Time-resolved fluorescence study of the single tryptophan in thiocyanate and azide derivatives of horseradish peroxidase: Implication for a p H-induced conformational change in the heme cavity

Detailed pH-dependent steady state and picosecond time-resolved tryptophan fluorescence studies on thiocyanate and azide complexes of horseradish peroxidase have been carried out. The fluorescence decay of the single tryptophan in these species was fitted to a discrete three exponential model. Maximum entropy method analysis also gave three distinct regions of lifetime distributions. The fast subnanosecond lifetime component was found to have > 97% amplitude contribution while other two longer lifetime components have small contributions. Small contributions from the nanosecond lifetime components possibly arise from apoprotein impurity or some small amount of disordered heme conformer of the protein. pH dependence of the fast picosecond lifetime components was found to show a systematic behavior which has been interpreted in the light of obligatory conformation change associated with activation of the enzyme at low pH.     

Effect of tip functionalization on transport through vertically oriented carbon nanotube membranes

Ionic flux through a composite membrane structure, containing vertically aligned carbon nanotubes crossing a polystyrene matrix film, was studied as a function of chemical end groups at the entrance to carbon nanotubes' (CNTs) cores. Plasma oxidation during the membrane fabrication process introduced carboxylic acid groups on the CNTs' tips that were modified using carbodiimide mediated coupling between the carboxylic acid and an accessible amine groups of the functional molecule. Functionalization molecules included straight chain alkanes, anionically charged dye molecules, and an aliphatic amine elongated by polypeptide spacers. Functionalization was confirmed by FTIR spectroscopy, and areal functional density was estimated by transmission electron microscopy studies of thiol terminated sites decorated by nanocrystalline gold. The transport through the membrane of two different sized but equally charged molecules (ruthenium bipyridine [Ru-(bipy)3(2+)] and methyl viologen [MV2+]) was quantified in a U-tube permeation cell by UV-vis spectroscopy. Relative selectivity of the permeates varied from 1.7 to 3.6 as a function of tip-functionalization chemistry. Anionic charged functional groups sharply increased the flux of the cationic permeates. This effect was reduced at higher solution ionic strength consistent with shorter Debye screening length. The observed selectivities were consistent with a hindered diffusion model with functionalization at the CNT tip and not along the length of the CNT core.     

On pathwise behavior of multiserver queues

In this paper, by exploiting recent results on the pathwise behavior of the workload process in single server, work conserving queues of theG/G/1/ type, we show that the workload of multiserver, work conserving queues ofG/G/m/ (m<) (andG/G/) queues satisfies an o(t) growth condition, provided that the time average of the work brought into the system is less thanm form < (and finite form=).     

Micro-Raman investigations of myelins in aerosol-OT/water system

Surfactant outgrowth during dissolution as myelin figures, which happens on contact with water, is of prime importance in emulsification and detergency. Micro-Raman investigation of different lyotropic phases formed during dissolution of aerosol-OT (bis 2-ethylhexyl sulfosuccinate) in water during myelin formation reveals the flexible arrangement of the surfactant bilayers in myelin. The conformation around CC-CS bond and the hydrocarbon chains of aerosol-OT in the different liquid crystalline phases were identified from the fingerprints of CC-CS stretching, C-C stretching, C-H bending, and stretching frequencies. Existence of mixture of trans and gauche conformations around CC-CS bond and that of the hydrocarbon chains in myelin supports the fluid nature of bilayers by which it is made. Similar conformations of hydrocarbon chains in lamellar phase and in myelin support the concept of myelins being rolled up lamella. The observations are in line with the disorderness of the hydrocarbon chains in the bilayers of phospholipids that has been reported earlier. From the C-C stretching frequencies at the root of myelins, the kinked structure of the hydrocarbon chain is identified, and loose packing of molecules which would facilitate water transport across membranes is evident.