A Novel Biomimetic Zinc(II)−Fluorophore, Dansylamidoethyl−Pendant Macrocyclic Tetraamine 1,4,7,10-Tetraazacyclododecane (Cyclen)

On the basis of the chemical principle of carbonic anhydrase
(CA)−aromatic sulfonamide inhibitor
interaction, a dansylamidoethyl−pendant cyclen
(1-(2-(5-(dimethylamino)-1-naphthalenesulfonamido)ethyl)-1,4,7,10-tetraazacyclododecane, HL) has been synthesized as a novel type of
zinc(II)−fluorophore. The new ligand HL
forms very stable complexes (ML) with zinc(II), cadmium(II),
and copper(II) at physiological pH. The
potentiometric
and spectrophotometric pH-titration study disclosed the 1:1
metal(II) complexes stability constants log K(ML)
(=
log([ML]/[M][L])) to be 20.8 ± 0.1 for ZnL, 19.1 ±
0.1 for CdL, and >30 for CuL. The crystalline zinc(II)
complex
ZnL was isolated from aqueous solution at pH 7. The X-ray crystal
study of ZnL disclosed a five-coordinate, distorted
square-pyramidal structure with the deprotonated dansylamide
N- coordinating at the apical site. Crystals of
the
monoperchlorate salt of ZnL
(C22H35N6O6SClZn)
are orthorhombic, space group Pna21 (no. 33)
with a = 23.777(3)
Å, b = 12.744(5) Å, c = 9.092(3)
Å, V = 2755(2) Å3, Z = 4,
R = 0.032, and Rw = 0.047.
The zinc(II) complex
shows a maximum UV absorption band (λmax) at 323 nm (ε
5360) at 25 °C in aqueous solution. The fluorescent
maximum and the quantum yield (Φ) of ZnL vary with the solvent:  at
528 nm (Φ = 0.11) in H2O, 496 nm (0.53)
in MeOH, 489 nm (0.60) in EtOH, and 484 nm (0.44) in
CH3CN. Demetalation of ZnL with excess amount
of
EDTA yielded the metal-free ligand HL, which in pH 7.3 aqueous solution
has an excitation and a weak emission
fluorescence at 330 nm (ε 4950) and 555 nm (Φ = 0.03),
respectively. The copper(II) ion, to the contrary,
completely
quenches the fluorescence. The crystalline copper(II) complex
CuL (λmax 306 nm, ε 7630 in H2O) was
isolated as
its monoperchlorate salt. The zinc(II)-dependent fluorescence
with 5 μM HL at pH 7.3 is quantitatively responsive
to 0.1−5 μM concentration of zinc(II), which is unaffected by
the presence of mM concentration of biologically
important metal ions such as Na+, K+,
Ca2+, and Mg2+. The new ligand HL
forms a far more stable 1:1 zinc(II)
complex than any previous zinc(II)-fluorophore and is evaluated as
a new zinc(II)-fluorophore.