D" configurations

Eig. 1. Brass cathode tool (H) and anode workpiece (D) configurations for ECM where the heavy arrows indicate the direction of cathode feed at a feed rate of 0.02 m/s. (a) Initial positions where A is the initial interelectrode gap of height b, and (—>) is the direction of electrolyte flow (10 m/s). The appHed voltage is 10 V. (b) Einal positions showing the 0.4-mm steady-state gap between the electrodes. 

Polypeptides. These are a string of a-amino acids usually with the natural 5(L) [L-cysteine is an exception and has the R absolute configuration] or sometimes "unnatural" 7f(D) configuration at the a-carbon atom. They generally have less than -100 amino acid residues. They can be naturally occurring or, because of their small size, can be synthesised chemically from the desired amino acids. Their properties can be very similar to those of small proteins. Many are commercially available, can be custom made commercially or locally with a peptide synthesiser. They are purified by HPLC and can be used without further purification. Their purity can be checked as described under proteins. 

All of the structures shown in Figures 7.2 and 7.3 are D-configurations, and the D-forms of monosaccharides predominate in nature, just as L-amino acids do. These preferences, established in apparently random choices early in evolution, persist uniformly in nature because of the stereospecificity of the enzymes that synthesize and metabolize these small molecules. 

The next three steps—reduction of the /3-carbonyl group to form a /3-alcohol, followed by dehydration and reduction to saturate the chain (Figure 25.7) — look very similar to the fatty acid degradation pathway in reverse. However, there are two crucial differences between fatty acid biosynthesis and fatty acid oxidation (besides the fact that different enzymes are involved) First, the alcohol formed in the first step has the D configuration rather than the L form seen in catabolism, and, second, the reducing coenzyme is NADPH, although NAD and FAD are the oxidants in the catabolic pathway. 

All d" configurations with T ground terms give rise to magnetic moments which are lower for second- and third-row than for first-row transition elements and are temperature dependent, but in no case so dramatically as for low-spin d". ... 

The high-spin d configuration of Fe , like that of Mn , confers no advantage by virtue of CFSE (p. 1131) on any particular stereochemistry. Some examples of its consequent ability to adopt stereochemistries other than octahedral have just been mentioned and further examples are given in Table 25.3 (p. 1078). These cover the range of coordination numbers from 3 to 8. 

A partial explanation, at least, can be provided for the relative abundances and ease of formation of the above stereochemical varieties of Ni complexes. It can be seen from Table 26.6 that the CFSEs of the d configuration, unlike those of the d configuration (e.g. Co ), favour an... 

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