Static and dynamic correlations in water at hydrophobic interfaces

We study the static and dynamic properties of the water-density fluctuations in the interface of large nonpolar solutes. With the help of extensive molecular dynamics simulations of TIP4P water near smooth spherical solutes, we show that for large solutes, the interfacial density profile is broadened by capillary waves. For purely repulsive solutes, the squared width of the interface increases linearly with the logarithm of the solute size, as predicted by capillary-wave theory. The apparent interfacial tension extracted from the slope agrees with that of a free liquid–vapor interface. The characteristic length of local density fluctuations is ≈0.5 nm, measured along the arc, again consistent with that of a free liquid–vapor interface. Probed locally, the interfacial density fluctuations exhibit large variances that exceed those expected for an ideal gas. Qualitatively consistent with theories of the free liquid–vapor interface, we find that the water interface near large and strongly nonpolar solutes is flickering, broadened by capillary-wave fluctuations. These fluctuations result in transitions between locally wet and dry regions that are slow on a molecular time scale.     

Theory of hydrophobicity: Transient cavities in molecular liquids

Observation of the size distribution of transient cavities in computer simulations of water, n-hexane, and n-dodecane under benchtop conditions shows that the sizes of cavities are more sharply defined in liquid water but the most-probable-size cavities are about the same size in each of these liquids. The calculated solvent atomic density in contact with these cavities shows that water applies more force per unit area of cavity surface than do the hydrocarbon liquids. This contact density, or “squeezing” force, reaches a maximum near cavity diameters of 2.4 Å. The results for liquid water are compared to the predictions of simple theories and, in addition, to results for a reference simple liquid. The numerical data for water at a range of temperatures are analyzed to extract a surface free energy contribution to the work of formation of atomic-size cavities. Comparison with the liquid-vapor interfacial tensions of the model liquids studies here indicates that the surface free energies extracted for atomic-size cavities cannot be accurately identified with the macroscopic surface tensions of the systems.     

Unraveling quantum mechanical effects in water using isotopic fractionation

When two phases of water are at equilibrium, the ratio of hydrogen isotopes in each is slightly altered because of their different phase affinities. This isotopic fractionation process can be utilized to analyze water's movement in the world's climate. Here we show that equilibrium fractionation ratios, an entirely quantum mechanical property, also provide a sensitive probe to assess the magnitude of nuclear quantum fluctuations in water. By comparing the predictions of a series of water models, we show that those describing the OH chemical bond as rigid or harmonic greatly overpredict the magnitude of isotope fractionation. Models that account for anharmonicity in this coordinate are shown to provide much more accurate results because of their ability to give partial cancellation between inter- and intramolecular quantum effects. These results give evidence of the existence of competing quantum effects in water and allow us to identify how this cancellation varies across a wide-range of temperatures. In addition, this work demonstrates that simulation can provide accurate predictions and insights into hydrogen fractionation.     

Static versus dynamic exercise: effects on Doppler echocardiographic indices of left ventricular performance.

In order to compare the effects of static exercise with those of dynamic exercise on the Doppler echocardiographic measurements of ascending aortic blood flow velocity and acceleration, Doppler echocardiography was performed with sustained handgrip exercise and with supine bicycle exercise in 12 normal subjects, 12 patients with coronary artery disease, and 7 patients with heart failure. In normal subjects: peak velocity decreased by 16 +/- 11% with handgrip from the resting value and increased by 49 +/- 19% with bicycle exercise (p less than 0.01); mean acceleration decreased by 6 +/- 30% with handgrip and increased by 162 +/- 83% with bicycle exercise (p less than 0.01). In patients with coronary artery disease: peak velocity declined by 9 +/- 14% with handgrip and increased by 19 +/- 18% with bicycle exercise (p less than 0.01); mean acceleration increased by 13 +/- 27% with handgrip and by 41 +/- 33% with bicycle exercise (NS). In patients with congestive heart failure: peak velocity decreased by 19 +/- 13% with handgrip and increased by 5 +/- 17% with bicycle exercise (p less than 0.01); mean acceleration decreased by 12 +/- 23% with handgrip and by 4 +/- 37% with bicycle exercise. A marked increase in afterload stress induced by static exercise presumably offsets the moderately increased contractility and accounts for the decline of peak velocity and mean acceleration with static exercise both in normals and cardiac patients. In contrast, marked increase in contractile state along with little change in afterload with dynamic exercise results in markedly increased peak velocity and mean acceleration in normal subjects.(ABSTRACT TRUNCATED AT 250 WORDS)     

Listeriosis in Iceland, 1978-2000: a description of cases and molecular epidemiology

The purpose of our study was to review all cases of listeriosis in Iceland during the period 1978-2000 and to analyse the genetic relatedness of their isolates. Case records of all patients in Iceland with listeriosis during the period were reviewed and the isolates compared using serotyping and pulsed-field gel electrophoresis (PFGE) using SmaI, AseI and ApaI restriction enzymes. Forty cases of listeriosis were diagnosed during the period, resulting in a mean annual incidence of 6.9 cases per million and a case fatality rate of 33%. In the first 5 y of the study only serotype 4b was observed; subsequently serotypes 1/2a and 1/2b appeared and serotype 4b declined in prevalence. PFGE yielded 24 different genotypes with 7 clusters of indistinguishable genotypes, each comprising 2-6 cases. During 1992-95 the annual incidence of listeriosis in Iceland rose to 15 cases per million. This was largely due to 2 clusters, 1 of 3 cases and the other of 6. No cases of listeriosis were diagnosed during 1998-2000. Our data show an increased number of cases within clusters in the latter half of the period. At the same time, food processing and distribution has become increasingly centralized in Iceland, suggesting an increased risk of listeriosis outbreaks.     

Static and dynamic lengths of neutrophil microvilli

Containing most of the L-selectin and P-selectin glycoprotein ligand-1 (PSGL-1) on their tips, microvilli are believed to promote the initial arrest of neutrophils on endothelium. At the rolling stage following arrest, the lifetimes of the involved molecular bonds depend on the pulling force imposed by the shear stress of blood flow. With two different methods, electron microscopy and micropipette manipulation, we have obtained two comparable neutrophil microvillus lengths, both ≈0.3 μm in average. We have found also that, under a pulling force, a microvillus can be extended (microvillus extension) or a long thin membrane cylinder (a tether) can be formed from it (tether formation). If the force is ≤34 pN (± 3 pN), the length of the microvillus will be extended; if the force is >61 pN (± 5 pN), a tether will be formed from the microvillus at a constant velocity, which depends linearly on the force. When the force is between 34 pN and 61 pN (transition zone), the degree of association between membrane and cytoskeleton in individual microvilli will dictate whether microvillus extension or tether formation occurs. When a microvillus is extended, it acts like a spring with a spring constant of ≈43 pN/μm. In contrast to a rigid or nonextendible microvillus, both microvillus extension and tether formation can decrease the pulling force imposed on the adhesive bonds, and thus prolonging the persistence of the bonds at high physiological shear stresses.     

Functional decline and mortality in long-term care settings: Static and dynamic approach

Background/PurposeFunctional impairment is known to be associated with higher mortality risk and adverse health outcomes. However, little is known about whether functional decline could predict mortality among the elderly in the long-term care setting.MethodsThis is a prospective cohort study in two veteran homes in northern Taiwan with active use of the minimum data set (MDS). Evaluation tools retrieved from the MDS, including MDS Resource Utilization Group-III for Activities of Daily Living (RUG-III ADL), MDS Cognitive Scale, MDS Social engagement, triggers for resident assessment protocol (RAP) and Pain scale, were utilized for the analysis.ResultsA total of 1125 male participants were included in this study. The mean age of the participants was 83.1 ± 5.1 years, and 65 (5.8%) developed physical functional decline within a 6-month period. Participants with functional decline [odds ratio (OR) 2.305, 95% confidence interval (CI) 1.002–5.303], poor baseline functional status (OR 1.116, 95% CI 1.002–1.242), positive RAP triggers for dehydration (OR 13.857, 95% CI 3.07–62.543), and underlying chronic lung diseases (OR 2.279, 95% CI 1.149–4.522), depression (OR 2.994, 95% CI 1.161–7.721), and cancer (OR 3.23, 95% CI 1.078–9.682) were more likely to have an additional 12-month mortality. By contrast, Parkinsonism (OR 3.875, 95% CI 1.169–12.841), increase in sum of RAP triggers (OR 6.096, 95% CI 2.741–13.562), and positive RAP triggers for cognitive loss (OR 3.164, 95% CI 1.612–6.212) and mood (OR 2.894, 95% CI 1.466–5.71) are strong predictors for functional decline within 6 months.ConclusionPhysical function decline within 6 months predicted the subsequent 1-year mortality, whereas increased sum of RAP triggers and positive trigger for cognitive loss and mood were associated with functional decline.     

INAUGURAL ARTICLE by a Recently Elected Academy Member:Static and dynamic friction in sliding colloidal monolayers

In a pioneer experiment, Bohlein et al. realized the controlled sliding of two-dimensional colloidal crystals over laser-generated periodic or quasi-periodic potentials. Here we present realistic simulations and arguments that besides reproducing the main experimentally observed features give a first theoretical demonstration of the potential impact of colloid sliding in nanotribology. The free motion of solitons and antisolitons in the sliding of hard incommensurate crystals is contrasted with the soliton–antisoliton pair nucleation at the large static friction threshold F_s when the two lattices are commensurate and pinned. The frictional work directly extracted from particles’ velocities can be analyzed as a function of classic tribological parameters, including speed, spacing, and amplitude of the periodic potential (representing, respectively, the mismatch of the sliding interface and the corrugation, or “load”). These and other features suggestive of further experiments and insights promote colloid sliding to a unique friction study instrument.     

Solar water splitting in a molecular photoelectrochemical cell

Artificial photosynthesis and the production of solar fuels could be a key element in a future renewable energy economy providing a solution to the energy storage problem in solar energy conversion. We describe a hybrid strategy for solar water splitting based on a dye sensitized photoelectrosynthesis cell. It uses a derivatized, core–shell nanostructured photoanode with the core a high surface area conductive metal oxide film––indium tin oxide or antimony tin oxide––coated with a thin outer shell of TiO₂ formed by atomic layer deposition. A “chromophore–catalyst assembly” 1, [(PO₃H₂)₂bpy)₂Ru(4-Mebpy-4-bimpy)Rub(tpy)(OH₂)]⁴⁺, which combines both light absorber and water oxidation catalyst in a single molecule, was attached to the TiO₂ shell. Visible photolysis of the resulting core–shell assembly structure with a Pt cathode resulted in water splitting into hydrogen and oxygen with an absorbed photon conversion efficiency of 4.4% at peak photocurrent.Photosynthesis uses the energy of the sun with water as the reducing agent to drive the reduction of carbon dioxide to carbohydrates with oxygen as a coproduct through a remarkably complex process. At photosystem II, a subsystem imbedded in the thylakoid membrane where O₂ is produced, light absorption, energy migration, electron transfer, proton transfer, and catalysis are all used in multiple stepwise chemical reactions which are carefully orchestrated at the molecular level (1, 2).Photosynthesis solves the problem of energy storage by biomass production but with low solar efficiencies, typically <1%. In artificial photosynthesis with solar fuels production, the goal is similar but the targets are either hydrogen production from water splitting, Eq. 1, or reduction of carbon dioxide to a carbon-based fuel, Eq. 2 (3, 4). Different strategies for solar fuels have evolved (5, 6). In one, direct bandgap excitation of semiconductors creates electron–hole pairs which are then used to drive separate half-reactions for water oxidation (2H₂O → O₂ + 4H⁺ + 4e⁻) and water/proton reduction (2H⁺ + 2e⁻ → H₂) (7–9).Here, we report a hybrid strategy for solar water splitting, the dye sensitized photoelectrosynthesis cell (DSPEC). It combines the electron transport properties of semiconductor nanocrystalline thin films with molecular-level reactions (10). In this approach, a chromophore–catalyst molecular assembly acts as both light absorber and catalyst. It is bound to the surface of a “core–shell,” nanostructured, transparent conducting oxide film. The core structure consists of a nanoparticle film of either tin-doped indium oxide (nanoITO), or antimony-doped tin oxide (nanoATO), deposited on a fluoride-doped tin oxide (FTO) glass substrate. The shell consists of a conformal TiO₂ nanolayer applied by atomic layer deposition (ALD). The resulting “photoanode,” where water oxidation occurs, is connected to a Pt cathode for proton reduction to complete the water splitting cell.A diagram for the photoanode in the DSPEC device is shown in Fig. 1. It illustrates the interface and the structure of chromophore–catalyst assembly 1, [(PO₃H₂)₂bpy)₂Ru_a(4-Mebpy-4′-bimpy)Ru_b(tpy)(OH₂)]⁴⁺ ((PO₃H₂)₂bpy is 4,4′-bisphosphonato-2,2′-bipyridine; 4-Mebpy-4′-bimpy is 4-(methylbipyridin-4′-yl)-N-benzimid-N′-pyridine; tpy is 2,2′:6′,2″-terpyridine)). It also illustrates the dynamic events that occur at the derivatized interface following light absorption by the surface-bound chromophore.Open in a separate windowFig. 1.Illustrating interface binding, assembly structure, and a kinetic scheme following MLCT excitation of the chromophore in the assembly [((PO₃H₂)₂bpy)₂Ru_a(4-Mebpy-4′-bimpy)Ru_b(tpy)(OH₂)]⁴⁺ (1) on core–shell nanoITO/TiO₂. Excitation of the chromophore is followed by electron injection, electron migration through the core–shell structure, and intraassembly electron transfer oxidation of the catalyst.Synthesis, characterization, and water oxidation catalysis by assembly 1 on nanoITO, abbreviated as , have been investigated and the results are described in a parallel publication (11). The mechanism, established earlier for single-site molecular catalysts (12, 13), is shown in Scheme 1. Oxidative activation by stepwise 3e⁻/2H⁺ oxidation of the catalyst gives the intermediate which is reactive toward water oxidation. It undergoes rate-limiting O-atom transfer to a water molecule in the surrounding solvent to give the peroxide intermediate, . This intermediate undergoes further oxidation and loss of O₂ and reenters the catalytic cycle. In Scheme 1, “RDS” denotes the rate-determining step.Open in a separate windowScheme 1.Mechanism of electrocatalytic water oxidation by 1 in 0.1 M HClO₄ (11).As illustrated in Fig. 1, on oxide semiconductor surfaces, metal-to-ligand charge transfer (MLCT) excitation of the chromophore in 1 results in excited state formation and electron injection into the conduction band of the semiconductor, Eq. 3. The absorption spectrum of the surface-bound assembly is shown in Fig. S1. It is dominated in the visible region by an absorption at λ_max = 450 nm with ε ∼ 19,000 M^-1⋅cm⁻¹ (pH dependent) arising from overlapping MLCT absorptions at the chromophore and, to a lesser degree, at the catalyst.Nanosecond transient absorption measurements on 1 and the model chromophore, [Ru(4,4′-(PO₃H₂bpy)(bpy)₂]²⁺ (RuP) in the core–shell nanoITO/TiO₂ structure depicted in Fig. 2, in 0.1 M HClO₄ with excitation at 425 nm and monitoring at 450 nm, show that injection, Eq. 3, occurs within 20 ns after the laser pulse. Recovery of the bleach at 450 nm following the laser flash by back electron transfer, Eq. 4, occurs with complex kinetics (14, 15), but on a noticeably longer average timescale for 1 compared with the model RuP, consistent with hole transfer to the catalyst.The reaction sequence in Eqs. 3 and 4, the mechanism of water oxidation by the assembly in Scheme 1, and the microscopic events illustrated in Fig. 1 reveal the underlying challenges to efficient DSPEC water splitting: 1) Three single-photon, single-electron excitation events are required for a single oxidative activation cycle to reach the key precursor intermediate, . 2) Under ambient sunlight in the nanostructured films, the rate of solar insolation is 1–2 s⁻¹, which limits the rate of water oxidation to 0.25–0.5 s⁻¹ per O₂ given the stoichiometry of the reaction (16, 17). With back electron transfer occurring on a timescale of μs to ms, and three injection–oxidation cycles required to activate the catalyst, its reactive form is present in trace amounts under ambient solar illumination and efficiencies for water splitting are negligible. Photolysis of assembly 1 on mesoscopic thin films of TiO₂ results in negligible photocurrents above background.Open in a separate windowFig. 2.Kinetics of absorbance changes probed at 450 nm following 3.2-mJ, 425-nm pulsed laser excitation for RuP- and 1-derivatized FTO|nanoITO/TiO₂ electrodes immersed in argon-degassed, aqueous pH 4.6 solutions with 0.5 M LiClO₄ and 20 mM acetate/acetic acid buffer at 22 ± 1 °C. The first 10 µs displayed here were collected in a separate experiment from the longer timescale (i.e., out to 400 µs) data for improved data quality. Both data sets are presented together here (dark gray, RuP; black, 1) with the fits shown in red and blue, respectively.

Table 1.

Best-fit parameters to the triexponential decay function shown below obtained from transient absorption kinetic measurements with 450-nm monitoring at 22 ± 1 °C with 3.2-mJ, 425-nm excitation for RuP- and 1-derivatized FTO|nanoITO/TiO₂ electrodes immersed in aqueous pH 4.6 solutions with 0.5 M LiClO₄ and 20 mM acetate/acetic acid buffer

Static versus dynamic distensibility of the carotid artery in humans

In clinical studies, the elastic behavior of central arteries is usually assessed by measuring dynamic distensibility. In this study, we aimed to investigate how dynamic and static distensibility of the common carotid artery (D(dyn) and D(stat), respectively) are related in 28 healthy volunteers of 20-71 years. The carotid diameter and its change with the pressure pulse were measured using an ultrasound echo-tracking device. Arterial blood pressure was measured by Finapres and carotid pressure was determined by applanation tonometry. D(dyn) was determined at rest using the pressure pulse, while D(stat) was determined during pressor responses induced by handgrip or cold pressor test. Data are given as mean +/- 1 SD. In younger subjects (<35 years), D(stat) did not differ from D(dyn) (7.0 +/- 3.4 vs. 6.5 +/- 2.1 x 10(-3) x mm Hg(-1), respectively), whereas in older subjects (>35 years), D(stat) was significantly higher than D(dyn) (3.8 +/- 1.4 vs. 2.1 +/- 0.9 x 10(-3) x mm Hg(-1), p < 0.001). For all subjects, D(stat) and D(dyn) decreased with increasing age and mean arterial pressure (MAP). Using stepwise multiple regression analysis, the strongest predictor of D(stat) proved to be MAP, while that of D(dyn) was age. D(stat) was found to be linearly related to the hysteresis loop area of the pressure-diameter relation (r = 0. 94), i.e. to vessel wall viscosity. It is concluded that, with increasing age, static distensibility overestimates the distension capacity of large arteries.     

Molecular dynamics studies of heterogeneous dynamics and dynamic crossover in supercooled atomic liquids

Supercooled liquids near the glass transition exhibit the phenomenon of heterogeneous relaxation; at any specific time, a nominally homogeneous equilibrium fluid undergoes dynamic fluctuations in its structure on a molecular distance scale with rates that are very different in different regions of the sample. Several theoretical and simulation studies have suggested a change in the nature of the dynamics of fluids as they are supercooled, leading to the concept of a dynamic crossover that is often associated with mode coupling theory. Here, we will review the use of molecular dynamics computer simulation methods to investigate heterogeneous dynamics and dynamic crossovers in models of atomic liquids.     

Static and dynamic longitudinal structural analyses of cognitive changes in old age

BACKGROUND: Among the main data-analytical advances of recent decades are Latent Growth Models (LGM) and Multilevel Models (MLM) for the analysis of longitudinal data. OBJECTIVE: We discuss the relative advantages and disadvantages of the two analytical methods and offer some practical guidelines concerning the choice between LGM and MLM based on (a). completeness and balance of the data, (b). theoretical functional form of change examined, (c). examination of the error structure, (d). theoretical relations among differential level effects and differential change effects, and (e). role of time-invariant covariates. METHODS: To discuss LGM and MLM, we provide illustrations from applications to the Berlin Aging Study (BASE) and the Swiss Interdisciplinary Longitudinal Study on the Oldest Old (SWILSO-O). RESULTS: As predicted by two-component theories of lifespan cognition, performance on a vocabulary test (indicator of broad crystallized intelligence) did not decline over time, while scores on a digit letter test (indicator of broad fluid intelligence) decreased over 6 years. Differential level effects were obtained on both variables, while average and differential change effects were obtained only for the digit letter test. In a second set of analyses, we tested the error-free effect that a broad fluid intelligence indicator exerted on the latent yearly change in a broad crystallized intelligence indicator, and vice versa. In both data sets we obtained strong evidence for a more reliable effect of the fluid indicator on the change in the crystallized indicator. This evidence provided support for the dedifferentiation hypothesis of cognitive abilities in very old age. CONCLUSIONS: New insights into cognitive aging phenomena can be gained with proper applications of LGM and MLM. We posit that the choice between LGM and MLM and their specification rests on theoretical and empirical motives to be defined a priori.     

Static and dynamic upper airway obstruction in sleep apnea: role of the breathing gas properties

Increased upper airway collapsibility in the sleep apnea/hypopnea syndrome (SAHS) is usually interpreted by a collapsible resistor model characterized by a critical pressure (Pcrit) and an upstream resistance (Rup). To investigate the role played by the upstream segment of the upper airway, we tested the hypothesis that breathing different gases would modify Rup but not Pcrit. The study was performed on 10 patients with severe SAHS (apnea-hypopnea index: 59 +/- 14 events/hour) when breathing air and helium-oxygen (He-O2) during non-REM sleep. The continuous positive airway pressure that normalized flow (CPAPopt) was measured. Rup and Pcrit were determined from the linear relationship between maximal inspiratory flow VImax and nasal pressure (PN):VImax = (PN - Pcrit)/Rup. Changing the breathing gas selectively modified the severity of dynamic (CPAPopt, Rup) and static (Pcrit) obstructions. CPAPopt was significantly (p = 0.0013) lower when breathing He-O2 (8.44 +/- 1.66 cm H2O; mean +/- SD) than air (10.18 +/- 2.34 cm H2O). Rup was markedly lower (p = 0.0001) when breathing He-O2 (9.21 +/- 3.93 cm H2O x s/L) than air (15.92 +/- 6.27 cm H2O x s/L). Pcrit was similar (p = 0.039) when breathing He-O2 (4.89 +/- 2.37 cm H2O) and air (4.19 +/- 2.93 cm H2O). The data demonstrate the role played by the upstream segment of the upper airway and suggest that different mechanisms determine static (Pcrit) and dynamic (Rup) upper airway obstructions in SAHS.