Condensation reactions biological

Dissolved humic substances (DHS) are the main constituents of the dissolved organic carbon (DOC) pool in surface waters (freshwaters and marine waters), groundwaters, and soil porewaters and commonly impart a yellowish-brown color to the water system. Despite the different origins responsible for the main structural characteristics of DHS, they all constitute refractory products of chemical and biological degradation and condensation reactions from plant or animal residues and play a crucial role in many biogeochemical processes. 

Naturally lignins that have been modified by chemical reactions, e.g. condensed or sulfonated by pulping, are far less susceptible to biological decay than natural lignins, because the weak, readily hydrolyzable benzyl aryl ether bonds or the readily oxidisable benzyl alcohols of guaiacyl-glycerol units have been sulfonated or transformed by condensation reactions into strong carbon-carbon bonds. 

Other indices measure a secondary stage of oxidation, such as the anisidine value (ANV), pointing to formation of carbonyl compounds, capable of undergoing condensation reactions with p-anisidine, and the thiobarbituric acid reactive substance (TBARS) pointing to the presence of malondialdehyde (MDA) in particular. In biological systems, TBARS is of widespread use as a measure for the extent of oxidation damage. Another test for stability of oils to oxidation is based on the development of acidity as secondary product, for example, standards using the Rancimat equipment or a similar setup. 

Despite the apparent simplicity of their structures, the chemistry of the nicotinamide ring in NAD+ and NADP+ is surprisingly complex.128 129 NAD+ is extremely unstable in basic solutions, whereas NADH is just as unstable in slightly acidic media. These properties, together with the ability of NAD+ to undergo condensation reactions with other compounds, have sometimes caused serious errors in interpretation of experiments and may be of significance to biological function. 

Virtually all types of metal ions have been complexed with macrocyclic ligands.2-7 Complexes of transition metal ions have been studied extensively with tetraaza macrocycles (Chapter 21.2). Porphyrin and porphyrin-related complexes are of course notoriously present in biological systems and have been receiving considerable investigative attention (Chapter 22).8 Macrocyclic ligands derived from the Schiffbase and template-assisted condensation reactions of Curtis and Busch also figure prominantly with transition metal ions.6,7 The chemistry of these ions has been more recently expanded into the realm of polyaza, polynucleating and polycyclic systems.9 Transition metal complexes with thioether and phosphorus donor macrocycles are also known.2... 

Similar oxygen stabilized ethylene dications were proposed in several types of superacid-catalyzed condensation reactions involving 1,2-dicarbonyl compounds. For example, 2,3-butanedione condenses in high yield with benzene and the superelectrophile (35) is considered to be the key intermediate, because the monoprotonated species (78) is not sufficiently electrophilic to react with benzene (eq 17).35 Several biologically important a-ketoacids were also found to generate superelectrophiles... 

A second class of ThDP-dependent enzymes, that perform the biological equivalent of the benzoin condensation reaction, interconvert sugar phosphates of different chain lengths. In the pentose shunt transketolase catalyzes the reaction shown in Scheme 10,... 

The use of carbodiimides as mediators in biological condensation reactions is of considerable importance. Sheehan received a Nobel price for the first total synthesis of phe-noxymethyl penicillin using DCC to effect the /3-lactam ring closure reaction. Diiso-propylcarbodiimide was used two years later to achieve the ring closure reaction in the... 

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