Biomedical subjects

M Y Choi

Publications and source records attributed to M Y Choi.

At least 19 recordsLinked to original sources

PubMed2006

Rotational dynamics of a diatomic solute in the room-temperature ionic liquid 1-ethyl-3-methylimidazolium hexafluorophosphate.

Reorientational time correlation functions C(l)(t)( identical withP(l)[cos theta(t)]) for a diatomic solute in 1-ethyl-3-methylimidazolium hexafluorophosphate (EMI(+)PF(6) (-)) are analyzed via molecular dynamics computer simulations, where <...> denotes an equilibrium ensemble average, P(l) the lth order Legendre polynomial and theta(t) the angle between the solute orientation at time t and its initial direction. Overall results are indicative of heterogeneous dynamics in EMI(+)PF(6) (-). For a small nondipolar solute, C(l)(t) are well-described as stretched exponential functions in wide time ranges. One striking feature is that after rapid initial relaxation, C(2)(t) decays more slowly than C(1)(t). As a result, the correlation time associated with the former is considerably longer than that with the latter. This is ascribed to solvent structural fluctuations, which allow large-amplitude solute rotations. As the solute size grows, relaxation of C(l)(t) approaches exponential decay.

Journal Article↗

PubMed2006

Health status and resonance in a model for living organisms under periodic stress and healing.

We apply the dynamic model for failures to a living organism under periodic stress and study how the health status of the organism evolves. It is found that without healing, the average fraction of intact cells decays either stepwise to zero or to a constant value far from zero, depending on the peak value of the periodic stress. As the parameter measuring the healing probability is raised from zero, the fraction exhibits oscillating behavior, reminiscent of periodic synchronization. The power spectrum at the stress frequency at first increases with the healing parameter, then decreases, which may be called healing resonance. We also study the time evolution of the system in the case that the healing parameter varies periodically with time and observe a transition from the unhealthy state to the healthy one as the healing frequency increases. This suggests how to adjust the frequency of medical treatment to the optimum.

Animals↗

PubMed2005

Unilateral recession-resection in children with exotropia of the convergence insufficiency type.

PURPOSE: The surgical success rates for intermittent exotropia of the convergence insufficiency type have been reported to be variable, and most were studied retrospectively in adults. The purpose of this study was to evaluate prospectively the long-term surgical results of unilateral lateral rectus (LR) muscle recession and medial rectus (MR) muscle resection in children with intermittent exotropia of the convergence insufficiency type. METHODS: A total of 14 children with intermittent exotropia greater at near than at distance by 10 prism diopters (PD) or more were included in this prospective study. The amounts of resection and recession were based on near and distance deviation, respectively. Minimum follow-up was 1 year (mean 26.6 months; range, 12-68 months) after surgery. The paired t-test was used to compare preoperative and postoperative measurements of the angle of deviation at distance and near, near-distance difference. RESULTS: Significant postoperative reduction was achieved in terms of mean distance exodeviation, from 22.5 PD to 9.1 PD (P=0.000), and mean near exodeviation from 33.8 PD to 13.6 PD (P=0.000). Mean near-distance difference reduced from 11.3 PD preoperatively to 4.6 PD postoperatively (P=0.000). Fresnel prism was used temporarily to treat postoperative esotropia in only one patient for postoperative 6 months. CONCLUSIONS: Unilateral surgery biased to MR strengthening more than LR weakening in children with intermittent exotropia of the convergence insufficiency type, was found to successfully reduce both distance and near deviation and to collapse near-distance differences with a low risk of long-term postoperative esotropia.

Child↗

PubMed2005

Glucose metabolism and oscillatory behavior of pancreatic islets.

A variety of oscillations are observed in pancreatic islets. We establish a model incorporating two oscillatory systems of different time scales: One is the well-known bursting model in pancreatic cells and the other is the glucose-insulin feedback model which considers direct and indirect feedback of secreted insulin. These two are coupled to interact with each other in the combined model, and two basic assumptions are made on the basis of biological observations: The conductance gK(ATP) for the ATP-dependent potassium current is a decreasing function of the glucose concentration whereas the insulin secretion rate is given by a function of the intracellular calcium concentration. Obtained via extensive numerical simulations are complex oscillations including clusters of bursts, slow and fast calcium oscillations, and so on. We also consider how the intracellular glucose concentration depends upon the extracellular glucose concentration, and examine the inhibitory effects of insulin.

Adenosine Triphosphate↗

PubMed2005

Collective synchronization in spatially extended systems of coupled oscillators with random frequencies.

We study collective behavior of locally coupled limit-cycle oscillators with random intrinsic frequencies, spatially extended over d -dimensional hypercubic lattices. Phase synchronization as well as frequency entrainment are explored analytically in the linear (strong-coupling) regime and numerically in the nonlinear (weak-coupling) regime. Our analysis shows that the oscillator phases are always desynchronized up to d=4 , which implies the lower critical dimension dP(l) =4 for phase synchronization. On the other hand, the oscillators behave collectively in frequency (phase velocity) even in three dimensions (d=3) , indicating that the lower critical dimension for frequency entrainment is dF(l)=2 . Nonlinear effects due to the periodic nature of limit-cycle oscillators are found to become significant in the weak-coupling regime: So-called runaway oscillators destroy the synchronized (ordered) phase and there emerges a fully random (disordered) phase. Critical behavior near the synchronization transition into the fully random phase is unveiled via numerical investigation. Collective behavior of globally coupled oscillators is also examined and compared with that of locally coupled oscillators.

Action Potentials↗

PubMed2005

The long-term result of slanted medial rectus resection in exotropia of the convergence insufficiency type.

PURPOSE: To evaluate the long-term results of slanted medial rectus (MR) resection for intermittent exotropia (X(T)) of the convergence insufficiency type. METHODS: In all, 10 patients with an X(T) greater at near than at distance by 10 prism diopters (PD) or more were included in this prospective study. Patients received slanted bilateral MR resection. The upper edge of the MR was resected according to the distance exodeviation and the lower edge of the MR was resected according to near exodeviation. The postoperative follow-up period was between 6 and 62 months with a mean of 38.9 months. The paired t-test was used to compare: mean distance angle of deviation preoperatively and postoperatively; mean near angle of deviation preoperatively and postoperatively; and mean near-distance exodeviation difference preoperatively and postoperatively. RESULTS: Bilateral slanted MR resections reduced mean exodeviation at distance from 23.0+/-7.2 to 16.3+/-5.4 PD (P=0.03); mean exodeviation at near from 34.3+/-7.7 to 24.6+/-6.9 PD (P=0.01); and mean near-distance difference from 11.4+/-2.6 to 8.3+/-3.5 PD (P=0.04). At the final follow-up examination, all patients demonstrated an exodeviation of 10 PD or more at distance and near, and the exodeviation difference between distance and near deviation was within 10 PD in five of the 10 patients. Three patients had an esodeviation at distance after surgery, but all resolved within 4 weeks. CONCLUSIONS: Bilateral slanted MR resections in patients with X(T) of the convergence insufficiency type resulted in undercorrection in all patients.

Adolescent↗

PubMed2005

Multiple isomers of uracil-water complexes: infrared spectroscopy in helium nanodroplets.

Infrared laser spectroscopy is used to show that four structural isomers of the uracil-water binary complex are formed in helium nanodroplets. The assignment of the infrared spectra is aided by measurements of vibrational transition moment angles (VTMAs) for various vibrational modes of these complexes. The experimental results are in excellent agreement with ab initio calculations, which had previously predicated the existence of the same four isomers. The results suggest that the relative abundances of the various isomers formed in helium droplets have more to do with the widths of the valleys in the potential surface that funnel into a particular local minimum than on the associated energetics.

Cold Temperature↗

PubMed2005

Slow relaxation in the Ising model on a small-world network with strong long-range interactions.

We consider the Ising model on a small-world network, where the long-range interaction strength J2 is in general different from the local interaction strength J1, and examine its relaxation behaviors as well as phase transitions. As J(2)/J(1) is raised from zero, the critical temperature also increases, manifesting contributions of long-range interactions to ordering. However, it becomes saturated eventually at large values of J(2)/J(1) and the system is found to display very slow relaxation, revealing that ordering dynamics is inhibited rather than facilitated by strong long-range interactions. To circumvent this problem, we propose a modified updating algorithm in Monte Carlo simulations, assisting the system to reach equilibrium quickly.

Journal Article↗

PubMed2005

Multiple tautomers of cytosine identified and characterized by infrared laser spectroscopy in helium nanodroplets: probing structure using vibrational transition moment angles.

Infrared laser spectroscopy in helium nanodroplets is used to identify and characterize several distinct tautomers of cytosine. The experimentally observed species correspond to the lowest-energy structures obtained from ab initio calculations, also reported here. The assignment of the infrared vibrational bands in the spectra is aided by the measurement of the corresponding vibrational transition moment angles, which are also calculated using ab initio methods. In the present study we confirm the existence of three primary tautomers and provide tentative assignments for even higher-energy forms of cytosine in helium nanodroplets.

Algorithms↗

PubMed2005

A molecular dynamics computer simulation study of room-temperature ionic liquids. I. Equilibrium solvation structure and free energetics.

Solvation in 1-ethyl-3-methylmidazolium chloride and in 1-ethyl-3-methylimidazolium hexafluorophosphate near equilibrium is investigated via molecular dynamics computer simulations with diatomic and benzenelike molecules employed as probe solutes. It is found that electrostriction plays an important role in both solvation structure and free energetics. The angular and radial distributions of cations and anions become more structured and their densities near the solute become enhanced as the solute charge separation grows. Due to the enhancement in structural rigidity induced by electrostriction, the force constant associated with solvent configuration fluctuations relevant to charge shift and transfer processes is also found to increase. The effective polarity and reorganization free energies of these ionic liquids are analyzed and compared with those of highly polar acetonitrile. Their screening behavior of electric charges is also investigated.

Journal Article↗

PubMed2005

A molecular dynamics computer simulation study of room-temperature ionic liquids. II. Equilibrium and nonequilibrium solvation dynamics.

The molecular dynamics (MD) simulation study of solvation structure and free energetics in 1-ethyl-3-methylimidazolium chloride and 1-ethyl-3-methylimidazolium hexafluorophosphate using a probe solute in the preceding article [Y. Shim, M. Y. Choi and H. J. Kim, J. Chem. Phys. 122, 044510 (2005)] is extended to investigate dynamic properties of these liquids. Solvent fluctuation dynamics near equilibrium are studied via MD and associated time-dependent friction is analyzed via the generalized Langevin equation. Nonequilibrium solvent relaxation following an instantaneous change in the solute charge distribution and accompanying solvent structure reorganization are also investigated. Both equilibrium and nonequilibrium solvation dynamics are characterized by at least two vastly different time scales--a subpicosecond inertial regime followed by a slow diffusive regime. Solvent regions contributing to the subpicosecond nonequilibrium relaxation are found to vary significantly with initial solvation configurations, especially near the solute. If the solvent density near the solute is sufficiently high at the outset of the relaxation, subpicosecond dynamics are mainly governed by the motions of a few ions close to the solute. By contrast, in the case of a low local density, solvent ions located not only close to but also relatively far from the solute participate in the subpicosecond relaxation. Despite this difference, linear response holds reasonably well in both ionic liquids.

Journal Article↗

PubMed2004

Collective phase synchronization in locally coupled limit-cycle oscillators.

We study collective behavior of locally coupled limit-cycle oscillators with scattered intrinsic frequencies on d -dimensional lattices. A linear analysis shows that the system should always be desynchronized up to d=4 . On the other hand, numerical investigation for d=5 and d=6 reveals the emergence of the synchronized (ordered) phase via a continuous transition from the fully random desynchronized phase. This demonstrates that the lower critical dimension for the phase synchronization in this system is d(l) =4 .

Journal Article↗

PubMed2004

Factors that predict better synchronizability on complex networks.

While shorter characteristic path length has in general been believed to enhance synchronizability of a coupled oscillator system on a complex network, the suppressing tendency of the heterogeneity of the degree distribution, even for shorter characteristic path length, has also been reported. To see this, we investigate the effects of various factors such as the degree, characteristic path length, heterogeneity, and betweenness centrality on synchronization, and find a consistent trend between the synchronization and the betweenness centrality. The betweenness centrality is thus proposed as a good indicator for synchronizability.

Computer Simulation↗

PubMed2003

Stability and ensemble inequivalence in a globally coupled system.

We consider a system of globally coupled rotors, described by a set of Langevin equations, and examine the stability of the incoherent phase. The corresponding Fokker-Planck equation, providing a unified description of microcanonical and canonical ensembles, bears a few solutions, depending upon the ensemble. It is found that the stability of each solution varies differently with the temperature, revealing the inequivalence between the two ensembles. This also suggests a physical explanation of the quasistationarity observed in recent numerical results.

Journal Article↗

PubMed2003

Phase transition in the Ising model on a small-world network with distance-dependent interactions.

We study the collective behavior of an Ising system on a small-world network with the interaction J(r) proportional to r(-alpha), where r represents the Euclidean distance between two nodes. In the case of alpha=0 corresponding to the uniform interaction, the system is known to possess a phase transition of the mean-field nature, while the system with the short-range interaction (alpha-->infinity) does not exhibit long-range order at any finite temperature. The Monte Carlo simulations are performed at various values of alpha, and the critical value alpha(c) beyond which the long-range order does not emerge is estimated to be 0. Thus, concluded is the absence of a phase transition in the system with the algebraically decaying interaction r(-alpha) for any nonzero positive value of alpha.

Journal Article↗

PubMed2003

Optimal size of a complex network.

We investigate the response behavior of an Ising system, driven by an oscillating field, on a small-world network, with particular attention to the effects of the system size. The responses of the magnetization to the driving field are probed by means of Monte Carlo dynamic simulations with the varied rewiring probability. It is found that at low and high temperatures the occupancy ratio, measuring how many spins follow the driving field, behaves monotonically with the system size. At intermediate temperatures, on the other hand, the occupancy ratio first grows and then reduces as the size is increased, displaying a resonancelike peak at a finite value of the system size. In all cases, further increase of the size eventually leads to saturation to finite values; the size at which saturation emerges is observed to depend on the temperature, similarly to the correlation length of the system.

Journal Article↗

PubMed2003

Light and scanning electron microscopic evaluation of Glyde File Prep in smear layer removal.

AIM: To evaluate the effectiveness of Glyde File Prep used in conjunction with sodium hypochlorite irrigation in the removal of smear layer produced during root canal instrumentation. METHODOLOGY: Thirty-nine extracted human teeth with single root canals were used. Access cavities were prepared and the teeth divided into three groups of 13 teeth each. Each group was treated by one of the three different regimes of irrigation and conditioning during root canal instrumentation. Group A: 0.5 mL of 1% NaOCl irrigation after each file size with an additional final irrigation of 10 mL 1% NaOCl; group B: 0.5 mL of 1% NaOCl irrigation after each file size with an additional final irrigation of 10 mL 17% EDTA; group C: Glyde File Prep coated on each instrumentation file used in conjunction with 0.5 mL 1% NaOCl irrigation after each file size and an additional final irrigation of 10 mL 1% NaOCl. The teeth were then longitudinally grooved and sectioned. Root canal cleanliness was evaluated with the aid of a Nikon light microscope (x40 and x100) and scanning electron microscope (x1000 and x3000). The debris scores obtained at three canal regions were compared statistically within the same group and among different groups using repeated measurements of analysis of variance (anova) with Bonferroni adjustments and anova with posthoc Tukey HSD, respectively. RESULTS: The canals treated with EDTA and Glyde File Prep were significantly cleaner than those treated with NaOCl alone. The apical region of the root canals generally displayed more residual smear layer, but the difference was not significant. CONCLUSIONS: Used in conjunction with NaOCl irrigation, Glyde File Prep was effective in removing smear layer produced during root canal instrumentation.

Analysis of Variance↗

PubMed2002

Dynamic instabilities induced by asymmetric influence: prisoners' dilemma game in small-world networks.

A two-dimensional small-world-type network, subject to spatial prisoners' dilemma dynamics and containing an influential node defined as a special node, with a finite density of directed random links to the other nodes in the network, is numerically investigated. It is shown that the degree of cooperation does not remain at a steady state level but displays a punctuated equilibrium-type behavior manifested by the existence of sudden breakdowns of cooperation. The breakdown of cooperation is linked to an imitation of a successful selfish strategy of the influential node. It is also found that while the breakdown of cooperation occurs suddenly, its recovery requires longer time. This recovery time may, depending on the degree of steady state cooperation, either increase or decrease with an increasing number of long-range connections.

Behavior↗