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Alkyl Transfer to Metal Thiolates: Kinetics, Active Species Identification, and Relevance to the DNA Methyl Phosphotriester Repair Center of Escherichia coli Ada

The Ada protein of Escherichia coli employs a [Zn(S-cys)4]2 site to repair deoxyribonucleic acid alkyl phosphotriester lesions. The alkyl group is transferred to a cysteine thiolate in a stoichiometric reaction. We describe a functional model for this chemistry in which a thiolate of [(CH3)4N]2[Zn(SC6H5) 4] accepts a methyl group from (CH3O)3PO. The thiolate salt (CH3)4N(SC6H5) is also active in methyl transfer, but the thiol C6H5SH fails to react. Conductivity measurements and kinetic studies demonstrate that [(CH3)4N]2[Zn(SC6H5) 4] forms ion pairs in dimethyl sulfoxide (DMSO) solution (KIP = 13 ± 4 M-1) which exhibit diminished reactivity. The reaction of [Zn(SC6H5)4]2- with (CH3O)3PO is first order with respect to each reagent. A second-order rate constant for this reaction, KZn. was determined to be (1.6 ± 0.3) × 10-2 M-1 s-1. From kinetic data and equilibria studies, all reactivity of [(CH3)4N]2[Zn(SC6H5) 4] toward (CH3O)3PO could be attributed to dissociated thiolate. Metal complexes representing alternative protein sites were prepared and displayed the following kinetic trend of methyl transfer ability: [(CH3)4N]2[Zn(SC6H5)4] > [(CH3)4N]2[Co(SC6H5) 4] ≈[(CH3)4N]2[Cd(SC6H 5)4] > [(CH3)4N][Zn(SC6H5) 3(MeIm)] > [Zn(SC6H5)2(MeIm)2], where Melm = 1-methylimidazole. These results are consistent with a dissociated thiolate being the active species and suggest that a similar mechanism might apply to alkyl phosphotriester repair by Ada.

Synthesis, Properties, and Molecular and Crystal Structures of (Me4N)4 (E = S, Se; M = Zn, Cd): Molecular Supertetrahedral Fragments of the Cubic Metal Chalcogenide Lattice

The complexes 4-(Me4N+) (3M, M = Zn, Cd) and 4-(Me4N+) (4M, M = Zn, Cd) are formed in 80-100percent yield by the reactions of sulfur or selenium with the adamantanoid cages 2-(Me4N+) (2M, M = Zn, Cd).Complexes 3M and 4M do not degrade to the metal chalcogenides, which also are not formed in the preparative reactions under varied conditions.All four complexes 3M and 4M are molecular, with the same (μ3-E)4M10S16 (E = S, Se) core structure, which is a supertetrahedral fragment of the cubic (sphalerite) ME lattice.In terms of expanding polyhedra the molecular structure is octahedro-M6-tetrahedro-(μ3-E)4-truncated tetrahedro-(μ-SPh)12-tetrahedro-M4-tetrahedro-(SR)4, with 3-E)2(μ-SPh)2> coordination at the six inner metal atoms and coordination at the four outer metal atoms.The three-coordinate chalcogenide ions are located at the centers of the hexagonal faces of the (μ-S)12 truncated tetrahedron.By inversion at the sulfur atoms of the 12 bridging thiolate ligands, 186 molecular configurational isomers can occur.Three different crystal structure determinations of 3M (one Zn, two Cd) have revealed the occurence of two isomers with 4 molecular symmetry.Crystal structure A (M = Cd), space group I4, contains one isomer, while the other isomer occurs in crystal structure B (M = Zn), space group P421c, and crystal structure C (M = Cd), space group I42m, which is a mirror disordered form of structure B.Small distortions of the core geometry from idealized 43m (Td) symmetry can be traced to weak repulsions between the phenyl substituents on the surfaces of the molecular anions.Low-frequency infrared and Raman data for 2M, 3M, 4M, and (Me4N)2 are interpreted empirically.Crystal data for A: I4 a = 20.946 (2) Angstroem, c = 14.779 (2) Angstroem, Z = 2(xC112H128Cd10S20N4), 2971 reflections (I> 3?(I)), Cu Kα, R (Rw) = 0.033 (0.045).B: P421c, a = 19.783 (4) Angstroem, c = 16.871 (5) Angstroem, Z = 2(x C112H128Zn10S20N4), 1140 reflections, Mo Kα, R (Rw) = 0.044 (0.054).C: I42m, a = 20.140 (2) Angstroem, c = 16.896 (1) Angstroem, Z = 2(x C112H128Cd10S20N4), 916 reflections, Mo Kα, R (Rw) = 0.042 (0.051).