Fast Relaxation Component Research Articles

Electronic structures of single-walled carbon nanotubes determined by NMR

Single-walled carbon nanotubes were studied by (13)C nuclear magnetic resonance (NMR). Two types of (13)C nuclear spins were identified with different spin-lattice relaxation rates. The fast-relaxing component, assigned to metallic tubes, followed the relaxation behavior expected in metals, and the density-of-states at the Fermi level increased with decreasing tube diameter. The slow-relaxing component has a significantly lower density-of-states at the Fermi level. Exposure to oxygen has a substantial effect on relaxation rates of both components.

Investigation of nonelastic response of semicrystalline polymers at high strain levels

The nonelastic behavior at high strains of three semicrystalline polymers [i.e., nylon-6, poly(ethylene terephthalate), and poly(ethylene 2,6-naphthalenedicarboxylate] was investigated. For all materials, room temperature tensile strain recovery tests revealed the existence of two components of nonelastic deformation: a fast-relaxing component (called anelastic) and a slow-relaxing component (usually called plastic). A strain recovery master curve could be constructed for each material from the strain recovery data obtained at various temperatures. The shift factors versus temperature relationship for the strain recovery master curves allowed us to evaluate an activation energy for the nonelastic strain recovery process. These data were then compared with the activation energy for the glass-transition process evaluated by dynamic mechanical measurements at low strain. The aim of this comparison was to investigate the influence of viscoelasticity on the nonelastic deformation recovery. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 78: 1664–1670, 2000

Recovery of post-yielding deformations in semicrystalline poly(ethylene-terephthalate)

In this paper, aspects of the non-elastic deformation of semicrystalline poly(ethylene-terephthalate) (PET) films were studied from strain recovery and differential scanning calorimetric tests. The results show the existence of two components of non-elastic deformation, i.e. a fast-relaxing component (called anelastic) and a slow-relaxing component (usually called plastic). These strain components are both reversible and distinguished only on the basis of their different recovery times at temperatures far below the glass transition. A strain recovery master curve was built from the results of recovery tests at increasing recovery temperature. The shift-factor for the strain recovery master curve was then compared with the shift-factor for the construction of the dynamic storage modulus master curve obtained in linear regime (small strain). The aim of this comparison was to investigate the viscoelastic nature of yielding and post-yielding behavior in a semicrystalline polymer.

Diurnal changes in chlorophyll a fluorescence, CO(2)-exchange and organic acid decarboxylation in the tropical CAM tree Clusia hilariana.

Crassulacean acid metabolism (CAM) plants are dependent on the organic acids that accumulate overnight in the vacuoles as a source of CO(2) during the daylight deacidification period, when stomata are closed and high irradiances generally prevail. We performed an integrative analysis of diurnal changes in gas exchange, chlorophyll fluorescence parameters and organic acid decarboxylation to understand the adjustments in photochemical and non-photochemical processes during the different CAM phases in Clusia hilariana Schlecht., a dominant tree species in the sandy coastal plains of southeastern Brazil. A linear relationship was obtained between the quantum yields of photochemical and non-photochemical quenching, irrespective of the CAM phase and prevailing irradiance. Degradation of malic and citric acids during the midday stomatal closure period could lead to potential CO(2) fixation rates of 23 &mgr;mol m(-2) s(-1), whereas CO(2) losses, measured as CO(2) evolution, corresponded to about 3% of this value. Thus, decarboxylation of malate and citrate provided high internal CO(2) concentrations during phase III of CAM, even though the stomata were closed, allowing optimal utilization of light energy, as indicated by the non-saturating electron transport rates (ETR) in the light response curves, with highest rates of ETR occurring at midday in the diurnal curves. At the transition from phase III to IV of CAM, depletion of internal CO(2) sources and low stomatal conductances, which restricted the supply of exogenous CO(2), reduced the demand for photochemical energy to drive carbon assimilation. This was compensated by increases in thermal energy dissipation as indicated by higher rates of non-photochemical quenching, while high irradiances still prevailed. Shifts in the CAM phases and changes in protective thermal dissipation potential allowed C. hilariana to match changes in PPFD patterns for leaves of different orientations. Evidence that most of the decline in photochemical efficiency was probably related to the fast-relaxing component of non-photochemical quenching is provided by the high values of the quantum yield of photosystem II after 20 min of relaxation in darkness, and an almost complete recovery after sunset. These adjustments in photosynthetic machinery minimized the danger of photo-inhibition in C. hilariana, which is commonly found in fully exposed habitats.

Correlations between chlorophyll fluorescence quenching parameters and photosynthesis in a segregating Lycopersicon esculentum × L. peruvianum population as measured under constant conditions

Chl fluorescence of mature leaves in low-temperature treated plants was studied under identical measuring conditions in a segregating population of the F3 offspring of a cross between a chilling-tolerant and a chilling-sensitive tomato species. Through recombination of genes involved in photosynthesis, the population revealed a wide, continuous variability of photosynthetic capacity from plants performing much worse to those performing better than the parental lines of the cross. In the parental species, a nearly linear correlation was observed between photochemical chl fluorescence quenching (qP) and O2 evolution over a wide temperature range. Across the F3 generation, still a weak correlation between the two parameters was found at 20 °C, but not at 10 °C, when measured under identical conditions. This indicates that the fraction of open reaction centres could at least in part be adjusted to the photosynthetic capacity of the individual genotype. However, the correlation was so weak, that the previously suggested use of qP as a selection criterion for chilling tolerance of photosynthesis in breeding programs is regarded as doubtful, as long as photosynthesis rates are not measured in addition. Quantum efficiency of Photosystem II (ΦPSII) was strongly dependent on qP both at 20 and at 10 °C measuring temperature and depended on the quantum efficiency of open reaction centres (F′v/F′m) at 20, but not at 10 °C. F′v/F′m, in turn, correlated negatively with the processes of energy dissipation by the mechanisms of non-photochemical quenching (qN), i.e. its fast-relaxing component (qF) and photoinhibitory quenching (qI).

μ+SR study of heavy-fermion magnetism in Ce(Cu1-xNix)2Ge2

We report muon spin relaxation (μ+SR) studies on the magnetic phase diagram of Ce(Cu1-xNix)2Ge2 polycrystals for 0.5≤ x ≤ 0.8. A sharp magnetic transition, evidenced by the appearance of a fast Gaussian relaxation component σ, has been observed in the x = 0.5 alloy at 4.0 K in zero applied field. The average local field at the stopping sites of the muons, extracted from σ, exhibits a linear temperature dependence. We associate these features with an incommensurate spin density wave (SDW) ordering. Magnetic ordering, either long range or short range, and signatures of non-Fermi liquid behaviour have not been observed down to 2.0 K at x = 0.8.

Hydrocarbon chain packing in the micellar core of surfactants studied by 1 H NMR relaxation

H-1 NMR spin-lattice and spin-spin relaxation of different types (cationic cetyltrimethyl ammonium bromide, anionic sodium dodecyl sulfonate and nonionic Triton X-100) of surfactants in water solution were studied. Simulation of the decay curves of proton relaxation shows that the spin lattice relaxation of all the samples exhibits exponentially, while the spin-spin relaxation for several protons on the hydrophobic chains forming the micellar core is bi-exponential. The fast relaxing component is attributed to the part of the segments of the hydrophobic chain, situated near or on the surface of the micellar core, while the slower relaxing component is attributed to the rest part staying in the interior. The latter exchanges with the former in equilibrium. Thus, a part of each certain segment of the hydrophobic chain has an opportunity to stay in the surface layer of the micellar core and spend some time on the interface experiencing hydrophilic environment. Generally, the protons on the methylene carbon of the hydrophobic chain nearest to the polar head have more chance to spend time in the hydrophilic environment. However, it seems to be dependent on the chemical structure of the surfactant molecule. Large size of the polar group of CTAB shows steric hindrance on the packing of the hydrophobic chain. Quantitative results are given. The fact, that the fraction of slow relaxing protons on the hydrophilic ethylene oxide long chain of Triton X-100 dominates over that of fast relaxing protons, and that their T-2 values are larger than those of the protons on the hydrocarbon chain in the interior of the micellar core, suggests that the ethylene oxide chain does not participate in the formation of the micellar core.

Photosynthetic light utilization and xanthophyll cycle activity in the galactolipid deficient dgd1 mutant of Arabidopsis thaliana

Photochemical and nonphotochemical light utilization was studied in the digalactosyl diacylglycerol-deficient dgd1 mutant of Arabidopsis thaliana. While the contents of the photosynthetic metabolites and carbohydrates analyzed were found to be unchanged, chlorophyll fluorescence quenching and xanthophyll-cycle activity were distinctly different in the mutant. A decrease in the quantum yield of photosystem II electron transport under non-saturating photon flux densities in the dgd1 mutant was fully accounted for by an increase in nonradiative energy dissipation, measured as nonphotochemical fluorescence quenching (qN). Furthermore, the mutant showed a decreased amplitude of the fast relaxing component of qN considered to reflect high-energy state quenching, but displayed an increased slowly relaxing component of qN ascribed to photoinhibition. The slowly relaxing qN component was correlated to persisting amounts of antheraxanthin and zeaxanthin, still present even after prolonged dark exposure of previously illuminated leaves. Violaxanthin deepoxidation was found to be accelerated in the dgd1 mutant, but the proportion of violaxanthin that becomes deepoxidized was similar to that of the wild type. Our data suggest that xanthophyll-cycle operation is involved in the reduction of the photosystem II quantum yield in the dgd1 mutant. The results are discussed in terms of the altered thylakoid membrane organization of the dgd1 mutant.

One- and two-photon-excited time-resolved photoluminescence investigations of bulk and surface recombination dynamics in ZnSe

Measurements of femtosecond time-resolved one- and two-photon-excited photoluminescence of bulk ZnSe enable us to distinguish the surface and bulk contributions to recombination dynamics. A photoluminescence lifetime of several nanoseconds or longer is measured for the bulk. A fast relaxation component with a decay time constant τT of a few tens of picoseconds observed in one-photon-pumped time-resolved spectra is identified as the result of diffusion and rapid surface recombination. A one-dimensional model taking into account surface nonradiative recombination and carrier diffusion is able to describe the observed behavior. The temperature dependence of τT shows good agreement with the theory. At room temperature, a surface recombination velocity S=5.8×105 cm s−1 and a diffusion constant D=10 cm2 s−1 are found using this model.

Interactive effects of photon fluence rates and drought on CAM-cycling in Delosperma tradescantioides (Mesembryanthemaceae).

The ability of photosynthesis and CAM to acclimate to low (220 µmol m-2 s-1 ; LL) and relatively high (550 µmol m-2 s-1 ; HL) photosynthetic photon flux densities (PPFD) was investigated in the CAM-cycling species Delosperma tradescantioides by means of CO2 gas exchange and chlorophyll fluorescence analysis. Furthermore, the influence of short-term drought on malic acid accumulation and the activity of photosystem II (PSII) was studied to assess the possible interactions between drought and the prevailing PPFD in this species. HL plants showed features of sun versus shade acclimation relative to LL plants. Nocturnal malic acid accumulation (Δ-malate) and leaf water content also tended to be higher in HL plants. Irrespective of the PPFD during growth, the weak Δ-malate doubled within 3 days of drought. Despite largely restricted CO2 uptake, photosynthetic activity as estimated from fluorescence analysis declined only ca 5%. After 7 days of drought, when plants showed CAM-idling and Δ-malate had decreased again, potential carbon assimilation was still ca 84% of that in well-watered plants and remained relatively constant throughout the day. Decarboxylation of malic acid accounted for ca 23% of potential assimilation assuming total oxidation of a maximum portion of this organic acid. Drought did not affect predawn maximum photochemical efficiency (Fv /Fm ). Nonphotochemical quenching (qN) increased (24%) in response to desiccation and resulted in a more or less constant reduction state of PSII. This increase in qN resulted mainly from the change in its fast-relaxing component (qNF), while the slow component (qNS) was significant only at or above saturating PPFD in both HL and LL plants. The photon response characteristics of PSII, which differed between LL and HL plants, were unaffected by short-term drought. Photon harvesting and photon use were always adjusted to guarantee a low reduction state of PSII. Results suggest that in both LL and HL plants CAM-cycling may help to stabilize photosynthesis but to a large extent by other means than simply providing internally derived CO2 .

Salt effects on the structure and internal dynamics of superhelical DNAs studied by light scattering and Brownian dynamics

Using laser light scattering, we have measured the static and dynamic structure factor of two different superhelical DNAs, p1868 (1868 bp) and simian virus 40 (SV40) (5243 bp), in dilute aqueous solution at salt concentrations between 1 mM and 3 M NaCl. For both DNA molecules, Brownian dynamics (BD) simulations were also performed, using a previously described model. A Fourier mode decomposition procedure was used to compute theoretical light scattering autocorrelation functions (ACFs) from the BD trajectories. Both measured and computed autocorrelation functions were then subjected to the same multiexponential decomposition procedure. Simulated and measured relaxation times as a function of scattering angle were in very good agreement. Similarly, computed and measured static structure factors and radii of gyration agreed within experimental error. One main result of this study is that the amplitudes of the fast-relaxing component in the ACF show a peak at 1 M salt concentration. This nonmonotonic behavior might be caused by an initial increase in the amplitudes of internal motions due to diminishing long-range electrostatic repulsions, followed by a decrease at higher salt concentration due to a compaction of the structure.

MuSR/μ + SR studies in KDP and DKDP

The temperature dependence of muon interactions has been studied in ferroelectric KDP ( H2KPO4) and DKDP ( D2KPO4) using conventional μSR and muon spin resonance spectroscopy. In longitudinal field measurements, a fast relaxing component and a slow relaxing component were observed. The slow relaxing component is attributed to diamagnetic muons. The muon spin resonance measurements indicate that the fast relaxing component results from some muonium like species: either normal or anomalous. In zero field and weak longitudinal field μSR (0–100 G), a remarkable peak in the fast relaxing component is observed around 220 K in both KDP and DKDP. An additional feature is also seen around 300 K. The amplitude of the resonance measurement has a broad minimum around 200 K which corresponds to the maximum in the relaxation rate in longitudinal field (100 G). The temperature dependence of the muonium relaxation rate in KDP is almost identical to that of DKDP. The diamagnetic fraction also shows almost no difference in relaxation rate or asymmetry for DKDP and KDP.

Geometrical confinement and cooperativity in supercooled liquids studied by solvation dynamics.

The molecular dynamics of supercooled liquids confined to the mesopores of sol-gel glasses are interpreted in terms of the spatial competition between the average length scale of cooperativity assumed to increase with time and the geometrical confinement on scales between 2.5 and 7.5 nm. Opposed to relating the fast and slow relaxation components to spatially distinct regimes, the picture of a mesoscopically uniform but cooperative relaxation as stimulated by the theoretical work of J\"ackle is proposed. It will be demonstrated that rationalizing the data along these lines leads to a consistency with the theory while relaxing some of the conceptual problems encountered in previous interpretations. The data supports the idea that the relaxation in restricting geometries proceeds like in the bulk liquid until the length scale \ensuremath{\xi}(t)\ensuremath{\propto}log(t) of cooperativity reaches the pore size leading to nonergodic behavior within the experimental time scale. For a characteristic cooperativity length ${\ensuremath{\xi}}_{\mathit{c}}$ the data analysis yields ${\ensuremath{\xi}}_{\mathit{c}}$(T)\ensuremath{\propto}T and ${\ensuremath{\xi}}_{\mathit{c}}$(${\mathit{T}}_{\mathit{g}}$)\ensuremath{\approxeq}3 nm. The solvation dynamics results under study are compared to analogous dielectric relaxation data. \textcopyright{} 1996 The American Physical Society.

Control of Photosystem II in spinach leaves by continuous light and by light pulses given in the dark

The light-induced induction of components of non-photochemical quenching of chlorophyll fluorescence which are distinguished by different rates of dark relaxation (qNf, rapidly relaxing and qNs, slowly relaxing or not relaxing at all in the presence brief saturating light pulses which interrupt darkness at low frequencies) was studied in leaves of spinach.After dark adaptation of the leaves, a fast relaxing component developed in low light only after a lag phase. Quenching increased towards a maximum with increasing photon flux density. This 'fast' component of quenching was identified as energy-dependent quenching qE. It required formation of an appreciable transthylakoid ΔpH and was insignificant when darkened spinach leaves received 1 s pulses of light every 30 s even though zeaxanthin was formed from violaxanthin under these conditions.Another quenching component termed qNs developed in low light without a lag phase. It was not dependent on a transthylakoid pH gradient, decayed exponentially with a long half time of relaxation and was about 20% of total quenching irrespective of light intensity. When darkened leaves were flashed at frequencies higher than 0.004 Hz with 1 s light pulses, this quenching also appeared. Its extent was very considerable, and it did not require formation of zeaxanthin. Relaxation was accelerated by far-red light, and this acceleration was abolished by NaF.We suggest that qNs is the result of a so-called state transition, in which LHC II moves after its phosphorylation from fluorescent PS II to nonfluorescent PS I. This state transition was capable of decreasing in darkened leaves the potential maximum quantum efficiency of electron flow through Photosystem II by about 20%.

1H spin-lattice relaxation parameters in the length of human intestine resected for cancer.

1H spin-lattice relaxation curves were acquired for samples of intestinal adenocarcinoma (B) and of uninvolved tissue at the upper (A) and lower (C) resection margin of lengths of intestine taken at surgery from 20 patients. Each sample showed a wide distribution of relaxation times with the order of 90% of the signal in a single peak at long times. Several different single-parameter relaxation times computed from discrete-exponential analysis showed that most of the relaxation times for C and B are in the upper two-thirds of the range of times for A. The mean time for the tumor is about 10% longer (with p < 0.01) than for the upper resection margin. The difference between the tumor and the lower resection margin is not significant. Distribution width parameters associated with A and C were significantly larger than those associated with the tumors. Two-exponential fits indicate that the fast-relaxing component represents a smaller signal fraction for the tumor B than for A or C.

Spin-Lattice Relaxation Times of Water in Polarized and Depolarized Rabbit Vagus Nerves

Spin-lattice relaxation times (T 1) of intracellular water of the nonmyelinated fibers of rabbit cervical vagus nerve were measured using a paramagnetic shift reagent, s-FDF. Spin-lattice relaxation decay curves were composed of the fast (T 1,F) and slow (T 1,S) relaxation components. The mean values of T 1,F and T 1,S in the polarized nerve fibers at 25°C were 0.12 ± 0.04 and 0.61 ± 0.13 sec., respectively, and their fractions were 0.71 ± 0.07 and 0.29 ± 0.07, respectively (n = 21). On the other hand, those values in the depolarized nerve fibers were 0.16 ± 0.01 sec., 0.77 ± 0.17 sec., 0.77 ± 0.10 and 0.23 ± 0.10, respectively (n = 15). T 1,F and T 1,S for the depolarized nerve fibers were significantly elongated (P < 0.01).

Computer simulation of solvation dynamics in several model solvents

The effect on solvation dynamics and energetics of the length and charge distribution of a model linear solvent has been determined. Solvent clustering around the ion is independent of the solvent molecule length if the solvent dipole terminus is attracted to the charged solute. In contrast, if they repel, the structure around the solute ion depends on the length of the solvent molecules and on the solute charge. The calculated solvation dynamics has two components and depends on solvent structure. The fraction of the fast relaxation component decreases with the length of the solvent molecules.

Strain sensitivity and turnover rate of low force cross-bridges in contracting skeletal muscle fibers in the presence of phosphate

Inorganic phosphate (Pi) decreases the isometric tension of skinned skeletal muscle fibers, presumably by increasing the relative fraction of a low force quaternary complex of actin, myosin, ADP, and Pi (A.M.ADP.Pi). At the same time, Pi gives rise to a fast relaxing mechanical component as detected by oscillations at 500 Hz. To characterize the dynamic properties of this A.M.ADP.Pi complex, the effect of Pi on the tension response to stretch was investigated with rabbit psoas fibers. A ramp stretch applied in the presence of 20 mM Pi increased tension more than in the control solution (0 mM Pi) but reduced the fast relaxing component to the control level. Thus, a stretch seems to convert the low force, fast relaxing A.M.ADP.Pi complex to a high force, slow relaxing form. However, the Pi-induced enhancement of the tension response was not observed until the fibers were stretched more than 0.4% of their length, suggesting that a critical cross-bridge extension of approximately 4 nm is required for this conversion. The rate constant of the attachment/detachment of this low force complex was estimated from the velocity dependence of the enhancement. It was approximately 10 s-1, in marked contrast to the A.M.ADP.Pi complex under low salt, relaxed conditions (approximately 10,000 s-1). The enhancement of the tension response was not observed when isometric tension was reduced by lowering free calcium, implying that calcium and Pi affect different steps in the actomyosin ATPase cycle during contraction.

Light-Induced Changes Among Fast-Relaxing Hydrogens in Plasma-Deposited Hydrogenated Amorphous Silicon

ABSTRACTLight-induced rearrangements in a-Si:H,D during and after illumination have been studied by deuteron magnetic resonance (DMR). A film sample was deposited on a 230°C Al substrate by plasma decomposition of S1H4 and D2. The high quality film contained 4 at.% H and 4 at. % D. DMR spectra of fast-relaxing components were examined at 700 msec or less after radio frequency saturation. At 30 K these showed a broad central signal (primarily nanovoid-trapped HD and D2), part of which shifted paramagnetically during illumination. No comparable shifts were shown by spectral components from Si-bonded hydrogen or from microvoid-contained dense fluid hydrogen. The observed post-illumination shifts vanished upon 150°C dark anneal and reflected light-induced metastable magnetic changes in the surroundings of hydrogen trapped in nanovoids.

Muonium formation and migration in CsMgCl3

The μ+SR were measured on CsMgCl3 single crystals to elucidate the muonium formation and migration in isostructural magnetic halides. More than 60% of incident μ+ were found to form muonium and the fraction of the muonium shows little temperature dependence. A part of muonium located near Mg2+ ions with zero nuclear dipole moments gives a fast relaxing component in the spectra. It was concluded from the decrease of asymmetry and the peak of the relaxation rate of the recovered asymmetry that muonium migrates above 90 K.