Green synthesis limitations

How do we measnre success in the area of green chemistry The limited availability of raw materials, combined with environmental concerns, reqnire the inclusion of a new parameter to measure the quality of a chemical process. Trost has referred to this as atom economy (Trost, 1991). Often, however, these parameters are used to compare a new synthesis against what had been done previously, a comparison that, not snrprisingly, nsnally favors the more recent work. 

A word here on the toxicity of V compds The compds act chiefly as irritants to the con-junctivae and respiratory tract, turning the tongue green. Exptl biochemical studies show that V compds inhibit cholesterol synthesis and the activity of the enzyme cholinesterase. Prolonged exposure to V compds may lead to fatal pulminary involvement. The Threshold Limiting Values of these compds, in particular the highly toxic pentoxide, are dust, 0.5mg/CM fumes, 0.05mg/CM (Refs 7,16 18)... 

Cysteine is formed in plants and in bacteria from sulfide and serine after the latter has been acetylated by transfer of an acetyl group from acetyl-CoA (Fig. 24-25, step f). This standard PLP-dependent (3 replacement (Chapter 14) is catalyzed by cysteine synthase (O-acetylserine sulfhydrase).446 447 A similar enzyme is used by some cells to introduce sulfide ion directly into homocysteine, via either O-succinyl homoserine or O-acetyl homoserine (Fig. 24-13). In E. coli cysteine can be converted to methionine, as outlined in Eq. lb-22 and as indicated on the right side of Fig. 24-13 by the green arrows. In animals the converse process, the conversion of methionine to cysteine (gray arrows in Fig. 24-13, also Fig. 24-16), is important. Animals are unable to incorporate sulfide directly into cysteine, and this amino acid must be either provided in the diet or formed from dietary methionine. The latter process is limited, and cysteine is an essential dietary constituent for infants. The formation of cysteine from methionine occurs via the same transsulfuration pathway as in methionine synthesis in autotrophic organisms. However, the latter use cystathionine y-synthase and P-lyase while cysteine synthesis in animals uses cystathionine P-synthase and y-lyase. 

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