The elusive benzocyclobutenylidene: a combined computational and experimental attempt.

Ab initio and density functional theory calculations predict that benzocyclobutenylidene (1) has a singlet ground state in contrast to the parent phenylcarbene and many other simply substituted arylcarbenes. Calculations also predict that 1 should lie in a relatively deep potential well, while its triplet state is 14.5 kcal mol(-)(1) higher in energy. However, attempts to observe 1 directly by photolysis of two different nitrogenous precursors were not successful. Irradiation of diazobenzocyclobutene (7) (lambda > 534 nm or lambda > 300 nm) or azibenzocyclobutene (10) (lambda > 328 nm) in Ar matrixes at 10 K leads to the formation of the strained cycloalkyne 7-methylenecyclohepta-3,5-dien-1-yne (3). (13)C-Labeled 3 was also prepared in a similar manner. There is very good agreement between experimental IR spectra and computationally derived harmonic vibrational frequencies for 3 and [(13)C]-3 and excellent agreement between observed and calculated isotopic shifts. Prolonged short-wavelength irradiation converts 3 into benzocyclobutadiene (5). Phenylacetylene (6) and benzocyclobutadiene dimer (11) were identified as products arising from flash vacuum pyrolysis of diazirine 10 at 500 degrees C.     

Reciprocity of steric and stereoelectronic effects in the collagen triple helix

In previous work, we demonstrated that 4-fluoroproline residues can contribute greatly to the conformational stability of the collagen triple helix, and that this stability arises from stereoelectronic effects that fix the pucker of the pyrrolidine ring and thereby preorganize the backbone properly for triple-helix formation. Here, we take a reciprocal approach, demonstrating that the steric effect of a 4-methyl group confers stability similar to that from a 4-fluoro group in the opposite configuration. Such fundamental interplay between steric and stereoelectronic effects is heretofore unknown in proteins-natural or synthetic-and provides a new means to modulate conformational stability.     

Magnetic properties of YbNi5 from 170Yb Mössbauer and magnetisation measurements

From Yb¹⁷⁰ M?ssbauer measurements on YbNi₅, we establish: the Yb³⁺ ground state Kramers doublet is well isolated and has uniaxial anisotropy, the compound magnetically orders at 0.55 K, the Yb³⁺ moments are aligned along the hexagonal axis, they have a saturated value of 3.9μ_B and a thermal variation close to a mean field S = 1/2 law. We obtain the strength of the Yb³⁺–Yb³⁺ coupling which we find is considerably smaller than that between the rare earths (R³⁺) in the other RNi₅. From a study of the spin dynamics, we find that dynamic short range order is present just above the ordering temperature and that above 2 K, the relaxation rate of the paramagnetic Yb³⁺ spins follows a T-linear Korringa law with a relatively pronounced slope which we link to a high density of states at the Fermi level. Magnetic susceptibility and magnetisation measurements establish the ferromagnetic nature of the Yb³⁺–Yb³⁺ coupling. We comment on the Yb³⁺ crystal field properties.     

Optimum concentration of trifluoroacetic acid for reversed-phase liquid chromatography of peptides revisited

Trifluoroacetic acid (TFA) remains the dominant mobile phase additive for reversed-phase high-performance liquid chromatography (RP-HPLC) of peptides after more than two decades since its introduction to this field. Generally, TFA has been employed in a concentration range of 0.05-0.1% (6.5-13 mM) for the majority of peptide separations. In order to revisit the question as to whether such a concentration range is optimum for separations of peptide mixtures containing peptides of varying net positive charge, the present study examined the effect of varying TFA concentration on RP-HPLC at 25 and 70 degrees C of three groups of synthetic 10-residue synthetic peptides containing either one (+1) or multiple (+3, +5) positively charged groups. The results show that the traditional range of TFA concentrations employed for peptide studies is not optimum for many, perhaps the majority, of peptide applications. For efficient resolution of peptide mixtures, particularly those containing peptides with multiple positive charges, our results show that 0.2-0.25% TFA in the mobile phase will achieve optimum resolution. In addition, the use of high temperature as a complement to such TFA concentration levels is also effective in maximizing peptide resolution.     

Preparative reversed-phase liquid chromatography of proteins from rabbit skeletal troponin,a multi-protein complex

A reversed-phase high-performance liquid chromatography protocol for purification of all proteins in a multi-protein (TnI, TnC, TnT, tropomyosin) complex from rabbit skeletal muscle has been developed, enabling efficient purification of sample amounts ranging from 43 mg of protein complex on a standard analytical column, to 1400 mg on a column of 21.2 mm I.D. and finally, to 5700 mg on a column of 50 mm I.D. Due to problems associated with scale-up procedures for these proteins (e.g. aggregation and/or solubility issues), an initial sample fractionation was devised whereby 50% of the TnC component was precipitated with acetonitrile prior to sample introduction on the RPLC column. By subsequently taking advantage of sample overload conditions to enhance the displacement effect between sample components, coupled with very slow gradient conditions (0.1% acetonitrile/min), we were able to achieve excellent protein separations at high yields of purified proteins.     

Perfluorinated membranes as catalyst supports

The perfluorinated polymer Nafion and porous PTFE/Nafion composite membranes have been employed as supports for nickel complexes or for platinum and palladium metal particles. The resultant materials have been employed as catalysts in various olefin conversion processes. Supported platinum and palladium metal systems were evaluated as catalysts for the hydrogenation of cyclohexene. Rates of reaction are better than those of commercially available catalysts; turnover numbers in excess of 6000 have been obtained with no poisoning apparent. Catalysts may be regenerated many times. The reduction rate approaches a limit at high pressures of hydrogen and has an activation energy of 13 kJ mol^?1 in neat cyclohexene. Nafion was employed as a strong acid cocatalyst to activate and then support a nickel complex catalyst. The resultant catalyst was active for double-bond-shift isomerization.     

Mössbauer study of super and semi-conducting samples of57Fe-doped (La2−y Sr y ) CuO4−δ

From Mössbauer and resistivity measurements on (La_2?y Sr _y ) CuO_4?δ samples doped with 5/1000⁵⁷Fe, three successive phases are observed depending on they value: a semiconducting antiferromagnetic phase fory<0.02; a semiconducting phase with two-dimensional spin-glass Cu magnetic order for 0.02<y<0.07; and a superconducting phase fory>0.07. The Mössbauer data show that a Cu spin-glass order survives in the superconducting phase up toy～0.15. Fory>0.15, a different type of magnetic ordering is observed on⁵⁷Fe, which is clearly governed by the iron impurities interacting through the Cu sublattice.