Biochemical stabilization, carbon

The toxicity of organic fluorine compounds depends on their biochemical stability and the toxicity of metabolites. The carbon-fluorine bond is highly resistant to biochemical degradation and, as a consequence, perfluoroearbons are physiologically inactive. However, if a fluorochemical is metabolized, the toxicity of the metabolized fluorocompound depends on the structure of the metabolites. Monofluorinated alkanoic acids are toxic when the j8-oxidation mechanism can produce monofluoroacetic acid [2]. Steric hindrance and branching of the alkane chain reduce toxicity (Table 10.1). 

The pentagon stabilization has been found in a biochemical phenomenon [80], The hydrogen on the thiazolium ring 9 (Scheme 7) is easily ionized to afford the corresponding carbene 10, a key catalyst in enzymatic reactions for which thiamine (vitamin B-1,11) pyrophosphate is the cofactor. The pentagon stability is expected to contribute to this unusual deprotonation. A lone pair generated on the carbon atom in 10 can similarly delocalize through the vicinal C-N and C-S a bonds in a cyclic manner. 

The fundamental biochemical function of pyridoxal-5 -phosphate is the formation of aldimines with a-amino acids that stabilize the development of carbanionic character at the a and /3 carbons of a-amino acids in intermediates, such as those in figures 10.4b and c. Enzymes acting alone cannot stabilize these carbanions and so cannot, by themselves, catalyze reactions requiring their formation as intermediates. 

Rumpel, C., Eusterhues, K., and Kogel-Knabner, I. (2004b). Location and chemical composition of stabilized organic carbon in topsoil and subsoil horizons of two acid forest soils. Soil Biol. Biochem. 36,177-190. 

Mclnerney M. and Bolger T. (2000b) Temperature, wetting cycles and soil texture effects on carbon and nitrogen dynamics in stabilized earthworm casts. Soil Biol. Biochem. 32(3), 335-349. 

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