Replacement sulfhydryl

Sulfhydryl protons usually exchange at a low rate so that at room temperature they are coupled to protons on adjacent carbon atoms (7—8 Hz). They do not exchange rapidly with hydroxyl, carboxylic, or enolic protons on the same or on other molecules thus, separate peaks are seen. However, exchange is rapid enough that shaking the solution for a few minutes with deuterium oxide replaces sulfhydryl protons with deuterium. The absorption range for aliphatic sulfhydryl protons is 81.6 to 8 1.2 for aromatic sulfhydryl protons, 8 3.6 to 8 2.8. Concentration, solvent, and temperature affect the position within these ranges. 

The biochemical basis for the toxicity of mercury and mercury compounds results from its ability to form covalent bonds readily with sulfur. Prior to reaction with sulfur, however, the mercury must be metabolized to the divalent cation. When the sulfur is in the form of a sulfhydryl (— SH) group, divalent mercury replaces the hydrogen atom to form mercaptides, X—Hg— SR and Hg(SR)2, where X is an electronegative radical and R is protein (36). Sulfhydryl compounds are called mercaptans because of their ability to capture mercury. Even in low concentrations divalent mercury is capable of inactivating sulfhydryl enzymes and thus causes interference with cellular metaboHsm and function (31—34). Mercury also combines with other ligands of physiological importance such as phosphoryl, carboxyl, amide, and amine groups. It is unclear whether these latter interactions contribute to its toxicity (31,36). 

Lead (soft, Pb2+) Injuries to peripheral nervous system, disturbs heme synthesis and affects kidneys Pb2+ may replace Ca2+ with loss of functional and structural integrity. Reacts with sulfhydryl groups, replaces Zn2+ in 8-aminolevulinic acid dehydratase. 

Cysteine is derived from serine by replacing the hydroxyl group with the sulfhydryl group. It is the second protein amino acid that contains sulfur. 

A similar process may take place in the reduction of polar compounds with single bonds. A halogen, hydroxy, sulfhydryl or amino derivative by accepting an electron dissociates into a radical and an anion. In aprotic solvents the two radicals combine. In the case of halogen derivatives the result is Wurtz synthesis. In the presence of protons the anion is protonated and the radical accepts another electron to form an anion that after protonation gives a hydrocarbon or a product in which the substituent has been replaced by hydrogen. 

Also called mercaptans, these compounds, with the generic formula RSH, are sulfur analogs of alcohols, just as HjS is of HjO. The O atom of the OH has been replaced by an S atom, and the — SH group is denominated sulfhydryl. 

The oxidation and reduction steps are coupled through the binding of G3P to a sulfhydryl group on an enzyme that already has bound a NAD+ molecule. The enzyme facilitates a H (hydride ion) shift from G3P to reduce the NAD+ to NADH. The oxidation of G3P results in the formation of a high-energy thioester bond that is subsequently replaced by an orthophosphate group to yield the 1,3-BGP. 

The P. chlororaphis B23 NHase is the ferric enzyme [50], which has been characterized in detail [51,52]. (i) The NHase is the first known nonheme iron enzyme containing a typical low-spin Fe(III) site, (ii) the axial position of the Fe(III) site in the enzyme may be occupied by aquo and sulfhydryl groups, and (iii) aliphatic nitrile substrates directly bind to the Fe(III)-active center through H20-substrate replacement. The NHase also seems to contain pyrroloquinoline quinone (PQQ) or a PQQ-like compound [53]. 

Replacement of a halogen atom in the 5-position of a pyrazole by nucleophiles is facilitated by the presence of an electron-withdrawing substituent, such as the phenylazo group, in the 4-position, where the halogen may be replaced by amino, alkylamino, dialkylamino, arylamino, sulfhydryl, and other such groups.664-660... 

The halogen atom of quaternary salts of 3- and 5-halogeno-l-phenylpyrazoles may be replaced easily at 80-100° by hydroxyl, alkoxyl, sulfhydryl, thioalkyl, amino, alkylamino, dialkylamino, arylamino, or cyano groups.565,675-683 The chlorine atom of quaternary salts of N-substituted 5-chloropyrazoIes may be replaced by bromine541,550,579,684,685 or by iodine265,550, 662,684-887 by heating... 

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