Time-of-flight secondary ion mass spectrometry of matrix-diluted oligo- and polypeptides bombarded with slow and fast projectiles: Positive and negative matrix and analyte ion yields, background signals, and sample aging

Human angiotensin II, chain B of bovine insulin, and porcine insulin were determined by time-of-flight secondary ion mass spectrometry under impact of approximately 25 keV Xe+ and SF5+ ion beams and approximately 100 MeV 252Cf fission fragments. Matrix-embedded samples, dissolved in a large surplus of alpha-cyano-4-hydroxycinnamic acid, were prepared by nebulizer spray deposition, neat samples by the droplet technique. It is shown that the status of the sample can be assessed by evaluating the matrix-specific features of the mass spectra. The beneficial effect of matrix isolation was small for angiotensin but large for the insulin samples, which did not show parent peaks from neat material. Negative ion yields under SF5+ impact were up to a factor of 50 higher than with Xe+. For positive secondary ions, the enhancement was much smaller. The mass spectra produced by slow ion beams or fast fission fragments were qualitatively similar. Quantitative differences include the following: with fast projectiles the yields were about 10-30 times higher than with slow ions, but similar for negative ion emission under SF5+ bombardment; the analyte-to-matrix yield ratios were higher with slow ions and up to 250 times higher than the molar analyte concentration; for analyte ions the peak-to-background ratios were higher using slow projectiles; the fraction of carbon-rich collisionally formed molecular ions was much higher with fast projectiles. Sample aging in vacuum for up to five weeks strongly reduced the yield of protonated analyte molecules ejected by slow ion impact, but not of deprotonated species. Hence protonation seems to correlate with sample "wetness" or the presence of volatile proton-donating additives.     

The friction coefficient of a Lennard-Jones fluid from the random force autocorrelation function determined as a memory function by molecular dynamics calculations

A recent molecular dynamics (MD) study showed that the friction coefficient of a simple fluid is obtainable by the integral over the autocorrelation function (ACF) of the total force of a Brownian-type particle. The results indicated that mass ratios 50M/m200 of the massive and the light particle suffice to yield accurate friction coefficients. Complementarily, we calculate the random force ACF of the light particle, which is the memory function force of the ACF of the velocity apart from a constant factor, for all the states of the Lennard-Jones system investigated previously. A detailed comparison is presented of the memory function, the total force ACF of the fluid particle, and the total force ACF of the massive particle. The MD results confirm quantitatively our theoretical predictions: (i) on a time scale corresponding to the dynamics of the massive particle the total force ACF of that particle approximates well the memory function, while there are slight differences between them on a short time scale, (ii) the total force ACF of the liquid particle deviates significantly from the memory function already after extremely short time and is thus completely useless for the determination of the friction coefficient, (iii) using the total force ACF of a heavy particle for the determination of the friction constant with mass ratios ofM/m=50 up to 200, the pseudo plateau value of the time integral is often not very noticeable, as the memory function is only approximated and the total force ACF of the massive particle has a negative part at medium times. In those cases the integration has to be extended to include the negative part.     

Photoconductivity and photoluminescence of a-Si:H at low temperature

We have studied the temperature dependence of the photoluminescence steady state and transient photoconductivity of a-Si:H down to 4 K. Below 50 K we observe photoconductivity with $\text{ημτ ? 10}{}^{\mathrm{?11}}\text{cm}{}^2\text{/}\text{V}$, which is independent of temperature and varies only little with the defect density in the films. We propose that this conduction arises predominantly from the drift of the photoexcited electrons and holes during thermalization in the extended states prior to the localization in states below the mobility edge. An important implication of the present data is that, even at helium temperatures, a considerable part of the carriers recombines in a non-geminate process.     

Gradient Consistency for Integral-convolution Smoothing Functions

Chen and Mangasarian (Comput Optim Appl 5:97–138, 1996) developed smoothing approximations to the plus function built on integral-convolution with density functions. X. Chen (Math Program 134:71–99, 2012) has recently picked up this idea constructing a large class of smoothing functions for nonsmooth minimization through composition with smooth mappings. In this paper, we generalize this idea by substituting the plus function for an arbitrary finite max-function. Calculus rules such as inner and outer composition with smooth mappings are provided, showing that the new class of smoothing functions satisfies, under reasonable assumptions, gradient consistency, a fundamental concept coined by Chen (Math Program 134:71–99, 2012). In particular, this guarantees the desired limiting behavior of critical points of the smooth approximations.     

Microarray technology as a universal tool for high-throughput analysis of biological systems

Over the last years microarray technology has become one of the principal platform technologies for the high-throughput analysis of biological systems. Starting with the construction of first DNA microarrays in the 1990s, microarray technology has flourished in the last years and many different new formats have been developed. Peptide and protein microarrays are now applied for the elucidation of interaction partners, modification sites and enzyme substrates. Antibody microarrays are envisaged to be of high importance for the high-throughput determination of protein abundances in translational profiling approaches. First cell microarrays have been constructed to transform microarray technology from an in vitro technology to an in vivo functional analysis tool. All of these approaches share a common prerequisite: the solid support on which they are generated. The demands on this solid support are thereby as manifold as the applications themselves. This review is aimed to display the recent developments in surface chemistry and derivatization, and to summarize the latest developments in the different application areas of microarray technology.