Protection of the indole nucleus of tryptophan in solid-phase peptide synthesis with a dipeptide that can be cleaved rapidly at physiological pH

The synthesis of a new derivative of tryptophan Fmoc-Trp(Boc-Sar-Sar)-OH is described. Fmoc-Trp(Boc-Sar-Sar)-OH is introduced into peptides by solid-phase peptide synthesis and during treatment with TFA at the end of the synthesis, the Boc group is cleaved and the peptide is obtained with the indole nucleus modified with the sarcosinyl-sarcosinyl (Sar-Sar) moiety. This modification will introduce a cationic charge that improves the solubility of the peptide during HPLC purification. The Sar-Sar moiety is cleaved upon exposure to physiological pH. The Boc-Sar-Sar group might, therefore, also find use in the design of pro-drugs of indole-containing compounds.     

Monitoring the aggregation of single casein micelles using fluorescence microscopy

The aggregation of casein micelles (CMs) induced by milk-clotting enzymes is a process of fundamental importance in the dairy industry for cheese production; however, it is not well characterized on the nanoscale. Here we enabled the monitoring of the kinetics of aggregation between single CMs (30-600 nm in diameter) by immobilizing them on a glass substrate at low densities and subsequently imaging them with fluorescence microscopy. We validated the new method by a quantitative comparison to ensemble measurements of aggregation. Single-particle statistics allowed us to observe for the first time several heterogeneities in CM aggregation. We observed two types of CM growth: a slow increase in the size of CMs and a stepwise increase attributed to interactions between aggregates preformed in solution. Both types of growth exhibit a lag phase that was very heterogeneous between different CMs, suggesting significant differences in their composition or structure. Detailed size histograms of CMs during aggregation also revealed the presence of two distinct subpopulations with different growth amplitudes and kinetics. The dependence of these distinct nanoscale processes/parameters on aggregation conditions is not accessible to bulk measurements that report only ensemble-average values and may prove important to an in-depth understanding of CM aggregation.     

Quantitation of surface-bound proteins on biochips using MALDI-TOF MS

We report on a novel method for the quantitation of proteins specifically bound on a ligand-presenting biochip by matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometry (MS). The bound protein was digested by trypsin, and the resulting peptide fragments were analyzed by MALDI-TOF MS in the presence of an isotope-labeled internal standard (IS). The IS has the same sequence as a reference peptide (RP) of the target protein digest, but a different molecular weight. The absolute amount of the specifically bound protein on a biochip is then quantitated by comparison of mass intensities between the RP and the IS. Because they have the same molecular milieu, the mass intensities of these two analytes represent the real amounts of analytes on the chip. As a model system, we tested glutathione s-transferase (GST) and a GST-fusion protein, which were captured on glutathione-presenting biochips. We observed that the glutathione densities on biochips showed a good correlation with the absolute quantity of the proteins. We believe that our method will provide an alternative to currently existing tools for the absolute quantitation of surface-bound proteins.     

Size control and catalytic activity of bio-supported palladium nanoparticles

The development of nanoparticles has greatly improved the catalytic properties of metals due to the higher surface to volume ratio of smaller particles. The production of nanoparticles is most commonly based on abiotic processes, but in the search for alternative protocols, bacterial cells have been identified as excellent scaffolds of nanoparticle nucleation, and bacteria have been successfully employed to recover and regenerate platinum group metals from industrial waste. We report on the formation of bio-supported palladium (Pd) nanoparticles on the surface of two bacterial species with distinctly different surfaces: the gram positive Staphylococcus sciuri and the gram negative Cupriavidus necator. We investigated how the type of bacterium and the amount of biomass affected the size and catalytic properties of the nanoparticles formed. By increasing the biomass:Pd ratio, we could produce bio-supported Pd nanoparticles smaller than 10nm in diameter, whereas lower biomass:Pd ratios resulted in particles ranging from few to hundreds of nm. The bio-supported Pd nanoparticle catalytic properties were investigated towards the Suzuki-Miyaura cross coupling reaction and hydrogenation reactions. Surprisingly, the smallest nanoparticles obtained at the highest biomass:Pd ratio showed no reactivity towards the test reactions. The lack of reactivity appears to be caused by thiol groups, which poison the catalyst by binding strongly to Pd. Different treatments intended to liberate particles from the biomass, such as burning or rinsing in acetone, did not re-establish their catalytic activity. Sulphur-free biomaterials should therefore be explored as more suitable scaffolds for Pd(0) nanoparticle formation.     

Copper-catalyzed highly regioselective silylcupration of terminal alkynes to form α-vinylsilanes

A highly regioselective synthesis of branched vinylsilanes through silicon-copper additions to terminal alkynes catalyzed by copper(I) was developed using methanol as additive. The corresponding vinylsilanes were obtained with excellent branched to linear selectivity of up to 99/1 in good yields.     

Determination of hexavalent chromium (Cr(VI)) in plastics using organic-assisted alkaline extraction

An organic-assisted alkaline extraction method was developed for the determination of hexavalent chromium (Cr(VI)) in plastics. The solubilization of polymer as a pre-step of the alkaline extraction provided good extraction efficiency of Cr(VI) from the sample. The optimization of the experimental conditions affecting the extraction and UV–vis spectrophotometric analysis was accomplished by evaluating the recovery rate of Cr(VI) through the analysis of Cr(VI) in in-house polymer reference materials (RMs). With the proposed method, most of the Cr(VI) in polymers was released within a short extraction time of 30 min and the Cr(III)-DPCO complex can be kept stable for 24 h. The heating for the extraction of the Cr(VI) was not necessary. The optimal pH of the final solution was fixed at 2.0. The proposed extraction method was applied successfully for the determination of Cr(III) and Cr(VI) in spiked samples. The practical applicability of this new method was evaluated through the analysis of Cr(VI) in in-house polymer RMs. The good linearity was demonstrated at desired concentrations of the range 0–3.3 mg L⁻¹. The detection limits were quite low, varying from 0.0061 to 0.0285 mg L⁻¹. The recovery of Cr(VI) was between 97 and 106%, and the relative standard deviation (R.S.D.) was below 6%.     

Synthesis of a difunctional orthogonal coupler for the preparation of carbohydrate-functionalized sP(EO-stat-PO) hydrogels

The synthesis and characterization of a new difunctional coupler (4) based on trimethylolpropane (TMP) are described. The coupler is used to connect biologically active N-acetylglucosamine (GlcNAc) on amino-reactive microtiter plates and on star-shaped poly[(ethylene oxide)-stat-(propylene oxide)] hydrogel layers in microtiter plates. The biological activity of immobilized carbohydrates is determined using an enzyme-linked lectin assay.     

On the Poisson distribution of lengths of lattice vectors in a random lattice

We prove that the volumes determined by the lengths of the non-zero vectors ±x in a random lattice L of covolume 1 define a stochastic process that, as the dimension n tends to infinity, converges weakly to a Poisson process on the positive real line with intensity ${\frac{1}{2}}$ . This generalizes earlier results by Rogers (Proc Lond Math Soc (3) 6:305?C320, 1956, Thm. 3) and Schmidt (Acta Math 102:159?C224, 1959, Satz 10).     

A q-analogue of the FKG inequality and some applications

Let L be a finite distributive lattice and μ: L → ℝ⁺ a log-supermodular function. For functions k: L → ℝ⁺ let

Perfect Matchings in Total Domination Critical Graphs

A graph is total domination edge-critical if the addition of any edge decreases the total domination number, while a graph with minimum degree at least two is total domination vertex-critical if the removal of any vertex decreases the total domination number. A 3 _t EC graph is a total domination edge-critical graph with total domination number 3 and a 3 _t VC graph is a total domination vertex-critical graph with total domination number 3. A graph G is factor-critical if G − v has a perfect matching for every vertex v in G. In this paper, we show that every 3 _t EC graph of even order has a perfect matching, while every 3 _t EC graph of odd order with no cut-vertex is factor-critical. We also show that every 3 _t VC graph of even order that is K _1,7-free has a perfect matching, while every 3 _t VC graph of odd order that is K _1,6-free is factor-critical. We show that these results are tight in the sense that there exist 3 _t VC graphs of even order with no perfect matching that are K _1,8-free and 3 _t VC graphs of odd order that are K _1,7-free but not factor-critical.