Wunshain Fann

Academia Sinica, Taipei, Taipei, Taiwan

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Publications (117)280.17 Total impact

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    ABSTRACT: A ubiquitin mutant with two Cys mutations, m[C]q/S65C, was site-specifically labeled with two dye molecules, Alexa Fluor 488 (donor) and Alexa Fluor 594 (acceptor), due to the different reactivity of these two Cys residues. This doubly dye-labeled ubiquitin has lower structural stability than wild-type ubiquitin. Taking advantage of this decreased stability, conformational heterogeneity of this protein under nondenaturing condition was observed at the single-molecule level using single-paired Förster resonance energy transfer (FRET) by trapping the protein in agarose gel. Three conformational populations corresponding to folded (E (ET) ≈ 0.95), loosely packed (E (ET) ≈ 0.72), and unfolded (E (ET) ≈ 0.22) structures, and the structural transitions between them were observed. Our results suggest that agarose immobilization is good for observing structural dynamics of proteins under native condition.
    Biophysics of Structure and Mechanism 11/2011; 41(2):189-98. · 2.44 Impact Factor
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    ABSTRACT: We performed experiments and simulations to investigate the influence of hydrodynamic interaction on the diffusion dynamics of circular and linear λ-DNA confined in nanoslits. Contrary to the common assumption that intrachain hydrodynamic interaction (HI) is completely screened when polymers are confined in channels with height h smaller than the radius of gyration R(g), it is found that the HI is partially screened and approaches complete screening only for R(g)≪h. For λ-DNA, the HI becomes nearly completely screened only when the channel height is smaller than the Kuhn length. In addition, the dynamics of linear and circular λ-DNA in very strong confinement is shown to be independent of the chain topology.
    Physical Review E 09/2011; 84(3 Pt 1):031917. · 2.31 Impact Factor
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    ABSTRACT: Kinetic measurement of protein folding is limited by the method used to trigger folding. Traditional methods, such as stopped flow, have a long mixing dead time and cannot be used to monitor fast folding processes. Here, we report a compound, 4-(bromomethyl)-6,7-dimethoxycoumarin, that can be used as a "photolabile cage" to study the early stages of protein folding. The folding process of a protein, RD1, including kinetics, enthalpy, and volume change, was studied by the combined use of a phototriggered caging strategy and time-resolved photoacoustic calorimetry. The cage caused unfolding of the photolabile protein, and then a pulse UV laser (∼10(-9) s) was used to break the cage, leaving the protein free to refold and allowing the resolving of two folding events on a nanosecond time scale. This strategy is especially good for monitoring fast folding proteins that cannot be studied by traditional methods.
    Proteins Structure Function and Bioinformatics 11/2010; 78(14):2973-83. · 3.34 Impact Factor
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    ABSTRACT: The requirements of a microscope differ greatly from field to field and with the sample being studied. Organic thin films are highly sensitive to environmental conditions being rapidly photobleached in the presence of oxygen. In this paper we introduce the design of a low cost near field optical microscope suitable for environmentally controlled studies. We demonstrate that reducing ambient pressure to ∼10 mTorr (1.3 Pa) of nitrogen (a pressure accessible with a mechanical pump) significantly reduces photo-bleaching on a typical experimental timescale. In addition, we introduce a novel non-optical feedback control mechanism combining a piezoelectric diaphragm with a tuning fork. The morphology of thin films of MEH-PPV spin cast from the polymer dissolved in chlorobenzene is investigated. These films are shown to be featureless on a 100 nm length scale in terms of topology, PL intensity and PL spectrum both before and during photo-bleaching. As far as we are aware these are the first reported thin films of MEH-PPV grown in which ∼100 nm sized domains are absent. We believe that the ability to control environment will not only allow better optical characterization of thin films but also opens up the possibility of the production of novel two dimensional organic photonic crystals through a combination of mechanical disruption, photo-bleaching and photo-conjugation of polymers.
    Journal of the Chinese Chemical Society. 06/2010; 57(3B).
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    ABSTRACT: ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.
    ChemInform 01/2010; 27(6).
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    ABSTRACT: Fis protein can bend DNA chain with length much shorter than its persistence length. We applied single-molecule fluorescence resonance energy transfer method to probe these conformational changes. A broad distribution of end-to-end distances correlates well with the molecular dynamics simulation. The flexibility of DNA upon Fis binding is attributed to the breakages of hydrogen bonds between base pairs. DNA kinks at specific sites, instead of continuous bending. The loosening of DNA structures might have biological implications for the functions of Fis-proteins as transcription cofactors.
    Chemical Physics Letters 01/2010; 500:318-322. · 2.15 Impact Factor
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    ABSTRACT: To investigate the local environment’s effect on the lifetime and quantum yield of extended polymer chains in the absence of intra- and interchain aggregation, short, rodlike polymers of poly(2,5-di-n-octyloxy-1,4-phenylenevinylene) (DO-PPV) were dissolved in chloroform and then embedded in a polystyrene matrix. The fluorescence lifetime was found to increase by 45% in moving from the solution to the matrix form. By using the absorption and emission spectra of the chloroform solution to estimate the radiative and nonradiative rate constants for the polymer in solution, along with calculations based on an exciton model, the corresponding decay rate constants for the polymer embedded in the matrix were obtained. The close agreement between the calculated and experimental values of fluorescent lifetime in the matrix proved the applicability of the exciton model used. On the basis of the model, the average quantum yield of isolated polymers in the matrix was calculated to be a factor of 2 higher than in solution—an effect arising from a 59% decrease in the nonradiative rate constant and, to a smaller extent, from a 20% increase in the radiative decay rate due to the different dielectric constants of the environments. These results suggest that by extending and isolating single luminescent polymers, high quantum yield devices are possible.
    Journal of Physical Chemistry C - J PHYS CHEM C. 10/2009; 113(43).
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    ABSTRACT: A series of six 2,3-diphenyl-4-neopentyl-1-silacyclobut-2-enes with different 1,1-substituents has been prepared and characterized by single-crystal X-ray crystallography. These compounds possess a cis-stilbene-like chromophore involving also the four-membered ring, and exhibit a photophysical behavior similar to that of previously reported 1,2-diphenyl-cycloalkenes. This chromophore system is confirmed by a theoretical investigation of the electronic structure and excitation spectra. The absorption and photoluminescence of selected derivatives were studied in solution, as solid powder samples, and in doped-polymer thin films. In well-dissolved solution, the silacyclobutenes show only very weak fluorescence emission (quantum yield approximately 0.1%), due to competition with photochemical and non-radiative photophysical relaxation. When the solubility is degraded in a poor (aqueous) solvent, the formation of nanoscale aggregates leads to a significant enhancement factor in the emission intensity, due to the suppression of the photoreactivity in the more rigid molecular environment, although the quantum yield still remains below a few percent. In the solid-state, however, photoreactivity is completely suppressed leading to fluorescence quantum efficiencies of 8-23% depending on the 1,1-substituents, which demonstrates these compounds' potential as chromophores for condensed-phase luminescence applications. Two dominant competing photochemical reactions have been identified in solution (for excitation in the lowest-energy absorption band, >260 nm), which are analogous to related (sila-)cyclobutenes and stilbenoids. The first involves ring-opening due to cleavage of the 1,4-Si-C bond to form metastable silabutadienes, which was confirmed by isolating the stereospecifically formed allylsilane which results from a secondary reaction with trapping agents such as methanol or water. The second photochemical reaction involves ring closure of the 2,3-diphenyl substructure to form a dihydrophenanthrene analogue, which was confirmed by isolating the phenanthrene derivative that results following subsequent hydrogen abstraction in the presence of oxygen. Measurements of the silacyclobutenes in doped-polymer thin films reveal a spectroscopic behavior ranging from that in solution to the nano-aggregate case as the silacyclobutene dopant concentration is increased.
    Chemistry 09/2009; 15(34):8625-45. · 5.93 Impact Factor
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    ABSTRACT: Six different kinds of coherently aligned porphyrin-appended polynorbornenes derived from 5,6-endo-fused N-arylpyrrolidenonorbornenes have been synthesized. Pi-pi interactions between the pendant groups are essential for dictating the photophysical properties of the polymers and the mechanism for the stereoselective formation of polymers. Splitting of the Soret band of polymers 2a-c, which have alkyl-substituted porphyrin pendant groups, suggests strong exciton coupling between chromophores. No splitting of the Soret band is observed for polymers 2d-f, which have tetraaryl substituents on the porphyrin moiety. Significant fluorescence quenching is found in polymers 2a-e, whereas only slightly reduced quantum yield is observed for 2f. Time-resolved fluorescence measurements also indicate a similar trend. The AFM image of 2d on graphite shows aggregation to form a two-dimensional, ordered pattern.
    Chemistry 05/2009; 15(23):5719-28. · 5.93 Impact Factor
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    ABSTRACT: Fluorescent nanodiamond (FND) contains nitrogen-vacancy defect centers as fluorophores. The intensity of its fluorescence can be significantly enhanced after deposition of the particle (35 or 140 nm in size) on a nanocrystalline Ag film without a buffer layer. The excellent photostability (i.e. neither photobleaching nor photoblinking) of the material is preserved even on the Ag film. Concurrent decrease of excited state lifetimes and increase of fluorescence intensities indicate that the enhancement results from surface plasmon resonance. Such a fluorescence enhancement effect is diminished when the individual FND particle is wrapped around by DNA molecules, as a result of an increase in the distance between the color-center emitters inside the FND and the nearby Ag nanoparticles. A fluorescence intensity enhancement up to 10-fold is observed for 35 nm FNDs, confirmed by fluorescence lifetime imaging microscopy.
    Physical Chemistry Chemical Physics 04/2009; 11(10):1508-14. · 3.83 Impact Factor
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    Macromolecules 03/2009; 42:1770. · 5.52 Impact Factor
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    ABSTRACT: The number of negatively charged nitrogen-vacancy centers ( N- V)<sup>-</sup> in fluorescent nanodiamond (FND) has been determined by photon correlation spectroscopy and Monte Carlo simulations at the single particle level. By taking account of the random dipole orientation of the multiple ( N- V)<sup>-</sup> fluorophores and simulating the probability distribution of their effective numbers (N<sub>e</sub>) , we found that the actual number (N<sub>a</sub>) of the fluorophores is in linear correlation with N<sub>e</sub> , with correction factors of 1.8 and 1.2 in measurements using linearly and circularly polarized lights, respectively. We determined N<sub>a</sub>=8±1 for 28 nm FND particles prepared by 3 MeV proton irradiation.
    Applied Physics Letters 02/2009; · 3.79 Impact Factor
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    ABSTRACT: Fluorescence correlation spectroscopy (FCS) has been extensively applied to study the kinetics and photophysics of molecules as well as interactions between molecules by extracting information from the fluctuation of signals. In particular, single molecule applications of FCS promise the greatest amounts of information. Ideally, one would like to carry out FCS in real-time; however, due to the time-consuming nature of the correlation process, performing the correlation in real-time is totally nontrivial. Generally an expensive hardware correlator or a TCSPC board is required for this purpose. Recently highly-efficient algorithms based on multi-tau method have been proposed to build up a software correlator. In this work, we set forth an innovative algorithm capable of realizing the real-time correlation, without turning to the multi-tau method. This algorithm takes advantage of the low count rate generally existing in the FCS experiments, directly using the time interval between each photon its adjacent photon to efficiently update the correlation function. Based on this efficiency, it is possible to build a low-cost software correlator with just an ordinary counter board. We practically demonstrate the feasibility by setting up this correlator to measure the diffusion motion of rhodamine 6G in water using FCS. The algorithm was validated by duplicating the signal from the photon detector and sending it to both the ordinary counter board with our software correlator and a commercial correlator simultaneously. The perfect coincidence of the correlation curves from these two correlators and the real-time display of the correlation function indicate the validity and practicability of our approach.
    Proc SPIE 01/2009;
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    ABSTRACT: Amyloid plaques, formed from amyloid beta (Abeta) peptides (mainly Abeta40 or Abeta42), are one of the most important pathological characteristics of Alzheimer's disease. Here, a single D-form proline substitution in the 40-amino-acid Abeta40 peptide can totally change the aggregation behavior of this peptide. The residue immediately preceding each glycine in Abeta40 (S8, V24, I32, and V36) was individually replaced by D-form proline ((D)Pro). The resulting (D)P-G sequence (the (D)Pro residue and the following Gly residue) was designed as a "structural clip" to force the formation of a bend in the peptide, as this sequence has been reported to be a strong promoter of beta-hairpin formation. The mutant peptide with Val24-to-(D)Pro substitution, named V24P, formed a new amyloid-like beta-aggregate at high peptide concentration. The aggregate has most of the characteristics of amyloid fibrils, except fibril morphology. Moreover, the mutant peptide V24P, when mixed with Abeta40, can attenuate the cytotoxicity of Abeta40.
    Journal of Molecular Biology 12/2008; 385(4):1257-65. · 3.91 Impact Factor
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    ABSTRACT: DNA dynamics in microchannels of height H may be categorized into three regimes: I. H > Rg, II. Rg > H > lp, and III. lp > H, where Rg and lp are the DNA radius of gyration and persistence length, respectively. Dynamics of DNA molecules in regime I has been extensively studied in recent theory and experiments. It has been shown that the intra-chain hydrodynamic interactions (HI) may strongly affect how DNA behaves under a microfluidic flow. In contrast, intra-chain HI is believed to be screened in regime III, as suggested by recent studies of Doyle et al. In this work, we investigate the role of HI in regime II and how it affects DNA of different conformation, specifically for linear and circular chains. In order to capture the intra-chain HI, we employ lattice Boltzmann simulations for the fluid, coupled with coarse-grained dynamics simulations for the DNA. We find that regime II confinement affects the chain conformation and shape differently for linear and circular chains due to the degrees of freedom for chain ends. The different chain conformations also lead to different chain dynamic.
    11/2008;
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    ABSTRACT: To study conformational changes within a single protein molecule, sp-FRET (single pair fluorescence resonance energy transfer) is an important technique to provide distance information. However, incorporating donor and acceptor dyes into the same protein molecule is not an easy task. Here, we report a strategy for the efficient double-labeling of a protein on a solid support. An ubiquitin mutant with two Cys mutations, one with high solvent accessibility and the other with low solvent accessibility, was constructed. The protein was bound to magnetic beads and reacted with the dyes. The first dye reacted with the side-chain of the Cys with the high solvent accessibility and the second with the other Cys under partially denaturing conditions. Using this method, we can easily label two dyes in a site-specific way on ubiquitin with a satisfied yield. The labeling sites for donor and acceptor dyes can be easily swapped.
    Bioconjugate Chemistry 07/2008; 19(6):1124-6. · 4.58 Impact Factor
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    ABSTRACT: Fluorescent nanodiamond is a new nanomaterial that possesses several useful properties, including good biocompatibility, excellent photostability and facile surface functionalizability. Moreover, when excited by a laser, defect centres within the nanodiamond emit photons that are capable of penetrating tissue, making them well suited for biological imaging applications. Here, we show that bright fluorescent nanodiamonds can be produced in large quantities by irradiating synthetic diamond nanocrystallites with helium ions. The fluorescence is sufficiently bright and stable to allow three-dimensional tracking of a single particle within the cell by means of either one- or two-photon-excited fluorescence microscopy. The excellent photophysical characteristics are maintained for particles as small as 25 nm, suggesting that fluorescent nanodiamond is an ideal probe for long-term tracking and imaging in vivo, with good temporal and spatial resolution.
    Nature Nanotechnology 05/2008; 3(5):284-8. · 31.17 Impact Factor
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    ABSTRACT: A convenient protocol to fabricate an organic-inorganic hybrid system with covalently bound light-harvesting chromophores (stilbene and terphenylene-divinylene) and an electron acceptor (titanium oxide) is described. Efficient energy- and electron-transfer processes may take place in these systems. Covalent bonding between the acceptor chromophores and the titania/silica matrix would be important for electron transfer, whereas fluorescence resonant energy transfer (FRET) would strongly depend on the ratio of donor to acceptor chromophores. Time-resolved spectroscopy was employed to elucidate the detailed photophysical processes. The coupling of FRET and electron transfer was shown to work coherently to lead to photocurrent enhancement. The photocurrent responses reached a maximum when the hybrid-material thin film contained 60 % acceptor and 40 % donor.
    Chemistry - An Asian Journal 04/2008; 3(3):578-84. · 4.57 Impact Factor
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    ABSTRACT: Two new silylene-spaced copolymers containing three kinds of different chromophores in regioregular manner were synthesized by rhodium-catalyzed hydrosilylation of a bisalkyne with a bis-silyl hydride. The photophysical and electrochemical properties of these polymers and the corresponding monomeric models were investigated. Time-resolved fluorescence spectroscopy has been employed for detailed kinetic analyses. The chromophores in these polymers have been shown to exhibit light harvesting to collect light energy, energy transfer, and efficient charge separation. The overall efficiencies involving both FRET and PET in these silylene-spaced vinylarene copolymers can be up to 96%.
    Macromolecules. 03/2008; 41(8).
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    ABSTRACT: Dialkylsilylene-spaced divinylarene copolymers having alternating donor and acceptor chromophores are designed and synthesized. The rates of photoinduced electron transfer (PET) depend on the nature of the substituent on silicon. A replacement of the methyl substituent by the bulky isopropyl group has been shown to enhance the rate of PET. The results are rationalized in terms of the Thorpe−Ingold effect, which would bring the neighboring donor and acceptor chromophores in closer proximity. The STM images of the isopropyl-substituted copolymers are more folded than those of the methyl-substituted analogues, and these results are consistent with the outcome of photophysical studies of these copolymers.
    Macromolecules. 11/2007; 40(26).

Publication Stats

940 Citations
280.17 Total Impact Points

Institutions

  • 1994–2010
    • Academia Sinica
      • • Institute of Biological Chemistry
      • • Institute of Atomic and Molecular Sciences
      Taipei, Taipei, Taiwan
  • 2009
    • Yuan Ze University
      Taoyuan City, Taiwan, Taiwan
    • Tunghai University
      • Department of Physics
      Taichung, Taiwan, Taiwan
  • 1995–2009
    • National Taiwan University
      • • Department of Chemistry
      • • Institute of Polymer Science and Engineering
      • • Department of Physics
      Taipei, Taipei, Taiwan
  • 2007
    • National Sun Yat-sen University
      Kao-hsiung-shih, Kaohsiung, Taiwan
  • 2001
    • National Tsing Hua University
      • Department of Chemical Engineering
      Hsinchu, Taiwan, Taiwan