Bong Lee

Time-resolved fluorescence anisotropy studies on the interaction of biologically active polycations with phospholipid membranes

Abstract Interaction of a polymeric in-chain quaternary ammonium salt with phospholipid bilayer membranes was studied by time-resolved fluorescence spectroscopy with the aid of a picosecond time-rcorrelated single photon counting system (instrument response function, 70 ps FWHM). Particular attention was paid to a phenomenon of polycation-induced fluidization of the membranes, which was well evaluated by time-resolved fluorescence anisotropy measurements. The fluorescence anisotropy, r ( t ), of 1,6-diphenyl-1,3,5-hexatriene (DPH) embedded in the membranes was analyzed based on the wobbling-in-cone model. Strong interaction was observed between the polycation and a negatively charged phosphatidic acid membrane as demonstrated by a large decrease of residual polarization value on adding the polycation to the acidic phospholipid dispersion. This means that the cone angle of the wobbling-in-cone motion of the DPH molecule increases by the addition of the polycaton, indicative of the polycation-induced fluidization of the acidic membrane. On the other hand, r ( t ) of DPH embedded in a zwitterionic phosphatidylcholine membrane was not affected significantly by the addition of the polycation, which is presumably due to non-binding of the polycation to the zwitterionic membrane. Our results clearly indicate that the polycationic disinfectant interacts strongly with negatively charged membranes, inducing fluidization of the membranes.

Fluorescence behaviour of cyanobiphenyl liquid crystal molecules in liquid crystal/polymer composite films

The fluorescence behaviour of the liquid crystal, 4-cyano-4-pentylbiphenyl (5CB), in composite thin films prepared by the photopolymerization of 5CB/diacrylate mixtures, was investigated by means of three different excitation methods, in which the total-internal-reflection or surface-limited excitation method was used for analysis of the fluorescence from an ultra-thin interface layer (100nm) in contact with the substrate surface, whereas the fluorescence from the interior bulk was analysed by the through-film excitation method. It was found that intensity ratios of the monomer and excimer emissions of 5CB are significantly lower in the interface layer than in the interior bulk, depending upon photopolymerization conditions as well as upon the structures of the diacrylates used. Scanning electron microscopic observations and light-scattering measurements of some typical composite films showed possible relationships between morphological features and fluorescence characteristics depending upon the diacryla...

Photoreactivity and emission properties of liquid-crystalline twin dimers containing cinnamic acid moieties at both ends of ethylene glycol spacers. I. Synthesis, thermotropic properties and photoreactivity

Abstract Twin dimers containing 4-methoxycinnamic acid moieties at both ends of various lengths of ethylene glycol spacers (nEGMC) were found to exhibit liquid-crystalline phase behaviour, while those with cinnamic acid moieties (nEGC) at both ends did not show the liquid-crystalline phase behaviour. The liquid-crystalline phase behaviour of nEGMC depended on the length of the ethylene glycol spacers, n. Photoirradiation of nEGMC at >300nm resulted in the formation of photodimers through cyclobutane ring formation at the C=C part of the 4-methoxycinnamic acid moieties. The photoreaction was explored by following a decrease in vC=C (FTIR), formation of the photodimers (GPC) and structural analysis of the photoproducts (NMR). It was found that the morphology of the systems strongly affected the photoreactivity and the head-to-head photodimer was predominantly formed. The photoreactivity of nEGMC is discussed in relation to the morphology of the systems.

Time‐resolved observation of excitation hopping between two anthryl moieties attached to both ends of polystyrene chains

Excitation hopping between two 9‐anthryl moieties attached to both ends of polystyrene chains with various molecular weights has been explored by picosecond time‐resolved fluorescence anisotropy measurements. The measurements were performed at 77 K in the rigid matrix of 2‐methyltetrahydrofuran (MTHF) or methylcyclohexane (MCH) in order to exclude the effect of rotational diffusion of the chromophores to the fluorescence anisotropy. The hopping behavior was found to be very sensitive to such factors as molecular weight (Mn), solvent, and concentration of the samples. In MTHF (good solvent), the excitation hopping was only observed for a sample with Mn∼104; on the other hand, it occurred unequivocally for samples with Mn∼105 in MCH (poor solvent), as evidenced by a clear decay of the fluorescence anisotropy, r(t). Furthermore, the excitation hopping was an intramolecular event only at very low concentration (∼10−7 M based on polymer chain), and at 10−6 M, an intermolecular excitation hopping was observed. ...

Photoreactivity and emission properties of liquid-crystalline twin dimers containing cinnamic acid moieties at both ends of ethylene glycol spacers: II. emission properties

Abstract Emission properties of liquid-crystalline twin dimers containing 4-methoxy-cinnamic acid moieties at both ends of various ethylene glycol derivatives (nEGMC), where n denotes the number of the ethylene glycol units in the spacers, have been explored by steady-state and time resolved fluorescence spectroscopy in relation to the morphology of the system. Characteristic emissions were observed in the twin dimers, particularly in 2EGMC and 5EGMC, which were ascribed to excimer emission, and the energy gap between the locally excited state (monomer emission) and the excimer state and could be correlated to the difference in the spatial orientation of the mesogenic 4-methoxycinnamate groups adopted in nEGMC in the ground state. The picosecond time resolved fluorescence spectroscopy revealed no observed rise in the excimer emission in any sample at any temperature, indicating that the excimer is most probably formed by direct excitation of the pair wise arrangement of the 4-methoxycinnamate moieties in ...

EXCITATION HOPPING BETWEEN TWO IDENTICAL CHROMOPHORES ATTACHED TO BOTH ENDS OF ALKANES

Excitation hopping behavior between two identical chromophores has been directly observed by time-resolved fluorescence anisotropy (r(t)) measurements with the aid of a picosecond time-correlated single photon counting system. In order to explore the excitation hopping behavior in purely isolated two identical chromophoric systems, we used α, ω-bis(2-naphthyl)-n-alkanes, Nap(CH 2 ) n Nap [Nn, where n = 3, 5, 7 and 12], and α, ω-bis(9-anthryl)-n-alkanes, An(CH 2 ) m An [Am, where m = 3, 5, 7, 10, 12, 14 and 18], as well as their model compounds with a single chromophore, 2-ethylnaphthalene (EN) and 9-ethylanthracene (EA). It was demonstrated that r(t) for the model compounds did not change with time during their lifetimes while in Nn and Am with m ≥ 7 the initial decay of r(t) was clearly observed, indicating that excitation hopping takes place between the two chromophores attached to both ends of alkanes. Conformational analysis was performed on the basis of potential energy calculations and the results were used to analyze the decay profiles of r(t).