Thiol‐disulfide redox equilibria of glutathione metaboloma compounds investigated by tandem mass spectrometry

The thiol group of cysteine plays a pivotal role in structural and functional biology. We use mass spectrometry to study glutathione‐related homo‐ and heterodimeric disulfides, aiming at understanding the factors affecting the redox potentials of different disulfide/thiol pairs. Several electrospray ionization (ESI)‐protonated disulfides of cysteamine, cysteine, penicillamine, N‐acetylcysteine, N‐acetylpenicillamine, γGluCySH, HSCyGly, and glutathione were analyzed on a triple quadrupole instrument to measure their energy‐resolved tandem mass spectra. Fission of the disulfide bond yields RSH*H⁺ and RS⁺ ions. The logarithm of the intensity ratio of the RS⁺/RSH*H⁺ fragments in homodimeric disulfides is proportional to the normal reduction potential of their RSSR/RSH pairs determined by nuclear magnetic resonance (NMR) in solution, the more reducing ones yielding the higher ratios. Also in some R₁S‐SR₂ disulfides, the ratio of the intensities of the RSH + H⁺ and RS⁺ ions of each participating thiol shows a linear relationship with the Nernst equation potential difference of the corresponding redox pairs. This behavior allows us to measure the redox potentials of some disulfide/thiol pairs by using different thiol‐reducing probes of known oxidoreductive potential as reference. To assist understanding of the fission mechanism of the disulfide bond, the fragments tentatively identified as ‘sulfenium’ were themselves fragmented; accurate mass measurement of the resulting second‐generation fragments demonstrated a loss of thioformaldehyde, thus supporting the assigned structure of this elusive intermediate of the oxidative stress pathway. Understanding this fragmentation process allows us to employ this technique with larger molecules to measure by mass spectrometry the micro‐redox properties of different disulfide bonds in peptides with catalytic and signaling biological activity. Copyright © 2008 John Wiley & Sons, Ltd.     

Competitive analysis of a dispatch policy for a dynamic multi-period routing problem

We analyze an on-line algorithm (dispatch policy) for a dynamic multi-period routing problem. The objective is to minimize the total cost over all periods. We show that the competitive ratio of this policy for instances with customers located on the non-negative real line is .     

Ultrastructural and functional characterization of circulating hemocytes from the freshwater crayfish Astacus leptodactylus: cell types and their role after in vivo artificial non-self challenge

The freshwater crayfish Astacus leptodactylus (Eschscholtz, 1823) is an important aquacultured decapod species as well as an invasive species in some European countries. In the current investigation we characterized the different classes of circulating blood cells in A. leptodactylus by means of light and electron microscopy analysis and we explored their reaction to different latex beads particles in vivo by total and differential cell counts at 0.5, 1, 2 and 4h after injections. We identified hemocytes by granule size morphometry as hyaline hemocytes with no or rare tiny granules, small granule hemocytes, unimodal medium diameter granule hemocytes and both small and large granule containing hemocytes. The latter granular hemocytes showed the strongest phenoloxidase l-DOPA reactivity both in granules and cytoplasm. A. leptodactylus respond to foreign particles with strong cellular immune responses. All treatments elicited a total hemocyte increase with a conspicuous recruitment of large granule containing hemocytes. All hemocyte types mounted some phagocytic response but the small granule hemocytes were the only ones involved in phagocytic response to all foreign particles with the highest percentages. These results (1) depict the variability in decapod hemocyte functional morphology; (2) identify the small granule hemocyte as the major phagocytic cell; (3) suggest that the rather rapid recruitment of large granule hemocyte in all treatments plays a relevant role by this hemocyte type in defense against foreign particles, probably in nodule formation.     

Dynamic instabilities of microwaves generated with optoelectronic oscillators

We introduce a time-domain model to study the dynamics of optoelectronic oscillators. We show that, due to the interaction between nonlinearity and time delay, the envelope amplitude of ultrapure microwaves generated by optoelectronic oscillators can turn unstable when the gain is increased beyond a given critical value. Our analytical predictions are confirmed by numerical simulations and experiments.     

The vibrational spectra of the cyanide ligand revisited. Bridging cyanides

The nu(CN) vibrational spectra of cyanide groups bridging two metal atoms present a confused picture. Factors relevant to the interpretation of the available data are reviewed. Some mechanisms for frequency change, relative to the corresponding terminal species, are made more quantitative than previously described, and others are highlighted for the first time. The kinematic effect is much less important than previously assumed. It seems that an effect responsible for the major part of the frequency increase upon bridging commonly observed is the cation electric field, together with concomitant relaxation (the IVSE model). However, a contribution may well come from the change in the sigma bonding in the CN unit.     

Dynamics of cell membrane passive depolarization: a phase portrait

Does a persistent blockage of the ionic pumps bring cell membrane voltage to zero? This apparently trivial question of basic cellular Biology stirred up an intriguing problem of nonlinear dynamics. A 3-ion model based on continuity and charge conservation proves that membrane voltage actually sets on a negative value, meaning that chemical equilibrium is never reached, rather an inversion of the Na+ concentration gradient occurs, usually hours after the blockage of the pumps. Experimental tests carried out with PC12 cells incubated with Oubaine for a period of 24 hours show an increase of cytosolic Na+ of about 266 mM/l with respect to a control sample. The result is compatible with an inversion of the Na+ gradient, which eventually brings the membrane voltage to a negative value. Reactivation of the Na+-K+ pumps even after a prolonged period of blockage (late repolarization) should lead to repolarization and revival of the cell. In the 3D space of the ionic concentrations, the dynamics of passive depolarization reveals an intriguing topology, all trajectories being stacked in parallel planes, each set ending to a unique fixed point via an infinitely dense set of lines. The dynamics of repolarization has a different phase portrait, especially in the case of late repolarization. Thus, a sequence of depolarization- repolarization cycles may result in a path wandering in the phase space, or in a closed loop, depending on the timing of the sequence.     

Inference and Learning in a Latent Variable Model for Beta Distributed Interval Data

Latent Variable Models (LVMs) are well established tools to accomplish a range of different data processing tasks. Applications exploit the ability of LVMs to identify latent data structure in order to improve data (e.g., through denoising) or to estimate the relation between latent causes and measurements in medical data. In the latter case, LVMs in the form of noisy-OR Bayes nets represent the standard approach to relate binary latents (which represent diseases) to binary observables (which represent symptoms). Bayes nets with binary representation for symptoms may be perceived as a coarse approximation, however. In practice, real disease symptoms can range from absent over mild and intermediate to very severe. Therefore, using diseases/symptoms relations as motivation, we here ask how standard noisy-OR Bayes nets can be generalized to incorporate continuous observables, e.g., variables that model symptom severity in an interval from healthy to pathological. This transition from binary to interval data poses a number of challenges including a transition from a Bernoulli to a Beta distribution to model symptom statistics. While noisy-OR-like approaches are constrained to model how causes determine the observables’ mean values, the use of Beta distributions additionally provides (and also requires) that the causes determine the observables’ variances. To meet the challenges emerging when generalizing from Bernoulli to Beta distributed observables, we investigate a novel LVM that uses a maximum non-linearity to model how the latents determine means and variances of the observables. Given the model and the goal of likelihood maximization, we then leverage recent theoretical results to derive an Expectation Maximization (EM) algorithm for the suggested LVM. We further show how variational EM can be used to efficiently scale the approach to large networks. Experimental results finally illustrate the efficacy of the proposed model using both synthetic and real data sets. Importantly, we show that the model produces reliable results in estimating causes using proofs of concepts and first tests based on real medical data and on images.     

Asymptotic Resonance, Interaction of Modes and Subharmonic Bifurcation

We study the existence of small amplitude oscillations near elliptic equilibria of autonomous systems, which mix different normal modes. The reference problem is the Fermi-Pasta-Ulam β-model: a chain of nonlinear oscillators with nearest-neighborhood interaction. We develop a new bifurcation approach that locates secondary bifurcations from the unimodal primary branches. Two sufficient conditions for bifurcation are given: one involves only the arithmetic properties of the eigenvalues of the linearized system (asymptotic resonance), while the other takes into account the nonlinear character of the interaction between normal modes (nonlinear coupling). Both conditions are checked for the Fermi-Pasta-Ulam problem.