Academic literature on the topic 'Bi-polar functions'

Unlike other group transfer reactions in biochemistry, the actions of nitrogen transferring enzymes do not follow a single unifying chemical principle. Nitrogen-transferring enzymes catalyze aminotransfer, amidotransfer, and amidinotransfer. An aminotransferase catalyzes the transfer of the NH2 group from a primary amine to a ketone or aldehyde. An amidotransferase catalyzes the transfer of the anide-NH2 group from glutamine to another group. These reactions proceed by polar reaction mechanisms. Aminomutases catalyze 1,2-intramolecular aminotransfer, in which an amino group is inserted into an adjacent C—H bond. The action of lysine 2,3-aminomutase, described in chapter 7, is an example of an aminomutase that functions by a radical reaction mechanism. Tyrosine 2,3-aminomutase also catalyzes the 2,3-amino migration, but it does so by a polar reaction mechanism. In this chapter, we consider NH2-transferring enzymes that function by polar reaction mechanisms. Transaminases or aminotransferases are the most extensively studied pyridoxal-5'-phosphate (PLP)–dependent enzymes, and many aminotransferases catalyze essential steps in catabolic and anabolic metabolism. In the classic transaminase reaction, aspartate aminotransferase (AAT) catalyzes the fully reversible reaction of L-aspartate with α-ketoglutarate according to fig. 13-1 to form oxaloacetate and L-glutamate. Like all aminotransferases, AAT is PLP dependent, and PLP functions in its classic role of providing a reactive carbonyl group to function in facilitating the cleavage of the α-H of aspartate and the departure of the α-amino group of aspartate for transfer to α-ketoglutarate (Snell, 1962). PLP in the holoenzyme functions in essence to stabilize the α-carbanions of L-aspartate or L-glutamate, the major biological role of PLP discussed in chapter 3. The functional groups of the enzyme catalyze steps in the mechanism, such as the 1,3-prototropic shift of the α-proton to C4' of pyridoxamine 5'-phosphate (PMP). The steady-state kinetics corresponds to the ping pong bi bi mechanism shown at the bottom of fig. 13-1. This mechanism allows L-aspartate to react with the internal aldimine, E=PLP in fig. 13-1, to produce an equivalent of oxaloacetate, with conversion of PLP to PMP at the active site (E.PMP), the free, covalently modified enzyme in the ping pong mechanism.

In their seminal work The Empire Writes Back Ashcroft et al. (1989) identify code-switching between two or more codes in post-colonial literary texts as ‘the most common method of inscribing alterity’ (p.72). Ashcroft (2001) further develops the idea of installing cultural distinctiveness in the text and posits that, together with a wide range of other linguistic devices (e.g. neologisms, ethno-rhythmic prose), the use of code-switching – whether between the variants of the same language or between languages – has a metonymic function to inscribe cultural difference. In this chapter, I will examine the hybrid nature of post-colonial literary texts through the concepts of nativisation (Kachru, 1982a, 1982b, 1984, 1986, 1987, 1995) and indigenisation (Zabus, 1991, 2007). I will then focus on code-switching, adopting Myers-Scotton’s (1993) approach of matrix language vs. embedded language and considering that ‘EL [embedded language] material of any size, from a single morpheme or lexeme to several constituents, may be regarded as CS [code-switching] material’ (p.5). I will analyse examples of code-switching taken from modern Ghanaian English-language novels and short stories, and I will argue that a synecdochic relationship exists between the code-switched embedded language and the culture it originates from. I will contend that it is along the metonymic gap thus created by language variance that readers can be expected to be divided. I will briefly examine the types of authorial assistance that can be provided in order to make the text accessible to the reader, and I will illustrate, in Sperber and Wilson’s (1995) relevance theoretical framework, how different groups of readers cope with code-switched language left in the texts untranslated and/or unexplained. I will argue that by withdrawing assistance from the reader, the author makes it manifest that he concedes ‘the importance of meanibility’ (Ashcroft, 2001, p.76) and opts for the inscription of difference. I will conclude that the metonymic gap is not a simple bi-polar concept between coloniser and colonised culture but a multi-layered entity where the readers’ position in relation to the gap is indicative of their ability to interpret code-switched language unaided. Full appreciation of the writer’s meanings is shown by those readers who share both the writer’s cultural and linguistic experience. Other readers may be able to cross the metonymic gap to various degrees, but for them code-switched language will be the symbol of the writer’s difference of experience.

In this study, a free vibration analysis of a polymer electrolyte membrane fuel cell (PEMFC) is performed by modelling the PEMFC as a composite plate structure. The membrane, gas diffusion electrodes, and bi-polar plates are modelled as composite material plies. Energy equations are derived based on the Mindlin plate theory, and natural frequencies and mode shapes of the PEMFC are calculated using finite element modelling. A parametric study is conducted to investigate how the natural frequency varies as a function of thickness, Young’s modulus, and density for each component layer. It is obse

The electrical contact resistance between gas diffusion layers and bi-polar flow channel plates is one of the important factors contributing to the operational voltage loss in fuel cells. Effective analysis and design of fuel cells therefore need to account for the contact resistance in deriving the polarization curve for the cell. Despite its significance, relatively scant work is reported in the open literature on the measurement and modeling of the contact resistance in fuel cell systems, and the present work aims to fill this void. Experimental data are reported for the first time to show

Abstract The increasing popularity of flip-chips brings new challenges to those who must perform device analysis (1). Its ability to accommodate high pin-count and high bandwidth microprocessors, DSPs and complex logic devices is increasing the demand for this technology. Conventional e-beam and mechanical probing techniques currently allow quick and efficient analysis of conventional semiconductor devices. When the surface of the device is not exposed, however, conventional analysis techniques are insufficient and new techniques must be developed. Conventional packaging technologies allow des