title: Zinc coordination to the bapbpy ligand in homogeneous solutions and at liposomes: zinc detection via fluorescence enhancement.
abstract: In this work, the complexation of the bapbpy ligand to zinc dichloride is described (bapbpy = 6,6′-bis(2-aminopyridyl)-2,2′-bipyridine). The water-soluble, colorless complex [Zn(bapbpy)Cl]Cl·2H2O (compound 2·H2O) was synthesized; its X-ray crystal structure shows a mononuclear, pentacoordinated geometry with one chloride ligand in apical position. Upon excitation of its lowest-energy absorption band (375 nm) compound 2 shows intense emission (Φ = 0.50) at 418 nm in aqueous solution, and an excited state lifetime of 5 ns at room temperature. Photophysical measurements, DFT, and TD-DFT calculations prove that emission arises from vibronically coupled Ligand-to-Ligand Charge Transfer singlet excited states, characterized by electron density flowing from the lone pairs of the non-coordinated NH bridges to the π* orbitals of the pyridine rings. Monofunctionalization of the ligand with one long alkyl chain was realized to afford ligand 3, which can be inserted into dimyristoylphosphatidylglycerol (DMPG) or dimyristoylphosphatidylcholine (DMPC) unilamellar vesicles. For negatively charged DMPG membranes the addition of a zinc salt to the vesicles leads to an enhancement of the fluorescence due to zinc coordination to the membrane-embedded tetrapyridyl ligand. No changes were observed for the zwitterionic DMPC lipids, where binding of the Zn ions does not take place. A modest binding constant was found (5 × 10(6) M(−1)) for the coordination of zinc cations to bapbpy-functionalized DMPG membranes, which allows for the detection of micromolar zinc concentrations in aqueous solution. The influence of chloride concentration and other transition metal ions on the zinc binding was evaluated, and the potential of liposome-supported metal chelators such as ligand 3 for zinc detection in biological media is discussed.
authors.affiliation: Leiden Institute of Chemistry, Gorlaeus Laboratories, Leiden University, P.O. Box 9502, Leiden, 2300 RA, The Netherlands.
authors.forename: Elwin
authors.initials: E
authors.lastname: Molenbroek
authors.forename: Natan
authors.initials: N
authors.lastname: Straathof
authors.forename: Sebastian
authors.initials: S
authors.lastname: Dück
authors.forename: Zahid
authors.initials: Z
authors.lastname: Rashid
authors.forename: Joop H
authors.initials: JH
authors.lastname: van Lenthe
authors.forename: Martin
authors.initials: M
authors.lastname: Lutz
authors.forename: Aurore
authors.initials: A
authors.lastname: Gandubert
authors.forename: Robertus J M
authors.initials: RJ
authors.lastname: Klein Gebbink
authors.forename: Luisa
authors.initials: L
authors.lastname: De Cola
authors.forename: Sylvestre
authors.initials: S
authors.lastname: Bonnet
mesh_terms: D015082:2,2'-Dipyridyl / Q000031:analogs & derivatives* / Q000737:chemistry; D000631:Aminopyridines / Q000737:chemistry*; D002712:Chlorides / Q000737:chemistry*; D005453:Fluorescence; D008024:Ligands; D008081:Liposomes / Q000737:chemistry*; D008958:Models, Molecular; D015394:Molecular Structure; D009942:Organometallic Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D011789:Quantum Theory; D012996:Solutions; D017967:Zinc Compounds / Q000737:chemistry*
chemical_list: C581978:6,6'-bis(2-aminopyridyl)-2,2'-bipyridine; D000631:Aminopyridines; D002712:Chlorides; D008024:Ligands; D008081:Liposomes; D009942:Organometallic Compounds; D012996:Solutions; D017967:Zinc Compounds; D015082:2,2'-Dipyridyl; C016837:zinc chloride