Biomedical subjects

M Ketelaars

Publications and source records attributed to M Ketelaars.

7 recordsLinked to original sources

PubMed1999

Unraveling the electronic structure of individual photosynthetic pigment-protein complexes

Low-temperature single-molecule spectroscopic techniques were applied to a light-harvesting pigment-protein complex (LH2) from purple photosynthetic bacteria. The properties of the electronically excited states of the two circular assemblies (B800 and B850) of bacteriochlorophyll a (BChl a) pigment molecules in the individual complexes were revealed, without ensemble averaging. The results show that the excited states of the B800 ring of pigments are mainly localized on individual BChl a molecules. In contrast, the absorption of a photon by the B850 ring can be consistently described in terms of an excitation that is completely delocalized over the ring. This property may contribute to the high efficiency of energy transfer in these photosynthetic complexes.

Journal Article↗

PubMed1998

Response preparation and control of movement sequences.

Two experiments investigated the response complexity effect using elbow extension/flexion movements. In the first experiment, RT for an extension movement was significantly less than RT for an extension/flexion movement. However, this difference in RT was not evident when participants were asked to pause at the reversal of the extension/flexion for approximately 260 ms. The second experiment manipulated the duration of the pause between these movements and also measured the electromyographical (EMG) activity of the triceps and biceps muscles. When the pause was reduced to 75 ms participants were not able to program the flexion portion of the movement at the reversal, forcing them to preprogram this movement; hence, increasing their premotor reaction time.

Arm↗

PubMed1997

Strain dependence of the elastic properties of force-producing cross-bridges in rigor skeletal muscle.

Stretch and release experiments carried out on skinned single fibers of frog skeletal muscle under rigor conditions indicate that the elastic properties of the fiber depend on strain. For modulation frequencies below 1000 Hz, the results show an increase in Young's modulus of 20% upon a stretch of 1 nm/half-sarcomere. Remarkably, the strain dependence of Young's modulus decreases at higher frequencies to about 10% upon a 1-nm/half-sarcomere stretch at a modulation frequency of 10 kHz. This suggests that the cause of the effect is less straightforward than originally believed: a simple slackening of the filaments would result in an equally large strain dependence at all frequencies, whereas strain-dependent properties of the actin filaments should show up most clearly at higher frequencies. We believe that the reduction of the strain dependence points to transitions of the cross-bridges between distinct force-producing states. This is consistent with the earlier observation that Young's modulus in rigor increases toward higher frequencies.

Animals↗

PubMed1996

Interaction of Photobacterium leiognathi and Vibrio fischeri Y1 luciferases with fluorescent (antenna) proteins: bioluminescence effects of the aliphatic additive.

The kinetics of the bacterial bioluminescence reaction is altered in the presence of the fluorescent (antenna) proteins, lumazine protein (LumP) from Photobacterium or the yellow fluorescence proteins (YFP) having FMN or Rf bound, from Vibrio fischeri strain Y1. Depending on reaction conditions, the bioluminescence intensity and its decay rate may be either enhanced or strongly quenched in the presence of the fluorescent proteins. These effects can be simply explained on the basis of the same protein-protein complex model that accounts for the bioluminescence spectral shifts induced by these fluorescent proteins. In such a complex, where the fluorophore evidently is in proximity to the luciferase active site, it is expected that the on-off rate of certain aliphatic components of the reaction should be altered with a consequent shift in the equilibria among the luciferase intermediates, as recently elaborated in a kinetic scheme. These aliphatic components are the bioluminescence reaction substrate, tetradecanal or other long-chain aldehyde, its carboxylic acid product, or dodecanol used as a stabilizer of the luciferase peroxyflavin. No evidence can be found for the protein-protein interaction in the absence of the aliphatic component.

Bacterial Proteins↗

PubMed1996

Reorientational properties of fluorescent analogues of the protein kinase C cofactors diacylglycerol and phorbol ester.

The reorientational properties of the fluorescently labelled protein kinase C (PKC) cofactors diacylglycerol (DG) and phorbol ester (PMA) in vesicles and mixed micelles have been investigated using time-resolved polarised fluorescence. The sn-2 acyl chain of DG was replaced by diphenylhexatriene- (DPH) propionic acid, while a dansyl labelled analogue of phorbol ester was used. The extent of ordering of DPH-DG in vesicles turned out to be slightly different from that of the control choline lipid DPH-PC. Addition of PKC to vesicles containing 30 mole% brain PS considerably slowed down the DPH-DG anisotropy decay. This was not observed when DPH-DG was replaced by DPH-PC. Analysis of the fluorescence anisotropy decays of these DPH-lipids in micelles polyoxyethylene-9-laurylether mixed with 10 mole% of the essential phosphatidylserine allowed estimation of their lateral diffusion, orientation distribution and reorientational dynamics within the micelles. Addition of PKC resulted in a significantly slower decay of the fluorescence anisotropy of both DPH-DG and DPH-PC even in the absence of calcium, indicating a calcium independent complexation of PKC with the PS containing micelles. Addition of calcium resulted in a further reduction of the decay of anisotropy of DPH-DG but not of DPH-PC indicating that the Ca2+ dependent immobilisation is cofactor-specific. Similar specific interactions with PKC resulted in a slower decay of dansylated PMA when calcium and PS were present.

Animals↗

PubMed1996

Nuclear magnetic resonance study of the conformation and dynamics of beta-casein at the oil/water interface in emulsions.

A (13)C and (31)P nuclear magnetic resonance (NMR) study has been carried out on beta-casein adsorbed at the interface of a tetradecane/water emulsion. (13)C NMR spectra show signals from the carbonyl, carboxyl, aromatic, and C alpha carbons in beta-casein, well resolved from solvent resonances. Only a small fraction of all carbon atoms in beta-casein contribute to detectable signals; intensity measurements show that the observable spectrum is derived from about 30 to 40 amino acid residues.(31)P NMR spectra show signals from the five phosphoserines on the hydrophilic N-terminal part of the protein. Analysis of T(1) relaxation times of these nuclei, using the model free approach for the spectral density function and the line shape of the alpha-carbon region, indicates that a large part of the protein is in a random coil conformation with restricted motion and a relatively long internal correlation time. The NMR results show that the conformation and dynamics of the N-terminal part of beta-casein are not strongly altered at the oil/water interface, as compared to beta-casein in micelle-like aggregates in aqueous solution.

Adsorption↗

PubMed1993

On the time course and accuracy of spatial localization: basic data and a two-process model.

This article addresses the question how fast and accurate the location of a single stimulus can be perceived. In Experiment 1, we measured localization performance in a task which required subjects to perceive and report the location of a single target stimulus ('*' sign) presented in one square of an imaginary 25 x 19 grid. Two factors were varied: stimulus duration and stimulus eccentricity. Stimulus duration was manipulated by employing a backward masking stimulus. Ten intervals (stimulus onset asynchronies) separated target and masking stimulus: 25, 50, 75, 100, 125, 150, 200, 250, 300, and 350 ms. Stimulus eccentricity was manipulated by presenting the target stimulus at five different distances from the fixation point. The observer localized the target stimulus by moving the cursor from the middle of the grid (the initial fixation point) to the perceived target location by pressing the 'arrow' keys on the keyboard. Localization performance showed to be typically related to stimulus duration. That is, two components could be distinguished: The first component represented an initial steep rise in localization performance during the first 50 ms of stimulus duration; the second component represented a gradual rise in localization performance after 50 ms, reaching maximal performance at about 300 ms. We interpreted these two localization performance functions as reflecting the operation of two systems, namely the attentional system for the initial strong increase and the eye movement system for the subsequent gradual increase. In Experiment 2, we measured saccadic eye response latencies to clarify the role of eye movements in localization performance. It was found that in 98.4% of all trials saccades were executed, and, moreover, that saccadic eye response latency decreased with increasing stimulus duration. In Experiment 3, we compared localization performance in the absence and presence of eye movements and demonstrated that localization performance for stimulus durations up to 50 ms was independent of eye movements. Overall, the present findings were interpreted as evidence in support of a two-process model of localization performance in which a shift of attention is followed by a rapid eye movement toward the target location. In line with a continuous flow conception of visual information processing, our model assumes that location information takes time to develop in the visual system; hence, an observer's localization response may be based on qualitatively different processes operating on qualitatively different kinds of information. In case of short duration stimuli, information conveyed by transient cells is used by the attentional system to shift attention toward the target location; this results in course location information being available.(ABSTRACT TRUNCATED AT 400 WORDS)

Adult↗