Formation of spanning water networks on protein surfaces via 2D percolation transition

The formation of spanning hydrogen-bonded water networks on protein surfaces by a percolation transition is closely connected with the onset of their biological activity. To analyze the structure of the hydration water at this important threshold, we performed the first computer simulation study of the percolation transition of water in a model protein powder and on the surface of a single protein molecule. The formation of an infinite water network in the protein powder occurs as a 2D percolation transition at a critical hydration level, which is close to the values observed experimentally. The formation of a spanning 2D water network on a single rigid protein molecule can be described by adapting the cluster analysis of conventional percolation studies to the characterization of the connectivity of the hydration water on the surface of finite objects. Strong fluctuations of the surface water network are observed close to the percolation threshold. Our simulations also furnish a microscopic picture for understanding the specific values of the experimentally observed hydration levels, where different steps of increasing mobility in the hydrated powder are observed.     

Extremal hexagonal chains

Some extremal properties of the linear chainL _h ofh hexagons are pointed out. In the class of all hexagonal chains withh hexagons,L _h has minimumK,Z andx ₁ values, as well as maximum W and values;K = number of perfect matchings,Z = number of independent edge sets (Hosoya index),x ₁ = largest graph eigenvalue,W = Wiener index, = number of independent vertex sets (Merrifield-Simmons index). The extremality ofL _h with respect toZ, andx ₁ is demonstrated here for the first time.Dedicated to Frank Harary, teacher, inspirer and friend, pioneer, champion and proprietor of graph theory, on the occasion of his 70th birthday.     

On the mechanism of THF catalyzed vinylic lithiation of allylamine derivatives: structural studies using 2-D and diffusion-ordered NMR spectroscopy

N-Lithio-N-(trialkylsilyl)allylamines can be deprotonated in the presence of ethereal solvents exclusively at the cis-vinylic position to yield 3,N-dilithio-N-(trialkylsilyl)allylamines under mild conditions. Low temperature (1)H and (7)Li NMR ((1)H NOESY, TOCSY, (1)H/(7)Li HSQC, and DO-NMR) studies on the solution structure of 3,N-dilithio-N-(tert-butyldimethylsilyl)allylamine identified three major aggregates in THF (monomer, dimer and tetramer), but the aggregate structures failed to explain the solvent dependence and regiochemical outcome of the reaction. Low temperature (1)H NMR (NOESY, TOCSY, DO-NMR) studies on the solution structure of N-lithio-N-(tert-butyldimethylsilyl)allylamine in the presence of nBuLi identified amide/nBuLi mixed aggregates in both the ethereal solvent THF (1:1 dimer) and the hydrocarbon solvent toluene (1:3 tetramer). Addition of 2 equiv of THF to toluene solutions induces the formation of the same THF solvated 1:1 dimer as observed in neat THF. NMR evidence suggests that in THF the mixed aggregate has close contact between the olefin and the beta-CH(2) of nBuLi, while in the absence of THF, the allyl chain appears to be pointed away from the nearest nBuLi residues.     

Transition metal methylene complexes : III. Methylene (CH2), a carbonyl analogous ligand; crystal structure of μ-methylenebis(carbonyl-η5-cyclopentadienylrhodium)(Rh—Rh

μ-Carbonylbis(carbonyl-ν⁵-cycopentadienylrhodium)(Rh—Rh) reacts with N-methyl-and N-ethyl-N-nitrosourea in boiling benzene to yield the dinuclear, diamagnetic, neutral rhodium complexes μ-methylene- (A) and μ-ethylidenebis(carbonyl-η⁵-cyclopentadienylrhodium)(Rh—Rh) (B), respectively. Deuterium labelled experiments prove the origin of the metal-stabilized methylene ligand to be the alkyl group of the organic precursor. This new method of preparation of transition metal—methylene complexes may be used as an alternative to the commonly used diazo method; the latter method was shown to work with diazodiethylmalonate and dicarbonyl-η⁵-cyclopentadienylrhodium, the reaction yielding μ-bis(ethoxycarbonyl)methylenebis(carbonyl-η⁵-cyclopentadienylrhodium)(Rh—Rh).Compound A crystallizes in the triclinic system, P$\text{1}$, and with cell constants of a 803.42(5), b 909.98(6), c 938.81(2) pm, α 74.402(3), β81.923(3), and γ 83.685(6)°. The unit cell volume and the calculated density are 651.6 ų and 2.069 g cm^-3, for one molecule in the asymmetric unit. The molecular geometry of μ-CH₂[η⁵-C₅H₅Rh(CO)]₂ was established from 2718 unique reflections collected with a computer-controlled diffractometer and refined to a final R(F) = 0.0379. The molecular parameters derived from the single-crystal X-ray study conform to a remarkable degree with those found for μ-CO[η⁵-C₅H₅Rh(CO)]₂. Thus, the bridging ligands CH₂ and CO seem to be analogous in their effects on the structural characteristics of the molecular framework of the two molecules.     

2-Oxocephems. Synthesis and Properties of 7-Acylamino-2-oxocephem-4-carboxylic Acids

A synthesis of (6 R, 7 R)-7-acylamino-2-oxocephem-4-carboxylic acids from penicillins is reported based on the formation of the 2-oxocephem system in an intramolecular Wittig condensation. An interesting steric effect of the acylamino grouping on the course of this key step was used to advantage. Some physical and biological properties of this new type of Δ³-cephem are described.     

Solid state and solution conformation of 2-pyridinecarboxylic acid hydrazides: a new structural motif for foldamers

X-Ray crystallography and NMR show a strong preference for trans conformers of N′-phenyl or N′-(2-pyridyl) 2-pyridinecarboxylic acid hydrazides, stabilized by an NHN_pyr. intramolecular hydrogen bond both in the solid state and in solution. This allows us to extrapolate that oligomers of this unit should adopt extended linear conformations.     

Silylstannation of terminal alkynes using a recyclable palladium(0) catalyst immobilised in an ionic liquid

Silylstannanes can be regioselectively added across terminal alkynes in a quantitative fashion in the presence of a palladium(0) catalyst immobilised in the [bmim]PF6 ionic liquid which can be recycled without loss of activity.     

Dendrimeric Gd(III) complex of a monophosphinated DOTA analogue: optimizing relaxivity by reducing internal motion

A marked increase of relaxivity has been observed upon rigidifying the internal frame of Gd-containing PAMAM dendrimers: the effect has been attained by either protonation of the dendrimer or by forming supramolecular adducts with cationic polyaminoacids.     

Synthesis of a homoleptic niobium(V) thiolate complex and the preparation of niobium sulfide via thio "sol-gel" and vapor phase thin-film experiments

Reaction of [Nb(NMe(2))(5)] with 10 equiv of 2,6-Me(2)C(6)H(3)SH in toluene results in the formation of red crystals of [Nb(SC(6)H(3)Me(2)-2,6)(5)]. Crystal structure analysis of [Nb(SC(6)H(3)Me(2)-2,6)(5)] showed that the niobium center adopts a distorted trigonal bipyramidal geometry. Niobium disulfide, NbS(2), has been successfully prepared via a thio "sol-gel" process using [Nb(SC(6)H(3)Me(2)-2,6)(5)] as the metal source. In contrast, vapor phase thin-film studies revealed that [Nb(SC(6)H(3)Me(2)-2,6)(5)] functions as a single-source precursor to NbS films.