MOLECULAR PACKING IN TRIACYL-sn-GLYCEROLS: INFLUENCES OF ACYL CHAIN LENGTH AND UNSATURATION

The molecular packing in triacylglycerols having different acyl chains has been examined by differential scanning calorimetry, powder X-ray diffraction and vibrational spectroscopy (infrared and Raman) techniques. In the triacylglycerols examined, the acyl chain length, unsaturation or the position of substitution on the glycerol were changed systematically to observe their influence on the molecular packing in different polymorphic forms. Variation in the 3-acyl chain length of 1,2-dipalmitoyl-3-acyl-sn-glycerols (PPX) influenced the molecular packing along the long axis in the stable polymorphic forms. Three different modes of packing were observed. If X ≤ 4, the compounds packed in a bilayer structure similar to diacylglycerols, or if X ≥ 10 and ≤ 16 the compounds packed in a bilayer structure but similar to mono acid triacylglycerols. However for intermediate 3-acyl chain lengths, as in PP6 and PP8 the stable packing can occured only through chain segregation resulting in a trilayer structure. In the triacylglycerols containing unsaturated acyl chains, 1,2-dioleoyl-3-acyl-sn-glycerols (00X) and 1,3-dioleoyl-Z-acyl-sn-glycerols (0X0) the stable polymorphic forms packed in a trilayer structure where the odd acyl chains segregated and formed a middle layer. In a metastable hexagonal packing (α-phase) the long range ordering is minimal. Because of this lack of specific chain-chain interaction the 3-short acyl chain compounds of PPX packed in a unimolecular length structure (except PP2) whereas the 3-long acyl chain compounds packed in a bilayer structure. In orthorhombic perpendicular and triclinic parallel packing where the specific chain-chain interaction is increased, the end plane methyl packing and the glycerol conformation played important roles in the formation of bi-, tri- and hexalayer structures. The driving force in the formation of these different structures is to minimize the crystal defects created by the odd acyl chains and to enhance the specific chain-chain interactions. The presence of an odd acyl chain influenced the lateral chain packing as well, e.g., the stability of the orthorhombic perpendicular packing is enhanced by the presence of an odd acyl chain and even in some cases it is favored over the triclinic parallel packing. The odd acyl chain at the 1- or 3position of -sn-glycerol stabilized the orthorhombic perpendicular packing. This indicates the glycerol conformation is probably perpendicular to the layer plane and thus is different from the monoacid triacylglycerols.     

Manganese-enhanced magnetic resonance microscopy of mineralization

Paramagnetic manganese (II) can be employed as a calcium surrogate to sensitize magnetic resonance microscopy (MRM) to the processing of calcium during bone formation. At high doses, osteoblasts can take up sufficient quantities of manganese, resulting in marked changes in water proton T(1), T(2) and magnetization transfer ratio values compared to those for untreated cells. Accordingly, inductively coupled plasma mass spectrometry (ICP-MS) results confirm that the manganese content of treated cell pellets was 10-fold higher than that for untreated cell pellets. To establish that manganese is processed like calcium and deposited as bone, calvaria from the skull of embryonic chicks were grown in culture medium supplemented with 1 mM MnCl(2) and 3 mM CaCl(2). A banding pattern of high and low T(2) values, consistent with mineral deposits with high and low levels of manganese, was observed radiating from the calvarial ridge. The results of ICP-MS studies confirm that manganese-treated calvaria take up increasing amounts of manganese with time in culture. Finally, elemental mapping studies with electron probe microanalysis confirmed local variations in the manganese content of bone newly deposited on the calvarial surface. This is the first reported use of manganese-enhanced MRM to study the process whereby calcium is taken up by osteoblasts cells and deposited as bone.     

Different Epidemic Models on Complex Networks

Models for diseases spreading are not just limited to SIS or SIR. For instance, for the spreading of AIDS/HIV, the susceptible individuals can be classified into different cases according to their immunity, and similarly, the infected individuals can be sorted into different classes according to their infectivity. Moreover, some diseases may develop through several stages. Many authors have shown that the individuals＇ relation can be viewed as a complex network. So in this paper, in order to better explain the dynamical behavior of epidemics, we consider different epidemic models on complex networks, and obtain the epidemic threshold for each ease. Finally, we present numerical simulations for each case to verify our results.     

On-line identification of diastereomeric dibenzo[a,l]pyrene diol epoxide-derived deoxyadenosine adducts by capillary electrophoresis-fluorescence line-narrowing and non-line narrowing spectroscopy.

A capillary electrophoretic method for the separation and on-line identification of closely related analytes using low-temperature fluorescence spectroscopy is reported for the eight diastereomeric deoxyadenosine (dA) adducts derived from dibenzo[a,l]pyrene diol epoxide (DB[a,l]PDE). Electrophoretic separation of stereoisomers was accomplished by application of a mixed surfactant buffer [dioctyl sulfosuccinate (DOSS) and Brij-S], which was below the critical micelle concentration (CMC) due to the high concentration (approximately 25%) of organic solvent. Addition of multiple surfactant additives to the separation buffer provided electrophoretic resolution, which was unattainable under single surfactant conditions. It is shown that the CE-separated analyte zones could be identified on-line via low-temperature (4.2 K) fluorescence non-line narrowing and fluorescence line-narrowing (FLN) spectroscopy. In addition, it was determined that in CE buffer trans-syn-,cis-syn- and cis-anti-DB[a,l]PDE-14-N6dA diastereomeric adducts exist mostly with the -dA and DB[a,l]P moiety in an "open"-type conformation while the trans-anti-DB[a,l]PDE-14-N6dA adducts exist in two different conformations whose relative distribution depends on matrix composition. The above conformations have also been revealed by selective laser excitation. Thus, the low-temperature methodology not only provides fingerprint structure via vibrationally resolved 4.2 K fluorescence spectra for adduct identification, but also provides conformational information on the spatial relationship of the carcinogen and dA moiety. These results, taken together with those for DB[a,l]P-DNA adducts formed in standard glasses and mouse epidermis exposed to DB[a,l]P, support our earlier findings that DB[a,l]P-derived adducts exist in different conformations [Jankowiak et al., Chem. Res. Toxicol. 11 (1998) 674]. Therefore, the combination of the separation power of CE and spectral selectivity of low-temperature fluorescence spectroscopy at NLN and FLN conditions provides a powerful methodology which should prove useful for identification of closely related DNA adducts formed at low levels in biological systems.     

One-phonon spectra in absorption and fluorescence spectra of molecular impurities: breakdown of mirror symmetry

Conditions for breakdown of the structural and intensity mirror symmetry relationship between the one-phonon spectra in the absorption and fluorescence spectra of molecular impurities are established. A criterion is given for determining when the contribution of quadratic coupling to the zero-phonon bandwidth, for a given linear electron— phonon coupling strength, is expected to be minimal.     

Electric field modulation of galvanomagnetic properties of mesoscopic graphite

Electric field effect devices based on mesoscopic graphite are fabricated for galvanomagnetic measurements. Strong modulation of magnetoresistance and Hall resistance as a function of the gate voltage is observed as the sample thickness approaches the screening length. Electric field dependent Landau level formation is detected from Shubnikov-de Haas oscillations. The effective mass of electron and hole carriers has been measured from the temperature dependent behavior of these oscillations.     

The effect of gain and absorption on surface plasmons in metal nanoparticles

The compensation of loss in metal by gain in interfacing dielectric has been demonstrated in a mixture of aggregated silver nanoparticles and rhodamine 6G dye. An increase of the quality factor of surface plasmon (SP) resonance was evidenced by the sixfold enhancement of Rayleigh scattering. The compensation of plasmonic losses with gain enables a host of new applications for metallic nanostructures, including low- or no-loss negative-index metamaterials. We have also predicted and experimentally observed a suppression of SP resonance in metallic nanoparticles embedded in dielectric host with absorption. PACS 61.46.Df; 73.20.Mf; 78.67.Bf     

Measurement of rail cross-section with solid-state sensor arrays

The application of light sectioning to the accurate measurement of a rail cross-section, from a train moving at high speed, is described. The intensity distribution through the thickness of the illuminating light sheet is deliberately graded, allowing the shape of the object to be recovered more accurately by interpolation of the sampled image. Rail profiles are extracted with a definition approximately ten-times better than that of the sampled image.