Electronic Energy Transfer in a Single Donor–Acceptor Pair with Triplet–Triplet Absorption

Optics and Spectroscopy - The protocol of dynamic decoupling using a sequence of resonance RF pulses with different shapes in a system of dipole-coupled electron/nuclear spins with inhomogeneous...     

Triplet‐Triplet Electron Excitation Energy Transfer in the Gas Phase for the Carbazol–Diacetyl Donor‐Acceptor Pair

For the carbazol–diacetyl donoracceptor pair, the features of exothermal triplettriplet (TT) transfer of electron excitation energy in the gas phase upon excitation of carbazol by the nitrogen laser radiation which is not absorbed by diacetyl have been investigated. The luminescence spectra of carbazol in the gas phase have been studied. It is shown that carbazol phosphorescence in the gas phase is absent. Therefore, for estimating the TT transfer efficiency and its temperature dependence, in the kinetic approximation the dependences of the intensity of timeresolved delayed fluorescence of carbazol and sensitized phosphorescence of diacetyl on diacetyl pressure have been analyzed. It has been found that in the gas phase the features of the electronic structure of the molecules under investigation are responsible for the low efficiency of transfer (2·10^–4 at 453 K), which increases by an order of magnitude with increasing temperature in the 453–513 K range despite the increasing influence of the back transfer.     

Utility and Considerations of Donor–Donor Energy Migration as a Fluorescence Method for Exploring Protein Structure-Function

This review aims at surveying the use of electronic energy transport between chemically identical fluorophores (i.e. donors) in studies of various protein systems. Applications of intra- and interprotein energy migration are presented that make use of polarised steady-state and time-resolved fluorescence spectroscopic techniques. The donor-donor energy migration (DDEM) and the partial donor-donor energy migration (PDDEM) models for calculating distances between donor groups are exposed together with the most recent development of an extended Forster theory (EFT). Synthetic fluorescence depolarisation data that mimic time-correlated single photon counting experiments were generated using the EFT, and then further re-analysed by the different models. The results obtained were compared with the known parameters used to generate EFT data. Aspects on how to adopt the EFT in the analyses of time-correlated single photon counting experiments are also presented, as well as future aspects on using energy migration for examining protein structure.     

Spin Chemistry Investigation of Peculiarities of Photoinduced Electron Transfer in Donor–Acceptor Linked System

Photoinduced intramolecular electron transfer in linked systems, (R,S)- and (S,S)-naproxen-N-methylpyrrolidine dyads, has been studied by means of spin chemistry methods [magnetic field effect and chemically induced dynamic nuclear polarization (CIDNP)]. The relative yield of the triplet state of the dyads in different magnetic field has been measured, and dependences of the high-field CIDNP of the N-methylpyrrolidine fragment on solvent polarity have been investigated. However, both (S,S)- and (R,S)-enantiomers demonstrate almost identical CIDNP effects for the entire range of polarity. It has been demonstrated that the main peculiarities of photoprocesses in this linked system are connected with the participation of singlet exciplex alongside with photoinduced intramolecular electron transfer in chromophore excited state quenching.     

On the Quantitative Treatment of Donor–Donor Energy Migration in Regularly Aggregated Proteins

An algorithm is presented that quantitatively accounts for donor–donor energy migration (DDEM) among fluorophore-labeled proteins forming regular aggregates. The DDEM algorithm is based on Monte Carlo and Brownian dynamics simulations and applies to calculation of fluorescence depolarisation data, such as the fluorescence anisotropy. Thereby local orientations, as well as reorienting motions of the fluorescent group are considered in the absence and presence of DDEM and among, in principle, infinitely many proteins as they form regular aggregates. Here we apply the algorithm for calculating and illustrating the DDEM and the time-resolved fluorescence anisotropy under static as well as dynamic conditions within helical, linear and circular aggregate structures. A principal approach of the DDEM algorithm for analysing protein aggregates is also outlined.     

Mechanisms of Fluorescence Quenching in Donor—Acceptor Labeled Antibody—Antigen Conjugates

The cyanine dyes Cy5 and Cy5.5 are presented as a new long wavelength-excitable donor-acceptor dye pair for homogeneous fluoroimmunoassays. The deactivation pathways responsible for the quenching of the fluorescence of the antibody-bound donor are elucidated. Upon binding of the donor dye to the antibodies at low dye/protein ratios, its fluorescence quantum yield rises to unity. Higher dye/protein ratios lead to progressive aggregation of the dyes, which results in quenching of monomer fluorescence due to resonance energy transfer (RET) from the monomers to the nonfluorescent dimers. The dependence of the quenching efficiency on the labeling ratio is described quantitatively by assuming a Poisson distribution of the dyes over the antibodies. The maximum fluorescence intensity per antibody is obtained at a labeling ratio of 4. Upon formation of the antibody-antigen complex, electron transfer and RET to the antigen-bound acceptor dye occur. Steady-state and time-resolved fluorescence measurements reveal that approximately 50% of the donor quenching is due to RET, while the residual quenching effect is caused by the static quenching process.     

Light-Induced Spin Polarization in Porphyrin-Based Donor–Acceptor Dyads and Triads

The light-induced spin polarization generated by sequential electron transfer in an axially bound triad based on Al(III) porphyrin (AlPor) is discussed. In the triad, $\text {TTF} \!-\! \text {Ph} \!-\! \text {py}\!\to\!\text {AlPor}\! - \!\text {Ph}\! -\! \text {NDI}$ , the electron acceptor naphthalene diimide (NDI) is attached covalently to the Al(III) center, while the donor tetrathiafulvalene (TTF) coordinates to Al(III) via an appended pyridine (py) on the opposite face of the porphyrin ring. Excitation of the porphyrin at room temperature in solution leads to charge separation between the donor and acceptor. In the liquid crystalline solvent 5CB, a spin-polarized transient electron paramagnetic resonance spectrum of a weakly coupled radical pair is observed and is assigned to the state $ \text{TTF}^{\cdot + } \text{NDI}^{\cdot - } $ . In the absence of the donor, a spectrum of the triplet state of the strongly coupled radical pair $ \text{AlPor}^{ \cdot + } \text{NDI}^{ \cdot - } $ is obtained. The analysis of the spectra is described using a model developed by Kandrashkin et al. (Appl Magn Reson 15: 417–447, 1998). It is shown that in the triad the spectrum of $ \text {TTF}^{ \cdot + } \text {NDI}^{ \cdot - } $ shows evidence of the singlet–triplet mixing in the precursor $ \text{AlPor}^{ \cdot + } \text{NDI}^{ \cdot - } $ . At later time, singlet recombination leads to inversion of the spectrum, from which the singlet back reaction lifetime is estimated as 350 ns. The decay of the inverted spectrum yields a lifetime of 8.3 μs for the triplet back reaction lifetime.     

Luminescence of Donor‐Acceptor Pairs in Compounds of Yttrium and Scandium Oxides

The luminescence spectra of Sc₂O₃, Y₂O₃, and Y₂GeO₅ ceramics and thin films exposed to laser and cathode excitation were investigated. The investigation of the properties of longwave luminescence bands in Sc₂O₃ with maxima at 2.65, 2.35, and 2.05 eV, in Y₂O₃ with maxima at 2.60, 2.35, and 2.10 eV, and also in Y₂GeO₅ with maxima at 2.55, 2.25, and 2.00 eV point to the fact that they are caused by radiative recombination of the excited donoracceptor pairs Sc³⁺ (or Y³⁺)O^2–.     

Temperature Dependence of the Triplet–Triplet Energy Transfer Efficiencies in Carbonyl Vapors

The temperature dependences of the rate constants k _TT and the efficiencies _DA of triplet–triplet transfer of the energy of electronic excitation in a gas phase for a number of donor–acceptor pairs (benzophenone–diacetyl, anthraquinone–diacetyl, carbazole–diacetyl, and naphthalene–diacetyl) were studied. It is shown that for gas phase systems the k _TT and _DA constants can both increase and decrease with increase in the temperature over the range 360–510 K. To explain the temperature changes of the k _TT values, the experimental rate constants were compared with those calculated by a classical model (Marcus' equation). In the temperature range under study, the influence of the average vibrational energy <E _vib> of the donor and acceptor molecules was taken into account to estimate the free energy change G, which varied from 0.4 to –1.2 eV. It is established that the increase in k _TT with temperature for certain gas-phase systems and decrease for others represent the typical, for a gas-phase system, transition from a normal region (increase in k _TT) to an inverted one (decrease in k _TT) with monotonic variation of G, including the temperature increase of <E _vib> of the molecules.     

Fluorescence as a Probe of Energy Dependence of Electron–Molecule Interaction

The fluorescence intensity dependences on electron kinetic energy, or fluorescence excitation functions, for some aromatic (perylene and p-terphenyl) and heterocyclic (biphenylfuran, oxazoles, and oxadiazoles) compounds are studied. It is found that the addition of phenyl rings to the above molecules result in a moderate decrease in excitation function. On the other hand, introducing heteroatoms such as oxygen or nitrogen leads to a substantial decrease in excitation function.     

Single Molecule Fluorescence of Native and Refolded Peridinin–Chlorophyll–Protein Complexes

Single molecule spectroscopy was applied to study the optical properties of native and refolded peridinin-chlorophyll-protein (PCP) complexes. The native system is a trimer with six chlorophyll a (Chl a) molecules, while the refolded one contains two Chl a and resembles structurally and spectroscopically the PCP monomer. The fluorescence emission of single PCP complexes strongly broadens with increasing excitation power. Simultaneously, the distribution of fluorescence maximum frequencies is also broadened. These spectral changes are attributed to photoinduced conformational changes of the protein that influence the fluorescence of embedded chromophores. Comparison of fluorescence intensities measured for PCP complexes in two different solvents indicates that the native PCP trimers are preserved in EDTA Tris buffer, while in PVA polymer matrix only monomers are stable.     

Determination of the Composition and Thickness of Co–Ni/Cr System Films by X-Ray Fluorescence

A procedure of determining the composition and thickness of Co–Ni/Cr two-layer films on Polycor by means of x-ray fluorescence is developed. The correction factors that take into account the effects of the mutual influence of the elements in the given system are calculated. The density of the materials entering into the composition of the films is determined experimentally. The metrological characteristics of the procedure are established.     

Isolated (Quantum) Emitters Generated with the Participation of Donor–Acceptor Pairs in the ZnSe/ZnMgSSe Heterostructure

Measurements of low-temperature (5 K) microphotoluminescence reveal the existence of isolated (quantum) emitters in ZnSe/ZnMgSSe quantum well with a radiation line experiencing abrupt displacements of its spectral position by several meV within 1–10 min. The unusual properties of the detected emitters are explained on the basis of a system with a large dipole moment in its ground state, e.g., a single donor–acceptor pair.     

Laser Phase Determination and Transfer Function to Directly Measure the Temporal Structure of a Narrow Bandwidth Attosecond EUV Pulse

A laser phase determination method and a transfer function that includes a proportional term of a measured photoelectron energy spectrum are presented to directly measure the detailed temporal structure of a narrow bandwidth attosecond extreme-ultraviolet （EUV） pulse. The method is based on the spectrum measurement of an electron generated by EUV photo-ionization interacting with a femtosecond laser field. The results of the study suggest that measurements should be taken at 0° or 180° with respect to the linear laser polarization. The method has a temporal measurement range of about half a laser oscillation period. The temporal resolution also depends on the jitter and control precision of the laser and EUV pulses.     

Pulsed EPR Analysis of the Photo-Induced Triplet Radical Pair in the BLUF Protein SyPixD: Determination of the Protein–Protein Distance and Orientation in the Oligomeric Protein

A photo-induced radical pair of FADH^· and Y8^· and in BLUF protein SyPixD was studied by pulsed electron paramagnetic resonance (EPR) spectroscopy. Blue light illumination at 150 K for 30 min followed by cooling to 50 K during illumination induced the stable radical pair. The EPR signal has been characterized by a Pake doublet signal with complete S = 1 spin state. The radical pair was utilized as a probe to analyze the oligomer of SyPixD. The relative arrangement of PixD proteins in the complex was investigated by pulsed electron–electron double resonance (PELDOR) with the orientation selection. Based on the decameric structure in the crystal, the possible structure for the PELDOR results was discussed.     

The Electron－Hole Pair in a Single Quantum Dot and That in a Vertically Coupled Quantum Dot

The energy spectra of low-lying states of an exciton in a single and a vertically coupled quantum dots are studied under the influence of a perpendicularly applied magnetic field. Calculations are made by using the method of numerical diagonalization of the Hamiltonian within the effective-mass approximation. We also calculated the binding energy of the ground and the excited states of an exciton in a single quantum dot and that in a vertically coupled quantum dot as a function of the dot radius for different vaJues of the distance and the magnetic field strength.     

Donor–Donor Energy Migration (DDEM) for Determining Intramolecular Distances in Proteins. II. the Effect of Partial Labeling

By using site-specific mutagenesis it is possible to prepare a protein molecule that can be labeled with two identical fluorescent probes at different positions.^(1,2) To calculate intramolecular distances between the two fluorescent donors in a protein, a recently developed DDEM model can be applied.^(3,4) Here we have studied the influence of incomplete donor labeling on the calculated distances. For this purpose, the previous model has been extended and compared with experiments performed on three mutants (V106C, M266C, and V106C-M266C) of plasminogen activator inhibitor type 1 (PAI-1) labeled with N-(4,4-difluoro-5,7-dimethyl-4-bora-3a,4a-diaza-s-indacene-3-yl)methyl) iodo-acetamide (SBDY). The C of the residues to which the SBDYs are covalently linked are separated by 55.1 Å, as determined by X-ray diffraction on the wild-type PAI-1. To examine the reliability of the extracted parameters, synthetic data were generated and reanalyzed with the same model as used to analyze real experiments. It is concluded that, even for a low degree of labeled double mutant (60%), a distance of 54 ± 3 Å is found for both models.     

Energy and Charge Transfer from Guest to Host in Doped Organic Electroluminescent Devices

The luminescence properties of doped organic electroluminescent devices are explained by means of a Hamiltonian model.The results show that there is a corresponding relation between the amount of transferred charge and the change of the energy originating from charge transfer,and the relation can be influenced by dopant concentration.As the amountof transferred charge increases,the total energy decreases and the luminescence intensity increases.Therefore,we deduce that the energy transfer from guest to host may be derived from the charge transfer.For a given organic electroluminescent device,the maximum value of the conductivity can be observed in a specific dopant concentration.The calculated results show that the greater the transferred charges,the higher the conductivities in doped organic electroluminescent devices.The results agree basically with experimental results.     

Conformational Changes of the Troponin—Tropomyosin Complex on F-Actin Observed by Fluorescence Resonance Energy Transfer Measurements

Contraction of vertebrate striated muscle is regulated by the strong Ca²⁺-dependent interaction among troponin (Tn), tropomyosin (Tm), and actin on the thin filament. Using fluorescence resonance energy transfer (FRET), the interactions between Tm and the Tn complex or between Tm and the Tn subunit, TnI or TnC, with or without other troponin subunits, were characterized in the presence or absence of F-actin and Ca²⁺ ions. Cys-190 of Tm was selectively labeled with the acceptor probe, 4-dimethylaminophenylazophenyl 4-maleimide. Troponin was selectively labeled at position 9 or 133 of TnI and position 98 of TnC with a donor probe, 5-(2-iodoacetylaminoethyl)aminonaphtha lene 1-sulfonic acid. FRET measurements indicate that the interaction between TnI and Tm alone is very weak, but that in the presence of F-actin, TnI binds to the proper binding site on Tm even in the absence of TnT. The distances between Cys-190 of Tm on F-actin and Cys-9 or Cys-133 of Tnl or Cys-98 of TnC in the reconstituted Tn were determined to be 52.8, 53.7, Å and 56.5 Å, respectively, in the absence of Ca²⁺, indicating that the Tnl—TnC complex, the globular portion of Tn, is located near Cys-190 of Tm on the reconstituted thin filaments. Upon binding of Ca²⁺ to TnC, these distances increased by 5.6 and 1.4 Å or decreased by 5.4 Å, respectively. These Ca²⁺-induced changes in Tn—Tm seem to occur only when F-actin is present, suggesting that the stable complex formation of TnI with the outer domain of F-actin upon removal of Ca²⁺ is a very important event during inhibition.