Molecular Surface Density Research Articles

Cold gas and star formation in a merging galaxy sequence

We explore the evolution of the cold gas and star-formation activity during galaxy interactions, using a merging galaxy sequence comprising both pre- and post-mergers. Data for this study come from the literature but supplemented by new radio observations presented here. Firstly, we confirm that the star-formation efficiency (SFE) increases close to nuclear coalescence. At post-merger stages there is evidence that the SFE declines to values typical of ellipticals. This trend can be attributed to M(H_2) depletion due to interaction induced star-formation. However, there is significant scatter, likely to arise from differences in the interaction details of individual systems. Secondly, we find that the central molecular hydrogen surface density, increases close to the final stages of the merging of the two nuclei. Such a trend is also predicted by numerical simulations. Furthermore, there is evidence for a decreasing fraction of cold gas mass from early interacting systems to merger remnants, attributed to gas conversion into other forms. The evolution of the total-radio to blue-band luminosity ratio, reflecting the disk+nucleus star-formation activity, is also investigated. Although this ratio is on average higher than that of isolated spirals, we find a marginal increase along the merging sequence, attributed to the relative insensitivity of disk star-formation to interactions. However, a similar result is also obtained for the nuclear radio emission, although galaxy interactions are believed to significantly affect the activity in the central galaxy regions. Finally, we find that the FIR--radio flux ratio distribution of interacting galaxies is consistent with star-formation being the main energising source.

Annealed decanethiol monolayers on Au(111) – intermediate phases between structures with high and low molecular surface density

Self-assembled monolayers of decanethiol on Au(111) have been investigated using high resolution scanning tunneling microscopy (STM). After the annealing of the as-adsorbed densely packed c(2 3 ×4 3 )R30° structure and before the formation of a previously described low density (11.5× 3 ) stripe phase three different phases with intermediate molecular surface density have been observed. We present highly resolved and distortion-corrected STM images for both the (7.5× 3 ) stripe phase and the meshlike structure. We describe a mixed (7.5/11.5× 3 ) stripe phase that shows a switching of the stripe arrangement.

Localization and quantification of hydrophobicity: the molecular free energy density (MolFESD) concept and its application to sweetness recognition.

A method for the localization, the quantification, and the analysis of hydrophobicity of a molecule or a molecular fragment is presented. It is shown that the free energy of solvation for a molecule or the transfer free energy from one solvent to another can be represented by a surface integral of a scalar quantity, the molecular free energy surface density (MolFESD), over the solvent accessible surface of that molecule. This MolFESD concept is based on a model approach where the solvent molecules are considered to be small in comparison to the solute molecule, and the solvent can be represented by a continuous medium with a given dielectric constant. The transfer energy surface density for a 1-octanol/water system is empirically determined employing a set of atomic increment contributions and distance dependent membership functions measuring the contribution of the increments to the surface value of the MolFESD. The MolFESD concept can be well used for the quantification of the purely hydrophobic contribution to the binding constants of molecule-receptor complexes. This is demonstrated with the sweeteners sucrose and sucralose and various halogen derivatives. Therein the relative sweetness, which is assumed to be proportional to the binding constant, nicely correlates to the surface integral over the positive, hydrophobic part of the MolFESD, indicating that the sweetness receptor can be characterized by a highly flexible hydrophobic pocket instead of a localized binding site.

Cellular Uptake of PEO Surface-Modified Nanoparticles: Evaluation of Nanoparticles Made of PLA: PEO Diblock and Triblock Copolymers

Nanoparticles with either physically adsorbed or covalently bound poly(ethylene oxide) (PEO) coatings were produced from various combinations of poly(lactic acid) (PLA) and diblock or triblock copolymers of PLA and PEO. The particles were produced by the salting-out process and purified by the cross-flow filtration technique. The amount of PEO at the nanoparticle surface, as well as the residual amount of emulsifier polyvinyl alcohol) were assessed, with a good correlation with expected values. Stability of the nanoparticulate suspensions was studied at 4°C and after freezing under various conditions for up to 6 months. The nanoparticle redispersibility after storage was related to the thermal behavior of the PEO coatings. The in vitro cellular uptake of the different types of nanoparticles was compared by flow cytometry after incubation with human monocytes in serum and in plasma. The influence of the PEO molecular weight and surface density on the particle uptake was especially marked for the diblock and triblock copolymer formulations, with a decrease in uptake of up to 65% with one of the diblock copolymer formulations. Nanoparticles made of triblock copolymer with short PEO chains at their surface in the postulated “loop conformation” proved to be as resistant to cellular uptake as nanoparticles made of diblock copolymers with PEO chains in the “brush conformation”.

Liver uptake and hepato-biliary transfer of galactosylated proteins in rats are determined by the extent of galactosylation

The effect of molecular mass and surface density of galactose residues on hepatic uptake and subsequent biliary excretion of galactosylated proteins was investigated in rats. Several proteins with different molecular weights (15–70 kDa) and different numbers of galactose units were synthesized and radiolabeled with 111In. Galactosylated proteins were administered i.v. to anaesthetized rats and samples of plasma and bile were collected for 3 h. Liver was harvested at the end of the experiments and the radioactivity of all samples was measured. Galactosylated proteins accumulated primarily in the liver and 2–10% of the administered dose appeared in the bile, mainly in undegraded form. The hepatic uptake clearance (Cl liver) and biliary excretion rate constant ( k bile) of galactosylated proteins were calculated. No direct effect of molecular weight was observed, however, on increasing the galactose density, Cl liver increased from about 4 to 400 ml/h whereas k bile gradually decreased from about 0.057 to 0.007 (h −1). In conclusion, both hepatic uptake and biliary excretion of galactosylated proteins were found to be affected by the extent of galactosylation.

The Molecular Free Energy Surface Density (MOLFESD). Approach for a quantitative treatment of hydrophobicity

A method for the localization, the quantification, and the analysis of hydrophobicity of a molecule or a molecular fragment is presented which is based on the concepts that the overall hydrophobicity of a molecule can be obtained as a superposition of fragment contributions, and that the corresponding free energy can be calculated as an integral of the molecular free energy surface density (MOLFESD), ρ, over the molecular solvent accessible surface [1-5]. The MOLFESD is composed as a sum of atomic increment functions. The concept can be well used for the quantification of the hydrophobic contribution to the (relative) binding constants for molecule-receptor complexes. This is demonstrated with the sweetener sucrose and its various chlorine derivatives.

Study by infrared spectroscopy of the conformation of dipalmitoylphosphatidylglycerol monolayers at the air–water interface and transferred on solid substrates

A parallel investigation of the conformational changes of dipalmitoylphosphatidylglycerol (DPPG) monolayers induced by surface pressure has been studied in situ at the air water interface using polarization modulation external infrared reflection absorption spectroscopy (PM-IRRAS) and on solid substrate by attenuated total reflection (ATR) spectroscopy. For both the ATR and PM-IRRAS spectra of DPPG, the non-linear increase of the intensity of the antisymmetric methylene stretching band of DPPG with molecular surface density, indicates that the compression of the DPPG monolayer induces a change of the conformation and/or orientation of the acyl chains of the phospholipid. At surface pressures between 7 and 20 mN/m, the acyl chains of the DPPG monolayer are found to be more ordered on the germanium substrate than at the air–water interface. At higher surface pressures, the conformation of the lipid acyl chains of DPPG on both substrates are almost similar. In the liquid-condensed phase, the acyl chains are in all- trans conformation and their tilt angle with respect to the normal of the film is approximately 30°. Our results also suggest the presence of DPPG solid domains in the liquid-expanded phase. Investigation of polar head group region indicates that at low surface pressures, the carbonyl groups were more oriented on the water surface than the acyl chains. Finally, the present study shows that intermolecular hydrogen bonding probably occur between the glycerol hydroxyl and the phosphate or carbonyl groups of the phospholipid.

Flat Lying Pin-Stripe Phase of Decanethiol Self-Assembled Monolayers on Au(111)

Self-assembled monolayers with low molecular surface density of decanethiol on Au(111) have been prepared by annealing of the densely packed (√3 × √3)R30° structure. High-resolution scanning tunneling microscope (STM) imaging reveals the formation of a 11.5 × √3 stripe structure where the molecules are aligned along the substrate surface in double lamellae. We can deduce from the images that the molecules bind to specific sites at the Au(111) surface rather than adsorb as didecane disulfides. For the first time, a lattice structure for an alkanethiol stripe phase can be derived from the obtained real-space data.

DNA-containing organized molecular structure based on controlled assembly on supported monolayers

Based on the Langmuir–Blogett (LB) technique and molecular recognition, we reported in the present paper the controlled assembly of biotin-lipid/avidin/biotin-ssDNA/ssDNA multilayers. The ssDNA-protein and ssDNA–ssDNA interactions were directly monitored at supported monolayers in real-time using a surface plasmon resonance technique. Four oligonucleotides with 20 bp were synthesized, which included complementary and non-complementary ssDNA fragments. According to the quantitative analysis we obtained the molecular surface densities of the avidin layer, biotin-ssDNA layer and the complementary ssDNA layer are 2.1×10 10, 4.2×10 10 and 4.1×10 10, respectively; and the molar ratio among of them is approximately 1:2:2.

Surface behavior of myelin monolayers

Myelin can be spread as a stable monomolecular layer, with reproducible properties, at the air–water interface. The major lipids and proteins of myelin are represented in this monolayer in molar ratios similar to those in the original membrane. A well-defined collapse point of the myelin monolayer occurs at ca. 46 mN/m. At a surface pressure of ca. 20 mN/m, the surface pressure–molecular area isotherm of the myelin monolayer shows a change in its compressibility, exhibited as a diffuse but reproducible inflection with a clearly marked change of the surface compressional modulus; the surface potential–area curve shows a change of slope at the same surface pressure. The myelin monolayer shows considerable hysteresis during the first compression–decompression cycle; no detectable protein unfolding under expansion; and decreased hysteresis after the first cycle. The average molecular areas, the inflection at 20 mN/m, the variation of the surface potential per unit of molecular surface density, and the hysteresis properties of the myelin monolayer indicate that this membrane undergoes changes of intermolecular organization mostly ascribed to the protein fraction, above a lateral surface pressure of ca. 20 mN/m. The behavior is consistent with a surface pressure-dependent relocation of protein components in the film. This has marked effects on the stability, molecular packing, and dipolar organization of the myelin interface.

PEG-coated nanospheres from amphiphilic diblock and multiblock copolymers: Investigation of their drug encapsulation and release characteristics

The development of long-circulating carriers capable of continuously delivering drugs intravenously provides a potential approach to achieve targeting of drugs to specific sites of action. In this research, polyethylene glycol (PEG)-coated nanospheres were prepared in a single step process using amphiphilic diblock and multiblock copolymers, and their drug encapsulation properties and release characteristics were investigated in vitro. The results obtained indicate that the parameters controlling drug release characteristics of these nanospheres are the chemical composition of their hydrophobic core (i.e. polyester or polyanhydride), as well as the molecular weight and surface density of their hydrophilic (i.e. PEG) chains.

Formation of Liquid Crystalline Phases from a Langmuir Monolayer

We report the transformation of a two-dimensional monolayer at the air−water interface into three-dimensional liquid crystalline phases on continuous tuning of the surface molecular density. On compressing a 4‘-n-octyl-4-cyanobiphenyl monolayer, at room temperature, we obtain multilayer domains whose thicknesses range from a few layers to hundreds of layers. Above 28 °C, these domains undergo a phase transition and change to much thicker lenslike domains. On expansion, these domains revert to the monolayer. Although these domains originate from a monolayer, we find that they exhibit features typical of liquid crystals.

Molecular orientation of submonolayer rhodamine-6G on quartz substrates: A comparative study using reflection and transmission UV-Vis spectroscopy

Reflection and transmission UV-Vis spectroscopy have been applied to study the molecular orientation and surface density of rhodamine-6G molecules physisorbed on optically flat quartz (SiO2) substrates. Our results have shown that for the s-polarized excitation, the submonolayer of physisorbed rhodamine-6G dye molecules causes enhanced reflection in the wavelength region from 400 nm to 600 nm where the electronic transition takes place. For the p-polarized excitation, the reflection is enhanced when the angle of incidence is smaller than Brewster’s angle of quartz at 55.6°, and the reflection is reduced when the angle of incidence is larger than Brewster’s angle of quartz. An independent method has been established in this paper by which the molecular orientation can be determined accurately by carrying out optical measurements in both the reflection and transmission directions.

Langmuir-Blodgett film and nematic interface

The orientation induced by a Langmuir-Blodgett film on a Nematic Liquid Crystal (NLC) is theoretically analyzed. We show that the effective surface energy is due to different contributions connected with steric and van der Waals interactions between the nematic and the solid substrate. The analysis shows that the Langmuir-Blodgett film orientation depends on the surface density of the molecules. The initial homeotropic orientation may become unstable giving rise to a tilted film. The average orientation of the nematic molecules is also analyzed. We show that, in the event in which the steric interaction Nematic-Langmuir-Blodgett film is very large with respect to the dispersion interaction Nematic-Substrate, the nematic orientation coincides with the one of the film. On the contrary, when the two interactions are comparable, the orientation of the two media may differ. In particular, we analyze how the stable orientation depends on the surface molecular density of the film.

Theory of the structure of adsorbed block copolymers: Detailed comparison with experiment

We present a numerical, self-consistent field study of adsorbed diblock copolymers in thermal solvents, with a detailed and quantitative comparison with recent experiments performed on poly(dimethylsiloxane-block-styrene) copolymer spread as a monolayer at the free surface of ethyl benzoate [M. S. Kent, L. T. Lee, B. Farnoux, and F. Rondelez, Macromolecules 25, 6240 (1992); M. S. Kent, L. T. Lee, B. J. Factor, F. Rondelez, and G. S. Smith, J. Chem. Phys. 103, 2320 (1995)]. These neutron reflectivity experiments, for the first time, independently varied both the molecular weight and surface density, and probed the size and nature of the depletion layer at the surface. In the calculations, the polymer and solvent are characterized by realistic values of the pure component densities, the Kuhn length and the Flory interaction parameter. We examine the properties of the dangling block, specifically the depletion layer, the thickness of the brush, the maximum polymer concentration and its location, and the dependence of these properties on surface density and molecular weight. We obtain very good agreement with the experiments, especially for the functional dependences, which indicate that these systems are not in the asymptotic brush limit. We also argue that this conclusion applies to many experiments, some of which were previously thought to be in the asymptotic limit.

CO emission from the dwarf irregular galaxy NGC 6822

view Abstract Citations (24) References (45) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS CO Emission from the Dwarf Irregular Galaxy NGC 6822 Wilson, Christine D. Abstract Four regions of the Local Group dwarf irregular galaxy NGC 6822 have been mapped for CO emission. The regions observed were centered on bright H II regions and strong H I peaks. Of the 47 lines of sight observed, CO emission was detected at only three. The peak line strengths observed in NGC 6822 are intermediate between the line strengths observed at comparable spatial resolution in the Large and Small Magellanic Clouds. In contrast, the irregular galaxy IC 10, which has a metallicity very similar to NGC 6822, has CO line strengths that are an order of magnitude larger than those in NGC 6822. The product of the star formation rate per galaxy mass and the galaxy metallicity appears to be a better indicator of CO line strength than metallicity alone. However, these low-resolution observations cannot rule out the possibility that the line strength variations are simply due to differing molecular hydrogen surface densities. Within substantial uncertainties in the CO to H, conversion factor, the molecular gas mass detected at each position is comparable to what is seen in Galactic giant molecular clouds. Comparing these mass estimates with the Hα fluxes, the two largest H II regions in NGC 6822 appear to have star formation efficiencies similar to those of giant H II regions seen in other galaxies. Publication: The Astrophysical Journal Pub Date: May 1992 DOI: 10.1086/171332 Bibcode: 1992ApJ...391..144W Keywords: Carbon Monoxide; Dwarf Galaxies; Emission Spectra; H Ii Regions; Irregular Galaxies; Molecular Gases; Astronomical Maps; Astronomical Spectroscopy; Interstellar Gas; Local Group (Astronomy); Molecular Clouds; Astrophysics; GALAXIES: INDIVIDUAL NGC NUMBER: NGC 6822; GALAXIES: INTERSTELLAR MATTER; GALAXIES: IRREGULAR; ISM: H II REGIONS; ISM: MOLECULES full text sources ADS | data products SIMBAD (5) NED (1)

The molecular structure of a starburst - (C-13)O in the nucleus of Maffei 2

view Abstract Citations (30) References (61) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Molecular Structure of a Starburst: 13CO in the Nucleus of Maffei 2 Hurt, Robert L. ; Turner, Jean L. Abstract We present high-resolution (~6") maps of the ^13^C0 emission in the nucleus of the nearby spiral galaxy Maffei 2 made with the Owens Valley Millimeter Interferometer. The optically thin ^13^CO emission, a tracer of H_2_ gas density, indicates that the molecular cloud distribution in the nucleus of Maffei 2 is remarkably well-correlated with the starburst as traced by radio continuum emission. Evidently the molecular density distribution determines the spatial structure of this starburst. The observed range of ^13^CO emission implies H_2_ column densities of N_H2_ = 0.13-1.3 X 10^32^ cm^-2^, molecular surface densities of σ_H2_ = 200-2000 M_sun_ pc^-2^, and visual extinctions of A_v_ = 14-140 mag on size scales of ~150 pc. The molecular material forms a barlike structure 40" (1 kpc at an assumed distance of 5 Mpc) in extent, with a total H_2_ mass of M_H2_ = 2.3 x 10^8^ M_sun_. Two dynamical components are evident in the ^13^CO maps: a region within ~120 pc of the nucleus that appears to rotate rapidly as a solid body; then nearly Keplerian fall-off to a less-rapidly rotating region extending to a radius of ~500 pc. The most intense star formation activity appears in the inner solid body region of the rotation curve. Within the solid body region, gas accounts for ~40% of the estimated dynamical mass. The centroid of the molecular mass is offset by ~120 pc from the dynamical center of the galaxy. Since the dynamical time scale against winding by differential rotation is ~10^6^ yr, some mechanism must contribute to the longevity of this nuclear bar structure, possibly spiral density waves. Publication: The Astrophysical Journal Pub Date: August 1991 DOI: 10.1086/170373 Bibcode: 1991ApJ...377..434H Keywords: Galactic Nuclei; Galactic Structure; Interstellar Gas; Molecular Clouds; Spiral Galaxies; Starburst Galaxies; Astronomical Maps; Carbon Monoxide; Carbon 13; Star Formation; Astrophysics; GALAXIES: INDIVIDUAL NAME: MAFFEI 2; GALAXIES: INTERSTELLAR MATTER; GALAXIES: NUCLEI; STARS: FORMATION full text sources ADS | data products SIMBAD (1) NED (1)

Ganglioside GD3 lactones: polar head group-mediated control of the intermolecular organization

The individual properties and intermolecular organization of ganglioside GD3 and of two of its lactone forms (GD3Lactone I and GD3Lactone II) were studied in lipid monolayers. The formation of the first lactone ring in GD3Lactone I eliminates one negative charge and leads to a decrease of the molecular area at all surface pressures. The intermolecular dispersion energy and collapse pressure are higher in GD3Lactone I compared to those in the parent GD3. The surface potential per unit of molecular surface density and the resultant molecular dipole moment are increased in GD3Lactone I with respect to those in GD3 at comparable values of molecular area. In GD3Lactone I the molecular parameters suggest an oligosaccharide chain oriented similarly to that of GD3. On the average, this is perpendicular to the surface, and the resultant polar head-group dipole moment points away from the interface. In GD3Lactone II the negative charges are eliminated, resulting in considerably larger molecular areas than for GD3 and GD3Lactone I at all pressures. The intermolecular dispersion energy of GD3Lactone II is also greatly diminished and the collapse pressure is further increased compared to those of GD3Lactone I. However, the surface potential per unit molecular surface density and the resultant molecular dipole moment of GD3Lactone II are higher than in GD3 Lactone I at similar values of molecular areas. This is probably due to a positive polar head-group dipole moment contribution induced by the additional lactone ring in GD3Lactone II. These changes result from a distorted conformation of the oligosaccharide chain owing to the presence of fused carbohydrate rings which require a greater intermolecular spacing compared to GD3 and GD3Lactone I.

Molecular clouds without detectable CO

The clouds identified by Desert, Bazell, and Boulanger (DBB clouds) in their search for high-latitude molecular clouds were observed in the CO (J = 1-0) line, but only 13 percent of the sample was detected. The remaining 87 percent are diffuse molecular clouds with CO abundances of about 10 to the -6th, a typical value for diffuse clouds. This hypothesis is shown to be consistent with Copernicus data. The DBB clouds are shown to ben an essentially complete catalog of diffuse molecular clouds in the solar vicinity. The total molecular surface density in the vicinity of the sun is then only about 20 percent greater than the 1.3 solar masses/sq pc determined by Dame et al. (1987). Analysis of the CO detections indicates that there is a sharp threshold in extinction of 0.25 mag before CO is detectable and is derived from the IRAS I(100) micron threshold of 4 MJy/sr. This threshold is presumably where the CO abundance exhibits a sharp increase

Star formation, molecular clouds, and the origin of blue light from galaxies

The relationship between the blue luminosity and the molecular content of 66 isolated and weakly interacting spiral galaxies is examined, showing that blue luminosity is not an accurate star formation indicator. It is found that blue luminosity per unit molecular mass is negatively correlated with molecular surface density. The results are compared with those from the study of the relationship between FIR luminosity and CO luminosity. 23 refs.