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H Stiel

Publications and source records attributed to H Stiel.

15 recordsLinked to original sources
PubMed2005

Towards elucidating the energy of the first excited singlet state of xanthophyll cycle pigments by X-ray absorption spectroscopy.

The first excited singlet state (S(1)) of carotenoids (also termed 2A(g)(-)) plays a key role in photosynthetic excitation energy transfer due to its close proximity to the S(1) (Q(y)) level of chlorophylls. The determination of carotenoid 2A(g)(-) energies by optical techniques is difficult; transitions from the ground state (S(0), 1A(g)(-)) to the 2A(g)(-) state are forbidden ("optically dark") due to parity (g <-- //--> g) as well as pseudo-parity selection rules (- <-- //--> -). Of particular interest are S(1) energies of the so-called xanthophyll-cycle pigments (violaxanthin, antheraxanthin and zeaxanthin) due to their involvement in photoprotection in plants. Previous determinations of S(1) energies of violaxanthin and zeaxanthin by different spectroscopic techniques vary considerably. Here we present an alternative approach towards elucidation of the optically dark states of xanthophylls by near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The indication of at least one pi* energy level (about 0.5 eV below the lowest 1B(u)(+) vibronic sublevel) has been found for zeaxanthin. Present limitations and future improvements of NEXAFS to study optically dark states of carotenoids are discussed. NEXAFS combined with simultaneous optical pumping will further aid the investigation of these otherwise hardly accessible states.

Spectrophotometry, Ultraviolet↗
PubMed2004

Explosion characteristics of intense femtosecond-laser-driven water droplets.

An efficient acceleration of energetic ions is observed when small heavy-water droplets of approximately 20 microm diameter are exposed to ultrafast (approximately 40 fs) Ti:sapphire laser pulses of up to 10(19) W/cm2 intensity. Quantitative measurements of deuteron and neutron spectra were done, allowing one to analyze the outward and inward directed deuteron acceleration from the droplet. Neutron spectroscopy based on the D (d,n) fusion reaction was accomplished in four different spatial directions. The energy shifts of those fusion neutrons produced inside the exploding droplet reflect a remaining deuteron acceleration inside the irradiated droplet along the axis of the incident laser beam. The overall neutron yield of the microdroplets is relatively small as a result of the dominant outward directed acceleration of the deuterons with 1200 neutrons/shot. Relying on the "explosion-like" acceleration of such spherical droplet targets we have developed a spray target consisting of heavy-water microspheres with diameters of 150 nm . Both the high deuteron energies of up to 1 MeV resulting from the irradiation intensity of approximately 10(19) W/cm2 as well as the collisions between the deuterons and the surrounding spray delivered about one order of magnitude more neutrons than the single-droplet system. The approximately 6 x 10(3) neutrons per laser pulse from the spray can be attributed to an efficient deuteron release from a significantly smaller laser excited volume as from deuterium-cluster targets.

Journal Article↗
PubMed2001

Evaluation of the energetic position of the lowest excited singlet state of beta-carotene by NEXAFS and photoemission spectroscopy.

In carotenoids the lowest energetic optical transition belonging to the pi-electron system is forbidden by symmetry, therefore the energetic position of the S(1) (2(1)A(g)) level can hardly be assessed by optical spectroscopy. We introduce a novel experimental approach: For molecules with pi-electron systems the transition C1s-->2p(pi*) from inner-atomic to the lowest unoccupied molecular orbital (LUMO) appears in X-ray absorption near edge spectra (NEXAFS) as an intense, sharp peak a few eV below the carbon K-edge. Whereas the peak position reflects the energy of the first excited singlet state in relation to the ionization potential of the molecule, intensity and width of the transition depend on hybridization and bonding partners of the selected atom. Complementary information can be obtained from ultraviolet photoelectron spectroscopy (UPS): At the low binding energy site of the spectrum a peak related to the highest occupied molecular orbital (HOMO) appears. We have measured NEXAFS and UPS of beta-carotene. Based on these measurements and quantum chemical calculations the HOMO and LUMO energies can be derived.

Electron Probe Microanalysis↗
PubMed2001

Absolute extreme ultraviolet yield from femtosecond-laser-excited Xe clusters.

Large Xe clusters (10(5)-10(6) atoms per cluster) have been irradiated with ultrashort (50 fs) and high-intensity ( approximately 2 x 10(18) W/cm(2)) pulses from a Ti:sapphire multi-TW laser at 800 nm wavelength. Scaling and absolute yield measurements of extreme ultraviolet (EUV) emission in a wavelength range between 7 and 15 nm in combination with cluster target characterization have been used for yield optimization. Maximum emission as a function of the backing pressure and a spatial emission anisotropy covering a factor of two at optimized yields is discussed with a simple model of the source geometry and EUV-radiation absorption. Circularly polarized laser light instead of linear polarization results in a factor of 2.5 higher emission in the 11 to 15 nm wavelength range. This indicates the initial influence of optical-field ionization for the interaction parameter range used and contrasts to collisional heating that seems to influence preferentially higher ionization. Absolute emission efficiency at 13.4 nm of up to 0.5% in 2pi sr and 2.2% bandwidth has been obtained.

Anisotropy↗
PubMed2001

Towards time-resolved, coupled structure-function information on carotenoid excited state processes: X-ray and optical short-pulse double resonance spectroscopy.

A novel soft X-ray and optical short-pulse double resonance spectroscopic technique tailor-made to elucidate processes involving the optically forbidden S1 (2(1)A(g)) state of carotenoids in native biological samples (e.g., photosynthetic antenna complexes) is described. The principle relies on probing the near carbon K-edge absorption of the optically excited sample with soft X-rays generated by a laser-induced plasma. A first application concerns location of the 2(1)A(g) state of beta-carotene in vitro. Further applications are proposed.

Biophysics↗
PubMed2000

Pigment-protein architecture in the light-harvesting antenna complexes of purple bacteria: does the crystal structure reflect the native pigment-protein arrangement?

Structural analysis of crystallized peripheral (LH2) and core antenna complexes (LH1) of purple bacteria has revealed circular aggregates of high rotational symmetry (C8, C9 and C16, respectively). Quantum-chemical calculations indicate that in particular the waterwheel-like arrangements of pigments should show characteristic structure-sensitive spectroscopic behavior in the near infrared absorption region. Laser-spectroscopic data obtained with non-crystallized, isolated LH2 of Rhodospirillum molischianum are in line with a highly symmetric (C8) circular aggregate, but deviations have been found for LH2 of Rhodobacter sphaeroides and Rhodopseudomonas acidophila. For both the latter, C-shaped incomplete circular aggregates (as seen only recently in electron micrographs of crystallized LH1-reaction center complexes) may be a suitable preliminary model.

Bacterial Proteins↗
PubMed2000

Photophysical studies of the pheophorbide a dimer.

This work reports on a monomer-dimer equilibrium state of pheophorbide a in solution. A methodology for controlling the equilibrium constant by use of temperature and solvent variation is described. The absorption spectrum of the dimer is calculated, using different prepared equilibria of monomer and dimer in solution. We propose that these aggregates provide a good model for understanding the dimerization process in tetrapyrroles.

Absorption↗
PubMed2000

Photophysical consequences of coupling bacteriochlorophyll a with serine and its resulting solubility in water.

We investigated the dependence on solvents of optical absorption and emission of the bacteriochlorophyll a-serine (BChl-ser), a water soluble bacteriochlorophyll (BChl) derivative. Comparison between the experimental data and those collected for BChl in nonaqueous solvents shows that only a minor interaction takes place between serine and the macrocycle's pi-electron system. Nevertheless, the coupling with serine results in a small enhancement of the nonradiative relaxation rate from the first excited singlet state S1. In buffered aqueous solution (pH = 7.4), the Stokes shift of the BChl-ser fluorescence and its nonradiative relaxation rate are enhanced compared with those in nonaqueous solutions (Scherz, A., S. Katz, Y. Vakrat, V. Brumfeld, E. Gabelmann, D. Leupold, J. R. Norris, H. Scheer and Y. Salomon (1998) Photosynthesis: Mechanisms and Effects, Vol. V (Edited by G. Garab), pp. 4207-4212. Kluwer Academic, Dordrecht.), probably as a result of a hydrogen bonding between the BChl macrocycle and the water molecules. In aprotic solvents, without hydrogen bonds, the permanent dipole moment of the first excited singlet state in both BChl and BChl-ser is increased compared with the ground state by at least 2.5 Debye.

Bacteriochlorophylls↗
PubMed1996

Quantitative comparison of excited state properties and intensity- dependent photosensitization by rose bengal.

A quantitative (multistep) excitation-deactivation model of rose bengal (RB) has been developed which includes energy and electron transfer to oxygen and the possibility of photoproduct formation via higher excited triplet-singlet states. The model is based on previous measurements of non-linear absorption (NLA) and emission of RB with picosecond pulses at 532 nm and on NLA measurements with nanosecond pulses. A coupled rate equation and photon transport equation approach for non-linear light-matter interaction is used. The resulting term scheme with all relevant excited state parameters confirms that (i) in the first excited state of RB, relevant absorption at 532 nm takes place only in the triplet, and (ii) the previously reported intensity dependence of RB-sensitized enzyme inhibition is well modelled by the intensity-dependent RB-T1 population and (as the main process) subsequent energy transfer to form singlet oxygen.

Lasers↗
PubMed1996

Intensity-dependent enzyme photosensitization using 532 nm nanosecond laser pulses.

The intensity dependence of the rose bengal (RB)-photosensitized inhibition of red blood cell acetylcholinesterase has been studied experimentally and the results compared to a quantitative excitation/deactivation model of RB photochemistry. Red blood cell membrane suspensions containing 5 microM RB were irradiated with 532 nm, 8 ns laser pulses with energies between 1 and 98.5 mJ. A constant dose (7 J) was delivered to all samples by varying the total number of pulses. At incident energies greater than approximately 4.5 mJ/pulse, the efficiency for photosensitized enzyme inhibition decreased as the energy/pulse increased. The generation of RB triplet state was monitored as a function of laser energy and the triplet-triplet absorption coefficient was determined to be 1.9 x 10(4) M-1 cm-1 at 530 nm. The number of singlet oxygen molecules produced at each intensity was calculated from both the physico-mathematical model and from laser flash photolysis results. The results indicated that the photosensitized inhibition of acetylcholinesterase was exclusively mediated by singlet oxygen, even at the highest laser intensities employed.

Chemical Phenomena↗
PubMed1993

Photophysical properties of the photosensitizer pheophorbide a studied at high photon flux densities.

The photophysical properties of pheophorbide a (Pheo a) under two-step laser activation were investigated. For the first time quantum yields of higher excited state formation were calculated. It was shown that the quantum yields of the formation of these states depend strongly on the pulse duration of the excitation source. The nonlinear properties of Pheo a are quite different in dependence on the excitation wavelength but the quantum yields of the higher excited state formation in both cases (lambda exc = 337 nm and lambda exc = 667 nm) rise up increasing the photon flux density of the laser light up to 10(26) phot cm-2 s-1. A further increase of the photon flux density has no effect on the quantum yields.

Chlorophyll↗
PubMed1993

Spectroscopic properties of potential sensitizers for new photodynamic therapy start mechanisms via two-step excited electronic states.

Three substituted tetraazaporphyrins, octa-(4-tert-butylphenyl)-tetrapyrazinoporphyrazine, tetra-(4-tert-butyl)phthalocyanine and tetra-(4-tert-butyl)phthalocyanatomagnesium(t4-PcMg), were spectroscopically checked in solutions and liposomes with respect to suitability as potential sensitizers of a possible new start mechanism for photodynamic therapy (PDT) from a stepwise excited higher singlet state. This PDT start mechanism was recently proposed to overcome the problem of O2 (1delta2)-caused cutaneous phototoxicity in PDT. By means of absorption and fluorescence measurements as well as nonlinear absorption investigation, transient spectroscopy and lasing experiments, compound t4-PcMg was found to have the most appropriate properties: Based on a high fluorescence quantum yield (phiF1 = 0.84) and a very low crossing to the triplet (phiISC = 0.05), two higher excited states can be effectively populated both by two stepwise absorption transitions at 674nm consecutive absorption transitions at 684 and and 710 nm. Moreover, t4-PcMg incorporates into liposomes very well with spectroscopic properties similar to those in solution.

Indoles↗
PubMed1989

Distance between lysine 384 and heme of cytochrome P-450 LM2 (P-450 IIB4) studied by fluorescence energy transfer measurements.

The distance between FITC-modified lysine 384 of cytochrome P-450 LM2 and the active site, heme, was estimated by fluorescence energy transfer measurements. To avoid differential labelling of P-450 LM2 for protection of the alpha-amino group from FITC modification, deconvolution of measured fluorescence decay curves using a double exponential model was performed. A value of 2.7 nm was obtained for the distance FITC (lysine 384) - heme. This distance is too large to account for a direct electron tunneling from prosthetic group to prosthetic group at this interaction site between reductase and P-450 LM2.

Animals↗
PubMed1983

Modification of cytochrome P-450 with fluorescein isothiocyanate.

Fluorescein isothiocyanate (FITC) has been shown to be selectively attached to the N-terminus of cytochrome P-450 LM2. The N-demethylase activity of cytochrome P-450 LM2 reconstituted systems modified in this way was inhibited by 25%. As revealed by CD measurements the overall conformation as well as the immediate heme environment of cytochrome P-450 LM2 remained unchanged after attachment of the FITC molecule. The binding affinity of modified cytochrome P-450 LM2 toward benzphetamine and aniline and the cumene hydroperoxide- or H2O2-supported N-demethylation of benzphetamine are maintained. However, the introduction of the electron via NADPH-cytochrome P-450 reductase (EC 1.6.2.4) is impaired after modification of the alpha-amino group. The extent of reduced modified cytochrome P-450 LM2 in the cytochrome P-450 reductase-supported reduction reaction is diminished and the half-time of the reduction is increased. The diminished reducibility is ascribed to steric hindrance of groups directly involved in the interaction between cytochrome P-450 LM2 and NADPH-cytochrome P-450 reductase or to blocking of the charge-pair interactions between the alpha-amino group of P-450 LM2 and the respective negatively charged group of NADPH-cytochrome P-450 reductase. By energy-transfer measurements distances between the heme and the alpha-amino group of 2.65 and 3.97 nm for the oligomeric and the monomeric forms of P-450 LM2, respectively, have been determined.

Animals↗