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PubMed2005

Femtosecond fluorescence dynamics imaging using a fluorescence up-conversion microscope.

Femtosecond fluorescence dynamics imaging microscopy was performed. Femtosecond fluorescence dynamics images were constructed based on the "mean" fluorescence decay or rise time constants that were evaluated by the time-resolved intensity sampling using a fluorescence up-conversion microscope. This dynamics imaging microscopy was carried out for the organic microcrystals alpha-perylene and tetracene-doped anthracene microcrystal, and ultrafast dynamics in the organic microcrystals were clearly imaged in the two-dimensional manner. For the alpha-perylene microcrystal, the obtained dynamics images showed that the crystal edges exhibited relatively shorter free exciton and the Y-state lifetimes compared to the crystal center, reflecting the higher concentration of defects. For the tetracene-doped anthracene microcrystal, the image was constructed based on the time constant of excitation energy transfer from anthracene to tetracene. By experiments changing the doping ratio of tetracene in anthracene, it was concluded that the inhomogeneity observed in the dynamics image arises from the difference in the local concentration of tetracene in the mixed crystal.

Journal Article↗
PubMed2005

Nature of the water molecules in the palisade layer of a triton X-100 micelle in the presence of added salts: A solvation dynamics study.

The effect of added electrolytes on the nature of water molecules in the palisade layer of a Triton X-100 (TX-100) micelle has been investigated using solvation dynamics studies of C153 dye in the presence of different concentrations of NaCl, KCl, and CsCl salts. In all of the cases, the solvation dynamics is found to be biexponential in nature. It is seen that in the presence of added salts the solvation dynamics becomes slower. As previously reported (Charlton et al. J. Phys. Chem. B 2000, 104, 8327; Molina-Bolivar et al. J. Phys. Chem. B 2002, 106, 870), the presence of salt increases micellar hydration (and also size) for TX-100, mainly due to enhancement in the mechanically trapped water content in the palisade layer. Under normal circumstances, increased micellar hydration was expected to cause faster solvation dynamics (Kumbhakar et al. J. Phys. Chem. B 2004, 108, 19246), though in the present work, a reverse trend is in fact observed with the added salts. In accordance with solvation dynamics results, fluorescence anisotropy studies also indicate an increase in microviscosity for the palisade layer of the TX-100 micelle with the added salts. The present results have been rationalized assuming that the ions reside in the palisade layer, and due to the hydration of the ions, especially the cations, the water molecules in the palisade layer undergo a kind of clustering, causing the microviscosity to in fact increase rather than decrease as expected due to increased micellar hydration. A partial collapse of the surfactant chains due to their dehydration as caused by the hydration of the ions in the palisade layer may also add to the increase in microviscosity and the consequent retardation in relaxation dynamics in the presence of salts.

Journal Article↗
PubMed2006

Nonadiabatic trajectory studies of NaI(H2O)n photodissociation dynamics.

We have investigated the photodissociation dynamics of NaI(H(2)O)(n) [n = 1-4] clusters using the molecular dynamics with quantum transitions method and a quantum mechanics/molecular mechanics description of NaI(H(2)O)(n), which involves a semiempirical valence-bond approach to describe the NaI electronic structure and classical solvent-solvent and solute-solvent interaction potentials. Our simulation results show that the NaI(H(2)O)(n) excited-state population decay upon reaching the NaI curve-crossing region increases with cluster size due to the stabilization of the ionic branch of the NaI excited state by the surrounding water molecules, and the resulting increase in nonadiabatic transition probability. After reaching the curve-crossing region for the first time, however, the excited-state population decay resembles that of bare NaI because of rapid evaporation of 99% and 95% of the water molecules for NaI(H(2)O) and NaI(H(2)O)(n) [n = 2-4], respectively. This extensive evaporation is due to the reversed NaI polarity in the Franck-Condon region of the NaI first excited state, which causes strong repulsive NaI-H(2)O forces and induces rapid nonstatistical water evaporation, where product water molecules are formed more rotationally than translationally hot. A few water molecules (5% or less) remain transiently or permanently bound to NaI, forming long-lived clusters, when NaI remains predominantly ionic, i.e., remains in the excited state, after reaching the curve-crossing region. To connect simulation results with experiment, we have simulated femtosecond probe signals resulting from two-photon and one-photon excitation to the X and I NaI(+) probe states. In agreement with experimental findings, the probe signals resulting from the two-photon probe scheme, where excitation occurs from the covalent branch of the excited state, decay exponentially over the NaI first excited-state vibrational period, with very little evidence of long-time dynamics. The one-photon probe scheme (not used for experimental cluster studies) is shown to be less sensitive to solvation, in that excitation energies will remain similar over a range of cluster sizes, as the ionic branch of the excited state and the NaI(+) probe states are stabilized to the same extent by the presence of water molecules. The resulting probe signals are also more revealing of the NaI(H(2)O)(n) photodissociation dynamics than the two-photon probe signals, as they may allow monitoring of solvation effects on the NaI nonadiabatic dynamics and of successive evaporation of water molecules. Time-resolved photoelectron spectra provide limited additional information regarding the NaI(H(2)O)(n) photodissociation dynamics. A key consequence of the rapid water evaporation demonstrated here is that experimentally observed signals may arise from the photodissociation of much larger NaI(H(2)O)(n) parent clusters.

Computer Simulation↗
PubMed2006

Cytochrome c552 mutants: structure and dynamics at the active site probed by multidimensional NMR and vibration echo spectroscopy.

Spectrally resolved infrared stimulated vibrational echo experiments are used to measure the vibrational dephasing of a CO ligand bound to the heme cofactor in two mutated forms of the cytochrome c552 from Hydrogenobacter thermophilus. The first mutant (Ht-M61A) is characterized by a single mutation of Met61 to an Ala (Ht-M61A), while the second variant is doubly modified to have Gln64 replaced by an Asn in addition to the M61A mutation (Ht-M61A/Q64N). Multidimensional NMR experiments determined that the geometry of residue 64 in the two mutants is consistent with a non-hydrogen-bonding and hydrogen-bonding interaction with the CO ligand for Ht-M61A and Ht-M61A/Q64N, respectively. The vibrational echo experiments reveal that the shortest time scale vibrational dephasing of the CO is faster in the Ht-M61A/Q64N mutant than that in Ht-M61A. Longer time scale dynamics, measured as spectral diffusion, are unchanged by the Q64N modification. Frequency-frequency correlation functions (FFCFs) of the CO are extracted from the vibrational echo data to confirm that the dynamical difference induced by the Q64N mutation is primarily an increase in the fast (hundreds of femtoseconds) frequency fluctuations, while the slower (tens of picoseconds) dynamics are nearly unaffected. We conclude that the faster dynamics in Ht-M61A/Q64N are due to the location of Asn64, which is a hydrogen bond donor, above the heme-bound CO. A similar difference in CO ligand dynamics has been observed in the comparison of the CO derivative of myoglobin (MbCO) and its H64V variant, which is caused by the difference in axial residue interactions with the CO ligand. The results suggest a general trend for rapid ligand vibrational dynamics in the presence of a hydrogen bond donor.

Bacterial Proteins↗
PubMed2006

Temperature-dependent protein dynamics: a simulation-based probabilistic diffusion-vibration Langevin description.

An enduring challenge in the understanding of internal protein motions is the effective separation and characterization of diffusive and vibrational dynamical components. To address this problem, here nanosecond molecular dynamics trajectories of myoglobin in aqueous solution, performed over a range of temperatures between 120 and 300 K, are subjected to principal component analysis, and the coordinate autocorrelation functions of the resulting principal modes are interpreted using a model combining damped Langevin vibration within potential wells and barrier-crossing diffusion between them. Both the vibrational frequency and the fraction of the mean-square fluctuation arising from vibrational motion undergo transitions with temperature at about 180 K. In contrast, the vibrational friction remains linear with temperature. The diffusional component of the mean-square fluctuation increases dramatically at the dynamical transition. The heights of the energy barriers between the potential wells are estimated, and the associated diffusion constants are calculated using Kramers' rate theory. Model functions of the frequency dependence of the frictional and diffusional quantities are obtained. The dynamic structure factor from the full molecular dynamics trajectory is well reproduced by the model. Overall, the results indicate that a global description of nanosecond temperature-dependent diffusion and vibrational internal protein dynamics can be obtained by applying the results of the present diffusion-vibration model to the vibrational motions obtained from a normal-mode analysis.

Algorithms↗
PubMed2006

Ultrafast excited state dynamics of the perylene radical cation generated upon bimolecular photoinduced electron transfer reaction.

The ultrafast ground state recovery (GSR) dynamics of the radical cation of perylene, Pe(*+), generated upon bimolecular photoinduced electron transfer in acetonitrile, has been investigated using pump-pump-probe spectroscopy. With 1,4-dicyanobenzene as electron acceptor, the free ion yield is substantial and the GSR dynamics of Pe(*+) was found to depend on the time delay between the first and second pump pulses, Deltat(12), i.e., on the "age" of the ion. At short Deltat(12), the GSR dynamics is biphasic, and at Deltat(12) larger than about 500 ps, it becomes exponential with a time constant around 3 ps. With trans-1,2-dicyanoethylene as acceptor, the free ion yield is essentially zero and the GSR dynamics of Pe(*+) remains biphasic independently of Deltat(12). The change of dynamics observed with 1,4-dicyanobenzene is ascribed to the transition from paired to free solvated ion, because in the pair, the excited ion has an additional decay channel to the ground state, i.e., charge recombination followed by charge separation. The rate constants deduced from the analysis of these GSR dynamics are all fully consistent with this hypothesis.

Journal Article↗
PubMed2006

Multiple time scales in solvation dynamics of DNA in aqueous solution: the role of water, counterions, and cross-correlations.

Recent time domain experiments have explored solvation dynamics of a probe located inside a DNA duplex, in an effort to gain information, e.g., on the dynamics of water molecules in the DNA major and minor grooves and their environment. Multiple time constants in the range of a few picoseconds to several nanoseconds were obtained. We have carried out 15 ns long atomistic molecular dynamics simulations to study the solvation dynamics of bases of a 38 base-pair long DNA duplex in an aqueous solution containing counterions. We have computed the energy-energy time correlation function (TCF) of the four individual bases (A, T, G, and C) to characterize the solvation dynamics. All the TCFs display highly nonexponential decay with time. When the trajectories are analyzed with 100 fs time resolution, the TCF of each base shows initial ultrafast decay (with tau1 approximately equal 60-80 fs) followed by two intermediate components (tau2 approximately equal 1 ps, tau3 approximately equal 20-30 ps), in near complete agreement with a recent time domain experiment on DNA solvation. Interestingly, the solvation dynamics of each of the four different nucleotide bases exhibit rather similar time scales. To explore the existence of slow relaxation at longer times reported recently in a series of experiments, we also analyzed the solvation TCFs calculated with longer time trajectories and with a larger time resolution of 1 ps. In this case, an additional slow component with a time constant of the order of 250 ps is observed. Through an analysis of partial solvation TCFs, we find that the slow decay originates mainly from the interaction of the nucleotides with the dipolar water molecules and the counterions. An interesting negative cross-correlation between water and counterions is observed, which makes an important contribution to relaxation at intermediate to longer times.

Base Sequence↗
PubMed1998

Dynamics of Polymer Interdiffusion: The Ripple Experiment.

We explore the interdiffusion of oppositely labeled triblock polystyrene chains, HDH/DHD, during welding in the melt using dynamic secondary ion mass spectroscopy (DSIMS) and specular neutron reflectivity (SNR). The HDH chains have the central portion of the chain deuterated (D) approximately 50% while the two ends (H) each have approximately 25% protonation; the DHD is oppositely labeled, but each set of chains contains about 50% deuteration. During welding, the deuterium depth profile exhibits "ripples" whose characteristic features, such as the time and molecular weight dependent shape, amplitude, and position, are very sensitive to the microscopic details of the polymer dynamics. The ripple experiment is especially sensitive to the presence, or absence, of topological constraints and anisotropic motion of chains. The current work significantly extends the molecular weight range up to 400 000. This allows greater separation of the six key ripple features used in deciphering the correct polymer dynamics model at the polymer-polymer interface. The DSIMS and SNR experimental results are compared to theoretical predictions and ripple simulations for Rouse, polymer mode-coupling, reptation (with and without tube broadening), and other phenomenological dynamics models. The six ripple characteristics were found to be perfectly correlated and convincingly consistent with the predictions of the reptation dynamics model. The ripple results are in significant disagreement with the polymer mode-coupling model proposed by Schweizer and other tubeless models. We conclude that the reptation model, proposed by DeGennes in 1971 with parallel developments by Edwards, is the correct model to describe the dynamics of polymer interdiffusion.

Journal Article↗
PubMed1992

Influence of animation on dynamical judgments.

The motions of objects in the environment reflect underlying dynamical constraints and regularities. The conditions under which people are sensitive to natural dynamics are considered. In particular, the article considers what determines whether observers can distinguish canonical and anomalous dynamics when viewing ongoing events. The extent to which such perceptual appreciations are integrated with and influence common-sense reasoning about mechanical events is examined. It is concluded that animation evokes accurate dynamical intuitions when there is only 1 dimension of information that is of dynamical relevance. This advantage is lost when the observed motion reflects higher dimension dynamics or when the kinematic information is removed or degraded.

Adult↗
PubMed1997

Intrinsic and required dynamics of a simple bat-ball skill.

Three experiments investigated the coordination dynamics of a simple bat-and-ball skill: cyclically striking a ball suspended by a string with a pendular bat. The relative phase phi between the bat and ball is dictated by the potential function V(phi) = k sin phi and the difference delta omega in their uncoupled frequencies. For various delta omega, phi and its standard deviation were measured in the absence of any environmental restraints (intrinsic dynamics) and when the ball had to reach resistive or nonresistive targets at set distances (required dynamics). Results support the dynamical theory of coordination patterns (G. Schöner & J.A.S. Kelso, 1988a, 1988c), particularly the hypothesis that required dynamics are understandable as the addition of terms to the potential governing the intrinsic dynamics.

Biomechanical Phenomena↗
PubMed2003

The impact of chief executive officer personality on top management team dynamics:one mechanism by which leadership affects organizational performance.

This article explores 1 mechanism by which leader personality affects organizational performance. The authors hypothesized and tested the effects of leader personality on the group dynamics of the top management team (TMT) and of TMT dynamics on organizational performance. To test their hypotheses, the authors used the group dynamics q-sort method, which is designed to permit rigorous, quantitative comparisons of data derived from qualitative sources. Results from independent observations of chief executive officer (CEO) personality and TMT dynamics for 17 CEOs supported the authors' hypothesized relationships both between CEO personality and TMT group dynamics and between TMT dynamics and organizational performance.

Administrative Personnel↗
PubMed1991

On the origins of dynamical awareness.

An inquiry into the origins of dynamical awareness is conducted. Particular attention is given to a theory that postulates that impressions of dynamical quantities are derived from and structured by lawful physical relations. It is shown that impressions of dynamical quantities are not generally correlated with the values that these quantities take in the equations of motion but rather are highly correlated with simple ratios of kinematic quantities or with specific kinematic features that do not specify the underlying dynamics. It is argued that kinematic information, to the extent that it is used, is used heuristically, and its availability for dynamical analysis is constrained by general principles of organization. A formal analysis of the physical organization implicit in the specification of dynamical invariants is given and compared with types of perceptual organization that are observed.

Attention↗
PubMed2006

Qualitative and quantitative change in the dynamics of motor learning.

The experiments examined qualitative and quantitative changes in the dynamics of learning a novel motor skill (roller ball task) as a function of the manipulation of a control parameter (initial ball speed). The focus was on the relation between the rates of change in performance over practice time and the changing time scales of the evolving attractor dynamic. Results showed 3 different learning patterns to the changes in the dynamics as a function of practice that were mediated by the initial ball speed. Only participants who learned the task showed a bifurcation in coordination mode that was preceded by enhanced performance variability. The observed multiple time scales to motor learning are interpreted as the products of the dynamical stability and instability realized from (a) the continually evolving landscape dynamics due to bifurcations between attractor organization and (b) the transient phenomena associated with moving toward and away from fixed-point dynamics.

Adult↗
PubMed2004

Linkage between dynamics and catalysis in a thermophilic-mesophilic enzyme pair.

A fundamental question is how enzymes can accelerate chemical reactions. Catalysis is not only defined by actual chemical steps, but also by enzyme structure and dynamics. To investigate the role of protein dynamics in enzymatic turnover, we measured residue-specific protein dynamics in hyperthermophilic and mesophilic homologs of adenylate kinase during catalysis. A dynamic process, the opening of the nucleotide-binding lids, was found to be rate-limiting for both enzymes as measured by NMR relaxation. Moreover, we found that the reduced catalytic activity of the hyperthermophilic enzyme at ambient temperatures is caused solely by a slower lid-opening rate. This comparative and quantitative study of activity, structure and dynamics revealed a close link between protein dynamics and catalytic turnover.

Adenylate Kinase↗
PubMed2003

Fixed vs dynamic plate complications following multilevel anterior cervical corpectomy and fusion with posterior stabilization.

STUDY DESIGN: Comparison of fixed vs dynamic plate complications in cervical surgery. SETTING: : New York, USA. METHODS: Anterior cervical plate-related complications were evaluated following 66 anterior cervical corpectomy and fusion (ACF) with posterior stabilization (PWF) procedures performed in patients with ossification of the posterior longitudinal ligament (OPLL). Clinical data were comparable for both patient populations. Patients averaged between 52 and 53 years of age. The male to female ratio was approximately 2:1. Surgery addressed MR and CT documented multilevel OPLL in all patients accompanied by spondylosis and stenosis. Preoperatively average Nurick Grades ranged from 3.6 to 3.7. Anterior cervical corpectomies included an average of 2.6-3.0 vertebral bodies, while PWF covered seven levels. Fixed plates were applied in the initial 38 patients, while the latter 28 patients had dynamic plates (ABC, Aesculap, Tuttlingen, Germany) applied. Halo devices were used until fusion was documented on both X-ray and 2D-CT studies. Patients were followed-up for an average of 5.4 years in the fixed-plated groups, and 2.7 years in the dynamic-plated population. RESULTS: CT and dynamic X-ray confirmed that fusion occurred an average of 4.5-4.9 months postoperatively. Five (13%) fixed plates (Medtronic, Sofamor Danek, Memphis, TN, USA) failed warranting secondary surgery, while only one (3.6%) dynamic-plated patient developed a pseudarthrosis and required secondary posterior fusion. DISCUSSION/CONCLUSION: Higher failure rates follow multilevel ACF as compared with anterior diskectomy and fusion required to resect multilevel OPLL. Vaccaro et al observed a 9% failure rate following two-level ACFs and 50% failure rate following three-level ACFs performed with fixed plates. In this series, the plate extrusion rate was reduced to 3.6% when dynamic plates were applied.

Adult↗
PubMed2003

Universal non-diffusive slow dynamics in aging soft matter.

We use conventional and multispeckle dynamic light scattering to investigate the dynamics of a wide variety of jammed soft materials, including colloidal gels, concentrated emulsions, and concentrated surfactant phases. For all systems, the dynamic structure factor f(q,t) exhibits a two-step decay. The initial decay is due to the thermally activated diffusive motion of the scatterers, as indicated by the q(-2) dependence of the characteristic relaxation time, where q is the scattering vector. However, due to the constrained motion of the scatterers in jammed systems. the dynamics are arrested and the initial decay terminates in a plateau. Surprisingly, we find that a final, ultraslow decay leads to the complete relaxation of f(q,t), indicative of rearrangements on length scales as large as several microns or tens of microns. Remarkably, for all systems the same very peculiar form is found for the final relaxation of the dynamic structure factor: f(q,t) approximately exp[-(t/tau s)p], with p approximately equal to 1.5 and tau s approximately q(-1), thus suggesting the generality of this behavior. Additionally, for all samples the final relaxation slows down with age. although the aging behavior is found to be sample dependent. We propose that the unusual ultraslow dynamics are due to the relaxation of internal stresses, built into the sample at the jamming transition, and present simple scaling arguments that support this hypothesis.

Algorithms↗
PubMed1991

The effect of solution composition on microtubule dynamic instability.

The exchange of tubulin dimer into steady-state microtubules was studied over a range of solution conditions, in order to assess the effects of various common buffer components on the dynamic instability of microtubules. In comparison with standard buffer conditions (100 mM-Pipes buffer, pH 6.5, containing 0.1 mM-EGTA, 1.8 mM-MgC12 and 1 M-glycerol), the rate and extent of exchange, and thus of dynamic instability, are suppressed by increasing the concentration of glycerol above 2 M. Exchange is enhanced by the addition of further Mg2+ (up to 17 mM) or by the addition of Ca2+ (up to 0.4 mM). Phosphate ion (150 mM) has relatively little effect on the dynamic behaviour of microtubules, as judged by the exchange method. The findings are interpreted within the framework of the Lateral Cap model for microtubule dynamic instability, in terms of the effects of these changes on the intrinsic rate constants of the system. By contrast, the extent of tubulin exchange depends selectively on the value of the dissociation rate constant for tubulin-GDP. A decrease in the extent of exchange, and hence in dynamic activity, is associated with a decreased value for this rate constant, and vice versa. The results also show good agreement of predictions of the model in treating the observed variations in the dynamic properties of individual microtubules, induced by different solution conditions.

Alkaloids↗
PubMed2006

Dynamic assessment of school-age children's narrative ability: an experimental investigation of classification accuracy.

UNLABELLED: Two experiments examined reliability and classification accuracy of a narration-based dynamic assessment task. PURPOSE: The first experiment evaluated whether parallel results were obtained from stories created in response to 2 different wordless picture books. If so, the tasks and measures would be appropriate for assessing pretest and posttest change within a dynamic assessment format. The second experiment evaluated the extent to which children with language impairments performed differently than typically developing controls on dynamic assessment of narrative language. METHOD: In the first experiment, 58 1st- and 2nd-grade children told 2 stories about wordless picture books. Stories were rated on macrostructural and microstructural aspects of language form and content, and the ratings were subjected to reliability analyses. In the second experiment, 71 children participated in dynamic assessment. There were 3 phases: a pretest phase, in which children created a story that corresponded to 1 of the wordless picture books from Experiment 1; a teaching phase, in which children attended 2 short mediation sessions that focused on storytelling ability; and a posttest phase, in which children created a story that corresponded to a second wordless picture book from Experiment 1. Analyses compared the pretest and posttest stories that were told by 2 groups of children who received mediated learning (typical and language impaired groups) and a no-treatment control group of typically developing children from Experiment 1. RESULTS: The results of the first experiment indicated that the narrative measures applied to stories about 2 different wordless picture books had good internal consistency. In Experiment 2, typically developing children who received mediated learning demonstrated a greater amount of pretest to posttest change than children in the language impaired and control groups. Classification analysis indicated better specificity and sensitivity values for measures of response to intervention (modifiability) and posttest storytelling than for measures of pretest storytelling. Observation of modifiability was the single best indicator of language impairment. Posttest measures and modifiability together yielded no misclassifications. CONCLUSION: The first experiment supported the use of 2 wordless picture books as stimulus materials for collecting narratives before and after mediation within a dynamic assessment paradigm. The second experiment supported the use of dynamic assessment for accurately identifying language impairments in school-age children.

Abstracting and Indexing↗