Nonresonant Polarized Raman Spectra Calculations of Nitrogen-Doped Single-Walled Carbon Nanotubes: Diameter, Chirality, and Doping Concentration Effects.

Raman spectra of nitrogen-doped single-walled carbon nanotubes are calculated using the spectral moment's method combined with the bond polarizability model. The influence of the nanotube diameter and chirality is investigated. We also address the important question of the effect of the N-doping concentration, and we propose an equation to estimate the doping concentration from the knowledge of the tube diameter and the frequency of the radial breathing mode.

Computation of the infrared active modes in single-walled boron nitride nanotube bundles.

In this work, the infrared active modes are computed for homogeneous bundles of single-walled boron nitride nanotubes (BBNNTs), using the so-called spectral moments method. The dependence of the wavenumber on these modes in terms of diameters, lengths, and numbers of tubes, is investigated. To this end, use is made of a Lennard-Jones potential for describing the van der Waals interactions between tubes in a bundle. We find that, for a finite homogeneous bundle, additional modes appear as a specific signature. Finally, these results are useful for the interpretation of the experimental infrared spectra of BBNNTs.

Raman spectra of C60 dimer and C60 polymer confined inside a (10, 10) single-walled carbon nanotube.

A new set of C-C interball force constant was developed in order to reproduce the low wavenumber density of states measured by neutron scattering and the Raman spectra of the C(60) dimer and C(60) polymer chain. The nonresonant Raman spectra of the C(60) dimer and C(60) polymer confined inside a (10, 10) single-walled carbon nanotube were calculated in the framework of the bond-polarization theory by using the spectral moments method. The main changes of the Raman spectrum as a function of the organization of the C(60) molecules inside the nanotubes were identified. We found that the radial breathing modes of a (10, 10) single-walled carbon nanotube are more sensitive on the structure of the C(60) molecules than the G-modes. These predictions are useful to interpret the experimental Raman spectrum of fullerene peapods.

Raman-active modes in homogeneous and inhomogeneous bundles of single-walled carbon nanotubes.

In the present work, the non-resonant Raman-active modes are calculated for several diameters, chiralities and sizes for homogeneous and inhomogeneous bundles of single-walled carbon nanotubes (BWCNTs), using the spectral moment's method (SMM). Additional intense Raman-active modes are present in the breathing-like modes (BLM) spectra of these systems in comparison with a single fully symmetric A(1g) mode characteristic of isolated nanotubes (SWCNTs). The dependence of the wavenumber of these modes in terms of diameters, lengths and number of tubes was investigated. We found that, for a finite (in)homogeneous bundle, additional breathing-like modes appear as a specific signature.

Infrared spectroscopy of single-walled carbon nanotubes.

By using the spectral moments method, we calculate the infrared spectra of chiral and achiral single-walled carbon nanotubes (SWCNTs) of different diameters and lengths. We show that the number of the infrared modes, their frequencies, and intensities depend on the length and chirality of the nanotubes. Furthermore, the dependence of the infrared spectrum as a function of the size of the SWCNT bundle is analyzed. These predictions are useful to interpret the experimental infrared spectra of SWCNTs.

Implication of ATP and sodium in arachidonic acid incorporation by placental syncytiotrophoblast brush border and basal plasma membranes in the human.

The human placental syncytiotrophoblast is the main site of exchange of nutrients and minerals between the mother and her fetus. In order to characterize the placental transport of some fatty acids, we studied the incorporation of arachidonic acid, a fetal primordial fatty acid, in purified bipolar syncytiotrophoblast brush border (BBM) and basal plasma membranes (BPM) from human placenta. The basal arachidonic acid incorporation in BBM and BPM was time dependent and reached maximal values of 0.75+/-0.10 and 0.48+/-0.18 pmol/mg protein, respectively, after 2.5 min. The presence of adenosine triphosphate (ATP) (3 m m) increases significantly the maximal incorporation of arachidonic acid by sixfold (4.75+/-0.35 pmol/mg) and ninefold (4.40+/-0.84 pmol/mg) in BBM and BPM, respectively. Moreover, an increase in the arachidonic acid incorporation was also obtained in the presence of sodium where the values achieved 7.68+/-0.98 (10x) and 6.53 pmol/mg (13.6x) for BBM and BPM, respectively. We also showed that the combination of both Na(+)and ATP increases significantly the maximal incorporation of arachidonic acid in BPM to 7.89+/-0.15 pmol/mg protein, while in BBM it did not modify its incorporation (8.18+/-0.25 pmol/mg protein), as compared to the presence of sodium alone. Our results demonstrate that arachidonic acid is incorporated by both placental syncytiotrophoblast membranes, and is ATP and sodium-linked. However, different mechanisms seem to be involved in this fatty acid incorporation through BBM and BPM, since the presence of Na(+)or ATP increased it, while the association of these two elements increased it only in BPM. We also demonstrated by osmolarity experiments that both membranes bind arachidonic acid, potentially involving one or more fatty acids binding proteins.

Human syncytiotrophoblast NPY receptors are located on BBM and activate PLC-to-PKC axis.

Neuropeptide Y (NPY) is abundant in plasma and amniotic fluid of women throughout pregnancy, during which its involvement in placental hormonogenesis has been proposed. In accordance with its putative role, the aim of this study was to characterize the human placental syncytiotrophoblast receptivity to NPY. Thus we performed this study on brush-border membranes (BBM) and basal plasma membranes (BPM). Specific 125I-labeled NPY (125I-NPY) binding to BBM was rapid (20 min), saturable, with a maximum binding capacity of 604 +/- 100 fmol/mg protein, and of high affinity, with a dissociation constant of 11 +/- 3 nM. No saturable binding could be shown in BPM. The rank order of affinity of NPY and related peptides to compete for 125I-NPY binding sites was peptides YY (PYY) > NPY = [Leu31,Pro34]NPY > 13-36NPY >> pancreatic polypeptide (PP). It is noteworthy that PYY displaced only 45% of the binding sites. In BBM, both NPY and PYY were potent phospholipase C (PLC) stimulators, leading to a four- to fivefold increase of control phosphodiesterase activity. The latter effect could be prevented by preincubation of membranes with 5 microM U-73122, a known inhibitor of G protein-linked receptor activation of PLC-beta. Furthermore, 5 microM BIBP-3226, a Y1-receptor antagonist, shifted both dose-response curves to the right in a similar fashion for both peptides. In accordance with the PLC stimulation, both peptides also induced stimulation of protein kinase C (PKC) activity, which could be partially but additively prevented by U-73122 and LY-294002, a selective inhibitor of phosphatidyl-inositol-3 kinase (PI3K). Taken together, these data suggest that placental and blood-derived NPY binds to a mixed population of receptors composed of Y1 and Y3 subtypes on the maternal side of the syncytiotrophoblast, where it can mediate its physiological purposes via PLC-beta and PI3K activation, both of which lead to PKC activation. However, because BIBP-3226 antagonized both effects, the physiological relevance of the apparent Y3 fraction is still unsolved.