| 1. |
- Andreasson, Joakim, 1973, et al.
(author)
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Temperature and viscosity dependence of the triplet energy transfer process in porphyrin dimers
- 2002
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In: Photochemical & Photobiological Sciences. - : Springer Science and Business Media LLC. - 1474-905X .- 1474-9092. ; 1:2, s. 111-119
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Journal article (peer-reviewed)abstract
- The temperature and viscosity dependence of the triplet energy transfer (TET) process in porphyrin dimers has been studied. A zinc porphyrin (donor) and a free base porphyrin (acceptor) are covalently linked together by rigid bridging chromophores at a center-center distance of 25 Angstrom. Due to the large donor-acceptor distance and the weakness of the spin forbidden transitions involved, neither direct (through space) electron exchange nor Coulombic mechanisms are expected to contribute to the observed TET process. The results from transient absorption measurements at temperatures between room temperature and 80 K show that TET occurs with unexpectedly high efficiency in the systems connected by fully conjugated bridges and a pronounced temperature dependence of the process is observed. Comparison of the TET efficiencies in dimers connected by different bridging chromophores correlates well with a transfer reaction governed by a through bond exchange (superexchange) interaction. However, in high viscosity media the TET process is dramatically slowed down. This is attributed to a conformational gating of the TET process where the electronic coupling varies strongly with the relative orientation of the donor and the bridging chromophore. Further, the zinc porphyrin donor offers two distinct donor species, T-1A and T-1B. At room temperature, the TET rate constant of the T-1A Species is about two orders of magnitude larger than for the T-1B species. The dimers studied are well suited model systems for materials where the rate of the transfer reactions can be changed by external stimuli.
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| 2. |
- Kyrychenko, Alexander, et al.
(author)
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Sterically induced conformational relaxation and structure of meso-diaryloctaalkyl porphyrins in the excited triplet state: Experimental and DFT studies
- 2002
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In: Journal of Physical Chemistry B Materials. - : American Chemical Society (ACS). - 1089-5647 .- 1520-6106 .- 1520-5207. ; 106:48, s. 12613-12622
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Journal article (peer-reviewed)abstract
- The excited triplet state conformations of zinc and free base 5,15-diaryloctaalkylporhyrins are studied by experimental and computational (density functional theory, DFT) methods. From the observations of an unusual triplet state dynamics, i.e., fast nonradiative and biexponential decay, it has been suggested that these porphyrins exist in two distinguishable conformers in the lowest excited triplet state. X-ray crystallography and,DFT (B3LYP/6-31G(d)) optimization of the ground state show that the porphyrins are planar prior to excitation. However, in the excited triplet state, the planar structure relaxes to an out-of-plane distorted saddle-shaped conformer. This distorted conformer and the lowest triplet potential energy surface are characterized by DFT calculations. It is suggested that the conformational relaxation explains the unusual triplet dynamics of this class of porphyrins.
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| 3. |
- Pettersson, Karin, 1975, et al.
(author)
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Singlet Energy Transfer in Porphyrin-Based Donor-Bridge-Acceptor Systems: Interaction between Bridge Length and Bridge Energy
- 2006
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In: Journal of Physical Chemistry A. - : American Chemical Society (ACS). - 1089-5639 .- 1520-5215. ; 110:1, s. 310-318
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Journal article (peer-reviewed)abstract
- Singlet excitation energy transfer is governed by two donor-acceptor interactions, the Coulombic and exchange interactions giving rise to the Foerster and Dexter mechanisms, resp., for singlet energy transfer. In transfer between colliding mols. or between a donor (D) and acceptor (A) connected in donor-bridge-acceptor (D-B-A) system by an inert spacer (B), the distinction between these two mechanisms is quite clear. However, in D-B-A systems connected by a p-conjugated bridge, the exchange interaction between the donor and acceptor is mediated by the virtual low-lying excited states (unoccupied orbitals) of that bridge and, as a consequence, becomes much more long-range in character. Thus, the clear distinction to the Coulombic mechanism is lost. This so-called superexchange mechanism for singlet energy transfer has been shown to make a significant contribution to the energy transfer rates in several D-B-A systems, and its D-A distance as well as D-B energy gap dependencies have been studied. We here demonstrate that in a series of oligo-p-phenyleneethynylene (OPE) bridged porphyrin-based D-B-A systems with varying D-A distances the Foerster and through-bond (superexchange) mechanisms both make considerable contributions to the obsd. singlet energy transfer rates. The donor is either a zinc porphyrin or a zinc porphyrin with a pyridine ligand, and the acceptor is a free base porphyrin. By comparison to a homologous series where only the D-B energy gaps varies, a sepn. between the two energy transfer mechanisms was possible and, moreover, an interplay between distance and energy gap dependencies was noted. The distance dependence was shown to be approx. exponential with an attenuation factor b = 0.20 .ANG.-1. If the effect of the varying D-B energy gaps in the OPE series was taken into account, a slightly higher b-value was obtained. Ground-state absorption, steady-state, and time-resolved emission spectroscopy were used. The exptl. study is accompanied by time-dependent d. functional theory (TD-DFT) calcns. of the electronic coupling, and the exptl. and theor. results are in excellent qual. agreement (same distance dependence).
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| 4. |
- Vargas-Uribe, Mauricio, et al.
(author)
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Thermodynamics of Membrane Insertion and Refolding of the Diphtheria Toxin T-Domain
- 2015
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In: Journal of Membrane Biology. - : Springer Science and Business Media LLC. - 0022-2631 .- 1432-1424. ; 248:3, s. 383-394
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Journal article (peer-reviewed)abstract
- The diphtheria toxin translocation (T) domain inserts into the endosomal membrane in response to the endosomal acidification and enables the delivery of the catalytic domain into the cell. The insertion pathway consists of a series of conformational changes that occur in solution and in the membrane and leads to the conversion of a water-soluble state into a transmembrane state. In this work, we utilize various biophysical techniques to characterize the insertion pathway from the thermodynamic perspective. Thermal and chemical unfolding measured by differential scanning calorimetry, circular dichroism, and tryptophan fluorescence reveal that the free energy of unfolding of the T-domain at neutral and mildly acidic pH differ by 3-5 kcal/mol, depending on the experimental conditions. Fluorescence correlation spectroscopy measurements show that the free energy change from the membrane-competent state to the interfacial state is approximately -8 kcal/mol and is pH-independent, while that from the membrane-competent state to the transmembrane state ranges between -9.5 and -12 kcal/mol, depending on the membrane lipid composition and pH. Finally, the thermodynamics of transmembrane insertion of individual helices was tested using an in vitro assay that measures the translocon-assisted integration of test sequences into the microsomal membrane. These experiments suggest that even the most hydrophobic helix TH8 has only a small favorable free energy of insertion. The free energy for the insertion of the consensus insertion unit TH8-TH9 is slightly more favorable, yet less favorable than that measured for the entire protein, suggesting a cooperative effect for the membrane insertion of the helices of the T-domain.
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