The thiolate-alkylating protein (Ada protein) is a zinc protein that repairs the defective DNA by transferring methyl group on to itself. We have used the thiophenylphosphine ligand to provide sulfur-rich environment for model study. In this work, we have accidentally found that this zinc complex can activate CH2Cl2 to generate a methylenated complex (4). Carbon-halide activation is commonly used in organic synthesis. However activation of C-Cl bond is comparatively scarce compared to C-Br or C-I bonds. Varying the degree of deprotonation on the PS3 ligand, a simple zinc dimer (2) can be obtained instead of (4). The dimer (2) can even react with much milder alkyl-reagent, methylphosphotriester. Therefore (2) serves as a successful Ada protein model in this sense. Other related alkyl-transfer reactions using different ligand or zinc complexes were discussed to give insight of the methyl-transfer action of Ada protein.

In our previous studies on modeling methyl transfer protein, Ada protein, we found that zinc complexes of tri(2-thiophenyl)phosphine (PS3), (A), have similar methyl transfer behavior as its biological counter part. In order to probe the role of zinc in methylation process, we used tin and cadmium in model study to compare their chemistry relative to that of zinc. We found that all three metal complexes have similar chemistry and assume similar dimeric anion structure. For example, [Cd(PS3)]22-(1) and [Cd(SiPS3)]22-(2) have been successfully characterized crystallographically to possess the same structure as zinc dimer (A). However , in attempt to crystallize the tin analogue, the accidental oxidation product [SnIV(OH)2(SiPS3)]2, (5), was obtained. Its crystal structure gave clue to the mechanism of oxidation of the original tin dimer. The reactions with alkylating reagent of (1) have been compared with those of zinc dimer (A), and we found that the metal (Zn or Cd) center causes the dimers to produce different degree of methylation products toward different alkylating reagents. For the reactions with CH3I, the different degree of methylation between Cd and Zn dimers shows that the presence of zinc center has higher methylation selectivity and weaker reactivity.

We have successfully used the cadmium ion with (2-thiophenyl)phenylphosphine(PS2) or Bis(3-trimethylsilyl-2-thiophenyl)phenylphosphine(SiPS2) to synthesize [(PS2)Cd(TMEDA)](2) and [(SiPS2)Cd(TMEDA)](4) that were structurally similar to [(PS2)Zn(TMEDA)](1). We also obtained a series of zinc complxes [NEt4][(PS2)Zn(SC6H5)](6), [NEt4][{[(PS2)Zn](SC6H11)[(PS2)Zn]}](7) and [NEt4][(PS2)Zn(SCH2C6H5)](8) with systematically varied thiolates. From studying of the methylation reactions of these complexeswith methyliodine or trimethylphosphate, we found that the metal center(zinc or cadmium), ligand(PS2 or SiPS2) or the net charge (neutral ornegative) can influence the reaction time, selectivity of reaction site ormechanism. The zinc complexes (6), (7), and (8) are capable of completingthe catalytic cycle of mthylation-demethylation and hence a good model for related enzyme system.