Optical imaging of electrical carrier injection into individual InAs quantum dots

We image the micro-electroluminescence (EL) spectra of self-assembled InAs quantum dots (QDs) embedded in the intrinsic region of a GaAs p-i-n diode and demonstrate optical detection of carrier injection into a single QD. Tunneling of electrons and holes into the QDs at bias voltages below the flat-band condition leads to a spectrum of sharp EL lines from a small number of bright spots on the diode surface, characteristic of emission from individual QDs. We explain this behavior in terms of Coulomb interaction effects and the selective excitation of a small number of QDs within the ensemble due to preferential tunneling paths for carriers.     

Twin-like domains in chiral smectic C liquid crystals

In the chiral smectic C phase of liquid crystals with the phase transition N*-SmC*, texture development depending on the sample thickness is reported. In very thin samples, domains of rectangular-like shape are observed. As two possible tilts of smectic layers are possible for one anchoring direction, smectic layers inside a domain, called twin-like domains, are tilted with respect to layers in outer regions, similarly to crystalline planes in solid crystalline twins. An elastic model of such a twin domain is proposed and its energy determined.     

Thermodynamic study of indoxylsulfate interaction with human serum albumin and competitive binding with p-cresylsulfate

Indoxylsulfate (IS) and p-cresylsulfate (PCS) are natural compounds endowed with toxicity. These molecules are harmful to the environment and removed by the procedure of dialysis. Knowledge of their interaction with biologic compounds such as proteins and particularly human serum albumin (HSA) is limited. This study was therefore designed to determine the thermodynamic parameters of the interaction of IS with HSA and in competition with PCS. Results showed that IS binding is moderate (K _a = 1,750 ± 39 M^?1). The interaction is mainly electrostatic (?H° = ?36.2 ± 1.7 kJ mol^?1) and yields a modification of conformation upon binding (?_conf S° < 0). The thermodynamic parameters obtained at different temperatures show an enthalpy–entropy compensation process. Competition with PCS reveals that affinity for IS decreased by 36 %, with profound modification of the binding forces involved and a release of PCS from the binding site.     

Enrichment specificity of micro and nano‐sized titanium and zirconium dioxides particles in phosphopeptide mapping

Owning to their anion‐exchange properties, titanium and zirconium dioxides are widely used in phosphopeptide enrichment and purification protocols. The physical and chemical characteristics of the particles can significantly influence the loading capacity, the capture efficiency and phosphopeptide specificity and thus the outcome of the analyses. Although there are a number of protocols and commercial kits available for phosphopeptide purification, little data are found in the literature on the choice of the enrichment media. Here, we studied the influence of particle size on the affinity capture of phosphopeptides by TiO₂ and ZrO₂. Bovine milk casein derived phosphopeptides were enriched by micro and nanoparticles using a single‐tube in‐solution protocol at different peptide‐to‐beads ratio ranging from 1 : 1 to 1 : 200. Unsupervised hierarchical cluster analysis based on the whole set of Matrix Assisted Laser Desorption/Ionization time‐of‐flight mass spectra of the phosphopeptide enriched samples revealed 62 clustered peptide peaks and shows that nanoparticles have considerably higher enrichment capacity than bulk microparticles. Moreover, ZrO₂ particles have higher enrichment capacity than TiO₂. The selectivity and specificity of the enrichment was studied by monitoring the ion abundances of monophosphorylated, multiphosphorylated and non‐phosphorylated casein‐derived peptide peaks at different peptide‐to‐beads ratios. Comparison of the resulting plots enabled the determination of the optimal peptide‐to‐beads ratios for the different beads studied and showed that nano‐TiO₂ have higher selectivity for phosphopeptides than nano‐ZrO₂ particles. Copyright © 2013 John Wiley & Sons, Ltd.     

Liquid crystalline comb-like polyimides with biphenyl-based side groups

We have synthesized series of comb-like polyimides with mesogenic units in their side groups. Such comb-like polyimides were obtained by polycondensation of aromatic diamines bearing biphenyl-based mesogenic moieties with bicyclo[2,2,2]oct-7-ene 2,3,5,6-tetracarboxylic dianhydride (BCDA). The different diamines, with two lengths of spacer (with 6 and 11 methylene groups), were synthesized in three steps using as mesogenic groups: biphenyl, 4-cyanobiphenyl and 4-(2-methyl-1-butoxy)biphenyl. The synthesis of the polyimides was performed in two steps: polycondensation of a dianhydride with a diamine in N-methyl-2-pyrrolidone at room temperature giving the corresponding polyamic acid, followed by thermal cyclization into the corresponding polyimide. The comb-like polyimides were studied by X-ray diffraction between room temperature and 250°C. Two types of smectic structure were established: SmA₁ for the long spacer and SmC₁ for the short spacer.     

On Perfect Codes: Rank and Kernel

We establish upper and lower bounds on the rank and the dimension of the kernel of perfect binary codes. We also establish some results on the structure of perfect codes.     

Gas phase photo-formation and vacuum UV photofragmentation spectroscopy of tryptophan and tyrosine radical-containing peptides

Tryptophan (Trp(?)) and tyrosyl (Tyr(?)) radical containing peptides were produced by UV laser-induced electron detachment from a suitable precursor. Vacuum ultraviolet (VUV) action spectra of these radical peptides were recorded with synchrotron radiation in the 4.5-16 eV range, from which fragmentation pathways and yields are measured as a function of the VUV photon energy. An enhancement in photofragmentation yields of radical species by 1 order of magnitude with respect to nonradical peptides is demonstrated here for the first time. Photofragmentation spectra are compared with absorption spectra for model chromophores calculated in the frame of the time-dependent density functional theory (TDDFT). A qualitative agreement in the position of bands in the 6-8 eV region is observed between experimental photofragmentation and calculated absorption spectra. Photofragmentation spectra of peptide radicals can be useful to better assess the complex deactivation pathways that occur following the absorption of a VUV photon in biomolecular radical anions.     

Molecular and structural characteristics in toxic algae cultures of Ostreopsis ovata and Ostreopsis spp. evidenced by FTIR and FTNIR spectroscopy

In this article we investigated the compositional and structural characteristics of the principal biomolecules such as carbohydrates, proteins, lipids, nucleic acids and chlorophyll pigments present in biofilm cultures of Ostreopsis spp. and in batch cultures of Ostreopsis ovata. Our approach based on the use of infrared (FTIR) and near infrared (FTNIR) spectroscopy showed the marked differences existing between biofilm cultures and batch cultures. FTIR spectroscopy showed the higher contents of polysaccharides and chlorophyll pigments in O. ovata from batch cultures with respect to Ostreopsis spp. Second derivative FTIR spectroscopy showed different features concerning the secondary structure of proteins because in O. ovata samples the beta sheet and beta turn structures were observed whereas in Ostreopsis spp. samples the alpha helix structure was the most evident. FTNIR spectroscopy showed other structural differences observed existing between O. ovata and Ostreopsis spp. mainly related to hydrogen bond interactions determining more packed structures in the nucleus of O. ovata. In addition, the interpretation of FTIR and FTNIR spectral information was also supported by the application of two statistical methods, the independent component analysis (ICA) and the spectral cross correlation analysis (SCCA). ICA was used as spectral deconvolution technique to separate the effects of the interference bicarbonate ion from algal FTIR spectra so to verify the high similar qualitative composition of the three biofilm samples of Ostreopsis spp. At last, SCCA applied to FTIR and FTNIR spectra was useful to evidence some structural differences involving -CH and CH(2) groups of aliphatic chains in O. ovata and Ostreopsis spp. samples. Though preliminary, these results agree with some previous studies suggesting that the presence of different ecophysiological characteristics in O. ovata and Ostreopsis spp. depending on the parameters related to the condition growth.     

CE analysis and molecular characterisation of depurinated DNA samples

A DNA sample was partially degraded by scalar heat-acid treatments to study the extent of apurinic-apyrimidinic (A-P) lesions produced along the molecule. A CE-UV method allowed us to measure the rate of depurination at pH 5.0 and 70°C which was calculated to be 5.41×10(-6) s(-1) for adenine and 6.27×10(-6) s(-1) for guanine. CE identified depurination on treated samples when it occurred with a loss of >4% of the basic moieties. The molecular features of the A-P enriched samples were investigated by using molecular assays (agarose gel electrophoresis, UV spectrophotometry and quantitative PCR) and the consistency of the results of the STR typing were compared with the degree of depurination of the PCR template. The treated DNA samples showed molecular features such as fragmentation, altered OD(260) /OD(280) ratios and decreased ability of the quantitative PCR to synthesise the human target, related to the severity of depurination. A satisfactory correlation between the degree of damage and the amount of residual PCR-sensitive target sequences was also demonstrated (r(2) =0.9717). The conventional and mini-STR typing of the samples showed that the genetic outcome was influenced by a depurination damage that exceeded 4% when locus drop-outs and artefactual PCR results were evident. As the success of STR typing depends on the integrity of the DNA recovered from the samples, the CE-UV, physical and molecular assays described here are proposed as a set of useful methods in the analysis of certain forensic and clinical samples, for a critical evaluation of the outcome of the genetic testing.     

A Branching Random Walk Seen from the Tip

We show that all the time-dependent statistical properties of the rightmost points of a branching Brownian motion can be extracted from the traveling wave solutions of the Fisher-KPP equation. The distribution of all the distances between the rightmost points has a long time limit which can be understood as the delay of the Fisher-KPP traveling waves when the initial condition is modified. The limiting measure exhibits the surprising property of superposability: the statistical properties of the distances between the rightmost points of the union of two realizations of the branching Brownian motion shifted by arbitrary amounts are the same as those of a single realization. We discuss the extension of our results to more general branching random walks.