Novel properties of cooperative dinuclear zinc(II) ions: The selective recognition of phosphomonoesters and their P-O ester bond cleavage by a new dinuclear zinc(II) cryptate

A propanol-bridged octaazacryptand (26-hydroxy-1,4,7,10,13,16,19,22-octaazabicyclo [11.11.3]heptacosane, HL) has been synthesized from diethylenetriamine and [2-oxo-6-(aminomethyl)morpholyl]-N,N',N'-triacetic acid triethyl ester by refluxing in MeOH followed by BH3 . THF reduction. This octaazacryptand forms a novel dinuclear zinc(II) cryptate (Zn(2)L) (L = alkoxide form of HL) in aqueous solution. The X-ray crystal structure of the cryptate showed each zinc(II) ion in a distorted trigonal-bipyramidal environment involving two NH's and an alkoxide O- anion as equatorial donors, with tertiary amine and an NH stand in apical positions. Crystals of the triperchlorate salt of Zn(2)L (C19H43N8O13Cl3Zn2) are monoclinic, space group P2(1)/n (No. 14) with a = 15.037(5) Angstrom, b = 13.862(5) Angstrom, c = 15.780(4) Angstrom, beta = 90.29(2)degrees, Z = 4, and R = 0.109. Although the two zinc(II) ions (separated with a distance of 3.42 Angstrom) in the cryptate appeared to be coordinately saturated and hence were assumed unreactive, they work together to selectively interact with a phosphomonoester, 4-nitrophenyl phosphate dianion (NPP2-), and promote the cleavage of its P-O ester bond by nucleophilic attack of one of the apically coordinated NH's at pH 4.9-9.5 in aqueous solution. The reaction product was isolated as a phosphoramide derivative (Zn(2)L-PO3H) from aqueous solution at pH 3 and characterized by X-ray crystal analysis. Crystals of Zn(2)L-PO3H . 2H(2)O .(ClO4)(3)(C19H48N8O18Cl3Zn2) are monoclinic, space group Cc (No. 9) with a = 19.573(3) Angstrom, b = 12.454(4) Angstrom, c = 15.066(3) Angstrom, beta = 103.94(1)degrees, Z = 4, and R = 0.036. The structure of Zn(2)L-PO3H featured the two phosphoryl oxygens bound to each zinc(II) ion in place of the original two apical NH's. The kinetics were followed for liberation of 4-nitrophenol (the maximum second-order rate constant k(NPP) = (1.52 +/- 0.05) x 10(-3) M(-1) s(-1) at pH 5.9 and 35 degrees C). The rate vs pH profile with 5 mM Zn(2)L and 10 mM 4-nitrophenyl phosphate showed a bell-shaped curve with pK(2) of 5.2 and pK(2) of 6.3, which were assigned to the protonation constants for NPP2- + H reversible arrow HNPP- and NH (a freed apical donor in the associated reaction intermediate) + H+ reversible arrow HNH+, respectively. The most remarkable character of the present new zinc(II) cryptate was that it reacted only with phosphoester polyanions such as NPP2- and ATP(4-), but not with bis(4- nitrophenyl) phosphodiester monoanion, tris(4-nitrophenyl phosphotriester, or 4-nitrophenyl acetate. The present results may well be relevant to the significance of dinuclear metal centers in metallophosphatases such as alkaline monophosphatase.