Previously reported mono- and dinuclear Zn(II), Cu(II), and Ni(II) complexes of 1,4,7,10-tetrazacyclododecane ([12]aneN(4) or cyclen) with different heterocyclic spacers (triazine, pyridine) of various lengths (bi- and tripyridine) or an azacrown-pendant have been tested for the hydrolysis of bis(4-nitrophenyl)phosphate (BNPP) under physiological conditions (pH 7-9, 25 degrees C). All Zn(II) complexes promote the hydrolysis of BNPP under physiological conditions, while those of Cu(II) and Ni(II) do not have a significant effect on the hydrolysis reaction. The hydrolysis kinetics in buffered solutions (0.05 M Bis/Tris, TRIS, HEPES, or CHES, I = 0.1 M, NaCl) at 25 degrees C were determined by the initial slope method (product conversion <5%). Comparison of the second-order pH-independent rate constants (k(BNPP), M-1 s(-1)) for the mononuclear complexes ZnL1, ZnL3, and ZnL6, which are 6.1 x 10(-5), 5.1 x 10(-5), and 5.7 x 10-5, respectively, indicate that the heterocyclic moiety improves the rate of hydrolysis up to six times over the parent Zn([12]aneN(4)) complex (k(BNPP) = 1.1 x 10(-5) M-1 s(-1)). The reactive specie's is the Zn(II)-OH- complex, in which the Zn(II)-bound OH- acts as a nucleophile. For dinuclear complexes Zn2L.2, Zn(2)L4, and Zn(2)L5, the rate of reaction is defined by the degree of cooperation between the metal centers, which is determined by the spacer length. Zn2L2 and Zn2L4 possessing shorter spacers are able to hydrolyze BNPP 1 to 2 orders of magnitudes faster than Zn(2)L5. The second-order rate constants k of Zn(2)L4 and Zn(2)L2 at pH 7, 8, and 9 are significantly higher than those of previously reported related complexes. The high BNPP hydrolytic activity may be related to pi-siacking and hydrophobic interactions between the aromatic spacer moieties and the substrate. Complexes Zn(2)L4 and Zn(2)L2 show hydrolytic activity at pH 7 and 8, which allows for the hydrolysis of activated phosphate esters under physiological conditions.