Kryoskopie

Ohne Zusammenfassung     

Conformational change in unsolvated Trp-cage protein probed by fluorescence

We report the first direct measurements of the unfolding of a protein, Trp-cage, in the gas phase using laser-induced fluorescence of protein ions in a heated quadrupole ion trap. The changes in enthalpy and entropy associated with the observed conformational change are obtained by fitting a two-state model of protein unfolding to the fluorescence intensities plotted versus temperature. The enthalpy and entropy changes for the 2+ and 3+ charge states are greater than the values measured in solution and depend on charge state.     

Studies on the mechanism of B(C(6)F(5))(3)-catalyzed hydrosilation of carbonyl functions

The strong organoborane Lewis acid B(C(6)F(5))(3) catalyzes the hydrosilation (using R(3)SiH) of aromatic and aliphatic carbonyl functions at convenient rates with loadings of 1-4%. For aldehydes and ketones, the product silyl ethers are isolated in 75-96% yield; for esters, the aldehydes produced upon workup of the silyl acetal products can be obtained in 45-70% yield. Extensive mechanistic studies point to an unusual silane activation mechanism rather than one involving borane activation of the carbonyl function. Quantitative kinetic studies show that the least basic substrates are hydrosilated at the fastest rates; furthermore, increased concentrations of substrate have an inhibitory effect on the observed reaction rate. Paradoxically, the most basic substrates are reduced selectively, albeit at a slower rate, in competition experiments. The borane thus must dissociate from the carbonyl to activate the silane via hydride abstraction; the incipient silylium species then coordinates the most basic function, which is selectively reduced by [HB(C(6)F(5))(3)](-). In addition to the kinetic data, this mechanistic proposal is supported by a kinetic isotope effect of 1.4(5) for the hydrosilation of acetophenone, the observation that B(C(6)F(5))(3) catalyzes H/D and H/H scrambling in silanes in the absence of substrate, computational investigations, the synthesis of models for proposed intermediates, and other isotope labeling and crossover experiments.     

Vibrational lifetime of bond-center hydrogen in crystalline silicon

The lifetime of the stretch mode of bond-center hydrogen in crystalline silicon is measured to be T1 = 7.8+/-0.2 ps with time-resolved, transient bleaching spectroscopy. The low-temperature spectral width of the absorption line due to the stretch mode converges towards its natural width for decreasing hydrogen concentration C(H), and nearly coincides with the natural width for C(H) approximately 1 ppm. The lifetimes of the Si-H stretch modes of selected hydrogen-related defects are estimated from their spectral widths and shown to range from 1.6 to more than 37 ps.     

Proton-inclusive cross sections from 600 MeV proton-nucleus reactions

Proton-inclusive cross sections were measured for 600 MeV proton bombardment of ²⁷Al and ¹⁸¹Ta targets using the SREL synchrocyclotron. Proton energies were measured with a plastic scintillator time-of-flight system, and a 10.2 cm thick NaI detector was used for particle identification. The results of classical many-body and knock-on calculations are compared with the data.     

Maximum stress in a tensile strip with a large hole

Measurements were made to decide between two solutions of the title problem. In question was the limiting value of the stress-concentration factor at the edge of a large hole in a tensile strip as the hole diameter approached the strip width. The results indicate that the stress-concentration factor is near two and support one of the solutions with a minor qualification.     

The reactions of metastable [C5 H10 O]+˙ ions with the oxygen on the second carbon

Nearly all [C₅H₁₀O]⁺˙ isomers with the oxygen on the second carbon are shown to interconvert with each other and lose methyl and ethylene at the threshold for dissociation. The methyls contain the carbons from the 1- and 5-positions with about equal frequency, and C(3) or perhaps C(4) about half as often as either terminal carbon. CH₃ CH₂ CH₂ CO⁺ is formed by loss of the C(1) methyl and by loss of the C(5) methyl. Hydorgen transfer between C(5) and the oxygen and between the oxygen and C(4) are facile, and 1,2-hydrogen transfers between C(3) and C(4) occur with high frequency. Extensive skeletal rearrangements also take placae by 1,2-shifts between C(2), C(3) and C(4). We attribute the occurrence of teh three-center shifts between C(2), C(3) and C(4) to the presence of considerable charge density on C(2) and C(3) in many of the [C₅H₁₀O]⁺˙ isomers. The isomerizations of [C₅H₁₀O]⁺˙ can be considered a mixture of free radical and carbocation reactions. Strong similarities exist between the isomerizations of metastable [C₅H₁₀O]⁺˙ ions with the oxygen on the second carbon and those of isomers of ionized butanoic acid, methyl butanoate and n-butanal.