Cholesterol levels and activity of membrane bound proteins: characterization by thermal and electrochemical methods

The long-term goal of this investigation is to study the effects of increased cholesterol levels on the molecular activity of membrane-bound enzymes such as nitric oxide synthase, that are critical in the functioning of the cardiovascular system. In this particular investigation, we used differential scanning calorimetry (DSC) and dielectric thermal analysis (DETA) to study the effect of added cholesterol on melting/recrystallization and dielectric behavior, respectively, of phosphatidylcholine (PC) bilayered thin films. We also used electrochemical methods to investigate the effect of added cholesterol on the redox behavior of the oxygenase domain of nitric oxide synthase as a probe embedded in the PC films. The results show that added cholesterol in the PC films seems to depress the molecular dynamics as indicated by lowered current responses in the presence of cholesterol as well as a slight increase of the transition temperature in the overall two-phase regime behavior observed in PC–cholesterol films. These results are rationalized in the context of the general DSC and DETA behaviors of the PC–chol films.     

Multiple positive solutions of singular Dirichlet problems on time scales via variational methods

This paper is devoted to using variational techniques and critical point theory to derive some sufficient conditions for the existence of multiple positive solutions to a singular second order dynamic equation with homogeneous Dirichlet boundary conditions.     

Positive solutions in the sense of distributions of singular boundary value problems

We obtain positive solutions in the sense of distributions of singular boundary value problems using perturbation and variational methods.

     

Sandwich pairs for p-Laplacian systems

We solve boundary value problems for p-Laplacian systems using sandwich pairs.     

Quantum dot nanostructures for multi-band infrared detection

A dual-band (two-color) tunneling-quantum dot infrared photodetector (T-QDIP) structure, which provides wavelength selectivity using bias voltage polarity, is reported. In this T-QDIP, photoexcitation takes place in InGaAs QDs and the excited carriers tunnel through an AlGaAs/InGaAs/AlGaAs double-barrier by means of resonant tunneling when the bias voltage required to line up the QD excited state and the double-barrier state is applied. Two double-barriers incorporated on the top and bottom sides of the QDs provide tunneling conditions for the second and the first excited state in the QDs (one double-barrier for each QD excited state) under forward and reverse bias, respectively. This field dependent tunneling for excited carriers in the T-QDIP is the basis for the operating wavelength selection. Experimental results showed that the T-QDIP exhibits three response peaks at ～4.5 (or 4.9), 9.5, and 16.9 μm and selection of either the 9.5 or the 16.9 μm peak is obtained by the bias polarity. The peak detectivity (at 9.5 and 16.9 μm) of this detector is in the range of 1.0–6.0 × 10¹² Jones at 50 K. This detector does not provide a zero spectral crosstalk due to the peak at 4.5 μm not being bias-selectable. To overcome this, a quantum dot super-lattice infrared photodetector (SL-QDIP), which provides complete bias-selectability of the response peaks, is presented. The active region consists of two quantum dot super-lattices separated by a graded barrier, enabling photocurrent generation only in one super-lattice for a given bias polarity. According to theoretical predictions, a combined response due to three peaks at 2.9, 3.7, and 4.2 μm is expected for reverse bias, while a combined response of three peaks at 5.1, 7.8, and 10.5 μm is expected for forward bias.     

In situ Polymerization of Nanocomposites by TpTiCl2(Et) System: UHMWPE Filled with Carbon Nanotubes

Summary: Nanocomposites of ultra-high molecular weight polyethylene (UHMWPE) filled with multi-walled carbon nanotubes (CNT) were obtained by in situ polymerization of ethylene by TpTiCl₂Et. This novel catalytic complex activated with polymethylaluminoxane (P-MAO) (Al:Ti = 200) allowed to incorporate the CNT at different compositions (0,1-1 w/w %) into the UHMWPE matrix. The filler addition produced an important enhancement of the catalytic activity when it was compared to that of homogeneous ethylene polymerization carried out under the same experimental conditions (30 min; 1bar; 25 °C). This fact was attributed to in situ support of TpTiCl₂Et onto the CNT surface, which not only could induce the stabilization of the catalytic system but also allowed the growing of polymeric chains around the CNT structures. The characterization of these nanocomposites was carried out by Differential Scanning Calorimetry (DSC) and Thermogravimetric Analysis (TGA). Thermal analysis showed that the incorporation of CNT (at the tested concentrations) did not produce changes in the polymer thermal stability, as revealed by the initial degradation temperature values. However, the CNT produced a nucleating effect in the crystallization of UHMWPE as observed by DSC, independently of the filler content. The crystallization temperature of the obtained nanocomposites increased and the crystallinity degree slightly increased as well.     

A silica gel orthogonal high-performance liquid chromatography method for the analyses of steviol glycosides: novel tetra-glucopyranosyl steviol

A silica gel orthogonal method using acetonitrile: water was developed for the analyses of fractions rich in very polar steviol glycosides and resolve regions of co-elution of these compounds in reversed-phase. Additionally, we also used this normal phase analytical method to scale up the purification process of steviol glycosides. Using these approaches, one novel minor tetra-glucopyranosyl diterpene glycosides together with three known compounds were purified from a commercial Stevia rebaudiana leaf extract. Compound 1 was unambiguously elucidated as 13-[(2-O-β-D-glucopyranosyl-β-D-glucopyranosyl)oxy]ent-kaur-16-en-19-oic acid-(6-O-β-D-glucopyranosyl-β-D-glucopyranosyl) ester (rebaudioside Y) based on high-performance liquid chromatography retention times, tandem mass spectrometry dissociation pattern and 1D and 2D NMR experiments. Known compounds were isolated in gram quantities and identified as rebaudioside D, E and M.     

A genetically encoded infrared probe

An orthogonal tRNA/aminoacyl-tRNA synthetase pair has been evolved that makes it possible to selectively and efficiently incorporate para-cyanophenylalanine (pCNPhe) into proteins in E. coli at sites specified by the amber nonsense codon, TAG. Substitution of pCNPhe for histidine-64 in myoglobin (Mb) affords a sensitive vibrational probe of ligand binding. This methodology provides a useful infrared reporter of protein structure, biomolecular interactions, and conformational changes.