Observation of enhanced transmission for s-polarized light through a subwavelength slit

Enhanced optical transmission (EOT) through a single aperture is usually achieved by exciting surface plasmon polaritons with periodic grooves. Surface plasmon polaritons are only excited by p-polarized incident light, i.e. with the electric field perpendicular to the direction of the grooves. The present study experimentally investigates EOT for s-polarized light. A subwavelength slit surrounded on each side by periodic grooves has been fabricated in a gold film and covered by a thin dielectric layer. The excitation of s-polarized dielectric waveguide modes inside the dielectric film strongly increases the s-polarized transmission. A 25 fold increase is measured as compared to the case without the dielectric film. Transmission measurements are compared with a coupled mode method and show good qualitative agreement. Adding a waveguide can improve light transmission through subwavelength apertures, as both s and p-polarization can be efficiently transmitted.

Influence of tacrine on dopamine-induced reactions of the gastric smooth muscle of rats

The clinical usage of the cholinesterase inhibitor tacrine for treatment of Alzheimer's disease is accompanied by adverse effects on the gastrointestinal tract. These adverse effects are a result of the direct action of tacrine on the intestinal smooth muscles or of the modulation of certain neurotransmitters regulating gastrointestinal functions. Dopamine is a neurotransmitter that modulates gastrointestinal motility. This study was designed to examine in vitro the effects of tacrine on dopamine-induced changes in spontaneous activity of smooth muscle preparations from rat's gastric corpus. The mechanical activity was isometrically registered. Tacrine 1.10(-7)-1.10(-5) mol/l caused smooth muscle contraction, which was blocked by atropine 1.10(-6) mol/l. Tacrine 1.10(-4) mol/l provoked a relaxation resistant to atropine. Dopamine and D(2)-receptor antagonists haloperidol and R121 had no effect on tacrine-induced relaxation. Dopamine-induced contraction was concentration-dependent. It was blocked by D(2)-receptor antagonists haloperidol and R121 and by tacrine 1.10(-4) mol/l. In the presence of tacrine 1.10(-7)-10(-5) mol/l or atropine the dopamine-induced contraction was significant. The data obtained suggested that tacrine 1.10(-4) mol/l inhibited the dopamine effects on gastric corpus smooth muscles. The effect was probably not dependent on its anticholinesterase activity or not realized through direct influence on D(2)-dopamine receptors.

Electrostatic coupling to pH-titrating sites as a source of cooperativity in protein-ligand binding

This paper describes an alternative mechanism for the cooperative binding of charged ligands to proteins. The ligand-binding sites are electrostatically coupled to protein side chains that can undergo protonation and deprotonation. The binding of one ligand alters the protein's protonation equilibrium in a manner that makes the the binding of the second ligand more favorable. This mechanism requires no conformational change to produce a cooperative effect, although it is not exclusive of conformational change. We present a theoretical description of the mechanism, and calculations on three kinds of systems: A model system containing one protonation site and two ligand-binding sites; a model system containing two protonation sites and two ligand-binding sites; and calbindin D9k, which contains two Ca2+-binding sites and 30 protonation sites. For the one-protonation-site model, it is shown that the influence of the protonation site can only be cooperative. The competition of this effect with the anticooperative effect of ligand-ligand repulsion is studied in detail. For the two-protonation site model, the effect can be either cooperative or, in special cases, anticooperative. For calbindin D9k, the calculations predict that six protonation sites in or near the ligand-binding sites make a cooperative contribution that approximately cancels the anticooperative effect of Ca2+-Ca2+ repulsion, accounting for more than half of the total cooperative effect that is needed to overcome repulsion and produce the net cooperativity observed experimentally. We argue that cooperative mechanisms of the kind described here are likely when there is more than one ligand-binding site in a protein domain.

A seven-years experience in treatment of childhood ALL in Bulgaria

After analyzing nonsatisfactory therapeutic results in the 1970s and early 1980s, the 81-01 treatment protocol of the Dana-Farber Cancer Institute was initiated in 1987 in the Children's Oncohaematology Clinic in Sofia, Bulgaria. Two hundred thirty patients were enrolled with a period of observation of a minimum of 14 and a maximum of 97 months; the median age was 5.83 +/- 3.6 years. According to the original criteria, standard risk (SR) patients were 38.26% and high-risk (HR) patients 61.74%. The probability for event-free survival at the seventh year estimated with the Kaplan-Meier method for the total group was 0.67 +/- 0.04 (+/- SE) and 0.55 +/- 0.09 and 0.81 +/- 0.06 for HR and SR, respectively (P < .001). Improvement of therapeutic results in terms of remission failures, early deaths, patients lost to follow-up, and rate of relapses is discussed.

Computer simulation of excitation-contraction coupling in cardiac muscle. A study of the regulatory role of calcium binding to troponin C

The influence of a change of troponin concentration as well of a change of binding and dissociation of Ca2+ ions to the regulatory protein troponin C on the time course of isometric tension has been studied, using a mathematical model developed to investigate excitation-contraction coupling in cardiac muscle cells. The numerical simulations show that peak amplitude, rate of force development, time to peak tension and relaxation time depend significantly on the above parameters even in the case when the equilibrium dissociation constant remains unchanged. The obtained results might be useful for the planing of new experiments in the view of the fact that no similar data have been reported for cardiac muscle cells as yet.

Effects of GABAB receptor antagonists on learning and memory retention in a rat model of absence epilepsy

A variety of animal models of absence epilepsy have been described and among these exists a genetically susceptible strain of rat (genetic absence epilepsy rats of Strasbourg (GAERS)). These rats produce periods of behavioural arrest with simultaneous production of cortical spike and wave discharges (SWD). GABAB receptor antagonists suppress completely the production of these spike and wave discharges. GABAB receptor ligands have also been reported to affect cognitive performance in rodents. The present study examined the cognitive performance of GAERS and the influence of GABAB receptor antagonists on this activity. Rats were injected intraperitoneally once per day with saline or a GABAB receptor antagonist (CGP 36742 (3-amino-propyl-n-butyl-phosphinic acid) 100 mg/kg; CGP 56433 ([3-(1-(S)-[(3-(cyclohexylmethyl)hydroxy phosphinyl]-2-(S) hydroxy propyl] amino]ethyl]benzoic acid) ([3-{1-(S)-[{3-(cyclohexylmethyl)hydroxy phosphinyl]-2-(S) hydroxy propyl] amino]ethyl]benzoic acid) 1 mg/kg or CGP 61334 ([3-({[3-[(diethoxymethyl)hydroxy phosphinyl]propyl] amino}methyl]-benzoic acid (1 mg/kg). A two-way active avoidance test paradigm with negative reinforcement was used. Untreated GAERS performed significantly better than non-epileptic rats (P < 0.05) and this enhancement in cognitive performance was sustained in rats treated with the GABAB receptor antagonists.

Relationship between accessibility and reactivity of Lys, Met and Tyr in subtilisins DY and Carlsberg

The chemical reactivities of lysyl, tyrosyl and methionyl residues in subtilisin DY are compared with the computed accessibilities to the water molecules of the corresponding sensitive atoms in the crystal structure of subtilisin Carlsberg. A good correlation between accessibilities and reactivity was established for the nonionized specific reagents glyceraldehyde and chloramine T used for modification of lysine and methionine residues, respectively. In a few cases an apparent deviation from the general relationship was established. The disagreement may indicate that differences between crystal and solution structures exist with altered accessibilities of the atoms under consideration. However, when an electrically charged system takes part in the reaction and it is in low concentration, as in the case of tetranitromethane treatment where the phenoxylate anion reacts, the correlation is more complex. In this case not only the accessibility, but also the electrostatic field around the reacting group, should be taken into account for the explanation of the reaction observed.

Electrostatic properties of two porin channels from Escherichia coli

The electrostatic interactions in the channels of OmpF and PhoE porins from Escherichia coli were analysed on the basis of a macroscopic multi-dielectric model of the protein-membrane complex derived from the respective porin X-ray structures. The membrane was represented as layers of distinct dielectric constants corresponding to the aliphatic core and the polar head groups of the lipids. The pKa values of the titratable groups and the electrostatic field in the region of the channel were calculated by the finite difference technique. In spite of the differences in sequences and charge constellations, the calculated electrostatic properties of the two porins are similar in several aspects: (1) unusual titration behaviour (pKa below 7) was found for some groups of the cluster of basic residues at the constriction of the pore; (2) a number of acidic groups buried between the internal loop and the barrel wall are stabilized in their protonated forms at neutral pH; (3) there is a strong transverse electrostatic field in the channel characterized by a screw-like form. The strength of the field is greatest at the region of the constriction zone. This would facilitate the diffusion of solutes with a large dipole moment such as free amino acids. Differences between the electrostatic fields of OmpF and PhoE are mainly confined to that end of the pore that faces the cell exterior in vivo. In OmpF the electrostatic potential is close to zero in this region of the channel, whereas a positive potential was found in PhoE. It was shown that the experimentally observed difference in ion selectivity of the two porins can largely be attributed to this distinct electrostatic property.

Effect of insulin and glucagon on the mobility of ESR-probes incorporated in rat liver plasma membranes

1. The effect of different concentrations of insulin (INS) and glucagon (GLU) on the rotational mobility of a membrane-incorporated spin probe 2,2-6,6-tetramethyl-4-capriloyl piperidine-1-oxil (C7) was investigated by electron spin resonance (ESR) technique. 2. Two strong adenylate cyclase effectors guanosine 5'-triphosphate (GTP) and guanylyl 5'-imidodiphosphate [Gpp(NH)p], as well as an antioxidant 4-methyl-2,6-ditretbutilphenol (AO) and a nonprotein hormone prostaglandin E2 (PGE2) were used as reference effectors. 3. Applied effectors reduced by 30-82% the rotation correlation time (TR) of the rat liver plasma membranes spin probe C7. The effect was time-dependent and reached saturation 30-40 min after the effector application. 4. The kinetic- and concentration-dependent changes in TR were described by a simple phenomenological model. The apparent binding constants and the number of the apparent membrane binding sites of the effectors used were calculated using the model.