26 research outputs found
The generation of low-energy cosmic rays in molecular clouds
It is argued that if cosmic rays penetrate into molecular clouds, the total
energy they lose can exceed the energy from galactic supernovae shocks. It is
shown that most likely galactic cosmic rays interacting with the surface layers
of molecular clouds are efficiently reflected and do not penetrate into the
cloud interior. Low-energy cosmic rays ( GeV) that provide the primary
ionization of the molecular cloud gas can be generated inside such clouds by
multiple shocks arising due to supersonic turbulence.Comment: 11 pages, no figure
The chemistry of multiply deuterated molecules in protoplanetary disks. I. The outer disk
We present new models of the deuterium chemistry in protoplanetary disks,
including, for the first time, multiply deuterated species. We use these models
to explore whether observations in combination with models can give us clues as
to which desorption processes occur in disks. We find, in common with other
authors, that photodesorption can allow strongly bound molecules such as HDO to
exist in the gas phase in a layer above the midplane. Models including this
process give the best agreement with the observations. In the midplane, cosmic
ray heating can desorb weakly bound molecules such as CO and N. We find the
observations suggest that N is gaseous in this region, but that CO must be
retained on the grains to account for the observed DCO/HCO. This could
be achieved by CO having a higher binding energy than N (as may be the case
when these molecules are accreted onto water ice) or by a smaller cosmic ray
desorption rate for CO than assumed here, as suggested by recent theoretical
work.
For gaseous molecules the calculated deuteration can be greatly changed by
chemical processing in the disk from the input molecular cloud values. On the
grains singly deuterated species tend to retain the D/H ratio set in the
molecular cloud, whereas multiply deuterated species are more affected by the
disk chemistry. Consequently the D/H ratios observed in comets may be partly
set in the parent cloud and partly in the disk, depending on the molecule.Comment: Accepted for publication in ApJ. 48 pages, 8 figure
Increased lipophilicity and subsequent cell partitioning decrease passive transcellular diffusion of novel, highly lipophilic antioxidants
ABSTRACT Oxidative stress is considered a cause or propagator of acute and chronic disorders of the central nervous system. Novel 2,4-diamino-pyrrolo [2,3-d]pyrimidines are potent inhibitors of iron-dependent lipid peroxidation, are cytoprotective in cell culture models of oxidative injury, and are neuroprotective in brain injury and ischemia models. The selection of lead candidates from this series required that they reach target cells deep within brain tissue in efficacious amounts after oral dosing. A homologous series of 26 highly lipophilic pyrrolopyrimidines was examined using cultured cell monolayers to understand the structure-permeability relationship and to use this information to predict brain penetration and residence time. Pyrrolopyrimidines were shown to be a more permeable structural class of membrane-interactive antioxidants where transepithelial permeability was inversely related to lipophilicity or to cell partitioning. Pyrrole substitutions influence cell partitioning where bulky hydrophobic groups increased partitioning and decreased permeability and smaller hydrophobic groups and more hydrophilic groups, especially those capable of weak hydrogen bonding, decreased partitioning, and increased permeability. Transmonolayer diffusion for these membrane-interactive antioxidants was limited mostly by desorption from the receiver-side membrane into the buffer. Thus, in this case, these in vitro cell monolayer models do not adequately mimic the in vivo situation by underestimating in vivo bioavailability of highly lipophilic compounds unless acceptors, such as serum proteins, are added to the receiving buffer. A series of novel 2,4-diamino-pyrrolo[2,3-d]pyrimidines were described as potent inhibitors of iron-dependent lipid peroxidation, and proved to be cytoprotective in cell culture models of oxidative injury and neuroprotective in brain injury and ischemia models Structural determinants of permeability and partitioning are discussed for a series of structurally similar homologs. In addition, detailed studies were conducted concurrently with two radiolabled compounds from the pyrrolopyrimidine series representing different physicochemical, permeability, and cell partitioning attributes to discern the roles of protein binding and cell partitioning on permeation and to complement ongoing pharmacological and pharmacokinetic studies. The data proved useful in predicting which compounds were most likely to leave the blood and penetrate underlying tissue. In a companion paper, brain uptake dynamics and cellular penetration of these compounds are confirmed in viv
Rate coefficients for the endothermic reactions C+(^2P)+H2(D2)→CH^+(CD^+)+H(D) as functions of temperature from 400–1300 K
This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/106/24/10.1063/1.474093.We have measured the bimolecular rate coefficients for the reactions of C+(2P) with H2 and D2 as functions of temperature from 400 to 1300 K using a high temperatureflowing afterglow apparatus. The temperature dependences of these rate coefficients are accurately fit by the Arrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependences have been deduced by combining the present data with previous drift tube and ion beammeasurements. We found that reactant rotational energy and translational energy are equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neutral reactant to the v=1 state enhances the rate coefficients by a factor of ∼1000 for the reaction with H2 and by ∼6000 for the reaction with D2 at temperatures of 800 and 500 K, respectively. This vibrational enhancement is larger than the enhancement that would be produced if the same amount of energy were put into translational and/or rotational modes of the reactants. In addition, rate coefficients have been derived for the three-body association reaction of C+(2P) with H2 in a helium buffer over the temperature range 300–600 K