RESUMO
We have investigated the ability of a series of three related copper complexes Cu([9-11]aneN(3))X(2) to hydrolyze the activated phosphodiesters bis(4-nitrophenyl) phosphate (BNPP) and ethyl 4-nitrophenyl phosphate (ENPP). The compound Cu([10]aneN(3))Br(2) crystallizes in the monoclinic space group C2/c, a = 20.693(4) Å, b = 11.429(2) Å, c = 20.138(4) Å, beta = 104.78(3) degrees, V = 4605(2) Å(3), and Z = 16. The compound Cu([11]aneN(3))Br(2) crystallizes in the orthorhombic space group Pnma, a = 13.7621(10) Å, b = 8.7492(13) Å, c = 10.0073(10) Å, V = 1205.0(2) Å(3), and Z = 4. The structure of Cu([9]aneN(3))Cl(2) was previously reported (Inorg. Chem. 1980, 19, 1379). The crystal structures of the three complexes show a progression in geometry from square pyramidal to distorted trigonal bipyramidal as the size of the macrocycle increases. Larger macrocycles also result in the copper ion being pulled closer to the plane defined by the three ligand nitrogens, which in turn results in an increase in the sum of the three N-Cu-N angles from 249 degrees to 257 degrees to 278 degrees along with a concomitant decrease in the X-Cu-X angle. Significantly, the rate constant for the hydrolysis of BNPP by Cu([9-11]aneN(3))X(2) increases by nearly an order of magnitude as the ligand size increases from a nine-membered to an 11-membered ring. Correlations have been made between the catalytic ability of Cu([9-11]aneN(3))X(2) and the structural and electronic properties of the complexes. All three catalysts exist in a monomer-dimer equilibrium in solution, with the monomer being the catalytically active species. As the ligand size increases, the dimer formation constant (K(f)) decreases due to steric constraint, thereby increasing the concentration of active species and hence the rate of hydrolysis. The contributions of Lewis acidity and steric constraint on both substrate binding (K(2)) and P-O bond cleavage (k(3) or k(cat)) are less important than the dimerization equilibrium constant in determining the rate of the reaction.
RESUMO
By a combination of Q-band pulsed ENDOR (electron nuclear double resonance) and X-band ESEEM (electron stimulated echo envelope modulation) techniques, we have determined the hyperfine tensors for ethylene (C1) and cyano (C2) carbons and N, of [Ni(mnt)(2)](-), along with the quadrupole tensor for nitrogen. These measurements give pi electron spin densities of rho(C1) approximately 0.03 in the C1 2p(z)() orbital, rho(C2) < 0.003, rho(N) approximately 0.01, such that in total, approximately 0.15 of the spin resides on the ligand atoms C and N, while the rest resides in the NiS(4) core, giving rho(NiS(4)(-)) = 0.85. These results are compared with extended Hückel and density functional (BLYP) MO calculations, as well as with Xalpha calculations reported earlier.
RESUMO
The plasmid-encoded pco copper resistance operon in Escherichia coli consists of seven genes that are expressed from two pco promoters in response to elevated copper; however, little is known about how they mediate resistance to excess environmental copper. Two of the genes encode the soluble periplasmic proteins PcoA and PcoC. We show here that inactivation of PcoC, and PcoA to a lesser extent, causes cells to become more sensitive to copper than wild-type nonresistant strains, consistent with a tightly coupled detoxification pathway. Periplasmic extracts show copper-inducible oxidase activity, attributed to the multicopper oxidase function of PcoA. PcoC, a much smaller protein than PcoA, binds one Cu(II) and exhibits a weak electronic transition characteristic of a type II copper center. ENDOR and ESEEM spectroscopy of Cu(II)-PcoC and the (15)N- and Met-CD(3)-labeled samples are consistent with a tetragonal ligand environment of three nitrogens and one aqua ligand "in the plane". A weakly associated S-Met and aqua are likely axial ligands. At least one N is a histidine and is likely trans to the in-plane aqua ligand. The copper chemistry of PcoC and the oxidase function of PcoA are consistent with the emerging picture of the chromosomally encoded copper homeostasis apparatus in the E. coli cell envelope [Outten, F. W., Huffman, D. L., Hale, J. A., and O'Halloran, T. V. (2001) J. Biol. Chem. 276, 30670-30677]. We propose a model for the plasmid system in which Cu(I)-PcoC functions in this copper efflux pathway as a periplasmic copper binding protein that docks with the multiple repeats of Met-rich domains in PcoA to effect oxidation of Cu(I) to the less toxic Cu(II) form. The solvent accessibility of the Cu(II) in PcoC may allow for metal transfer to other plasmid and chromosomal factors and thus facilitate removal of Cu(II) from the cell envelope.