| Summary: | 博士 === 國立臺灣大學 === 化學研究所 === 96 === Part 1: Detection of Phospholipids Base on Chemosensing Ensemble
We have previously designed a fluorescence receptor for sensing of phosphate ions. This kind of receptor is established by incorporating four additional amido groups onto pyridine 2,6-biscarboxamide to provide a pseudo-tetrahedral cleft and multiple hydrogen bondings to hold phosphates in a 1:1 complexation stoichiometry. For sensing phospholipids, such as geranyl pyrophosphate (GPP), a modified receptor was synthesized by incorporation of long hydrocarbon chains to increase hydrophobic interaction with the lipid moiety of phospholipids, and two pyrene units were kept for the fluorescence readout.
In this study, a chemosensing ensemble system was applied by using coumarin derivatives as the indicator. Upon photoirradiation at 340 nm, a fluorescence resonance energy transfer (FRET) would occur due to the interaction of the coumarin with the pyrene unit. Because of high binding affinity of the receptor with GPP, addition of GPP would cause displacement of the coumarin indicator. Once the coumarin indicator was extruded out of the receptor cleft, the FRET would be diminished. Thus, phospholipid is readily detected by this method, and the binding constant is determined by the fluorescence changes during the titration.
Part 2: Synthesis of BPA–Zn Complex for Selective Binding with LysoPA.
A bis(2-pyridylmethyl) amine–Zn complex [Ph–(BPA–Zn)2, 48] is designed to contain a rigid benzene unit and dinuclei of Zn2+ ions. On binding with a phosphate ion, each zinc ion is four-coordinated with BPA and one oxygen of phosphate. Because a Zn2+ ion can have coordination number of five, the residual vacant orbital on the Ph–BPA–Zn–phosphate is available to incorporate an additional ligand. Based on this concept, both Zn2+ ions of BPA–Zn can bind synergistically to the hydroxyl group and the phosphate moiety of lysophosphatidic acid (LysoPA). The binding constants for LysoPA and phosphatidic acid (PA) are determined in a chemosensing ensemble system using coumarin derivative as the indicator. As our anticipation, the binding of Ph–(BPA–Zn)2 receptor with LysoPA is about 4–fold stronger than that with PA.
Part 3: Phospholipid-Induced Aggregation and Anthracene Excimer Formation: Application to sensing phospholipids.
Receptor 60 displays an unusual and selective fluorescence response to LysoPA in aqueous buffer solutions through a concentration-dependent formation of aggregates and fluorescent anthracene excimers. The aggregation behavior relies on the hydrophobic aliphatic chains in LysoPA, and the excimer emission results from excitation of the ground-state pre-associated anthracene dimer or clusters. The aggregate formation is consistent with the broadening and red-shift of their absorption and excitation titration spectra. The elevation of absorption might reflect the formation of microcrystal-like aggregates in the buffer solution. The proposed fluroescence sensing mechanism can also account for the observed changes in the fluorescence decay time. The anthracene excimer vs monomer emission would allow a more favorable ratiometric detection of LysoPA. Furthermore, this method may be used to detect the activity of transglycosylase based on the selective binding of receptor 60 with pyrophosphate mono-ester over the corresponding di-ester. It may serve as a new tool for screening the inhibitors against formation of bacterial cell wall.
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