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PubMed2005

Functional dissociation of amygdala-modulated arousal and cognitive appraisal, in Turner syndrome.

The amygdala is preferentially activated by facial expressions of fear. Right and left amygdala are hypothesized to play distinct, but complementary, roles that influence somatic and cognitive responses to facial expressions. Right amygdala activation is linked to autonomic arousal, and thus indirectly influences left hemisphere cognitive processing centres. Left amygdala activation is more closely associated with cognitive processing and differentiation of facial emotions. A double-dissociation between the functions of left and right amygdala is implied by lesion studies but supportive evidence is inconsistent, partly because patients with structural anteromedial temporal anomalies have experienced variable surgical procedures. A functional dissociation can be demonstrated between arousal and the cognitive appraisal of fearful faces in the condition of X-monosomy or Turner syndrome. Previous research found Turner syndrome women of normal verbal intelligence are seriously impaired in their ability cognitively to differentiate fearful from other facial expressions but they acquire fear conditioning normally, with enhanced autonomic responses. These findings supported the dissociation hypothesis, which was formally tested in a study of 12 X-monosomic and 12 control females who participated in functional magnetic resonance imaging during which simultaneous skin conductance recordings were acquired. Faces depicting fear or neutral emotions were presented to both case and control subjects in random order. Arousal to (fearful-neutral) faces was associated with transiently increased skin conductance responses and bilateral amygdala activation in both groups, but X-monosomic females had proportionately greater--and more persistent--right amygdala activation than controls. In both groups, cognitive accuracy correlated positively with differential activity of left fusiform gyrus. There was a significant correlation between the left fusiform and left medial amygdala activation only in normal females, and only in them did differential SCRs (to fearful-neutral faces) correlate positively with left fusiform responses. Arousal and cognitive appraisal functions of the amygdala can thus be functionally dissociated. X-monosomy selectively impairs explicit recognition of fearful faces in the presence of normal or enhanced autonomic reactivity, and is associated with a functional dissociation of activity in left amygdala and left fusiform gyrus. These findings imply X-linked genes are essential for binding somatic responses to the cognitive appraisal of emotional stimuli.

Adult↗
PubMed1993

Visualising the kinetics of dissociation of actinomycin from individual sites in mixed sequence DNA by DNase I footprinting.

We have investigated the kinetics of dissociation of actinomycin D from DNA by a variation of the footprinting technique. Complexes of actinomycin with a radiolabelled DNA fragment (tyrT) were dissociated by addition of a large excess of unlabelled calf thymus DNA and the mixture subjected to DNase I footprinting at subsequent intervals. The rates at which the footprints disappeared varied between the different binding sites. The dissociation was temperature dependent with average time constants of 30 s, 10 mins and 2 hours at temperatures of 37 degrees C, 20 degrees C and 4 degrees C respectively. The dissociation from a DNA fragment containing the synthetic insert T9GCA9 was significantly faster, with a half-life of about 1 min at 20 degrees C. In contrast, the dissociation of distamycin was too fast to measure (< 5 s) even at 4 degrees C.

Base Sequence↗
PubMed1981

Nucleosome dissociation at physiological ionic strengths.

Monomer nucleosomes purified on isokinetic sucrose gradients are shown to dissociate into component DNA and histones at physiological ionic strength upon dilution to a DNA concentration below 20 microgram/ml. The starting material is 11S, contains 145-190 BP DNA, and equimolar amounts of the four core histones with slightly less H1. Dilution of monomers in the presence of 0.14 M NaCl results in the rapid conversion of 10-40% of the 3H thymidine labeled material from 11S to 5S (5S is coincident with the S value of monomer length DNA). The proportion of nucleosomes which dissociate increases with increasing NaCl concentration between 0.15 M and 0.35 M and decreases with increasing DNA concentration above 1 microgram/ml. Recycling 11S monomers, which remain after dissociation, through a second dilution in salt generates an equivalent proportion of 5S material as seen after the initial dilution. Thus, the dissociation does not result from special properties of a subset of nucleosomes. An equilibrium between intact monomer and free DNA and histones appears to be rapidly established under the conditions described and the dissociated DNA will reassociate with histones to form 11S monomers if conditions of high DNA concentration and low ionic strength are established.

Animals↗
PubMed1986

Pseudo-atrial dissociation: a respiratory artifact.

Atrial dissociation is an electrocardiographic phenomenon characterized by the occurrence of two separate atrial rhythms. A patient is presented, showing electrocardiographic signs suggestive for atrial dissociation. The dissociated, non-conducted, atrial rhythm showing micro-oscillations after its P wave, is shown to be a respiratory artifact. It is concluded that the micro-oscillations seen on the surface electrocardiogram after a dissociated P wave are probably diagnostic for pseudo-atrial dissociation.

Aged↗
PubMed1999

Kinetic modeling of slow dissociation of bromosulphophthalein from albumin in perfused rat liver: toxicological implications.

Due to strong binding between organic anions and albumin, the kinetics of the binding process must be carefully considered in biologically-based models used for predictive toxicology applications. Specifically, the slow dissociation rate of an organic anion from the protein may lead to reduced availability of free anion in its flow through the capillaries of an organ. In this work, the effect of the dissociation rate of the anion bromosulphophthalein (BSP) from albumin was studied in isolated, perfused rat livers in the presence of albumin concentrations of 0.25, 1, and 4% (w/v) and an initial BSP concentration of 20 microM. The uptake of BSP from the perfusion medium was modeled using a biologically-based kinetic model of the sinusoidal and intracellular liver compartments. The best fit of the model to data resulted in the prediction of a slow dissociation rate constant for the BSP-albumin of between 0.097 and 0.133 s(-1). Assuming BSP and albumin to be in binding equilibrium in the sinusoidal space, with rapid binding-rate constants, as is often done, produced an unacceptable fit. These results indicate that the strong binding interaction between BSP and albumin, beyond keeping the concentration of free chemical low due to a small equilibrium dissociation constant, can also reduce uptake by an organ due to the slow release of BSP from the protein during passage through the capillaries. The implication of this dissociation-limited condition, when extrapolating to other doses and in-vivo situations, is discussed.

Animals↗
PubMed2000

Isoflurane increases the apparent agonist affinity of the nicotinic acetylcholine receptor by reducing the microscopic agonist dissociation constant.

BACKGROUND: Isoflurane increases the apparent agonist affinity of ligand-gated ion channels. This action reflects a reduction in the receptor's agonist dissociation constant and/or the preopen/open channel state equilibrium. To evaluate the effect of isoflurane on each of these kinetic constants in the nicotinic acetylcholine receptor, the authors analyzed isoflurane's actions on (1) the binding of the fluorescent agonist Dns-C6-Cho to the nicotinic acetylcholine receptor's agonist self-inhibition site and (2) the desensitization kinetics induced by the binding of the weak partial agonist suberyldicholine. METHODS: The dissociation constant for Dns-C6-Cho binding to the self-inhibitory site was determined using stopped-flow fluorescence spectroscopy. The values of the kinetic constants for agonist binding, channel gating, and desensitization were determined by modeling the suberyldicholine concentration-dependence of the apparent rate of desensitization. RESULTS: Isoflurane did not significantly alter the dissociation constant for Dns-C6-Cho binding to the self-inhibitory site even at a concentration as high as 1.5 mM, the highest concentration studied. At this concentration, isoflurane substantially reduced the dissociation constant for suberyldicholine binding to its channel opening site by 97% from 17 +/- 5 microM to 0.5 +/- 0.2 microM, whereas the preopen/open channel state equilibrium was reduced only from 19.1 to 5 +/- 1. CONCLUSIONS: Isoflurane increases the apparent agonist affinity of the nicotinic acetylcholine receptor primarily by reducing the agonist dissociation constant of the site responsible for channel opening rather than altering channel gating kinetics.

Algorithms↗
PubMed1991

Electromechanical dissociation in newborns treated with extracorporeal membrane oxygenation: an extreme form of cardiac stun syndrome.

OBJECTIVE: To recognize cardiac stun syndrome and electromechanical dissociation in patients receiving extracorporeal membrane oxygenation (ECMO), and to define patients at risk. DESIGN: Retrospective review. SETTING: Tertiary neonatal ICU. PATIENTS: Four newborn patients with cardiorespiratory failure who developed signs of cardiac stun syndrome and electromechanical dissociation early in the ECMO course. MEASUREMENTS AND MAIN RESULTS: Initially, these patients had metabolic acidosis, chest roentgenograms showing pulmonary granularity and moderate cardiomegaly, and symptoms of severe respiratory distress. Cardiac dysfunction was apparent after ECMO was begun, with poor perfusion, pale color, narrow pulse pressure, and tachycardia despite normovolemia. Within 1 to 2 hrs, electromechanical dissociation occurred manifested by the absence of pulse pressure, palpable pulse, cardiac sounds, and apical impulse while on 50% to 70% bypass. All patients survived. INTERVENTIONS: Patients received ECMO, calcium gluconate, sodium bicarbonate, and dobutamine. CONCLUSIONS: Patients with cardiac stun syndrome have symptoms similar to severe respiratory distress syndrome, and may require ECMO support. In the ECMO patient, cardiac stun syndrome and electromechanical dissociation can be confused with low circuit volume, pneumothorax, or cardiac tamponade. Early recognition of electromechanical dissociation may improve care and outcome. Cardiac stun syndrome can be treated successfully with ECMO.

Extracorporeal Membrane Oxygenation↗
PubMed1999

Simulations of fluid hydrogen: comparison of a dissociation model with tight-binding molecular dynamics.

We compare the results of two complementary approaches, tight-binding molecular-dynamics simulations and a dissociation model, for determining the characteristics of dense, fluid hydrogen at pressures extending to megabars and temperatures to 10 000 K. Two tight-binding models were examined: one parametrization emphasized crystalline, molecular, and fluid properties, the other focused more on the intricate molecular interactions involving up to four hydrogen atoms. The two tight-binding cases and the dissociation model agree reasonably well for a variety of properties, including the equation of state, dissociation degree, and proton pair-correlation functions. In simulations of recently reported laser-driven shock experiments, the tight-binding and dissociative models predict different maximum compressions of four and five, respectively. We discuss the sensitivities of the models as well as give estimates for the region of validity of the chemical picture (dissociation model) and the accuracy of the dynamical picture (tight-binding simulations) in cases where molecular hydrogen still dominates the physical behavior.

Journal Article↗
PubMed2000

N+ charge transfer and N+2 dissociation in N2 at swarm energies.

This paper reports a drift-tube-mass-spectrometer measurement of the relative abundances of N+ and N+2 in pure nitrogen, over a ratio of electric field to gas density, E/N, from 800 to 7200 Td [1 townsend (Td)=10(-17) V cm(2)]. A proposed charge transfer dissociation scheme between the above two ions and N2 allowed us to obtain spatial rate coefficients for charge transfer and dissociation over the E/N range 800-2800 Td. Using previously measured cross sections for the above processes, and assuming a Maxwellian distribution of ion velocities, we calculated the reaction coefficients, which were found to be in good agreement with our measured values. In particular, the present results support the trend toward fairly high charge transfer cross section values for N+ energies above 10 eV. In the overlap range between 2.4 and 7.2 kTd, our concentration ratio [N(+)/N(+)(2)] is about five times smaller than that measured previously from a diffuse Townsend discharge in which electron impact is involved in addition to N+2 collisional dissociation with N2, but has the same trend. Thus it seems that, besides N+2 dissociation by electron impact, collisional dissociation becomes important at elevated values of E/N. In connection with previous discharge work in nitrogen, the present study may help explain the enhanced cathode yields observed.

Journal Article↗
PubMed2000

Symmetry breakdown in ground state dissociation of HD+

Experimental studies of the dissociation of the electronic ground state of HD+ following ionization of HD by fast proton impact indicate that the H++D(1s) dissociation channel is more likely than the H(1s)+D+ dissociation channel by about 7%. This isotopic symmetry breakdown is due to the finite nuclear mass correction to the Born-Oppenheimer approximation which makes the 1ssigma state 3.7 meV lower than the 2psigma state at the dissociation limit. The measured fractions of the two dissociation channels are in agreement with coupled-channels calculations of 1ssigma to 2psigma transitions.

Journal Article↗
PubMed2001

Observation of accurate ion dissociation thresholds in pulsed field ionization-photoelectron studies.

We report the first observation, together with a mechanism for such an observation, of a steplike feature in the pulsed field ionization photoelectron measurement of CH4(C2H2), marking the 0 K dissociation threshold for the formation of CH3(+) + H(C2H(+) + H) from CH4(C2H2). The nonexistence of a step in the spectrum for C 2H4 at its dissociation threshold for C2H2(+) formation provides strong support for the proposed mechanism. This experiment shows that, for a range of molecules, where the ion dissociation lifetimes near the dissociation thresholds are <10(-7) s, pulsed field ionization photoelectron measurements will yield not only highly accurate ionization energies, but also 0 K dissociation thresholds.

Journal Article↗
PubMed2004

Water dissociation on Ru(001): an activated process.

It is shown using x-ray photoelectron spectroscopy that water is adsorbed either nondissociatively or partially dissociatively on Ru(001) under ultrahigh vacuum conditions. We found an activated dissociation process with a barrier slightly larger than that of desorption. A difference in dissociation barriers is found between H2O and D2O that explains the anomalous isotope effects in the thermal desorption. Previous theoretical and experimental disagreements can be rationalized based on electron or x-ray beam-induced dissociation of the water overlayer and an earlier underestimation of the dissociation barrier.

Journal Article↗
PubMed2002

Binding specificity and the ligand dissociation process in the E. coli biotin holoenzyme synthetase.

The binding of the Escherichia coli biotin holoenzyme synthetase to the two ligands, biotin and bio-5'-AMP, is coupled to disorder-to-order transitions in the protein. In the structure of the biotin complex, a "glycine-rich" loop that is disordered in the apo-enzyme is folded over the ligand. Mutations in three residues in this loop result in significant changes in the affinity of the enzyme for both biotin and bio-5'-AMP. The kinetic basis of these losses in the affinity resides primarily in changes in the unimolecular rates of dissociation of the complexes. In this work, isothermal titration calorimetry has been employed to examine the detailed thermodynamics of binding of three loop mutants to biotin and bio-5'-AMP. The energetic features of dissociation of the protein*ligand complexes also have been probed by measuring the temperature dependencies of the unimolecular dissociation rates. Analysis of the data using the Eyring formalism yielded entropic and enthalpic contributions to the energetic barrier to dissociation. The thermodynamic results coupled with the known structures of the apo-enzyme and biotin complex have been used to formulate a model for progression from the ground-state complex to the transition state in biotin dissociation. In this model, the transition-state is characterized by both partial disruption of noncovalent bonds and acquisition of some of the disorder that characterizes the glycine-rich loop in the absence of ligand.

Adenosine Monophosphate↗
PubMed2001

All together now: when dissociations between knowledge and action disappear.

Why do people sometimes seem to know things but fail to act appropriately on the basis of this knowledge? Such dissociations between knowledge and action often occur in infants and children, and in adults following brain damage. These dissociations have supported inferences about the organization of cognitive processes (e.g., separable knowledge and action systems) and their development (e.g., knowledge systems develop before action systems). The current study tested the basis for knowledge-action dissociations in a card-sorting task in which children typically correctly answer questions about sorting rules while sorting cards incorrectly. When questions and sorting measures were more closely equated for the amount of conflict that needed to be resolved for a correct response, children showed no systematic dissociation between knowledge and action. The results challenge standard interpretations of knowledge-action dissociations and support an alternative account based on graded knowledge representations.

Child, Preschool↗
PubMed1990

The dependence of detergent-induced cell dissociation in non-palmo-plantar stratum corneum on endogenous proteolysis.

We have recently shown that cell cohesion in plantar stratum corneum is mediated to a significant extent by protein structures, and that endogenous proteolysis plays an important role in desquamation in this tissue. This paper is a report of our investigations into whether similar mechanisms for cell cohesion and desquamation can be found in non-palmo-plantar stratum corneum. Biopsies of non-palmo-plantar human skin were incubated at 37 degrees C, pH 8, in a buffer with and without additions of detergents (a mixture of N,N-dimethyldodecylamine oxide and sodium dodecyl sulphate), ethylene diamine tetraacetate (EDTA), and the proteinase inhibitor aprotinin. Released cells were examined by phase contrast microscopy and counted. The incubated biopsies were examined by light microscopy. As has been previously shown by others, we found that in the presence of detergents there was a dissociation of stratum corneum cells. This dissociation was stimulated by EDTA and inhibited by aprotinin. After 36 h of incubation the entire stratum corneum and, on some parts of the biopsies, the stratum granulosum had dissociated. There was no evidence of cell dissociation in the spinous or basal epidermal layers. We conclude that the detergent-induced cell dissociation in non-palmo-plantar human stratum corneum is dependent on the action of proteinases present in the tissue on protein structures. These structures may be of significant importance for non-palmo-plantar stratum corneum cell cohesion.

Aprotinin↗
PubMed1992

Alpha IIB beta 3 integrin dissociation induced by EDTA results in formation of Birbeck granule-like structures in human blood platelets.

EDTA (5 mM at 37 degrees C and pH 7.4 for 30 min) induces morphologic changes in the platelet surface-connected canalicular system (SCCS), in particular the collapse of some portions of the canaliculi. These collapsed elements are made up of two parallel limiting membranes and a central irregular striated zone, a pentalaminar organization that clearly resembles that of Langerhans cell Birbeck granules (BG). Such BG-like structures are also seen between adjacent platelets in EDTA-treated platelet microaggregates. EDTA is known to induce an irreversible dissociation of the platelet membrane glycoprotein(GP)IIb-IIIa, the alpha IIb beta 3 platelet-specific intergrin, a calcium-dependent heterodimer that serves as an inducible receptor for fibrinogen and is essential for platelet aggregation. Hence, we looked for involvement of the GPIIb-IIIa in the formation of these BG-like modifications. We observed that these changes i) cannot be induced in type I Glanzmann's thrombasthenia, where the GPIIb-IIIa complexes are absent; ii) did not appear when human platelets were pre-incubated with MoAb anti-GPIIb-IIIa complex, which protected GPIIb-IIIa from EDTA-induced dissociation; iii) appeared only at alkaline pH and 37 degrees C, which corresponds to the range of pH and temperature where EDTA can dissociate the GPIIb-IIIa complexes; iv) are accompanied by the dissappearance on fluorescence flow cytometry analysis of the heterodimer specific epitopes; and v) do not appear in rat platelets at pH 7.4 where GPIIb-IIIa does not dissociate after EDTA treatment. Thus, the appearance of BG-like structures in the platelet SCCS is directly dependent on the EDTA-induced dissociation of the GPIIb-IIIa complexes. Furthermore, using gold-labeled MoAb concomitantly with the addition of EDTA, we observed that only GPIIb is present in the collapsed portions of the canaliculi. On Lowicryl thin sections essentially polyclonal antibodies to GPIIb labeled the central striated zone. All these observations lead us to suggest that homopolymers of GPIIb could be responsible for "zipping" of the SCCS and raise the question of the participation of a Langerhans cell integrin in the formation of Langerhans cell BG.

Blood Platelets↗
PubMed1984

Dissociation of the Factor-VIII complex during clotting: role of thrombin and phospholipids.

To study the dissociation of the two moieties of the factor-VIII complex during clotting, plasma, concentrate and serum were chromatographed on 4% agarose. In plasma and concentrate factor-VIII coagulant activity (VIII C), factor-VIII coagulant antigen (VIII C:Ag), and factor-VIII-related antigen (VIII R:Ag) eluted together in the void volume, but some VIII C:Ag eluted after the void volume. The amount of VIII C:Ag eluting after the void volume was made smaller by adding proteinase inhibitors. In serum nearly all VIII C:Ag eluted after the void volume. From factor-VIII complex immunoadsorbed by means of an antibody against VIII R:Ag no VIII C:Ag was dissociated by thrombin or by thrombin and physiologic CaCl2 concentrations. Radiolabelled human thrombin did not bind to the VIII C:Ag of immunoadsorbed factor-VIII complex. VIII C:Ag displaying VIII C was dissociated from immunoadsorbed factor-VIII complex by human brain thromboplastin or by phosphatidyl-serine. Our results suggest that VIII C:Ag and VIII R:Ag dissociate during clotting. This dissociation seems not to be mediated by thrombin, but may be mediated by phospholipids.

Antigens↗
PubMed1992

Determination of dissociation constants of some pharmaceutical compounds using derivative spectrophotometry.

Derivative spectrophotometry is applied to the determination of the dissociation constants (pKa) of some weakly dissociating compounds. The pKa value(s) can be calculated by measuring the amplitude of derivative maxima (D) at the selected wavelengths for the dissociated, undissociated and partially dissociated forms of the compound. A graphical technique, based on plotting the derivative spectrophotometric titration curves and interpolating pKa at D1/2 is also presented. Both methods are applied to pharmaceutical compounds with single and double dissociation constants. The results are in good agreement with the reported pKa values of the investigated compounds.

Chemistry, Pharmaceutical↗