Managing technology development for safety-critical systems

This paper presents a model that determines the optimal budget allocation strategy for the development of new technologies, for safety-critical systems, over multiple decision periods. The case of the development of a hypersonic passenger airplane is used as an illustration. The model takes into account both the probability of technology development success as a function of the allocated budget and the probability of operational performance of the final system. It assumes that the strategy is to consider (and possibly fund) several approaches to the development of each technology to maximize the probability of development success. The model, thus, decomposes the system's development process into multiple technology development modules (one for each technology needed), each involving a number of alternative projects. There is a tradeoff between development speed and operational reliability when the budget must be allocated among alternative technology projects with different probabilities of development success and operational reliability (e.g., an easily and quickly developed technology may have little robustness). The probabilities of development and operational failures are balanced by a risk analysis approach, which allows the decision maker to optimize the budget allocation among different projects in the development program, at the beginning of each budget period. The model indicates that by considering reliability in the R&D management process, the decision maker can make better decisions, optimizing the balance between development time, cost, and robustness of safety-critical systems.     

Analysis of an intact G-protein coupled receptor by MALDI-TOF mass spectrometry: molecular heterogeneity of the tachykinin NK-1 receptor

Integral membrane proteins are among the most challenging targets for biomedical research as most important cellular functions are tied to these proteins. To analyze intrinsically their structure/function, their transduction mechanism, or both, these proteins are commonly expressed in cultured cells as recombinant proteins. However, it is not possible to check whether these recombinant proteins are homogeneously or heterogeneously expressed. Owing to difficulties in their purification, very few mass spectrometry studies have been performed with those proteins and even less with G-protein coupled receptors. Here we have set up a procedure that is highly compatible with MALDI-TOF mass spectrometry to analyze an intact histidine-tagged G-protein coupled, namely, the tachykinin NK-1 receptor expressed in CHO cells, solubilized and purified using cobalt or nickel chelating magnetic beads. The metal-chelating magnetic beads containing the receptor were directly spotted on the MALDI plate for analysis. SDS-PAGE, combined with in-gel digestion analyzed by mass spectrometry, Western blot ((His)6 and FLAG M2 tags), photoaffinity labeling with a radioactive agonist, and Edman sequencing, confirmed the identity of the purified protein as the human tachykinin NK-1 receptor. Mass spectrometry study of both the glycosylated and deglycosylated intact protein forms revealed the existence of several receptor species that is tempting to correlate with the unusual pharmacological behavior of the receptor.     

Reaction of Zirconium Fluoride Monohydrate and Ammonium Bifluoride: Its Effect on Fluoride Glass Preparation and Quality

The products obtained from the roomtemperature reaction of ammonium bifluoride and zirconium fluoride monohydrate are ammonium heptafluorozirconate ((NH₄)₃ZrF₂), liquid water, and hydrogen fluoride. Ammonium bifluoride and zirconium fluoride monohydrate were reacted prior to glass batching, producing dry ammonium heptafluorozirconate which was used to prepare a high-quality ZBLAN fluoride glass.     

Sequence-dependent enrichment of a model phosphopeptide: a combined MALDI-TOF and NMR study

The goal of this study was to detect and quantify by MALDI-TOF MS the phosphorylation of a peptide containing the recognition motif of the Protein Kinase C (PKC). Such model peptide can be used as a phosphorylation probe to follow intracellular kinase/phosphatase activities. This study allowed us to establish relationships between sequence specificities and affinity for TiO(2) or IMAC media. The peptide has the sequence biotin-GGGGCFRTPSFLKK-NH(2) in which the serine residue can be phosphorylated. Enrichment of the corresponding phosphopeptide, by the dedicated IMAC and TiO(2) affinity chromatography methods, proved inefficient. By combining MALDI-TOF and NMR data, we first showed that the lack of affinity of the phosphopeptide for TiO(2) was partly related to the basic property of its peptide sequence. Furthermore, the peptide shows local structuration around the P(9)- S(10) segment, with formation of a salt bridge between the guanidinium group of the R(7) side chain and the phosphate moiety. In conjunction with an inadequate position of the {biotin-G(4)} N-terminal tag, this local structure could shield the phosphate group, preventing interaction with TiO(2). To improve TiO(2) affinity, the peptide sequence was modified accordingly. The new sequences retained the biological properties while their enrichment by IMAC or TiO(2) became possible.     

New Fluorination Reactions of Ammonium Bifluoride

This paper describes a novel route for the production of low-oxygen-containing fluorozirconate glasses. The process involves the use of a hitherto unknown solid-state chemical reaction between ammonium bifluoride and specific anhydrous and hydrated metal fluoride salts. The products are defined as ammonium fluoride–metal fluoride complexes. The kinetics of their formation and subsequent dissociation were followed by thermogravimetric analysis. It was observed that these complexes decompose with the evolution of HF above temperatures at which ammonium bifluoride decomposes and where its supply may be exhausted. These materials have been used to melt fluorozirconate glasses. The oxygen contents of these glasses have been dramatically reduced to approximately 10 ppm. The glass stability, as defined by the lack of devitrification on reheating, also appears to increase with a lowering of the oxygen content. This implies that the mechanism of nucleation and growth is predominantly heterogeneous and not homogeneous nucleation. Furthermore, the production of long lengths of crystal-free fluoride fibers for transcontinental optical communication systems appears to be a realistic goal, especially if the heterogeneities can be eliminated.     

Effect of Aluminum Fluoride on the Physical Properties and Stability of Fluorozirconate and Fluorozirco-Hafnate Glasses

The glass quality, physical properties, and thermal stability of fluorozirconate and fluorozirco-hafnate glasses were investigated as a function of the AIF₃ content. The AlF₃ concentration varied from 2 to 3.25 mol%. The ratio of the other components was constant, except for 50% substitution of ZrF₄ by HfF₄ in the fluorozirco-hafnate glasses. The physical properties of all the glasses were not affected strongly by the AlF₃ content. However, the fluorozirco-hafnate glasses were prone to precipitate out AlF₃ microcrystals for AlF₃ contents greater than 3 mol%. The thermal stability of the glasses revealed a compositional dependence only in the case of the fluorozirco-hafnate glasses. The thermal stability decreased as the AlF₃ content increased above 3 mol%. The authors concluded that the AlF₃ microcrystals nucleated other fluoride phases such as zirconium-barium fluoride crystals.     

Delays and safety in airline maintenance

Airline maintenance operations affect the potential for flight delays and can also affect flight safety if signals of technical problems are missed or misinterpreted. In this paper, we use a probabilistic risk analysis model, represented by an influence diagram, to quantify the effect of an airline's maintenance policy on delays, cancellations and in-flight safety. The model represents the leading edge (LE) sub-system of a commercial passenger jet and consists of three tiers: (1) a set of management decision variables (e.g. the level of qualification of maintenance personnel); (2) a ground model linking policy decisions and flight delays; and (3) an in-flight model, linking policy decisions, maintenance quality and flight safety. To illustrate this model, we use data adapted (for confidentiality reasons) from a study of an existing airline. Clearly, the LE devices of an airplane are not among the most safety-critical and the risk of an accident due to poor maintenance is extremely small, but non-zero. The same model can be used for other, more critical parts of the aircraft to support maintenance policy decisions in which the trade-off between delays and safety may be more pronounced.     

Isotope and affinity tags in photoreactive substance P analogues to identify the covalent linkage within the NK-1 receptor by MALDI-TOF analysis

Photoreactive analogues of substance P (biotin sulfone-spacer (amino pentanoic or Gly(3))-Arg-Pro-Lys-Pro-(pBzl)Phe-Gln-Phe-Phe-Gly-Leu-Met(O(2))NH(2)) with or without isotope (deuterium) labeling have been synthesized. Deuteriums were present on (d)-biotin or epibiotin sulfone (D(3)), on the Gly(3) spacer linker (D(6)), or on the Gly in position 9 of SP (D(2)). Therefore, peptide analogues could be either unlabeled or tri-, penta-, or hexadeuterated. Results obtained with the use of these peptide analogues show that (d)-biotin sulfone and epibiotin sulfone are not recognized with the same affinity by streptavidin, with (d)-biotin sulfone displaying better affinity for the protein. Photolabeling of the human NK-1 receptor with a 1:1 molar ratio of nondeuterated and deuterated photoreactive substance P (SP) analogues in position 5, followed by combined digestions, purification, and MALDI-TOF mass spectrometry analysis, made the identification of the domain of the receptor covalently linked by the photoreactive SP analogue easier. Indeed, doublets in mass spectra were specific for the covalent complex whereas single peaks could be attributed to contaminating species. This method is particularly suitable when minute amounts of complex have to be analyzed, as in the case of highly hydrophobic G-protein coupled receptors.