Mechanism of hydrogen production in [Fe–Fe]-hydrogenases: A quantum mechanics/molecular mechanics study

[Fe–Fe]-hydrogenases are a class of metalloenzymes that catalyze the production of H₂ from two protons and two electrons. Crystal structures for [Fe–Fe]-hydrogenases found in two species – Clostridium pasteurianum (CpI) and Desulfovibrio desulfuricans (DdH) – show very similar active sites. However, the catalytic mechanism has not as yet been fully clarified. We employed density functional theory (DFT) within a QM/MM method to investigate proposed mechanisms of hydrogen production by DdH and CpI hydrogenases and their dependence on the protein environment of the active sites. For each mechanism investigated, we found only minor differences between the CpI and DdH hydrogenases in terms of the intermediate active site structures, although one mechanism follows a lower energy path for DdH hydrogenase, while the other mechanism follows a lower energy path for the CpI hydrogenase. We note, however, that the high activation energy we calculated for a step unique to one of the mechanisms might preclude it, making the energy-path comparison for the two mechanisms unnecessary.     

Small cell carcinoma of gallbladder: report of two cases

Extrapulmonary small cell carcinoma has been reported from multiple sites, including the gallbladder. Small cell carcinoma of the gallbladder is a very rare tumor, found usually in elderly women and associated with cholelithiasis. It carries a grave prognosis, metastasizing early and causing death shortly after diagnosis. Treatment of metastatic disease with two different chemotherapeutic regimens has been shown to improve survival. To the best of our knowledge, this tumor has not been previously reported in a black individual, or in any subject less than 49 yr or more than 79 yr old. We report two cases: one is the first black and youngest reported case. The second is the oldest person reported with this rare malignancy. Radiological studies such as ultrasound and CT scan were useful in evaluating tumor spread and follow-up.     

APPLICATION OF QFT TO CONTROL SYSTEM DESIGN—AN OUTLINE FOR ENGINEERS

Control system design is an interdisciplinary and multistage process, and is not a sub-area of mathematics but rather an engineering endeavour. It behoves the engineer to reduce the real world control task to a tractable mathematical problem. The required simplifying assumptions and the attendant mathematical modelling effort require sound engineering knowledge and judgement. Furthermore, the mathematical solution of the control problem needs to be implemented, and the validity and applicability of the modelling assumptions need to be verified, by the engineer. Hence, in this paper the process of applying the QFT robust control system design method is presented from this broad perspective and it is shown that it is uniquely suited to address and solve engineering control system design problems. © 1997 by John Wiley & Sons, Ltd. This paper was produced under the auspices of the US Government and it is therefore not subject to copyright in the US.     

FUZZY SCHEDULING OF REGIONAL QFT CONTROLLERS

A novel optimization-based controller synthesis method is developed for nonlinear dynamic systems with structured parametric uncertainty. Fuzzy logic is used to smoothly schedule independently designed regional robust controllers over the plant's operational envelope. These linear controllers are synthesized using established conventional control design techniques, e.g., quantitative feedback theory. The resulting full envelope nonlinear dynamic controller handles complex dynamic systems which cannot otherwise be addressed by simple fuzzy logic control (FLC). An analytical representation of the membership functions of FLC allows the optimization to chose the location parameters of the regional controllers. The scheduled controller's valid region of operation is maximized, thus efficiently achieving full envelope operation, while guaranteeing pre-specified tracking performance. © 1997 by John Wiley & Sons, Ltd. This paper was produced under the auspices of the US Government and it is therefore not subject to copyright in the US.     

MODELLING AND CONTROL OF AN ELECTRO-HYDROSTATIC ACTUATOR

The control of a novel aerospace actuator is considered, viz., an electro-hydrostatic actuator. It offers a high degree of maintainability and combat survivability of the aircraft's flight control system because all the actuator's elements are collocated. The quantitative feedback theory robust control method is used to design a controller for this novel actuator. Parameter variation, sensor noise and flight condition variability are explicitly considered in the design process. The resulting design is not only robust with respect to actuator parameter variations and flight condition and insensitive to sensor noise, but, in addition, the controlled actuator's phase lag is significantly reduced, thus improving the performance of the overall flight control system. © 1997 by John Wiley & Sons, Ltd. This paper was prepared under the auspices of the U.S. Government and it is therefore not subject to copyright in the U.S.     

Wandering spleen with torsion in a geriatric patient. Report of an unusual case with a brief review of the clinical picture and management

Wandering spleen is a rare condition that is extremely uncommon in the elderly. A case of wandering spleen with torsion in a 73-year-old female is described in detail. Our patient's presentation is unusual because in 1972, during a total hysterectomy, she had no evidence of a wandering spleen. Preoperative technetium-99 radionuclide liver-spleen studies were utilized to minimize the potential for vascular difficulties. Splenectomy was the treatment of choice for this condition and was performed to avoid splenic infarction. A review of the literature and etiologic considerations are discussed.     

Linear-quadratic reversed Stackelberg differential games with incentives

In this technical note the linear-quadratic Stackelberg differential game with reversed information structure is considered. The leader is confined to stroboscopic (or snap-decision) strategies and necessary and sufficient conditions are then given for the leader to be able to impose, with the help of side payments, the (optimal) team solution.