Zn(2+) binding-enabled excited state
intramolecular proton transfer: a step toward new near-infrared fluorescent
probes for imaging applications.
Abstract
In order to facilitate the in vivo study of zinc-related biology, it is essential to develop a zinc-selective sensor that exhibits both near-infrared (NIR) emission and larger Stokes shift. A fluorescent sensor, Zinhbo-5, has been constructed by using bis(benzoxazole) ligand with 2, 2'-dipicolylamine (DPA) as metal ion receptor. In aqueous solution, Zinhbo-5 exhibits high sensitivity (K(d) = 2.58 nM(2) ) and selectivity for Zn(2+) cation, revealing about 14-fold fluorescence enhancement upon zinc binding to give green emission. Remarkably, Zn(2+) binding to Zinhbo-5 switches on the excited state intramolecular proton transfer (ESIPT), producing the desirable near-infrared region (over 710 nm) with large Stokes shift (ca. 240 nm). The new probe is demonstrated to be useful for in vivo imaging of the intracellular Zn(2+) ion. The Zinhbo-5 is also useful for detecting zinc ion distribution during the development of living zebrafish embryos.
Reference
Xu. Y, Liu. Q, Dou. B, Wright. B, Wang. J, Pang. Y. Adv
Healthc Mater. 2012, 4, 485
Results
and findings
-
In room temperature, Zinhbo-5 can bind Zn2+ in
ratio 2:1, while the complex 1:1 can be observed at high temperature (~85° C).
-
Mixing Zinbo-5 with Zn2+ gradually formed
complex ratio 2:1 as the kinetically favored product at room temperature, whose
conversion to the thermodynamically more stable complex 1:1 can only occur at a
higher temperature.
-
Not-polar solvents such as ether appeared to
disfavor the ESIPT mechanism. In DMSO (a polar aprotic solvent), Zn2+ could be
solvated to reduce the metal binding to phenoxide, thereby inhibit the ESIPT.
Whereas, the ESIPT signals were stronger in the protic solvents (EtOH and MeOH)
because the solvent interact with phenoxide by hydrogen bonding thereby
reducing the negative charge impact of the phenoxide oxygen and enhancing the
ESIPT
Citations
-
Xu et al. (2012) reported that the Zn2+
complex prefer forming 2:1 ratio ligand to metal at room temperature to 1:1,
even being stable form.
According
to Xu et al. (2012), the ESIPT mechanism can occur well when using protic solvents such as EtOH
and MeOH because the hydrogen bonding between these solvent and phenoxide
reduces the negative charge on the phenoxide oxygen leading to enha
