, the per cent of those frequently taken ill and is increased the number of healthy schoolchildren. In comparison with the traditional educative organization the five-day week programme has higher prophylactic effectiveness in the health protection of schoolchildren.

[Thermoregulation changes in secondary vocational and technical school students during the summer job experience]. / Promeni v termoregulatsiiata na uchenitsi ot SPTU po vreme na liatna proizvodstvena praktika.

Ninety pupils from a secondary polytechnical school for non-ferrous metallurgy, studying locksmith-mounter specialty, were examined during their summer industrial practice. The changes in body temperature, skin temperature and skin-electric resistance (SER) were studied. It was found that the diurnal and weekly variations in body and skin temperature and SER follow the diurnal and weekly biorhythm of these parameters and directly reflect the environment temperature changes and the duration and intensity of the work performed.

[Health status and morbidity in schoolchildren from villages in Silistra and Tutrakan Districts]. / Prouchvane na zdravnoto sustoianie i zabolevaemostta na uchenitsi ot niakoi sela na Silistrenski i Tutrakanski raion.

This is study of the health state and morbidity rate in a group comprising 408 schoolchildren from the Silistra and Tutrakan regions, aged 7 to 14 years. The results of the research demonstrate a slight lagging behind of the school-children's physical development relative to children of comparable ages from the urban areas. Their health state is estimated as good, and the somatic neurotic morbidity--rather low. The most frequently encountered affections include diseases of the upper airways, infectious and dental diseases. A considerable number of slightly hypochromic anemias are also recorded.