NMR spectra of a microcrystalline protein at 30 kHz MAS

Proteins are not always available in amounts desirable for solid-state magic-angle spinning (MAS) nuclear-magnetic resonance (NMR) spectroscopy. To maximize the signal-to-noise ratio achievable with small samples, the filling factor must be optimized by using small-diameter MAS rotors. These rotors have the added benefit of allowing higher radio frequency field amplitudes during polarization transfer steps and during decoupling periods as well as allowing higher spinning frequencies. We demonstrate the advantages of relatively fast MAS (30 kHz using a 2.5 mm rotor) compared to MAS at 12 kHz for the 10.4 kDa model protein Crh with 93 residues and show that the signal-to-noise ratio in two-dimensional correlation spectra can be significantly improved by taking advantage of optimized pulse sequences available with rapid MAS.     

Optimized ensemble Monte Carlo simulations of dense Lennard-Jones fluids

We apply the recently developed adaptive ensemble optimization technique to simulate dense Lennard-Jones fluids and a particle-solvent model by broad-histogram Monte Carlo techniques. Equilibration of the simulated fluid is improved by sampling an optimized histogram in radial coordinates that shifts statistical weight towards the entropic barriers between the shells of the liquid. Interstitial states in the vicinity of these barriers are identified with unprecedented accuracy by sharp signatures in the quickly converging histogram and measurements of the local diffusivity. The radial distribution function and potential of mean force are calculated to high precision.     

Hexaphosphapentaprismane: a new gateway to organophosphorus cage compound chemistry

Several independent synthetic routes are described leading to the formation of a novel unsaturated tetracyclic phosphorus carbon cage compound tBu4C4P6 (1), which undergoes a light-induced valence isomerization to produce the first hexaphosphapentaprismane cage tBu4C4P6 (2). A second unsaturated isomer tBu4C4P6 (9) of 1 and the bis-[W(CO)5] complex 13 of 1 are stable towards similar isomerization reactions. Another starting material for the synthesis of the hexaphosphapentaprismane cage tBu4C4P6 (2) is the trimeric mercury complex [(tBu4C4P6)Hg]3 (11), which undergoes elimination of mercury to afford the title compound 2. Single-crystal X-ray structural determinations have been carried out on compounds 1, 2, 9, 11, and 13.     

Structural integration of tellurium oxide into mixed-network-former glasses: connectivity distribution in the system NaPO(3)-TeO(2).

Sodium phosphate tellurite glasses in the system (NaPO(3))(x)(TeO(2))(1-) (x) were prepared and structurally characterized by thermal analysis, vibrational spectroscopy, X-ray photoelectron spectroscopy (XPS) and a variety of complementary solid-state nuclear magnetic resonance (NMR) techniques. Unlike the situation in other mixed-network-former glasses, the interaction between the two network formers tellurium oxide and phosphorus oxide produces no new structural units, and no sharing of the network modifier Na(2)O takes place. The glass structure can be regarded as a network of interlinked metaphosphate-type P(2) tetrahedral and TeO(4/2) antiprismatic units. The combined interpretation of the O 1s XPS data and the (31)P solid-state NMR spectra presents clear quantitative evidence for a nonstatistical connectivity distribution. Rather, the formation of homoatomic P--O--P and Te--O--Te linkages is favored over mixed P--O--Te connectivities. As a consequence of this chemical segregation effect, the spatial sodium distribution is not random, as also indicated by a detailed analysis of (31)P/(23)Na rotational echo double-resonance (REDOR) experiments.     

From a biogenetic scenario to a synthesis of the ABC ring of manzamine A

On the basis of a biogenetic proposal for explaining the biogenesis of manzamine A, the cycloaddition of dihydropyridinium salt 26 with diene derivative 5 leads to adducts 27. These adducts, as well as their related and previously described analogues 9, are now shown to be precursors of diene derivatives such as 10, 13, and 28. Treatment of diene 32 with sodium azide resulted in a one-step formation of the tricyclic imino derivative 34. This key intermediate was further transformed into tricyclic derivative 40, which possesses the essential features of the ABC ring of manzamine A.     

A novel solid support for the synthesis of 3-aminoalkylated oligonucleotides

A novel improved controlled pore glass (CPG) support based on the 2-(hydroxymethyl)-6-nitrobenzoyl (HMNB) protecting group was developed for the synthesis of 3^′-aminoalkylated oligonucleotides. The release of oligonucleotides with free 3^′-amino groups from the support is complete within 2 h at 55 °C in concentrated ammonia.     

Edge spreading lithography and its application to the fabrication of mesoscopic gold and silver rings

A new form of edge lithography, edge spreading lithography (ESL), has been demonstrated and applied to the formation of coinage metal rings. In this process, alkanethiols are delivered from a flat PDMS stamp to the surface of a metal film through a two-dimensional array of spherical silica colloids. The thiols further spread on the metal surface, forming highly ordered SAMs in the form of a ring pattern. Following lift-off of beads, the pattern in the SAMs can be transferred into the metal film through wet chemical etching, with SAMs serving as the resist. The dimensions of the rings can be readily controlled by several parameters such as the beads diameter, the concentration of the thiol solution, and the contact time between the stamp and the silica beads.     

Probing the Conformational Changes in the Enzymatic Hydrolysis of 2‐Acetamido‐2‐deoxy‐β‐D‐glucopyranosides

The isoquinuclidines 7 and 8 were synthesised and tested as inhibitors of hexosaminidases from jack beans and from bovine kidney. These isoquinuclidines mimick the ^1,4B‐conformer of a N‐acetyl‐glucosamine‐derived β‐d‐ glucopyranoside; they are competitive inhibitors with K_i values from 0.014 to 0.30 μM . The strong inhibition of these enzymes agrees with the hypothesis that the enzymatic hydrolysis of 2‐acetamido‐2‐deoxy‐β‐d‐ glucopyranosides proceeds via a boat‐like conformer with a pseudo‐axial scissile glycosidic bond and a pseudo‐axial acetamido substituent optimally oriented to effect an intramolecular substitution of the aglycon.     

Multiplexed hybridizations of positively charge-tagged peptide nucleic acids detected by matrix-assisted laser desorption/ionization time-of-flight mass spectrometry

Peptide nucleic acid (PNA) is a novel class of DNA analogues in which the entire sugar-phosphate backbone is replaced by a pseudopeptide counterpart. Owing to its neutral character and the consequent lack of electrostatic repulsion, PNA exhibits very stable heteroduplex formation with complementary nucleic acid that is essentially ionic strength independent and enables hybridization under minimum salt conditions. This feature as well as its superior ion stability and easy ionization compared to DNA renders PNA very attractive for hybridization-based matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOFMS) applications. We have developed an approach to DNA characterization that takes advantage of multiplexed PNA hybridizations analyzed by MALDI-TOFMS. Our motivation was the further development of oligonucleotide fingerprinting, an efficient technique for cDNA and genomic DNA library characterization. Through positive 'charge-tagging' of PNA the efficiency of detection in MALDI-TOFMS was considerably enhanced permitting an unparalleled degree of multiplexing. Results from the simultaneous hybridization of 21 charge-tagged PNA hexamer oligonucleotides showed that genomic DNA and cDNA clones are successfully characterized on the basis of their hybridization profiles. The degree of multiplexing achieved may render a significant increase in throughput and hence efficiency of oligonucleotide fingerprinting possible.     

Synthesis and Evaluation as Glycosidase Inhibitors of Isoquinuclidines Mimicking a Distorted β‐Mannopyranoside

Racemic and enantiomerically pure manno‐configured isoquinuclidines were synthesized and tested as glycosidase inhibitors. The racemic key isoquinuclidine intermediate was prepared in high yield by a cycloaddition (tandem Michael addition/aldolisation) of the 3‐hydroxy‐1‐tosyl‐pyridone 10 to methyl acrylate, and transformed to the racemic N‐benzyl manno‐isoquinuclidine 2 and the N‐unsubstituted manno‐isoquinuclidine 3 (twelve steps; ca. 11% from 10 ). Catalysis by quinine of the analogous cycloaddition of 10 to (?)‐8‐phenylmenthyl acrylate provided a single diastereoisomer in high yield, which was transformed to the desired enantiomerically pure D ‐manno‐isoquinuclidines (+)‐ 2 and (+)‐ 3 (twelve steps; 23% from 10 ). The enantiomers (?)‐ 2 and (?)‐ 3 were prepared by using a quinidine‐promoted cycloaddition of 10 to the enantiomeric (+)‐8‐phenylmenthyl acrylate. The N‐benzyl D ‐manno‐isoquinuclidine (+)‐ 2 is a selective and slow inhibitor of snail β‐mannosidase. Its inhibition strength and type depends on the pH (at pH 4.5: K_i=1.0 μM , mixed type, α=1.9; at pH 5.5: K_i=0.63 μM , mixed type, α=17). The N‐unsubstituted D ‐manno‐isoquinuclidine (+)‐ 3 is a poor inhibitor. Its inhibition strength and type also depend on the pH (at pH 4.5: K_i=1.2?10³ μM , mixed type, α=1.1; at pH 5.5: K_i=0.25?10³ μM , mixed type, α=11). The enantiomeric N‐benzyl L ‐manno‐isoquinuclidine (?)‐ 2 is a good inhibitor of snail β‐mannosidase, albeit noncompetitive (at pH 4.5: K_i=69 μM ). The N‐unsubstituted isoquinuclidine (?)‐ 2 is a poor inhibitor (at pH 4.5: IC₅₀=7.3?10³ μM ). A comparison of the inhibition by the pure manno‐isoquinuclidines (+)‐ 2 and (+)‐ 3 , (+)‐ 2 /(?)‐ 2 1 : 1, and (+)‐ 3 /(?)‐ 3 1 : 1 with the published data for racemic 2 and 3 led to a rectification of the published data. The inhibition of snail β‐mannosidase by the isoquinuclidines 2 and 3 suggests that the hydrolysis of β‐D ‐mannopyranosides by snail β‐mannosidase proceeds via a distorted conformer, in agreement with the principle of stereoelectronic control.