arXiv subjects

Jaeyoung Sung

Publications and source records attributed to Jaeyoung Sung.

13 recordsLinked to original sources

arXiv2026

A General Model for Continuous Time Principal-Agent Problem Under Hidden Action

In this paper, we study a general continuous-time Principal-Agent (PA) problem, where the agent privately makes effort and consumption decisions over time under a contract with payment schemes both in continuous time and in lump sums. In particular, we allow the continuous payment process to be a controlled diffusion, which is directly related to the pay-to-performance sensitivity (PPS) in the empirical literature. In solving the agent's problem, we propose a new sufficient condition that directly yields a solution to the agent's problem, without requiring a separate verification step for the solution obtained from the first-order approach. We also present an example which can be solved explicitly.

q-fin.MF↗

arXiv2026

Effects of Divalent Cations on Diffusion Dynamics of Biological Water Confined between Lipid Membranes

Biological water is an ionic solution containing both monovalent and divalent ions. However, the effects of divalent ions on the dynamics of biological water remain largely unknown. Here, we investigate how the transport dynamics of water molecules nanoconfined between lipid membranes depends on the concentration of calcium (Ca2+) and magnesium (Mg2+) ions by using molecular dynamics simulations and the generalized transport equation for biological water. We find that the diffusion coefficient of biological water monotonically increases with Ca2+ ion concentration but exhibits a largely opposite, non-monotonic dependence on Mg2+ concentration. The deviation of the water molecules' displacement distribution from the Gaussian also shows distinct dependence on the concentrations of Mg2+ and Ca2+. These contrasting behaviors originate from the different hydration radii of these divalent ions and their distinct effects on the interfacial structure and dynamics of biological water. The relaxation of the lateral displacement distribution of water molecules toward a Gaussian is determined by the time-correlation function of diffusion coefficient fluctuations, whose relaxation time increases with salt concentrations. The primary source of the lateral diffusion coefficient fluctuation is thermal motion of water molecules in the longitudinal direction, along which microscopic environments surrounding a water molecule, including the functional groups of lipid membrane and ion concentrations, drastically change.

cond-mat.soft↗

arXiv2024

Hyperphosphorylation-Induced Phase Transition in Vesicle Delivery Dynamics of Motor Proteins in Neuronal Cells

Synaptic vesicle transport by motor proteins along microtubules is a crucial active process underlying neuronal communication. It is known that microtubules are destabilized by tau-hyperphosphorylation, which causes tau proteins to detach from microtubules and form neurofibril tangles. However, how tau-phosphorylation affects transport dynamics of motor proteins on the microtubule remains unknown. Here, we discover that long-distance unidirectional motion of vesicle-motor protein multiplexes (VMPMs) in living cells is suppressed under tau-hyperphosphorylation, with the consequent loss of fast vesicle-transport along the microtubule. The VMPMs in hyperphosphorylated cells exhibit seemingly bidirectional random motion, with dynamic properties far different from VMPM motion in normal cells. We establish a parsimonious physicochemical model of VMPM's active motion that provides a unified, quantitative explanation and predictions for our experimental results. Our analysis reveals that, under hyperphosphorylation conditions, motor-protein-multiplexes have both static and dynamic motility fluctuations. The loss of the fast vesicle-transport along the microtubule can be a mechanism of neurodegenerative disorders associated with tau-hyperphosphorylation.

physics.bio-ph↗

arXiv2024

Transport Dynamics of Water Molecules Confined between Lipid Membranes

Water molecules confined between biological membranes exhibit a distinctive non-Gaussian displacement distribution, far different from bulk water. Here, we introduce a new transport equation for water molecules in the intermembrane space, quantitatively explaining molecular dynamics simulation results. We find the unique transport dynamics of water molecules stems from the lateral diffusion coefficient fluctuation caused by their longitudinal motion. We also identify an interfacial region where water possesses distinct physical properties, unaffected by changes in the intermembrane separation.

cond-mat.soft↗

arXiv2019

Product number counting statistics from stochastic bursting birth-death processes

Bursting and non-renewal processes are common phenomena in birth-death process, yet no theory can quantitatively describe a non-renewal birth process with bursting. Here, we present a theoretical model that yields the product number counting statistics of product creation occurring in bursts and of a non-renewal creation process. When product creation is a stationary process, our model confirms that product number fluctuation decreases with an increase in the product lifetime fluctuation, originating from the non-Poisson degradation dynamics, a result obtained in previous work. Our model additionally demonstrates that the dependence of product number fluctuation on product lifetime fluctuation varies with time, when product creation is a non-stationary process. We find that bursting increases product number fluctuation, compared to birth-processes without bursting. At time zero, in a burst-less birth process, product number fluctuation is unsurprisingly found to be zero, but we discover that, in a bulk creation process characterized by bursting, product number fluctuation is a finite value at time zero. The analytic expressions we obtain are applicable to many fields related to the study system population, such as queueing models and gene expression.

q-bio.MN↗

arXiv2019

Observation time dependent mean first passage time of diffusion and sub-diffusion processes

The mean first passage time, one of the important characteristics for a stochastic process, is often calculated assuming the observation time is infinite. However, in practice, the observation time, T, is always finite and the mean first passage time (MFPT) is dependent on the length of the observation time. In this work, we investigate the observation time dependence of the MFPT of a particle freely moving in the interval [-L,L] for a simple diffusion model and four different models of subdiffusion, the fractional diffusion equation (FDE), scaled Brown motion (SBM), fractional Brownian motion (FBM), and stationary Markovian approximation model of SBM and FBM. We find that the MFPT is linearly dependent on T in the small T limit for all the models investigated, while the large-T behavior of the MFPT is sensitive to stochastic properties of the transport model in question. We also discuss the relationship between the observation time, T, dependence and the travel-length, L, dependence of the MFPT. Our results suggest the observation time dependency of the MFPT can serve as an experimental measure that is far more sensitive to stochastic properties of transport processes than the mean square displacement.

cond-mat.stat-mech↗

arXiv2018

Frequency spectrum of biological noise: a probe of reaction dynamics in living cells

Even in the steady-state, the number of biomolecules in living cells fluctuates dynamically; and the frequency spectrum of this chemical fluctuation carries valuable information about the mechanism and the dynamics of the intracellular reactions creating these biomolecules. Although recent advances in single-cell experimental techniques enable the direct monitoring of the time-traces of the biological noise in each cell, the development of the theoretical tools needed to extract the information encoded in the stochastic dynamics of intracellular chemical fluctuation is still in its adolescence. Here, we present a simple and general equation that relates the power-spectrum of the product number fluctuation to the product lifetime and the reaction dynamics of the product creation process. By analyzing the time traces of the protein copy number using this theory, we can extract the power spectrum of the mRNA number, which cannot be directly measured by currently available experimental techniques. From the power spectrum of the mRNA number, we can further extract quantitative information about the transcriptional regulation dynamics. Our power spectrum analysis of gene expression noise is demonstrated for the gene network model of luciferase expression under the control of the Bmal 1a promoter in mouse fibroblast cells. Additionally, we investigate how the non-Poisson reaction dynamics and the cell-to-cell heterogeneity in transcription and translation affect the power-spectra of the mRNA and protein number.

physics.bio-ph↗

arXiv2018

Universal Transport Dynamics of Complex Fluids

Thermal motion in complex fluids is a complicated stochastic process but ubiquitously exhibits initial ballistic, intermediate sub-diffusive, and long-time non-Gaussian diffusive motion, unless interrupted. Despite its relevance to numerous dynamical processes of interest in modern science, a unified, quantitative understanding of thermal motion in complex fluids remains a long-standing problem. Here, we present a new transport equation and its solutions, which yield a unified quantitative explanation of the mean square displacement (MSD) and the non-Gaussian parameter (NGP) of various fluid systems. We find the environment-coupled diffusion kernel and its time correlation function are two essential quantities determining transport dynamics of complex fluids. From our analysis, we construct a general, explicit model of the complex fluid transport dynamics. This model quantitatively explains not only the MSD and NGP, but also the time-dependent relaxation of the displacement distribution for various systems. We introduce the concepts of intrinsic disorder and extrinsic disorder that have distinct effects on transport dynamics and different dependencies on temperature and density. This work presents a new paradigm for quantitative understanding of transport and transport-coupled processes in complex disordered media.

cond-mat.dis-nn↗

arXiv2017

Super-Gaussian, super-diffusive transport of multi-mode active matter

Living cells exhibit multi-mode transport that switches between an active, self-propelled motion and a seemingly passive, random motion. Cellular decision-making over transport mode switching is a stochastic process that depends on the dynamics of the intracellular chemical network regulating the cell migration process. Here, we propose a theory and an exactly solvable model of multi-mode active matter. Our exact model study shows that the reversible transition between a passive mode and an active mode is the origin of the anomalous, super-Gaussian transport dynamics, which has been observed in various experiments for multi-mode active matter. We also present the generalization of our model to encompass complex multi-mode matter with arbitrary internal state chemical dynamics and internal state dependent transport dynamics.

physics.bio-ph↗

arXiv2017

Comments on Nonrenewal Statistics in the Catalytic Activity of Enzyme Molecules at Mesoscopic Concentrations

It is well known in enzyme kinetics that the Michaelis-Menten (MM) equation is applicable only to enzymes in the steady state. We show that the result obtained in the previous work [Phys. Rev. Lett. 107, 218301 (2011)] is inconsistent with the MM equation, not because the authors considered the enzyme system at mesoscopic concentrations but because they considered the enzyme system in the non-stationary state. The substrate concentration dependence of the mean turnover time is, in fact, consistent with the MM equation in the steady state, regardless of the number of enzymes in the system.

physics.bio-ph↗

arXiv2005

Reply to Note on cond-mat/0510270: Jarzynski equation for adiabatically stretched rotor

Although the analysis in cond-mat/0510270 is correct, this doesn't mean Jarzynski relation holds always for an arbitrary process. There exists a sufficient and necessary condition for Jarzynski relation to hold for an adiabatic parameter switching process. In contradiction to recent assertions, the validity condition of Jarzynski relation for an adiabatic process is not always satisfied.

cond-mat.stat-mech↗

arXiv2005

Breakdown of the Jarzynski relation for an adiabatic stretching of an isotropic spring

We perform a theoretical test of Jarzynski relation for an adiabatic stretching of an isotropic spring, which is an exactly solvable model. It turns out that Jarzynski relation does not hold even when the entire infinite momentum space of the system state is taken into account, in contradiction to recent arguments supporting Jarzynski relation.

cond-mat.stat-mech↗

arXiv2005

Validity condition of the Jarzynski relation for a classical mechanical system

Recently, Jarzynski suggested a striking thermodynamic equation that relates free energy change of a system and work done on the system during arbitrary nonequilibrium processes, which has been believed to hold irrespective of detailed nature of the nonequilibrium process. However, we show here that the Jarzynski equation does not hold for an adiabatic process unless the phase-space extension of the system on completion of the adiabatic process coincides with that of the final equilibrium state of the system. This condition can be satisfied only when the adiabatic process does not change parameters on which the equilibrium phase-space extension of our system is dependent.

cond-mat.stat-mech↗