A new highly efficient three-component domino Heck-Diels-Alder reaction with bicyclopropylidene: rapid access to spiro[2.5]oct-4-ene derivatives

Bicyclopropylidene (1) was found to surpass even methyl acrylate (17 a) in its rate of undergoing carbopalladation with aryl- or alkenylpalladium species, leading to substituted allylidenecyclopropanes 5, 7 and 10, mostly in high yields (37-78 %). These dienes and cross-conjugated trienes react in a Diels-Alder mode with dienophiles to give spiro[2.5]octenes 18 a-Ph, 18 b-Ph and 18 a-Vin, respectively, in good yields (89, 69 and 65 %). The overall transformation can be achieved as a one-pot three-component reaction with a variety of dienophiles to furnish the domino Heck-Diels-Alder products 18 regioselectively in most cases in good to very high yields (49-100 %). The reaction of 1 with iodobenzene (2-Ph) and 17 a gave 18 a-Ph in virtually quantitative yield-also on a gram scale-using only 1 mol % of catalyst, and even bromobenzene (22) gave 18 a-Ph in 59 % yield. Bicyclopropylidene (1), in the presence of palladium acetate/triphenylphosphane underwent rearrangement to allylidenecyclopropane (5-H), which in turn dimerized (73 %) in the absence of other reaction partners, or could be trapped by diethyl fumarate (17 c) to give the Diels-Alder adduct 18 c-H in 45 % yield. The coupling of oligoiodobenzenes with 1 and subsequent cycloaddition could be extended to a multicomponent reaction. In this way, 1,4-diiodobenzene (37), 1 and an alkyl acrylate gave the products 38 of a twofold Heck-Diels-Alder reaction in up to 87 % yield, 1,3,5-triiodobenzene (39) reacted in up to 72 % yield and ultimately 1,2,4,5-tetraiodobenzene (41) gave the fourfold domino Heck-Diels-Alder product 42 in 47 % isolated yield, in a single operation in which 12 new carbon-carbon bonds were formed.     

Regioselectively Modified Cellulose and Chitosan Derivatives for Mono- and Multilayer Surface Coatings of Hemocompatible Biomaterials

In this study different synthetic strategies were developed and applied to introduce solely or in combination heparin/heparansulfate-like functional groups such as N-sulfo, O-sulfo, N-acetyl, and N-carboxymethyl groups into chitosan and cellulose with highest possible regioselectivity and completeness and defined distribution along the polymer chain. Completely substituted 6-amino-6-deoxycellulose and related derivatives were prepared from tosylcellulose (DS 2.02; C₆ 1.0) by nucleophilic substitution with azido groups only in the 6-position at 50 °C with subsequent reduction to amino groups and completely removing tosyl groups in the 2,3-position. 2,6-Di-O-sulfocellulose was prepared using the reactivity difference between C-2, C-6 and C-3 of cellulose. The reactivity difference between amino groups and hydroxyl groups was used to prepare various N-substituted derivatives. Partially 2,6-di-O-sulfated cellulose was obtained from trimethylsilylcellulose by the insertion of sulfurtrioxide into the Si–O ether linkage. Partially 3-O-sulfocellulose was synthesized by protecting C-2 and C-6 with trifluoroacetyl groups. A copper–chitosan complex was used to synthesize 6-O-sulfochitosan with a DS of 1.0 at C-6 and various partially 6-O-desulfonated products are possible. Using the phthalimido group to increase the solubility of chitosan in DMF, the regioselectivity of 3-O-sulfo groups was improved by regioselective 6-O-desulfonation of nearly complete 3,6-O-disulfochitosan. The platelet adhesion properties of immobilized regioselectively modified water-soluble derivatives on membranes have been tested in vitro. Some regioselectively modified chitosan and cellulose derivatives are potential candidates for the surface coatings of biomaterials if the regioselective reactions are somewhat further optimized.     

Detection of coexisting protein conformations in capillary zone electrophoresis subsequent to transient contact with sodium dodecyl sulfate solutions

Non-native conformations of proteins were generated by temporary contact with aqueous solutions of sodium dodecyl sulfate (SDS) and separated from the native state with capillary zone electrophoresis (CZE) in alkaline borate buffer deficient of SDS. Nine proteins at concentrations of 2.0 or 3.0 mg.L(-1) were compared in terms of their susceptibility to SDS. For superoxide dismutase and ferritin the tendency of unfolding was modest with < 25% of the protein being transformed to the non-native state at 10 mmol.L(-1) SDS. Highest susceptibility was observed for albumin, myoglobin (Mb), and hemoglobin with > 75% in the non-native state even at 2.0 mmol.L(-1) SDS. The influence of varying SDS concentrations on the conformational state of Mb was tested. Increasing the SDS concentration, circular dichroism revealed a reduction in alpha-helix, an increase in random coil, and an introduction of beta-sheet, which is absent in native structure. Modifications in the secondary structure were in agreement with distinct changes in the shape of the non-native Mb peak in CZE and make a gradual unfolding/refolding process with several coexisting molten globules instead of two-state transition of conformations most plausible for Mb. CZE was found to contribute to a further understanding of holo-Mb transformation towards a population of non-native conformations (i) by means of calculated peak area ratios of native to non-native states, which showed sigmoid transition, (ii) by detecting the release of the prosthetic heme group, and (iii) by changes in the effective electrophoretic mobility of the Mb-SDS peaks. Reconstituted holo-Mb forms differed in the Soret band around 410 nm, indicating diversity in the conformation of the heme pocket.     

Advances in the analysis of proteins and peptides by capillary electrophoresis with matrix-assisted laser desorption/ionization and electrospray-mass spectrometry detection

High throughput, outstanding certainty in peptide/protein identification, exceptional resolution, and quantitative information are essential pillars in proteome research. Capillary electrophoresis (CE) coupled to mass spectrometry (MS) has proven to meet these requirements. Soft ionization techniques, such as matrix-assisted laser desorption/ionization (MALDI) and electrospray ionization (ESI), have paved the way for the story of success of CE-MS in the analysis of biomolecules and both approaches are subject of discussion in this article. Meanwhile, CE-MS is far away from representing a homogeneous field. Therefore the review will cover a vast area including the coupling of different modes of CE (capillary zone electrophoresis, capillary isoelectric foscusing, capillary electrochromatography, micellar electrokinetic chromatography, nonaqueous capillary electrophoresis) to MS as well as on-line preconcentration techniques (transient capillary isotachophoresis, solid-phase extraction, membrane preconcentration) applied to compensate for restricted detection sensitivity. Special attention is given to improvements in interfacing, namely addressing nanospray and coaxial sheath liquid design. Peptide mapping, collision-induced dissociation with subsequent tandem MS, and amendments in mass accuracy of instruments improve information validity gained from MS data. With 2-D on-line coupling of liquid chromatography (LC) and CE a further topic will be discussed. A special section is dedicated to recent attempts in establishing CE-ESI-MS in proteomics, in the clinical and diagnostic field, and in the food sector.     

Point sets with distinct distances

For positive integersd andn letf _d (n) denote the maximum cardinality of a subset of then ^d -gird {1,2,...,n} ^d with distinct mutual euclidean distances. Improving earlier results of Erds and Guy, it will be shown thatf ₂ (n)c·n ^2/3 and, ford3, thatf _d (n)c _d ·n ^2/3 ·(lnn)^1/3, wherec, c _d >0 are constants. Also improvements of lower bounds of Erds and Alon on the size of Sidon-sets in {1²,22²,...,n ²} are given.Furthermore, it will be proven that any set ofn points in the plane contains a subset with distinct mutual distances of sizec ₁·n ^1/4, and for point sets in genral position, i.e. no three points on a line, of sizec ₂·n ^1/3 with constantsc ₁,c ₂>0. To do so, it will be shown that forn points in ² with distinct distancesd ₁,d ₂,...,d _t , whered _i has multiplicitym _i , one has _i=1 ^t m _i ² c·n _3.25 for a positive constantc. If then points are in general position, then we prove _i=1 ^t m _i ² c·n ₃ for a positive constantc and this bound is tight.Moreover, we give an efficient sequential algorithm for finding a subset of a given set with the desired properties, for example with distinct distances, of size as guaranteed by the probabilistic method under a more general setting.     

On Erdős-Rado numbers

In this paper new proofs of the Canonical Ramsey Theorem, which originally has been proved by Erd?s and Rado, are given. These yield improvements over the known bounds for the arising Erd?s-Rado numbersER(k; l), where the numbersER(k; l) are defined as the least positive integern such that for every partition of thek-element subsets of a totally orderedn-element setX into an arbitrary number of classes there exists anl-element subsetY ofX, such that the set ofk-element subsets ofY is partitioned canonically (in the sense of Erd?s and Rado). In particular, it is shown that $$2^{c1} .l^2 \leqslant ER(2;l) \leqslant 2^{c_2 .l^2 .\log l} $$ for every positive integerl≥3, wherec ₁,c ₂ are positive constants. Moreover, new bounds, lower and upper, for the numbersER(k; l) for arbitrary positive integersk, l are given.     

A new derivatization strategy for the analysis of phosphopeptides by precursor ion scanning in positive ion mode

Although numerous strategies have been devised to analyze protein phosphorylation, an abundant intracellular protein modification, there is still a need for different methods for the analysis of this modification. A method to both detect and localize the phosphorylation within a protein/peptide is especially required. In this paper, a new strategy is described, which makes use of beta-elimination/Michael addition reactions to introduce a functional group at the original site of phosphorylation, which gives rise to a dimethylamine-containing sulfenic acid derivative with a unique m/z value. This enables the detection of the phosphorylated species within peptide mixtures by sensitive and specific precursor ion scanning in positive ion mode. Working under acidic conditions in positive ion mode has the added advantage that subsequent normal peptide sequencing for the exact localization can be performed. No other peptide derived fragment ion is observed at the m/z value of the sulfenic acid derivative formed, thus specific precursor ion experiments can also be carried out on instruments with low fragment ion resolution and lends itself to LC-MS/MS approaches when skimmer fragmentation routines or triple quadrupole mass spectrometers are used.