Integral equation formulation for buoyancy-driven convection problems

An integral equation formulation for buoyancy-driven convection problems is developed and illustrated. Buoyancy-driven convection in a bounded cylindrical geometry with a free surface is studied for a range of aspect ratios and Nusselt numbers. The critical Rayleigh number, the nature of the cellular motion, and the heat transfer enhancement are computed using linear theory. Green's functions are used to convert the linear problem into linear Fredholm integral equations. Theorems are proved which establish the properties of the eigenvalues and eigenfunctions of the linear integral operator which appears in these equations.     

Permanently oriented antibody immobilization for digoxin determination with a flow-through fluoroimmunosensor

This paper reports a new flow-through fluoroimmunosensor, the function of which is based on antibodies immobilized on an inmunoreactor of controlled-pore glass (CPG), for determination of digoxin, used in the treatment of congestive heart failure and artery disease. The immunosensor has a detection limit of 1.20 microg L(-1) and provides high reproducibility (RSD=4.5% for a concentration of 0.0025 mg L(-1), and RSD=6.7% for 0.01 mg L(-1)). The optimum working concentration range was found to be 1.2 x 10(-3)-4.0 x 10(-2) mg L(-1). The lifetime of the immunosensor was about 50 immunoassays; if stored unused its lifetime can be extended to three months. A sample speed of about 10-12 samples per hour can be attained. Possible interference from substances with structures similar to digoxin (morphine, heroin, tebaine, codeine, pentazocine and narcotine) was investigated. No cross-reactivity was seen at the highest digoxin: interferent ratio studied (1:100). The proposed fluoroimmunosensor was successfully used to determine digoxin concentrations in human serum samples.     

Ultrafast intersystem crossing in 1-nitronaphthalene. An experimental and computational study

Previous studies have established that the major pathway for the first singlet excited state of 1-nitronaphthalene is intersystem crossing to the triplet manifold. In this contribution we present determinations of the decay of the S1 state of this compound in several solvents to establish the time scale of the multiplicity change as a function of the polarity and hydrogen-bonding ability of the solvent environment. The measurements were made with the femtosecond frequency up-conversion technique to follow the weak spontaneous molecular emission which precedes triplet formation. Our results show that in all environments the S1 lifetime is 100 fs or less, making 1-nitronaphthalene the organic compound with the fastest multiplicity change ever measured. We also show that the bathochromic shifts observed for the first absorption band imply changes in the relative energies of the singlet and triplet manifolds, which in turn manifest in a 2-fold increase of the fluorescence lifetime in cyclohexane compared with the polar solvents. Additionally, we performed excited-state calculations at the TD-DFT/ PBE0/6-311++G(d,p) level of theory with the PCM model for solvation. The TD-DFT theory identifies the presence of upper triplet states which can act as receiver states in this highly efficient photophysical pathway. Together, the experimental and theoretical results show that the dynamics of the S1 state in 1-nitronaphthalene represent an extreme manifestation of El-Sayed's rules due to a partial (n-pi*) character in the receiver triplets which are nearly isoenergetic with S1, determining a change in the molecular spin state within 100 fs.     

Excited state intramolecular proton transfer in Schiff bases. Decay of the locally excited enol state observed by femtosecond resolved fluorescence

Although the late (t>1 ps) photoisomerization steps in Schiff bases have been described in good detail, some aspects of the ultrafast (sub-100 fs) proton transfer process, including the possible existence of an energy barrier, still require experimental assessment. In this contribution we present femtosecond fluorescence up-conversion studies to characterize the excited state enol to cis-keto tautomerization through measurements of the transient molecular emission. Salicylideneaniline and salicylidene-1-naphthylamine were examined in acetonitrile solutions. We have resolved sub-100 fs and sub-0.5 ps emission components which are attributed to the decay of the locally excited enol form and to vibrationally excited states as they transit to the relaxed cis-keto species in the first electronically excited state. From the early spectral evolution, the lack of a deuterium isotope effect, and the kinetics measured with different amounts of excess vibrational energy, it is concluded that the intramolecular proton transfer in the S1 surface occurs as a barrierless process where the initial wave packet evolves in a repulsive potential toward the cis-keto form in a time scale of about 50 fs. The absence of an energy barrier suggests the participation of normal modes which modulate the donor to acceptor distance, thus reducing the potential energy during the intramolecular proton transfer.     

Human Neuroepithelial Cells Express NMDA Receptors

L-glutamate, an excitatory neurotransmitter, binds to both ionotropic and metabotropic glutamate receptors. In certain parts of the brain the BBB contains two normally impermeable barriers: 1) cerebral endothelial barrier and 2) cerebral epithelial barrier. Human cerebral endothelial cells express NMDA receptors; however, to date, human cerebral epithelial cells (neuroepithelial cells) have not been shown to express NMDA receptor message or protein. In this study, human hypothalamic sections were examined for NMDA receptors (NMDAR) expression via immunohistochemistry and murine neuroepithelial cell line (V1) were examined for NMDAR via RT-PCR and Western analysis. We found that human cerebral epithelium express protein and cultured mouse neuroepithelial cells express both mRNA and protein for the NMDA receptor. These findings may have important consequences for neuroepithelial responses during excitotoxicity and in disease.     

Ortho effects of a nitro group in 2′,3′ and 4′monosubstituted-trans-2,4-dinitrostilbenes on electron impact. IV

The mass spectral fragmentation patterns of thirteen 2′,3′ and 4′-R-trans-2,4-dinitrostilbenes obtained by electron impact have been studied. The main routes of fragmentation involves loss from the molecular ion due to the ortho effects of the 2-nitro substituents. Substitution on positions 2′,3′ and 4′ of stilbene moiety does not influence the fragmentation patterns.