Graph Partitions in Chemistry.

We study partitions (equitable, externally equitable, or other) of graphs that describe physico-chemical systems at the atomic or molecular level; provide examples that show how these partitions are intimately related with symmetries of the systems; and discuss how such a link can further lead to insightful relations with the systems' physical and chemical properties. We define a particular kind of graph partition, which we call Chemical Equitable Partition (CEP), accounting for chemical composition as well as connectivity and associate it with a quantitative measure of information reduction that accompanies its derivation. These concepts are applied to model molecular and crystalline solid systems, illustrating their potential as a means to classify atoms according to their chemical or crystallographic role. We also cluster materials in meaningful manners that take their microstructure into account and even correlate them with the materials' physical properties.

Synthesis and Modeling of Poly(L-lactic acid) via Polycondensation of L-Lactic Acid.

We present synthetic experiments of lactic acid (LA) polycondensation to produce poly(lactic acid) (PLA) as well as kinetic modeling calculations that capture the polymer molecular weight increase with time, given the initial concentrations. Tin-octoate-catalyzed polycondensation of (D,L)- or L-lactic acid was carried out in pre-dried toluene after azeotropic dehydration for 48-120 h at 130-137 °C. The polymerization was optimized by varying lactic acid and catalyst concentrations as well as the temperature. Gel permeation chromatography was used to experimentally follow the evolution of molecular weights and the products were characterized by NMR, TGA, DSC and IR. Under optimal conditions, PLLA with weight-average molecular weight (Mw) of 161 kDa could be obtained. The rate equations that describe polycondensation kinetics were recast in a condensed form that allowed very fast numerical solution and calculation of the number-average molecular weight with time. Deviations with respect to the experiment were minimized in a least-squares fashion to determine rate constants. The optimized kinetics parameters are shown to reproduce the experimental data accurately.

Molecular Dynamics Simulations of Essential Oil Ingredients Associated with Hyperbranched Polymer Drug Carriers.

Our work concerns the study of four candidate drug compounds of the terpenoid family, found as essential oil ingredients in species of the Greek endemic flora, namely carvacrol, p-cymene, γ-terpinene, and thymol, via the simulation method of molecular dynamics. Aquatic solutions of each compound, as well as a solution of all four together in realistic (experimental) proportions, are simulated at atmospheric pressure and 37 °C using an OPLS force field combined with TIP3P water. As verified, all four compounds exhibit a strong tendency to phase-separate, thereby calling for the use of carrier molecules as aids for the drug to circulate in the blood and enter the cells. Systems of two such carrier molecules, the hyperbranched poly(ethylene imine) (HBPEI) polyelectrolyte and hyperbranched polyglycerol (HPG), are examined in mixtures with carvacrol, the most abundant among the four compounds, at a range of concentrations, as well as with all four compounds present in natural proportions. Although a tendency of the terpenoids to cluster separately persists at high concentrations, promising association effects are observed for all drug-polymer ratios. HBPEI systems tend to form diffuse structures comprising small mixed clusters as well as freely floating polymer and essential oil molecules, a finding attributed to the polymer-polymer electrostatic repulsions, which here are only partially screened by the counterions. On the other hand, the electrically neutral HPG molecules cluster together with essential oil species to form a single nanodroplet. Currently, terpenoid-polymer clusters near lipid bilayer membranes are being studied to determine the propensity of the formed complexes to enter cell membranes.

Impact of a Single Nucleotide Polymorphism on the 3D Protein Structure and Ubiquitination Activity of E3 Ubiquitin Ligase Arkadia.

Single nucleotide polymorphisms (SNPs) are genetic variations which can play a vital role in the study of human health. SNP studies are often used to identify point mutations that are associated with diseases. Arkadia (RNF111) is an E3 ubiquitin ligase that enhances transforming growth factor-beta (TGF-ß) signaling by targeting negative regulators for degradation. Dysregulation of the TGF-ß pathway is implicated in cancer because it exhibits tumor suppressive activity in normal cells while in tumor cells it promotes invasiveness and metastasis. Τhe SNP CGT > TGT generated an amino-acid (aa) substitution of Arginine 957 to Cysteine on the enzymatic RING domain of Arkadia. This was more prevalent in a tumor than in a normal tissue sample of a patient with colorectal cancer. This prompted us to investigate the effect of this mutation in the structure and activity of Arkadia RING. We used nuclear magnetic resonance (NMR) to analyze at an atomic-level the structural and dynamic properties of the R957C Arkadia RING domain, while ubiquitination and luciferase assays provided information about its enzymatic functionality. Our study showed that the R957C mutation changed the electrostatic properties of the RING domain however, without significant effects on the structure of its core region. However, the functional studies revealed that the R957C Arkadia exhibits significantly increased enzymatic activity supporting literature data that Arkadia within tumor cells promotes aggressive and metastatic behavior.

Heart Failure and Atrial Fibrillation Modify the Associations of Nocturnal Blood Pressure Dipping Pattern With Mortality in Hemodialysis Patients.

Heart failure (HF), hypertension, and abnormal nocturnal blood pressure dipping are highly prevalent in hemodialysis patients. Atrial fibrillation (AF) and HF might be important mediators for the association of abnormal dipping patterns with worse prognosis. Thus, the aim of this study is to investigate the association of dipping with mortality in hemodialysis patients and to assess the influence of AF and HF. In total, 525 hemodialysis patients underwent 24-hour ambulatory blood pressure monitoring. All-cause and cardiovascular mortality served as end points. Patients were categorized according to their systolic dipping pattern (dipper, nondipper, and reverse dipper). Cox regression analysis was performed to determine the association between dipping pattern and study end points with dipping as reference. Subgroup analysis was performed for patients with and without AF or HF. In total, 185 patients with AF or HF and 340 patients without AF or HF were included. During a median follow-up of 37.8 months, 177 patients died; 81 from cardiovascular causes. Nondipping and reverse dipping were significantly associated with all-cause mortality in the whole cohort (nondipper: hazard ratio, 1.95 [1.22-3.14]; P=0.006; reverse dipper: hazard ratio, 2.31 [1.42-3.76]; P<0.001) and in patients without AF or HF (nondipper: hazard ratio, 2.78 [1.16-6.66]; P=0.02; reverse dipper: hazard ratio, 4.48 [1.87-10.71]; P<0.001) but not in patients with AF or HF. For cardiovascular mortality, associations were again significant in patients without AF or HF and in the whole cohort. The observed associations remained significant after adjustment for possible confounders. This study provides well-powered evidence for the association between abnormal dipping patterns and mortality in hemodialysis patients and suggests that HF or AF modifies this association.

Treatment-resistant hypertension in the hemodialysis population: a 44-h ambulatory blood pressure monitoring-based study.

BACKGROUND: Uncontrolled hypertension notwithstanding the use of at least three drugs or hypertension controlled with at least four drugs, the widely accepted definition of treatment-resistant hypertension (TRH), is considered as a common problem in the hemodialysis population. However, to date there is no estimate of the prevalence of this condition in hemodialysis patients. METHOD: We estimated the prevalence of TRH by 44-h ambulatory BP monitoring (ABPM) in 506 hemodialysis patients in 10 renal units in Europe included in the registry of the European Renal and Cardiovascular Medicine (EURECAm,), a working group of the European Association, European Dialysis and Transplantation Association (ERA EDTA). In a sub-group of 114 patients, we tested the relationship between fluid overload (Body Composition monitor) and TRH. RESULTS: The prevalence of hypertension with 44-h ABPM criteria was estimated at 85.6% (434 out of 506 patients). Of these, 296 (58%) patients were classified as uncontrolled hypertensive patients by 44-h ABPM criteria (≥130/80 mmHg). Two hundred and thirteen patients had uncontrolled hypertension while on treatment with less than three drugs and 210 patients were normotensive while on drug therapy (n = 138) or off drug treatment (n = 72). The prevalence of TRH was 24% (93 among 386 treated hypertensive patients). The prevalence of predialysis fluid overload was 33% among TRH patients, 34% in uncontrolled hypertensive patients and 26% in normotensive patients. The vast majority (67%) of hemodialysis patients with TRH had no fluid overload. CONCLUSION: TRH occurs in about one in four treated hypertensive patients on hemodialysis. Fluid overload per se only in part explains TRH and the 67% of these patients show no fluid overload.

The effects of nebivolol and irbesartan on postdialysis and ambulatory blood pressure in patients with intradialytic hypertension: a randomized cross-over study.

OBJECTIVES: Intradialytic hypertension is estimated at 5-15% of hemodialysis patients and is associated with poor prognosis. Studies on therapeutic interventions for this entity are extremely few. We aimed to evaluate the effects of nebivolol and irbesartan on peridialytic, intradialytic, and ambulatory BP in patients with intradialytic hypertension. METHODS: This is a pilot randomized-cross-over study in 38 hemodialysis patients (age: 60.4 ±â€Š11.1 years, men: 65.8%) with intradialytic hypertension (intradialytic SBP rise ≥10 mmHg at ≥4 over six consecutive sessions]. After baseline evaluation, patients were randomly assigned to nebivolol 5 mg and subsequently irbesartan 150 mg, or vice versa. Nineteen patients received a single drug-dose 1 h before hemodialysis and 19 received the drug for a week before evaluation. A 2-week wash-out period took place before the initiation of the second drug. Patients had three respective 24-h ambulatory BP measurements starting before a midweek session. RESULTS: In total, 20 (52.6%) patients received nebivolol first and 18 (47.4%) received irbesartan. Patients receiving a single dose of either drug had lower postdialysis BP (baseline: 160.2 ±â€Š17.8/93.2 ±â€Š13.6 mmHg; nebivolol: 148.0 ±â€Š20.8/84.5 ±â€Š13.1 mmHg, P = 0.013/P = 0.027; irbesartan 142.9 ±â€Š29.9/87.2 ±â€Š18.1 mmHg, P = 0.003/P = 0.104 for SBP and DBP, respectively). The 24-h BP presented a trend towards reduction, but was significant only for 24-h DBP in the nebivolol arm. Patients on weekly administration of either drug had lower postdialysis BP (baseline: 162.5 ±â€Š16.8/95.4 ±â€Š12.7 mmHg; nebivolol: 146.7 ±â€Š16.3/91.8 ±â€Š12.2 mmHg, P = 0.001/P = 0.235; irbesartan: 146.0 ±â€Š23.9/85.8 ±â€Š12.9 mmHg, P = 0.004/ P = 0.007, respectively), lower intradialytic BP and lower 24-h BP (baseline: 148.3 ±â€Š12.6/90.2 ±â€Š9.0 mmHg; nebivolol: 139.2 ±â€Š10.6/85.0 ±â€Š7.7 mmHg, P < 0.001/P = 0.001; irbesartan: 142.4 ±â€Š16.4/85.1 ±â€Š9.9 mmHg, P = 0.156/P = 0.030). No significant differences were observed in comparisons between the two drugs, with the exception of heart rate, being lower with nebivolol. CONCLUSION: Both nebivolol and irbesartan reduced postdialysis and 24-h BP in patients with intradialytic hypertension. Weekly administration had greater effect and nebivolol was numerically slightly more potent than irbesartan.

SAFT-γ Force Field for the Simulation of Molecular Fluids. 5. Hetero-Group Coarse-Grained Models of Linear Alkanes and the Importance of Intramolecular Interactions.

The SAFT-γ Mie group-contribution equation of state [ Papaioannou J. Chem. Phys. 2014 , 140 , 054107 ] is used to develop a transferable coarse-grained (CG) force-field suitable for the molecular simulation of linear alkanes. A heterogroup model is fashioned at the resolution of three carbon atoms per bead in which different Mie (generalized Lennard-Jones) interactions are used to characterize the terminal (CH3-CH2-CH2-) and middle (-CH2-CH2-CH2-) beads. The force field is developed by combining the SAFT-γ CG top-down approach [ Avendaño J. Phys. Chem. B 2011 , 115 , 11154 ], using experimental phase-equilibrium data for n-alkanes ranging from n-nonane to n-pentadecane to parametrize the intermolecular (nonbonded) bead-bead interactions, with a bottom-up approach relying on simulations based on the higher resolution TraPPE united-atom (UA) model [ Martin ; , Siepmann J. Phys. Chem. B 1998 , 102 , 2569 ] to establish the intramolecular (bonded) interactions. The transferability of the SAFT-γ CG model is assessed from a detailed examination of the properties of linear alkanes ranging from n-hexane ( n-C6H14) to n-octadecane ( n-C18H38), including an additional evaluation of the reliability of the description for longer chains such as n-hexacontane ( n-C60H122) and a prototypical linear polyethylene of moderate molecular weight ( n-C900H1802). A variety of structural, thermodynamic, and transport properties are examined, including the pair distribution functions, vapor-liquid equilibria, interfacial tension, viscosity, and diffusivity. Particular focus is placed on the impact of incorporating intramolecular interactions on the accuracy, transferability, and representability of the CG model. The novel SAFT-γ CG force field is shown to provide a reliable description of the thermophysical properties of the n-alkanes, in most cases at a level comparable to the that obtained with higher resolution models.

Evaluation of the tolerability and efficacy of sodium polystyrene sulfonate for long-term management of hyperkalemia in patients with chronic kidney disease.

PURPOSE: Sodium polystyrene sulfonate (SPS) is a cation-exchanging resin that has been widely used for several decades as first-line therapy of mild chronic hyperkalemia in patients with chronic kidney disease (CKD). However, evidence to prove the long-term tolerability and efficacy of SPS for the treatment of this condition is still missing. METHODS: In this retrospective, observational study, we enrolled 26 outpatients with stages 3-4 CKD who received oral therapy with low-dose SPS for mild chronic hyperkalemia in the Outpatient Nephrology clinic of our Department during 2010-2016. We obtained medical records on side effects potentially attributable to SPS use, and we analyzed the changes in serum electrolytes before and after the initiation of SPS therapy. RESULTS: Serum potassium levels fell from 5.9 ± 0.4 to 4.8 ± 0.5 mmol/l (P < 0.001) over a median follow-up of 15.4 months (range 3-27 months). SPS use was associated with a slight, but significant elevation in serum sodium levels (139.5 ± 2.9 vs 141.2 ± 2.4, P = 0.006), whereas serum calcium and phosphate remained unchanged before and after the initiation of SPS. We recorded ten episodes of recurrent serum potassium elevation ≥ 5.5 mmol/l, none of which required hospitalization or acute dialysis. No episode of colonic necrosis or any other serious drug-related adverse event was observed. SPS therapy was well-tolerated, since only 1 out of 26 patients discontinued SPS at 3 months due to gastrointestinal intolerance. CONCLUSION: This study suggests that low-dose SPS is well-tolerated and can effectively normalize elevated serum potassium over several weeks in CKD outpatients with mild chronic hyperkalemia.

Local structure of dilute aqueous DMSO solutions, as seen from molecular dynamics simulations.

The information about the structure of dimethyl sulfoxide (DMSO)-water mixtures at relatively low DMSO mole fractions is an important step in order to understand their cryoprotective properties as well as the solvation process of proteins and amino acids. Classical MD simulations, using the potential model combination that best reproduces the free energy of mixing of these compounds, are used to analyze the local structure of DMSO-water mixtures at DMSO mole fractions below 0.2. Significant changes in the local structure of DMSO are observed around the DMSO mole fraction of 0.1. The array of evidence, based on the cluster and the metric and topological parameters of the Voronoi polyhedra distributions, indicates that these changes are associated with the simultaneous increase of the number of DMSO-water and decrease of water-water hydrogen bonds with increasing DMSO concentration. The inversion between the dominance of these two types of H-bonds occurs around XDMSO = 0.1, above which the DMSO-DMSO interactions also start playing an important role. In other words, below the DMSO mole fraction of 0.1, DMSO molecules are mainly solvated by water molecules, while above it, their solvation shell consists of a mixture of water and DMSO. The trigonal, tetrahedral, and trigonal bipyramidal distributions of water shift to lower corresponding order parameter values indicating the loosening of these orientations. Adding DMSO does not affect the hydrogen bonding between a reference water molecule and its first neighbor hydrogen bonded water molecules, while it increases the bent hydrogen bond geometry involving the second ones. The close-packed local structure of the third, fourth, and fifth water neighbors also is reinforced. In accordance with previous theoretical and experimental data, the hydrogen bonding between water and the first, the second, and the third DMSO neighbors is stronger than that with its corresponding water neighbors. At a given DMSO mole fraction, the behavior of the intensity of the high orientational order parameter values indicates that water molecules are more ordered in the vicinity of the hydrophilic group while their structure is close-packed near the hydrophobic group of DMSO.

Ambulatory Pulse Wave Velocity Is a Stronger Predictor of Cardiovascular Events and All-Cause Mortality Than Office and Ambulatory Blood Pressure in Hemodialysis Patients.

Arterial stiffness and augmentation of aortic blood pressure (BP) measured in office are known cardiovascular risk factors in hemodialysis patients. This study examines the prognostic significance of ambulatory brachial BP, central BP, pulse wave velocity (PWV), and heart rate-adjusted augmentation index [AIx(75)] in this population. A total of 170 hemodialysis patients underwent 48-hour ambulatory monitoring with Mobil-O-Graph-NG during a standard interdialytic interval and followed-up for 28.1±11.2 months. The primary end point was a combination of all-cause death, nonfatal myocardial infarction, and nonfatal stroke. Secondary end points included: (1) all-cause mortality; (2) cardiovascular mortality; and (3) a combination of cardiovascular death, nonfatal myocardial infarction, nonfatal stroke, resuscitation after cardiac arrest, coronary revascularization, or hospitalization for heart failure. During follow-up, 37(21.8%) patients died and 46(27.1%) had cardiovascular events. Cumulative freedom from primary end point was similar for quartiles of predialysis-systolic BP (SBP), 48-hour peripheral-SBP, and central-SBP, but was progressively longer for increasing quartiles for 48-hour peripheral-diastolic BP and central-diastolic BP and shorter for increasing quartiles of 48-hour central pulse pressure (83.7%, 71.4%, 69.0%, 62.8% [log-rank P=0.024]), PWV (93.0%, 81.0%, 57.1%, 55.8% [log-rank P<0.001]), and AIx(75) (88.4%, 66.7%, 69.0%, 62.8% [log-rank P=0.014]). The hazard ratios for all-cause mortality, cardiovascular mortality, and the combined outcome were similar for quartiles of predialysis-SBP, 48-hour peripheral-SBP, and central-SBP, but were increasing with higher ambulatory PWV and AIx(75). In multivariate analysis, 48-hour PWV was the only vascular parameter independently associated with the primary end point (hazard ratios, 1.579; 95% confidence intervals, 1.187-2.102). Ambulatory PWV, AIx(75), and central pulse pressure are associated with increased risk of cardiovascular events and mortality, whereas office and ambulatory SBP are not. These findings further support that arterial stiffness is the prominent cardiovascular risk factor in hemodialysis.

Terms of endearment: Bacteria meet graphene nanosurfaces.

Microbial multidrug resistance poses serious risks in returning the human species into the pre-antibiotic era if it remains unsolved. While conventional research approaches to combat infectious diseases have been inadequate, nanomaterials are a promising alternative for the development of sound antimicrobial countermeasures. Graphene, a two-dimensional ultra-thin nanomaterial, possesses excellent electronic and biocompatibility properties, which position it in the biotechnology forefront for diverse applications in biosensing, therapeutics, diagnostics, drug delivery and device development. Yet, several questions remain unanswered. For instance, the way these nanosurfaces interact with the microbial entities is poorly understood. The mechanistic elucidation of this interface seems critical to determine the feasibility of applications under development. Are graphene derivatives appropriate materials to design potent antimicrobial agents, vehicles or effective diagnostic microsensors? Has the partition of major microbial resistance phenotypic determinants been sufficiently investigated? Can toxicity become a limiting factor? Are we getting closer to clinical implementation? To facilitate research conducive to answer such questions, this review describes the features of the graphene-bacterial interaction. An overview on paradigms of graphene-microbial interactions is expected to shed light on the range of materials available, and identify possible applications, serving the ultimate goal to develop deeper understanding and collective conscience for the true capabilities of this nanomaterial platform.

New effective method for quantitative analysis of diffusion jumps, applied in molecular dynamics simulations of small molecules dispersed in short chain systems.

Diffusion jumps of small molecules dispersed in chain molecules or other kinds of slow-moving matrices have already been observed in many previous simulations of such systems, and their treatment led to important qualitative conclusions. In the present work, a new, very simple yet effective method is described, allowing for both identification of individual penetrant jump events and their quantitative treatment in a statistical sense. The method is applied in equilibrium Molecular Dynamics simulations for systems of gaseous alkanes, methane through n-butane, including also a mixture of methane and n-butane, dispersed in n-decane or n-eicosane. Equilibration and attainment of a linear diffusion regime is confirmed by means of various criteria, and the jumps detection method is applied to all systems studied. The results obtained clearly show the existence of distinct jump events in all cases, although the average jump length is reduced with penetrant or liquid alkane molecular weight. The method allows one to determine the average jump length and the corresponding jumps frequency. On the basis of these results, it was possible to estimate a random walk type diffusion coefficient, D(s,jumps), of the penetrants, which was found to be substantially lower compared with the overall diffusion coefficient D(s,MSD) obtained by the mean square displacement method. This finding led us to assume that the overall penetrants' diffusion in the studied systems is a combination of longer jumps with a smoother and more gradual displacement, a result that confirms assumptions suggested in previous studies.

Slow intravenous iron administration does not aggravate oxidative stress and inflammatory biomarkers during hemodialysis: a comparative study between iron sucrose and iron dextran.

BACKGROUND/AIMS: Fast intravenous (i.v.) iron administration during hemodialysis (HD) is associated with the augmentation of oxidative stress and the increase in inflammatory biomarkers, which are also induced by the hemodialysis procedure itself. The aim of this study was to investigate if slow i.v. iron administration would aggravate the status of oxidative stress and inflammatory biomarkers during a hemodialysis session. METHODS: Twenty dialysis patients 30-92 years of age that were iron replete and had values for hemoglobin, transferrin saturation and serum ferritin among recommended goals were evaluated in three separate hemodialysis sessions. In the first session patients did not receive any iron treatment, whereas during the second and the third session patients received slow (60 min) i.v. infusions of 100 mg of iron sucrose and 100 mg of iron dextran, respectively. Blood samples were drawn before the hemodialysis session, 15 min after the end of iron administration and at the end of the hemodialysis session in all occasions, for the measurement of markers of oxidant stress (oxidized LDL and ischemia-modified albumin) and inflammation (high-sensitivity C-reactive protein, interleukin-6 and tumor necrosis factor-alpha). RESULTS: Oxidized LDL was not significantly altered during hemodialysis and this pattern was similar between the three occasions studied. In contrast, ischemia-modified albumin was significantly increased and this effect was also not different between the net hemodialysis and the occasions of iron administration. High-sensitivity CRP, IL-6 and TNF-alpha were all significantly elevated during hemodialysis and again both types of iron administration did not produce significant changes in this pattern. CONCLUSION: We did not find an increase in the markers of oxidation/inflammation studied, after slow i.v. iron administration during hemodialysis session.

Molecular dynamics simulation of structure, thermodynamic, and dynamic properties of poly(dimethylsilamethylene), poly(dimethylsilatrimethylene) and their alternating copolymer.

Molecular dynamics is used for the simulation of silicon-containing polymers with promising membrane material properties. An atomistic force field is developed for the description of bond bending, torsional angle variation, and nonbonded intra- and intermolecular interactions. Detailed ab initio quantum mechanics calculations on corresponding monomers that appeared recently in the literature are used for the parametrization of the bonded and nonbonded local intramolecular force field. For the intermolecular and nonbonded nonlocal intramolecular interactions, parameters are obtained from accurate force fields proposed in the literature for similar compounds. The force field is used subsequently for the calculation of thermodynamic, structure, and dynamic properties of two homopolymers, namely, poly(dimethylsilamethylene) and poly(dimethylsilatrimethylene), and their alternating copolymer. A wide range of temperatures and pressures is examined. Polymer systems of different molecular weights are simulated. Experimental data available for these polymers are very limited. In all cases, simulation results are in good agreement with these data. Furthermore, simulation results agree very well with empirical macroscopic correlations used widely for rubbery polymers for the properties under consideration.

Force field development for poly(dimethylsilylenemethylene) with the aid of ab initio calculations.

The molecular geometries, conformational energies, and zero-point energies of di(trimethylsilylene)methylene have been determined from high-level quantum chemistry calculations. The results are further used in the parametrization of a classical potential energy function suitable for performing simulations of the corresponding polymer, namely, poly(dimethylsilylenemethylene). Di(trimethylsilylene)methylene geometrical parameter optimizations for a proper location of the global minimum and other local minima, constrained at certain dihedral and bond angles, were performed at both the B3LYP/6-311G and MP2(full)/6-311G levels of theory. The global minimum configuration is slightly displaced from a perfectly staggered geometry, approximately by 16.0 degrees, at both levels of theory. Molecular mechanics and Monte Carlo calculations for isolated polymer chains together with molecular dynamics runs for the modeled dimer provide very good results in terms of conformational and thermodynamic properties.

Evaluation of solid sorbents for the determination of fenhexamid, metalaxyl-M, pyrimethanil, malathion and myclobutanil residues in air samples: application to monitoring malathion and fenhexamid dissipation in greenhouse air using C-18 or Supelpak-2 for sampling.

A methodology is described for greenhouse air analysis by sampling fenhexamid, pyrimethanil, malathion, metalaxyl-M and myclobutanil in solid sorbents. Pesticides were determined by gas chromatography with NP Detector. The trapping efficiency of XAD-2, XAD-4, Supelpak-2, Florisil and C-18 at different sampling conditions (rate, time and air humidity) and pesticides concentration levels has been evaluated. No breakthrough was observed in the range of concentration studied (0.10-75 microg of each pesticide). In almost all the cases good stability results were obtained. Personal pumps have been used with selected sorbents (Supelpak-2 and C-18) in order to sample malathion and fenhexamid in air of experimental greenhouse after their application in a tomato crop. The dissipation process of the analytes in various time periods after application has been studied. Malathion concentrations varied between 20.1 microg m(-3) just after application and 1.06 microg m(-3) 3 days later. Fenhexamid concentrations, determined by high performance liquid chromatography with UV detection, fall rapidly; after 12 h post-application being below 0.50 microg m(-3).