Neutral Fraction (neutral + fraction)

Distribution by Scientific Domains


Selected Abstracts


AN UNUSUAL POLYUNSATURATED C-27 HYDROCARBON FROM THE MARINE DINOFLAGELLATE PYROCYSTIS LUNULA

JOURNAL OF PHYCOLOGY, Issue 2001
Article first published online: 24 SEP 200
Leblond, J. D.1 & Chapman, P. J.2 1Department of Biology, Middle Tennessee State University, Murfreesboro, TN 37132. 2US EPA (NHEERL), Gulf Ecology Division, 1 Sabine Island Dr., Gulf Breeze, FL 32561 Studies of the lipids of different algal species have revealed a diversity of fatty acids, sterols, and hydrocarbons, of which several are considered useful biomarkers, with potential for characterizing phytoplankton community composition. To extend this approach and characterize the lipids and lipid classes of laboratory-cultured marine dinoflagellates, a silicic acid fractionation system was developed to obtain compositional data for sterols and hydrocarbons of over forty species. In the course of this work, a neutral fraction obtained from a lipid extract of Pyrocystis lunula was found to contain an abundant quantity of a long-chain polyunsaturated hydrocarbon, along with previously reported keto-steranes. The hydrocarbon molecular weight (364) and retention time obtained by gas chromatography/mass spectrometry analysis suggested a C27 compound, which was confirmed by reduction (Adams catalyst) to give the straight chain alkane, n-heptacosane. The presence of eight double bonds was established by deuteration to give a product with molecular weight 396. While the positions of double bonds have not been established, the carbon number of this hydrocarbon and the number of double bonds strongly suggest formation by decarboxylation of the recently described, long-chain polyunsaturated C28 fatty acid shown to be a constituent of phospholipids. This hydrocarbon was not found in any other genus of the examined dinoflagellates, and appears to be one of the first identifications of a hydrocarbon in this class of algae. The function(s) of this compound in P. lunula is currently unclear. [source]


Ly, leaks and reionization

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 4 2008
Longlong Feng
ABSTRACT Ly, absorption spectra of QSOs at redshifts z, 6 show complete Gunn,Peterson absorption troughs (dark gaps) separated by tiny leaks. The dark gaps are from the intergalactic medium (IGM) where the density of neutral hydrogen are high enough to produce almost saturated absorptions, however, where the transmitted leaks come from is still unclear so far. We demonstrate that leaking can originate from the lowest density voids in the IGM as well as the ionized apatches around ionizing sources using semi-analytical simulations. If leaks are produced in lowest density voids, the IGM must already be highly ionized, and the ionizing background should be almost uniform; in contrast, if leaks come from ionized patches, the neutral fraction of IGM should be still high, and the ionizing background is significantly inhomogeneous. Therefore, the origin of leaking is crucial to determining the epoch of inhomogeneous-to-uniform transition of the ionizing photon background. We show that the origin could be studied with the statistical features of leaks. Actually, Ly, leaks can be well defined and described by the equivalent width W and the full width of half-area WH, both of which are less contaminated by instrumental resolution and noise. It is found that the distributions of W and WH of Ly, leaks are sensitive to the modelling of the ionizing background. We consider four representative models: uniform ionizing background (model 0), the photoionization rate of neutral hydrogen ,H i and the density of IGM are either linearly correlated (model I), or anticorrelated (model II), and ,H i is correlated with high-density peaks containing ionizing sources (model III). Although all of these models can match to the mean of the observed effective optical depth of the IGM at z, 6, the distributions of W and WH are very different from each other. Consequently, the leak statistics provides an effective tool to probe the evolutionary history of reionization at z, 5,6.5. Similar statistics will also be applicable to the reionization of He ii at z, 3 [source]


LOFAR as a probe of the sources of cosmological reionization

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY: LETTERS (ELECTRONIC), Issue 1 2005
Saleem Zaroubi
ABSTRACT We propose use of the thickness of the ionization front as a discriminant between alternative modes of reionization in the early Universe, by stars or by miniquasars. Assuming a photoionization,recombination balance, we find that for miniquasar sources the transition from neutral to ionized intergalactic medium is extended and has two features. The first is a sudden steep increase in the neutral fraction with a typical width of 5,10 comoving megaparsecs, depending on the miniquasar power. The second feature is a long wing that represents a much slower transition from a neutral fraction of , 0.8 to 1. The angular resolution of LOFAR is expected to resolve these scales and will, therefore, play an important role in discriminating the hard sources of ionizing photons from the stellar ones. [source]


GC-MS analysis of penta- and tetra-cyclic triterpenes from resins of Pistacia species.

BIOMEDICAL CHROMATOGRAPHY, Issue 8 2005
Part II.
Abstract Pistacia species contain oleoresins with bioactive triterpenes. In this study triterpenes, including minor components, were identified and quantified in both neutral and acidic fractions of Pistacia terebinthus var. Chia resin, grown exclusively in Chios island (Greece), collected traditionally, as well as using stimulating agents (liquid collection). It was proved that these two resin samples were composed of several different minor triterpenes, while major constituents were similar but in different proportions. Compounds that differentiated two resin samples of P. lentiscus and P. terebinthus var. Chia, both traditionally collected, were detected, in order to identify the nature of resins present in archaeological materials. In the traditionally collected resin, 37 triterpenes were identified, 12 in the acidic and 25 in the neutral fraction. In the liquid collection resin 10 compounds were identified in the acidic and 23 in the neutral fraction, while 16 compounds were not contained in the traditionally collected resin. The main triterpenes in both resin samples collected traditionally and using stimulating agents were: isomasticadienonic acid (23.6 and 26.3% w[sol ]w of the triterpenic fraction, respectively), 28-norolean-17-en-3-one (16.3 and 17.5% w[sol ]w of the triterpenic fraction, respectively) and masticadienonic acid (5.8 and 6.0% w[sol ]w of the triterpenic fraction). In this study the qualitative and quantitative composition of triterpenes was compared in the Pistacia lentiscus and P. terebinthus var. Chia resin samples collected with the traditional and new liquid techniques, and also triterpenes in resins of P. terebinthus obtained by the traditional technique and using stimulating agents. The aim of the study was also to examine whether the collection technique influenced the triterpenes contained in P. terebinthus var. Chia resin samples. Copyright © 2005 John Wiley & Sons, Ltd. [source]


Neutral components from hexane extracts of Croton sellowii

FLAVOUR AND FRAGRANCE JOURNAL, Issue 1 2004
Sebastião F. Palmeira Jr
Abstract Qualitative analyses by GC,FID and GC,MS of the neutral fractions from hexane extract of the leaves, stems and roots of Croton sellowii were carried out. Sesquiterpenoids, n -alkanes, aliphatic esters and aliphatic alcohols and alkyl benzene derivatives were identi,ed. Caryophyllene oxide (46.8%) and trans -caryophyllene (40.8%) were most abundant in the leaves, while caryophyllene oxide (26.5%) and cubenol (16.7%) were the main constituents in the stems. In the roots, besides mesitylene (15.2%), the sesquiterpenes , -(7.6%), , -(6.9%) and , -eudesmol (5.5%) were the most abundant components. All identi,ed constituents are described for the ,rst time in this species. Copyright © 2003 John Wiley & Sons, Ltd. [source]


Structure Elucidation and Phytotoxicity of Ecdysteroids from Chenopodium album

CHEMISTRY & BIODIVERSITY, Issue 4 2005
Marina DellaGreca
The leaves of Chenopodium album were infused in H2O/MeOH. The extract treated with cold acetone gave heavy precipitation, which was removed by centrifugation. Solid material was fractionated into acidic and neutral fractions. The acidic material was subjected to different silica-gel column chromatographies, and then it was purified by reversed-phase HPLC to afford four known ecdysteroids and the new 3,,14, -dihydroxy-5, -pregn-7-ene-2,6,20-trione that were characterized by extensive spectroscopic investigation, especially by 1D- and 2D-NMR. Their effects on germination and growth of Lactuca sativa L. have been studied. The results are reported as percentage differences of germination, root elongation and shoot elongation, from the control at concentrations ranging from 10,4 to 10,7,M. [source]