Evaluating the conformational role of an allosteric CuII ion in anion recognition and catalysis by a tricopper complex

Structural and anion binding studies have provided an insight into the conformational role of an allosteric Cult ion in the trinuclear catalyst (L-2-2H)Cu-3(II). L-2 is a novel trinucleating ligand with pyridyl, pyrimidyl and amide donor groups. (L-2-2H)Cu (1) has been characterized by X-ray crystallography. The Cu ion is located at the allosteric site and coordinated by two amide N and two pyrimidine N atoms, the CuN4 plane is tetrahedrally distorted, and the complex adopts a helically twisted conformation. In contrast, the (L-2-2H)Cu-3(mu(4)-C2O4) subunit of the dodeca-nuclear complex [(L-2-2H)(4)Cu-12(mu(4)-C2O4)(2)(mu-OH)(4)(mu-Cl)(4)Cl-4(H2O)(2)] (5) is roof-shaped, and the allosteric Cu is located on the top of a square-based pyramid. The oxalate coligand is coordinated by the two catalytic Cu ions in an unusual 1,4-,mu-O,O bridging mode with an (OO)-O-... bite length of 2.6 Angstrom and a (CuCu)-Cu-... distance of 6.4 Angstrom. Intramolecular transesterification of the phosphodiester 2-(hydroxypropyl)-p-nitrophenyl phosphate (HPNP) by [(L-2-2H)Cu-3](4+) was investigated, in comparison with the closely related complex [(L-3-2H)Cu-3](4+) in which the ligand framework is somewhat less flexible. From a kinetic analysis of cleavage rate at varying HPNP concentrations, K-HPNP (the equilibrium constant for binding of HPNP to the complex) and k(cat) (first-order rate constant for cleavage of HPNP when bound to the complex) parameters were derived: K-HPNP = 190 M-1 ([(L-2-2H)Cu-3](4+)) and 305 M-1 ([(L-3-2H)Cu-3](4+)), k(cat) = 10 x 10(-3)s(-1) ([(L-2-2H)Cu-3](4+)) and 3.3 x 10(-3)s(-1) ([(L-3-2H)Cu-3](4+)). Anion binding constants of the complexes were determined by monitoring competitive inhibition of HPNP cleavage. The complexes have a high affinity to SO42-, HPO42-, and ReO4-, which appear to be of the appropriate size for bridging coordination, while smaller anions CH3CO2- and HCO3- are bound less efficiently. [(L-3-2H)Cu-3](4+) has a higher affinity than [(L-2-2H)Cu-3](4+) to HPNP but a lower affinity to the rather large anion ReO4-. This is interpreted as a consequence of the reduced flexibility of [(L-3-2H)Cu-3](4+), which slightly disfavours widening of the (CuCu)-Cu-... distance for incorporation of perrhenate. Similarly, the somewhat lower reactivity of [(L-3-2H)Cu-3](4+) is attributed to the larger energy gap between the ground state and sterically more demanding (and less efficiently stabilized) transition state.