Capillary electrophoresis/visible‐LED induced fluorescence of tryptophan: What's new?

Tryptophane (Trp) labelled by 3‐(4‐carboxybenzoyl)‐2‐quinolinecarboxaldehyde (CBQCA) is very difficult to identify using CE and fluorescence detection (480 nm). Why in this article some mass spectrometry experiments show that Trp is really labelled by CBQCA as Leucine (Leu)? If the maximum of UV absorption (λ_max) is the same between Leu‐CBQCA and Trp‐CBQCA, the molar extinction coefficient is around 2 fold higher for Trp‐CBQCA. The fluorescence of the Leu‐CBQCA derivative is 50 times more important than for Trp‐CBQCA. The addition of 7.5 mM of β‐cyclodextrin (β‐CD) was found to be a good mean to improve 2.1 fold the sensitivity of the Trp‐CBQCA fluorescence. Using a buffer containing SDS and β‐CD in CE, a LOD of 0.7 µM of L‐Trp can be reached and the ratio of the intensities between Leu, Isoleucine, Valine, Trp is 100, 21, 15, 1. Negative ESI^/MS and MS/MS of the labeled amino acids show that a loss of the carboxylate function takes place. In the presence of two enantiomers of Trp‐CBQCA, we have shown that this decarboxylation is not due to the derivatization process in the solution but rather occurs in the source of the mass spectrometer.     

Investigation of the reduced high-potential iron-sulfur protein from chromatium vinosum and relevant model compounds: a unified picture of the electronic structure of [Fe(4)S(4)](2+) systems through magnetic and optical studies

Magnetization measurements and variable temperature optical spectroscopy have been used to investigate, within the 4-300 K temperature range, the electronic structure of the reduced high-potential iron protein (HiPIP) from Chromatium vinosum and the model compounds (Cat)(2)[Fe(4)S(4)(SR)(4)], where RS(-) = 2,4,6-triisopropylphenylthiolate (1), 2,6-diphenylphenylthiolate (2), diphenylmethylthiolate (3), 2,4,6-triisopropylbenzylthiolate (4, 4'), 2,4,6-triphenylbenzylthiolate (5, 5'), 2,4,6-tri-tert-butylbenzylthiolate (6), and Cat(+) = (+)NEt(4) (1, 2, 3, 4', 5', 6), (+)PPh(4) (4, 5). The newly synthesized 2(2)(-), 3(2)(-), 5(2)(-), and 6(2)(-) complexes are, as 1(2)(-) and 4(2)(-), excellent models of the reduced HiPIPs: they exhibit the [Fe(4)S(4)](3+/2+) redox couple, because of the presence of bulky ligands which stabilize the [Fe(4)S(4)](3+) oxidized core. Moreover, the presence of SCH(2) groups in 4(2)(-), 5(2)(-), and 6(2)(-), as in the [Fe(4)S(4)] protein cores, makes them good biomimetic models of the HiPIPs. The X-ray structure of 2 is reported: it crystallizes in the orthorhombic space group Pcca with no imposed symmetry and a D(2)(d)()-distorted geometry of the [Fe(4)S(4)](2+) core. Fit of the magnetization data of the reduced HiPIP and of the 1, 2, 3, 4, 5, and 6 compounds within the exchange and double exchange theoretical framework leads to exchange coupling parameters J = 261-397 cm(-)(1). A firm determination of the double exchange parameters B or, equivalently, the transfer integrals beta = 5B could not be achieved that way. The obtained |B| values remain however high, attesting thus to the strength of the spin-dependent electronic delocalization which is responsible for lowest lying electronic states being characterized by delocalized mixed-valence pairs of maximum spin (9)/(2). Electronic properties of these systems are then accounted for by the population of a diamagnetic ground level and excited paramagnetic triplet and quintet levels, which are respectively J and 3J above the ground level. Optical studies of 1, 2, 4', 5', and 6 but also of (NEt(4))(2)[Fe(4)S(4)(SCH(2)C(6)H(5))(4)] and the isomorph (NEt(4))(2)[Fe(4)S(4)(S-t-Bu)(4)] and (NEt(4))(2)[Fe(4)Se(4)(S-t-Bu)(4)] compounds reveal two absorption bands in the near infrared region, at 705-760 nm and 1270-1430 nm, which appear to be characteristic of valence-delocalized and ferromagnetically coupled [Fe(2)X(2)](+) (X = S, Se) units. The |B| and |beta| values can be directly determined from the location at 10|B| of the low-energy band, and are respectively of 699-787 and 3497-3937 cm(-)(1). Both absorption bands are also present in the 77 K spectrum of the reduced HiPIP, at 700 and 1040 nm (Cerdonio, M.; Wang, R.-H.; Rawlings, J.; Gray, H. B. J. Am. Chem. Soc. 1974, 96, 6534-6535). The blue shift of the low-energy band is attributed to the inequivalent environments of the Fe sites in the protein, rather than to an increase of |beta| when going from the models to the HiPIP. The small differences observed in known geometries of [Fe(4)S(4)](2+) clusters, especially in the Fe-Fe distances, cannot probably lead to drastic changes in the direct Fe-Fe interactions (parameter beta) responsible for the delocalization phenomenon. These differences are however magnetostructurally significant as shown by the 261-397 cm(-)(1) range spanned by J. The cluster's geometry, hence the efficiency of the Femicro(3)-S-Fe superexchange pathways, is proposed to be controlled by the more or less tight fit of the cluster within the cavity provided by its environment.     

"Click chemistry" in zeolites: copper(I) zeolites as new heterogeneous and ligand-free catalysts for the Huisgen [3+2] cycloaddition

For the first time, copper(I)-exchanged zeolites were developed as catalysts in organic synthesis. These solid materials proved to be versatile and efficient heterogeneous, ligand-free catalytic systems for the Huisgen [3+2] cycloaddition. These cheap and easy-to-prepare catalysts exhibited a wide scope and compatibility with functional groups. They are very simple to use, easy to remove (by filtration), and are recyclable (up to three times without loss of activity). Investigations with deuterated alkynes and deuterated zeolites proved that this Cu(I)-zeolite-catalyzed "click" reaction exhibited a mechanism different from that reported for the Meldal-Sharpless version.     

Aqueous Ring-Opening Polymerization-Induced Self-Assembly (ROPISA) of N-Carboxyanhydrides

Reported here is the first aqueous ring-opening polymerization (ROP) of N-carboxyanhydrides (NCAs) using α-amino-poly(ethylene oxide) as a macroinitiator to protect the NCA monomers from hydrolysis through spontaneous in situ self-assembly (ISA). This ROPISA process affords well-defined amphiphilic diblock copolymers that simultaneously form original needle-like nanoparticles.     

Photoinduced Amyloid Fibril Degradation for Controlled Cell Patterning

Amyloid-like fibrils are a special class of self-assembling peptides that emerge as a promising nanomaterial with rich bioactivity for applications such as cell adhesion and growth. Unlike the extracellular matrix, the intrinsically stable amyloid-like fibrils do not respond nor adapt to stimuli of their natural environment. Here, a self-assembling motif (CKFKFQF), in which a photosensitive o-nitrobenzyl linker (PCL) is inserted, is designed. This peptide (CKFK-PCL-FQF) assembles into amyloid-like fibrils comparable to the unsubstituted CKFKFQF and reveals a strong response to UV-light. After UV irradiation, the secondary structure of the fibrils, fibril morphology, and bioactivity are lost. Thus, coating surfaces with the pre-formed fibrils and exposing them to UV-light through a photomask generate well-defined areas with patterns of intact and destroyed fibrillar morphology. The unexposed, fibril-coated surface areas retain their ability to support cell adhesion in culture, in contrast to the light-exposed regions, where the cell-supportive fibril morphology is destroyed. Consequently, the photoresponsive peptide nanofibrils provide a facile and efficient way of cell patterning, exemplarily demonstrated for A549, Chinese Hamster Ovary, and Raw Dual type cells. This study introduces photoresponsive amyloid-like fibrils as adaptive functional materials to precisely arrange cells on surfaces.     

From Chitin to Bioactive Chitooligosaccharides and Conjugates: Access to Lipochitooligosaccharides and the TMG‐chitotriomycin

The direct and chemoselective N‐transacylation of peracetylated chitooligosaccharides (COSs), readily obtained from chitin, to give per‐N‐trifluoroacetyl derivatives offers an attractive route to size‐defined COSs and derived glycoconjugates. It involves the use of various acceptor building blocks and trifluoromethyl oxazoline dimer donors prepared with efficiency and highly reactive in 1,2‐trans glycosylation reactions. This method was applied to the preparation of the important symbiotic glycolipids which are highly active on plants and to the TMG‐chitotriomycin, a potent and specific inhibitor of insect, fungal, and bacterial N‐acetylglucosaminidases.     

Carbo‐Quinoids: Stability and Reversible Redox‐Proaromatic Character towards Carbo‐Benzenes

The carbo‐mer of the para‐quinodimethane core is stable within in a bis(9‐fluorenylidene) derivative. Oxidation of this carbo‐quinoid with MnO₂ in the presence of SnCl₂ and ethanol affords the corresponding p‐bis(9‐ethoxy‐fluoren‐9‐yl)‐carbo‐benzene. The latter can be in turn converted back into the carbo‐quinoid by reduction with SnCl₂, thus evidencing a chemical reversibility of the interconversion between a pro‐aromatic carbo‐quinoid and an aromatic carbo‐benzene, and is reminiscent of the behavior of the benzoquinone/hydroquinone redox couple (in the red–ox opposite sense).     

Stereodivergent Organocatalytic Intramolecular Michael Addition/Lactonization for the Asymmetric Synthesis of Substituted Dihydrobenzofurans and Tetrahydrofurans

A stereodivergent asymmetric Lewis base catalyzed Michael addition/lactonization of enone acids into substituted dihydrobenzofuran and tetrahydrofuran derivatives is reported. Commercially available (S)‐(?)‐tetramisole hydrochloride gives products with high syn diastereoselectivity in excellent enantioselectivity (up to 99:1 d.r._syn/anti, 99 % ee_syn), whereas using a cinchona alkaloid derived catalyst gives the corresponding anti‐diastereoisomers as the major product (up to 10:90 d.r._syn/anti, 99 % ee_anti).     

‘Click chemistry’ in Cu-zeolites: a convenient access to glycoconjugates

Zeolites modified with Cu^I ions are efficient catalyst for ‘click’ reactions involving carbohydrates and aminoacid derivatives. Glycopeptides and oligosaccharides mimics as well as multivalent carbohydrate derivatives have been obtained in good to high yield using heterogeneous Cu^I-modified zeolite catalysts. Contrarily to expectation, pore sizes and internal shapes within zeolites were not a limitation and large glucosyl ditriazoles, disaccharide triazoles, and glucosylated triazolylaminoacids could easily be obtained. Such Cu^I-zeolite heterogeneous catalysts greatly facilitated products recovery, through an easy filtration-solvent evaporation sequence, thus offering a convenient alternative to current methods.     

Highly enantioselective electrophilic α-bromination of enecarbamates: chiral phosphoric acid and calcium phosphate salt catalysts

Metal-free chiral phosphoric acids and chiral calcium phosphates both catalyze highly enantio- and diastereoselective electrophilic α-bromination of enecarbamates to provide an atom-economical synthesis of enantioenriched vicinal haloamines. Either enantiomer can be formed in good yield with excellent diastereo- and enantioselectivity simply by switching the catalyst from a phosphoric acid to its calcium salt.