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 (Zn2L) (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
Zn2L
(C19H43N8O13Cl3Zn2)
are monoclinic, space group P21/n
(No. 14) with a = 15.037(5) Å, b =
13.862(5) Å, c
= 15.780(4) Å, β = 90.29(2)°, Z = 4,
and R = 0.109. Although the two zinc(II) ions
(separated with a distance of
3.42 Å) 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 (Zn2L−PO3H) from aqueous solution
at
pH 3 and characterized by X-ray crystal analysis. Crystals of
Zn2L−PO3H·2H2O·(ClO4)3
(C19H48N8O18Cl3Zn2)
are
monoclinic, space group Cc (No. 9) with a =
19.573(3) Å, b = 12.454(4) Å, c =
15.066(3) Å, β = 103.94(1)°, Z
= 4, and R = 0.036. The structure of
Zn2L−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 kNPP = (1.52 ±
0.05) × 10-3 M-1 s-1 at pH
5.9 and 35 °C). The rate vs pH profile with
5 mM Zn2L and 10 mM 4-nitrophenyl phosphate showed a
bell-shaped curve with pK1 of 5.2 and
pK2 of 6.3, which
were assigned to the protonation constants for NPP2- +
H ⇄ HNPP- and NH (a freed apical donor in the
associated
reaction intermediate) + H+ ⇄ 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 ATP4-, 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.