Dahl 2 Nucleophilic Reactions

The basic reaction involves transfer of sulfane sulfur from the donor (SCN) to the enzyme, forming a persulhde intermediate. The persulfide sulfur is transferred from the enzyme to a nucleophilic receptor (CN) to yield SCN. For most species, this enzyme activity is high in liver, kidney, brain, muscle, and olfactory mucosa (Himwich and Saunders, 1948 Dahl, 1989 Aminlari et al., 1994). The nasal metabolism of CN may have relevance to the toxicity of inhaled HCN. (3-Mercaptopymvate-cyanide transulfurases are present in blood, liver, and kidney, and catalyze the reaction ... 

Dahl suggested a kinetic model for the nucleophilic substitution reaction at tri-covalent phosphorus with a P-N bond with alcohols (Scheme 2.135) [41]. The reaction proceeds in the presence of an ammonium salt as a catalyst. In the first step, the phosphoramidite is protonated in a fast equilibrium (1). In the second step, replacement of the amino group takes place (2). This step is slow and is the rate-determining step. Regeneration of the catalyst in the last step closes the catalytic cycle (3). 

Dahl, B H., Nielsen, J, and Dahl, O. (1987) Mechanistic studies on the phosphoramidite coupling reaction in oligonucleotide synthesis I Evidence for nucleophilic catalysis by tetrazole and rate variations with the phosphorus substituents Nucl Acids Res 15, 1729-1743. 

---