Search PubMed⌕ Search

Biomedical subjects

F C De Schryver

Publications and source records attributed to F C De Schryver.

At least 19 recordsLinked to original sources
PubMed2005

Energy dissipation in multichromophoric single dendrimers.

Single-molecule spectroscopy of well-chosen dendritic multichromophoric systems allows investigation of fundamental photophysical processes such as energy or electron transfer in much greater detail than the respective ensemble measurements. In dendrimers with multiple chromophores, energy hopping and transfer to the chromophore with the energetically lowest S(1) state was observed. If more than one chromophore is in an excited state in one molecule, annihilation, either singlet-triplet or singlet-singlet, can occur. In the latter case, a higher singlet state is populated opening new deactivation pathways. In the presence of an electron donor, reversible electron transfer could be observed, and the rate constants of forward and backward electron transfer were established. The value of these rate constants fluctuates time-correlated with the rotational motion of the dendrimer arms and the mobility of the embedding matrix.

Dendrimers↗
PubMed2003

Direct proof of electron transfer in a rigid first generation triphenyl amine core dendrimer substituted with a peryleneimide acceptor.

The combination of nanosecond transient absorption experiments and single photon timing experiments proved the occurrence of an electron transfer process in the triphenyl amine core dendrimer, N1P1, by demonstrating the presence of an ion-pair absorption for N1P1 in solvents of medium polarity. By means of femtosecond transient absorption measurements the rise time of this ion-pair absorption dominated by the radical anion absorption could be determined, resulting in a value of 180 ps in MeTHF and 138 ps in THF. Furthermore, in femtosecond fluorescence upconversion as well as in monochromatic femtosecond transient absorption, a few ps component was resolved which was assigned to a vibrational and solvent relaxation process of the locally excited singlet state of the peryleneimide.

Journal Article↗
PubMed2001

Identification of different emitting species in the red fluorescent protein DsRed by means of ensemble and single-molecule spectroscopy.

The photophysics and photochemistry taking place in the DsRed protein, a recently cloned red fluorescent protein from a coral of the Discosoma genus, are investigated here by means of ensemble and single-molecule time-resolved detection and spectroscopic measurements. Ensemble time-resolved data reveal that 25% of the immature green chromophores are present in tetramers containing only this immature form. They are responsible for the weak fluorescence emitted at 500 nm. The remaining 75% of the immature green chromophores are involved in a fluorescence resonance energy transfer process to the red species. The combination of time-resolved detection with spectroscopy at the single-molecule level reveals, on 543-nm excitation of individual DsRed tetramers, the existence of a photoconversion of the red chromophore emitting at 583 nm and decaying with a 3.2-ns time constant into a super red one emitting at 595 nm and for which the decay time constant ranges between 2.7 and 1.5 ns. The phenomenon is further corroborated at the ensemble level by the observation of the creation of a super red form and a blue absorbing species on irradiation with 532-nm pulsed light at high excitation power. Furthermore, single-molecule experiments suggest that a similar photoconversion process might occur in the immature green species on 488-nm excitation.

Animals↗
PubMed2001

Polyphenylene dendrimers with perylene diimide as a luminescent core.

A novel synthesis is presented of a fourfold ethynyl-substituted perylene diimide dye 4, which acts as a core molecule for the buildup of polyphenylene dendrimers. Around the luminescent core 4, a first-generation (5), a second-generation (6), and a third-generation (7) polyphenylene dendritic environment consisting of pentaphenylbenzene building blocks are constructed. The dendrimers 5 and 6 are synthesized by an exclusively divergent route, whereas for 7, a combination of a divergent and convergent approaches is applied. Absorption and emission spectra of 5-7 in different solvents and in a film have been measured and compared to a nondendronized model compound 13. In solution, the internal chromophore is scarcely influenced by the dendritic scaffold; however, in the solid state, aggregation of the perylene diimide is prevented very effectively by the four rigid dendrons. Additionally, fluorescence quantum yields in solution have been determined for 5-7 and 13; they decrease as the number of generation increases.

Journal Article↗
PubMed2001

Single-molecule spectroscopy of a dendrimer-based host-guest system.

We report on a single-molecule study of a host-guest system that consists of a second-generation polyphenylene dendrimer and the cyanine dye Pinacyanol. The use of single-molecule spectroscopy enables us to obtain more detailed information on the properties of the host-guest system and can be used to confirm solution data. At low dye to dendrimer ratios the system is present as a one-to-one complex, while for higher ratios an ion-pair system is formed. Changes in the spectral properties of the single molecules are explained by differences in local polarisability. The difference of the triplet lifetimes of single free dye molecules and of associated ones is interpreted as deriving from a larger free volume for the dye molecules in the dendritic host relative to the rigid polymer matrix.

Journal Article↗
PubMed2001

Triplet states as non-radiative traps in multichromophoric entities: single molecule spectroscopy of an artificial and natural antenna system.

Energy transfer in antenna systems, ordered arrays of chromophores, is one of the key steps in the photosynthetic process. The photophysical processes taking place in such multichromophoric systems, even at the single molecule level, are complicated and not yet fully understood. Instead of directly studying individual antenna systems, we have chosen to focus first on systems for which the amount of chromophores and the interactions among the chromophores can be varied in a systematic way. Dendrimers with a controlled number of chromophores at the rim fulfill those requirements perfectly. A detailed photophysical study of a second-generation dendrimer, containing eight peryleneimide chromophores at the rim, was performed 'J. Am. Chem. Soc., 122 (2000) 9278'. One of the most intriguing findings was the presence of collective on/off jumps in the fluorescence intensity traces of the dendrimers. This phenomenon can be explained by assuming a simultaneous presence of both a radiative trap (energetically lowest chromophoric site) and a non-radiative trap (triplet state of one chromophore) within one individual dendrimer. It was shown that an analogue scheme could explain the collective on/off jumps in the fluorescence intensity traces of the photosynthetic pigment B-phycoerythrin (B-PE) (Porphyridium cruentum). The different values of the triplet lifetime that could be recovered for a fluorescence intensity trace of B-PE were correlated with different intensity levels in the trace, suggesting different chromophores acting as a trap as function of time.

Bile Pigments↗
PubMed2001

A new analysis method of single molecule fluorescence using series of photon arrival times: theory and experiment.

Up to now, single molecule fluorescence experiments were performed by dividing the time into a set of intervals and to observe the number of fluorescence photons arriving in each interval. It is obvious that the detected photons carry less information than the arrival times of the photons themselves. From the arrival times, one can still calculate the number of photons in any user-defined interval; whereas, when only the number of photons in an interval are recorded, information about their positions in time is lost. Therefore, we present a new analysis method of single molecule fluorescence data based on the positions in time of the detected fluorescence photons. We derive mathematically different statistical characteristics describing the single molecule fluorescence experiment assuming an immobilized molecule. The theory of point processes using the generating functionals formalism is ideally suited for a consistent description, linking the statistical characteristics of the excitation and detected photons to the statistical characteristics of the single motionless molecule. We then use computer-generated data sets mimicking the single molecule fluorescence experiment to explore the parametric estimation of mono- and bi-exponential single molecule impulse response functions (SMIRFs) via the following statistical characteristics: the probability density distributions (pdd) of the single and first photocount time positions in a user-defined detection interval, the probability distribution of the number of photocounts per user-defined detection interval, the time correlation function and the pdd of the time interval between two consecutive photocounts. It is shown that all of the above characteristics ensure a satisfactory recovery of the decay time of mono-exponential SMIRFs for a broad range of excitation intensities and widths of user-defined detection intervals. For bi-exponential SMIRFs, the selection of the experimental conditions is more critical and dependent on the detection procedure. At lower excitation intensities it is advantageous to use the pdds of the single and first photocount time occurrences in the user-defined detection interval. To show the practical usefulness of the new analysis method, series of photon arrival times from immobilized single molecules of DiI and rhodamine 6G were analyzed to estimate triplet lifetimes and intersystem crossing yields.

Biophysical Phenomena↗
PubMed2001

Polyphenylene dendrimers with different fluorescent chromophores asymmetrically distributed at the periphery.

A new synthetic approach leading to asymmetrically substituted polyphenylene dendrimers is presented. Following this method, polyphenylene dendrimers decorated with an increasing number of chromophores at the periphery have been obtained up to the second generation. Especially the synthesis of a polyphenylene dendrimer bearing three donor chromophores and one acceptor chromophore has been realized. Intramolecular energy transfer within this molecule is demonstrated by applying absorption and fluorescence measurements.

Journal Article↗
PubMed2001

Intramolecular energy hopping and energy trapping in polyphenylene dendrimers with multiple peryleneimide donor chromophores and a terryleneimide acceptor trap chromophore.

Intramolecular Förster-type excitation energy transfer (FRET) processes in a series of first-generation polyphenylene dendrimers substituted with spatially well-separated peryleneimide chromophores and a terryleneimide energy-trapping chromophore at the rim were investigated by steady-state and time-resolved fluorescence spectroscopy. Energy-hopping processes among the peryleneimide chromophores are revealed by anisotropy decay times of 50--80 ps consistent with a FRET rate constant of k(hopp) = 4.6 ns(-1). If a terryleneimide chromophore is present at the rim of the dendrimer together with three peryleneimide chromophores, more than 95% of the energy harvested by the peryleneimide chromophores is transferred and trapped in the terryleneimide. The two decay times (tau(1) = 52 ps and tau(2) = 175 ps) found for the peryleneimide emission band are recovered as rise times at the terryleneimide emission band proving that the energy trapping of peryleneimide excitation energy by the terryleneimide acceptor occurs via two different, efficient pathways. Molecular- modeling-based structures tentatively indicate that the rotation of the terryleneimide acceptor group can lead to a much smaller distance to a single donor chromophore, which could explain the occurrence of two energy-trapping rate constants. All energy-transfer processes are quantitatively describable with Förster energy transfer theory, and the influence of the dipole orientation factor in the Förster equation is discussed.

Journal Article↗
PubMed2001

An experimental comparison of the maximum likelihood estimation and nonlinear least-squares fluorescence lifetime analysis of single molecules.

Two procedures based on the weighted least-squares (LS) and the maximum likelihood estimation (MLE) method to confidently analyze single-molecule (SM) fluorescence decays with a total number (N) of 2,500-60,000 counts have been elucidated and experimentally compared by analyzing measured bulk and SM decays. The key observation of this comparison is that the LS systematically underestimates the fluorescence lifetimes by approximately 5%, for the range of 1,000-20,000 events, whereas the MLE method gives stable results over the whole intensity range, even at counts N less than 1,000, where the LS analysis delivers unreasonable values. This difference can be attributed to the different statistics approaches and results from improper weighting of the LS method. As expected from theory, the results of both methods become equivalent above a certain threshold of N detected photons per decay, which is here experimentally determined to be approximately 20,000. In contrast to the bulk lifetime distributions, the SM fluorescence lifetime distributions exhibit standard deviations that are sizably larger than the statistically expected values. This comparison proves the strong influence of the inhomogenuous microenvironment on the photophysical behavior of single molecules embedded in a 10-30-nm thin polymer layer.

Journal Article↗
PubMed2000

Scanning tunneling microscopy: a unique tool in the study of chirality, dynamics, and reactivity in physisorbed organic monolayers.

Scanning tunneling microscopy (STM) is applied to study organic monolayers, physisorbed at the liquid-graphite interface. Due to the very local nature of the probing, the structure of these adlayers has been imaged with very high detail. The high resolution allowed us to investigate the effect of molecular chirality on the monolayer formation and provided a unique way to study chemical reactions at the liquid-graphite interface. Making use of a fast scanning mode, dynamic processes in these adlayers have been visualized.

Absorption↗
PubMed1995

Photophysics of the fluorescent Ca2+ indicator Fura-2.

The photophysics of the complex forming reaction of Ca2+ and Fura-2 are investigated using steady-state and time-resolved fluorescence measurements. The fluorescence decay traces were analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution with BAPTA as Ca2+ buffer: k01 = 1.2 x 10(9)s-1, k21 = 1.0 x 10(11) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 2.2 x 10(7) s-1, and with EGTA as Ca2+ buffer: k01 = 1.4 x 10(9) s-1, k21 = 5.0 x 10(10) M-1 s-1, k02 = 5.5 x 10(8) s-1, k12 = 3.2 x 10(7) s-1. k01 and k02 denote the respective deactivation rate constants of the Ca2+ free and bound forms of Fura-2 in the excited state. k21 represents the second-order rate constant of binding of Ca2+ and Fura-2 in the excited state, whereas k12 is the first-order rate constant of dissociation of the excited Ca2+:Fura-2 complex. The ionic strength of the solution was shown not to influence the recovered values of the rate constants. From the estimated values of k12 and k21, the dissociation constant K*d in the excited state was calculated. It was found that in EGTA Ca2+ buffer pK*d (3.2) is smaller than pKd (6.9) and that there is negligible interference of the excited-state reaction with the determination of Kd and [Ca2+] from fluorimetric titration curves. Hence, Fura-2 can be safely used as an Ca2+ indicator. From the obtained fluorescence decay parameters and the steady-state excitation spectra, the species-associated excitation spectra of the Ca2+ free and bound forms of Fura-2 were calculated at intermediate Ca2+ concentrations.

Biophysical Phenomena↗
PubMed1995

Photophysics of the fluorescent K+ indicator PBFI.

The fluorescent indicator PBFI is widely used for the determination of intracellular concentrations of K+. To investigate the binding reaction of K+ to PBFI in the ground and excited states, steady-state and time-resolved measurements were performed. The fluorescence decay surface was analyzed with global compartmental analysis yielding the following values for the rate constants at room temperature in aqueous solution at pH 7.2: k01 = 1.1 x 10(9) s-1, k21 = 2.7 x 10(8) M-1s-1, k02 = 1.8 x 10(9) s-1, and k12 = 1.4 x 10(9) s-1. k01 and k02 denote the respective deactivation rate constants of the K+ free and bound forms of PBFI in the excited state. k21 represents the second-order rate constant of binding of K+ to the indicator in the excited state whereas k12 is the first-order rate constant of dissociation of the excited K(+)-PBFI complex. From the estimated values of k12 and k21, the dissociation constant Kd* in the excited state was calculated. It was found that pKd* (-0.7) is smaller than pKd (2.2). The effect of the excited-state reaction can be neglected in the determination of Kd and/or the K+ concentration. Therefore, intracellular K+ concentrations can be accurately determined from fluorimetric measurements by using PBFI as K+ indicator.

Benzofurans↗
PubMed1990

Photophysical properties of 2-nitro-5,10,15,20-tetra-p-tolylporphyrins.

Tetraarylporphyrins substituted with nitro groups at beta-pyrrolic positions are potential candidates for electron-accepting pigments in model systems for photosynthesis. The photophysics of 2-nitro-5,10,15,20-tetra-p-tolylporphyrin and its zinc analog have been studied in order to evaluate this potential. The ground state absorption spectrum, the triplet-triplet absorption spectrum, the fluorescence emission spectrum, and associated photophysical parameters have been determined. The molecules have short singlet lifetimes and anomalous temperature- and solvent-dependent emission spectra which are consistent with the formation of an intramolecular charge transfer state of the type P+.-NO2-. in which the nitro group is twisted about its bond to the porphyrin, relative to the ground state conformation.

Molecular Structure↗
PubMed1989

Fluorescence quenching with lindane in small unilamellar L,alpha-dimyristoylphosphatidylcholine vesicles.

The lateral mobility and lipid-water partition of the pesticide lindane was studied by fluorescence quenching of N-isopropylcarbazole (NIPC) and L,alpha-palmitoyl-beta-(N-carbazolyl) undecanoylphosphatidylcholine (PCUPC) in liposomes of dimyristoylphosphatidylcholine at 50 degrees C. In isotropic solvents the quenching reaction was highly inefficient. A scheme for dynamic quenching, in which the monomolecular quenching rate constant is small, was valid. In lipid bilayers the same scheme was applied to describe the quenching results but the rate constant of the back-reaction of the excited complex to quencher and excited probe was of comparable magnitude to the monomolecular quenching rate constant. This phenomenon results in biexponential decays of the fluorescent probe in the presence of quencher. All the rate constants of the scheme could be determined. Stern-Volmer plots at different membrane concentrations were obtained from fluorescence intensity and decay time measurements. From these plots the true bimolecular quenching rate constant, Kq, and the rate constant for lateral diffusion, kd, were determined: kq[NIPC] = 3.2 +/- 0.5 x 10(8) M-1 s-1, kq[PCUPC] = 1.9 +/- 0.4 x 10(8) M-1 s-1, kd[NIPC] = 6.6 +/- 0.8 x 10(8) M-1 s-1. The smaller value of kq compared to kd for the quenching reaction of NIPC with lindane indicates that this quenching reaction is not diffusion controlled. The lateral diffusion coefficient D of lindane was found to be 1.7 +/- 0.2 x 10(-6) cm2/s in dimyristoylphosphatidylcholine vesicles at 50 degrees C. The partition coefficient of lindane in these lipid bilayers is very high (greater than 2000).

Dimyristoylphosphatidylcholine↗
PubMed1989

Simultaneous analysis of single-photon timing data for the one-step determination of activation energies, frequency factors and quenching rate constants. Application to tryptophan photophysics.

A general global analysis of single-photon timing data is presented in which each fluorescence decay curve can be described by a different decay law. The model parameters can be held in common within one curve and/or between related curves. Any or all parameters can be kept fixed, or they may be variable to seek optimum values. This general analysis allows the determination of activation energies, frequency factors and quenching rate constants in one step. The construction of the global mapping table which relates parameters in one experiment to those in another is explained in detail. The use and performance of this general simultaneous analysis are examined using tryptophan fluorescence decays at pH 6.0 obtained at various emission wavelengths as a function of temperature and added solute quencher. The results show that tryptophan at pH 6.0 decays as a biexponential with decay times which are independent of the analysis wavelength. The decay component with the short lifetime has a deactivation rate constant of 1.4 x 10(9) s-1 independent of temperature. The decay component with the long lifetime has an activation energy of 28 kJ/mol and a frequency factor of 3 x 10(13) s-1; its temperature-independent decay rate constant equals 1 x 10(8) s-1. Recursion formulas for a computer program to estimate activation energies, frequency factors, and decay rate constants are provided.

Mathematics↗