Diffusional Behaviors Research Articles

A novel treatment of solute drag and solute trapping at a solid–liquid interface during rapid solidification of multicomponent alloys

AbstractIn response to the renewed interest in solute drag and solute trapping models fueled by their applications to additive manufacturing, a novel treatment is proposed to describe the diffusional behaviors of solute at a migrating solid–liquid interface during rapid solidification of multicomponent alloys. While the treatment is still built on irreversible thermodynamics and linear kinetic law, its novelty lies in breaking up the classical trans‐interface diffusional flux into two separate fluxes one is the transferred‐back flux with its ending point at the interface and the other is the bumping‐back flux with its starting point at the interface. This novel treatment entails three significant improvements in reference to the existing models. Firstly, it reveals that the degree of solute drag is dependent on the ratio of liquid diffusive speed over interface diffusive speed. Secondly, a novel relation between the distribution coefficient and interface velocity is derived. It amends the confusing behavior seen in Aziz’s without‐drag continuous growth model. Thirdly, the proposed treatment eliminates the need of prescribing the degree of solute drag parameter for the kinetic phase diagram calculation. The numerical solution to the proposed model is presented, and it is ready to be used for the kinetic phase diagram calculation.

Quantitative evaluation of oncogenic KRAS signaling and therapeutic effects of molecular-targeted drugs in living cells by single-molecule imaging.

76 Background: Oncogenic KRAS mutations pose challenges due to their constitutive activation and resistance to drugs in colorectal cancer. However, several aspects remain unresolved. Single-molecule imaging has revealed distinct behaviors of KRAS. Inactive KRAS wild-type (WT) molecules exhibit continuous lateral diffusion on the plasma membrane, while activated or oncogenic KRAS molecules display slower diffusion and transient trapping due to interactions with signaling molecules. These observations suggest that the overall signal intensity of KRAS within a cell is finely regulated by integrating short-term pulse signals during transient trapping. Methods: We conducted single-molecule observations of KRAS (WT, G13D, and G12D/C/V) in living colon cancer cells, both with and without molecular-targeted drugs, before and after EGF stimulation. We quantitatively analyzed their diffusional behavior to evaluate KRAS signaling and the therapeutic effects of molecular-targeted drugs. Results: The temporal fraction of trapped KRAS WT was highest at 2 minutes (6.4%) and decreased to pre-activation level (3.0%) at 5 minutes after EGF stimulation. Although the temporal fractions of trapped KRAS mutants were higher than those of KRAS WT both before stimulation (5%) and after stimulation (10%), their diffusional behaviors were mutation-specific. The temporal fraction of trapped KRAS G13D increased sharply, and those of KRAS G12D and G12C increased at a moderate rate, while that of KRAS G12V did not elevate after EGF stimulation. Cetuximab treatment significantly reduced the temporal fraction of trapped KRAS WT after stimulation, but only moderately decreased those of KRAS mutants. Notably, transient trappings of KRAS G12D and G12C were abrogated by KRAS inhibitors, and the combined therapy of KRAS inhibitors and cetuximab demonstrated additive effects. Conclusions: Our findings indicate that the mutation-specific diffusional behavior of oncogenic KRAS can be quantitatively assessed by single-molecule imaging. This method serves as a powerful tool to uncover the mechanisms underlying oncogenic KRAS signaling and evaluate the therapeutic efficacy of molecular-targeted drugs in living cells.

Diffusional and antimicrobial behaviors of some antimicrobial agents in active multilayer plastic films

AbstractIn this study, diffusional and antimicrobial properties of the active‐multilayer plastic films (LDPE/polyamide/LDPE‐2% antimicrobial agent) were assessed. To understand the diffusion of antimicrobials from the active plastic films containing nisin, potassium sorbate, silver substituted zeolite, or chitosan, the active‐multilayer plastic films has been put in different food simulating solutions. The food simulating solutions had various pH (3.0–7.0) and water activity (0.65–0.95) levels and were incubated at various storage temperatures (5–25°C). The release experiments for the combinations of the factors were set according to three factors face‐centered central composite design (CCF) of the response surface methodology (RSM). The diffusion rate constant (D) was calculated using Fick's law approach. The D of nisin and silver substituted zeolite was affected by pH or temperature, while aw is not a significant factor. However, Dps was found to be the highest and depended on all factors (4.45–10.04 × 10−14 cm2/s). Furthermore, no chitosan release was detected from the film matrix. The antimicrobial effectiveness of the films against microorganisms that were Staphylococcus aureus, Escherichia coli, Candida albicans, and Aspergillus niger varied according to the film type, which was verified by a challenge test on the kaşar cheese surface. Moreover, nisin and silver substituted zeolite incorporated films showed great potential to inhibit bacteria, yeast, and molds. All active films, which were studied, had featured tasks for being active food packaging material and diffused in allowance limits.

Short-range order localizing diffusion in multi-principal element alloys

The impact of chemical short-range order (SRO) on diffusion is computationally studied and theoretically analyzed in two classes of multi-principal elements alloys, fcc CrCoNi and bcc MoNbTa. We find the presence of SRO considerably reduces and localizes vacancy-mediated diffusion. The diffusivity reduction is induced by an increase of migration barrier (decreasing jump frequency) and enhanced diffusion correlation that lowers the effectiveness of atomic jumps. By sampling the diffusion pathways and associated energy barriers, a picture of SRO-stretched potential energy landscape is conceived that elucidates an increasing backward jumps and vacancy trapping effect stemming from chemical ordering. The results imply controlling the ordering in chemistry can act as a potential approach for manipulating diffusional behaviors in multi-principal elements alloys.

Diffusional behaviors and mechanical properties of Ni-Zn system

In this paper, the solid-solid diffusion reaction and growth kinetics of Ni-Zn system were investigated using diffusion couple technique. Three intermetallic compounds (IMCs), β1 (NiZn), γ (Ni2Zn11) and δ (Ni3Zn22) were observed at the diffusion zone with the annealing temperature range between 598 K and 673 K. The results indicate that the growth process of reaction zone was dominated by volume diffusion. And the growth constants, integrated diffusion coefficients, growth activation energies and diffusion activation energies of γ and δ were evaluated based on thickness of the phases, reaction temperatures and annealing times. The Young’s modulus and Vickers hardness of the three phases were evaluated from the displacement-loading curve by nanoindenter testing. The Young's modulus and hardness of the IMCs increased firstly and then decreased with the increasing of Zn concentration, where γ phase has the highest Young's modulus (183.3 GPa) and highest hardness (6.05 GPa) among the three phases. Finally, the bulk modulus (B), shear modulus (G), Young's modulus (E), and Poisson's ratio (v) of the Ni-Zn IMCs were calculated by the first-principles calculations. The tendency of calculated Young's modulus varying with the composition of Zn was consistent with that of experimental results.

Structure and Ion Conductivity Study of Argyrodite (Li5.5PS4.5Cl1.5) according to Cooling Method

All solid-state batteries (ASSBs) are now anticipated to be an ultimate solution to the persistent safety issues of conventional lithium-ion batteries (LIBs). Contemporary society’s expanding power demands and growing energy consumption require energy storage with greater reliability, safety and capacity, which cannot be easily achieved with current state-of-the-art liquid-electrolyte-based LIBs. In contrast, these conditions are expected to be met by implementing ASSBs with high-performance solid-state electrolytes (SSEs). In this work, we altered the microscopic structure and Li diffusional behaviors of argyrodites (Li6-xPS5-xCl1+x), which were precisely monitored with cooling protocols. It was shown that, at the cooling speed of -3 <sup>o</sup>C·h<sup>-1</sup>, as the cooling rate decreased, impurities in Li5.5PS4.5Cl1.5 such as LiCl and Li3PO4 gradually diminished and eventually disappeared. At the same time, differences in the lattice sizes of Li5.5PS4.5Cl1.5 crystallites gradually decreased, resulting in a single phase Li5.5PS4.5Cl1.5. It was also found that the Cl content of the 4d crystallographic sites increased, eventually contributing to the improvement in ionic conductivity. This work also revealed the effect of cooling rates on the crystallographic atomic arrangements, which became weaker as a decrease in x. The correlations between ionic conductivities and structural features were experimentally verified via XRD and solid-state NMR studies.

High-temperature and high-pressure NMR investigations of low viscous fluids confined in mesoporous systems

Abstract In this contribution, the relaxation and diffusional behaviors of low viscous fluids, water and methanol confined into mesoporous silica and controlled size pore glass were investigated. The engineered porous systems are relevant to geologically important subsurface energy materials. The engineered porous proxies were characterized by Brunauer–Emmett–Teller (BET) surface analyzer, nuclear magnetic resonance (NMR) spectroscopy, and electron microscopy (EM) to determine surface area, pore-wall protonation and morphology of these materials, respectively. The confined behavior of the low viscous fluids was studied by varying pore diameter, fluid-to-solid ratio, temperature, and pressure, and then compared to bulk liquid state. Both relaxation and diffusion behaviors for the confined fluids showed increasing deviation from pure bulk fluids as the fluid-to-solid ratio was decreased, and surface-to-volume ratio (S/V) was varied. Variable pressure deuteron NMR relaxation of confined D2O and confined methanol, deuterated at the hydroxyl or methyl positions, were performed to exploit the sensitivity of the deuteron quadrupole moment to molecular rotation. The methanol results demonstrated greater pressure dependence than those for water only in bulk. The deviations from bulk liquid behavior arise from different reasons such as confinement and the interactions between confined fluid and the nano-pore wall. The results of the present report give insight into the behavior of low viscosity fluid in nano-confined geometries under different state conditions.

Spatiotemporal Heterogeneity of κ-Carrageenan Gels Investigated via Single-Particle-Tracking Fluorescence Microscopy.

Hydrogels made of the polysaccharide κ-carrageenan are widely used in the food and personal care industry as thickeners or gelling agents. These hydrogels feature dense regions embedded in a coarser bulk network, but the characteristic size and behavior of these regions have remained elusive. Here, we use single-particle-tracking fluorescence microscopy (sptFM) to quantitatively describe κ-carrageenan gels. Infusing fluorescent probes into fully gelated κ-carrageenan hydrogels resulted in two distinct diffusional behaviors. Obstructed self-diffusion of the probes revealed that the coarse network consists of κ-carrageenan strands with a typical diameter of 3.2 ± 0.3 nm leading to a nanoprobe diffusion coefficient of ∼1–5 × 10–12 m2/s. In the dense network regions, we found a fraction with a largely decreased diffusion coefficient of ∼1 × 10–13 m2/s. We also observed dynamic exchange between these states. The computation of spatial mobility maps from the diffusional data indicated that the dense network regions have a characteristic diameter of ∼1 μm and show mobility on the second-to-minute timescale. sptFM provides an unprecedented view of spatiotemporal heterogeneity of hydrogel networks, which we believe bears general relevance for understanding transport and release of both low- and high-molecular weight solutes.

Diffusional behaviors and mechanical properties of Cu–Zn system

The growth behaviors, diffusion kinetics and mechanical properties of Cu–Zn intermetallic compounds (IMCs) have been studied. The formation of three IMCs β′(CuZn), γ(Cu5Zn8) and ε(CuZn5) has been verified in Cu–Zn diffusion zone between 563 and 653 K. The thickening of all three IMCs followed a parabolic growth law, indicating a diffusion-control mechanism. The growth constants and growth activation energy of IMCs were estimated. With the EPMA, the composition profiles along the diffusion direction were detected, then the diffusion coefficients and activation energy of diffusion were extracted for individual phases. The hardness, Young's modulus and creep stress index of the IMCs were obtained from the load-displacement curve in nanoindentation. Cu5Zn8 has the highest hardness of 6.81 GPa and highest Young's modulus of 186.76 GPa, respectively, while CuZn5 has the smallest creep stress index of 13.77.

Tensile Properties at Room and Elevated Temperatures and High-Cycle Fatigue Properties of Extruded AZ80 and TAZ711 Alloys

The microstructure, tensile properties at room and elevated temperatures, and high-cycle fatigue behavior of an extruded Mg-7Sn-1Al-1Zn (TAZ711) alloy were investigated in comparison with those of an Mg-8Al-0.5Zn (AZ80) alloy extruded under the same conditions. The extruded AZ80 alloy exhibited a fully recrystallized structure, whereas the extruded TAZ711 alloy had a bimodal structure consisting of coarse nonrecrystallized grains and fine recrystallized grains. The TAZ711 alloy had smaller recrystallized grains and a larger amount of fine dynamic precipitates than the AZ80 alloy. Both alloys showed the typical texture of extruded Mg alloys in which the basal planes of the grains are arranged parallel to the extrusion direction, but the texture intensity of the TAZ711 alloy was greater than that of the AZ80 alloy. The yield strength of the TAZ711 alloy was higher than that of the AZ80 alloy at room temperature because of the finer grain size, stronger texture intensity, and greater amount of fine precipitates in the TAZ711 alloy. However, at an elevated temperature of 150 °C, the yield strength of the TAZ711 alloy was lower than that of the AZ80 alloy because the climb-controlled dislocation creep, grain boundary sliding, and diffusional creep behaviors were more easily generated in the TAZ711 alloy owing to its finer grain structure. As the applied tensile strain rate was reduced, the decrease in the strength of the TAZ711 alloy was greater than that of the AZ80 alloy owing to the higher strain rate sensitivity of the former alloy. Although the yield strength of the TAZ711 alloy at room temperature was higher than that of the AZ80 alloy, the TAZ711 alloy exhibited lower high-cycle fatigue resistance, which can be attributed to the occurrence of fatigue cracks at the twins formed in the coarse non-recrystallized grains remaining in the TAZ711 alloy. Key words: magnesium alloy, extrusion, microstructure, tensile properties, fatigue resistance

Solid-State Diffusional Behaviors of Functional Metal Oxides at Atomic Scale

Metal/ metal oxides have attracted extensive research interesting due to their combination of functional properties and compatibility with industry process. The diffusion and thermal reliability become essential issues that requiring detailed study for developing the atomic scaled functional devices. In this work, the diffusional reaction behaviors are investigated that transformed piezoelectric ZnO into magnetic Fe­3O4 at atomic scale. The growth kinetics of metal oxide have systematically studied through macro to micro analysis. The growth rates are evaluated from morphology change, which can realize that the growth behavior is either a diffusion- or reaction- controlled process. Besides, the atom attachment on kink step are observed at atomic scale, which has important implications for thermodynamics in functional metal oxides. The faster growth planes simultaneously become smaller, which results in remaining of low surface energy planes. These results exhibit directly observing the atomic formation process of metal oxide via solid state diffusion. In addition, the nanofabricated method provides a novel approach to investigate metal oxide evolution, and sheds light on the diffusion reaction behavior. More importantly, the results and phenomena of this study provide significant inspiration on materials stability and reliability for the metal/oxide-based devices. Figure 1

Phase-field modeling of diffusional phase behaviors of solid surfaces: A case study of phase-separating LiXFePO4 electrode particles

We present a comprehensive phase-field model for simulating diffusion-mediated kinetic phase behaviors near the surface of a solid particle. The model incorporates elastic inhomogeneity and anisotropy, diffusion mobility anisotropy, interfacial energy anisotropy, and Cahn–Hilliard diffusion kinetics. The free energy density function is formulated based on the regular solution model taking into account the possible solute-surface interaction near the surface. The coherency strain energy is computed using the Fourier-spectral iterative-perturbation method due to the strong elastic inhomogeneity with a zero surface traction boundary condition. Employing a phase-separating LiXFePO4 electrode particle for Li-ion batteries as a model system, we perform parametric three-dimensional computer simulations. The model permits the observation of surface phase behaviors that are different from the bulk counterpart. For instance, it reproduces the theoretically well-established surface modes of spinodal decomposition of an unstable solid solution: the surface mode of coherent spinodal decomposition and the surface-directed spinodal decomposition mode. We systematically investigate the influences of major factors on the kinetic surface phase behaviors during the diffusional process. Our simulation study provides insights for tailoring the internal phase microstructure of a particle by controlling the surface phase morphology.

Dynamic and unique nucleolar microenvironment revealed by fluorescence correlation spectroscopy.

Organization and functions of the nucleolus is maintained by mobilities and interactions of nucleolar factors. Because the nucleolus is a densely packed structure, molecular crowding effects determined by the molecular concentrations and mobilities in the nucleolus should also be important for regulating nucleolar organization and functions. However, such molecular property of nucleolar organization is not fully understood. To understand the biophysical property of nucleolar organization, the diffusional behaviors of inert green fluorescent protein (GFP) oligomers with or without nuclear localization signals (NLSs) were analyzed under various conditions by fluorescence correlation spectroscopy. Our result demonstrates that the mobility of GFPs inside the nucleolus and the nucleoplasm can be represented by single free diffusion under normal conditions, even though the mobility in the nucleolus is considerably slower than that in the chromatin region. Moreover, the free diffusion of GFPs is found to be significantly size- and NLS-dependent only in the nucleolus. Interestingly, the mobility in the nucleolus is highly sensitive to ATP depletion, as well as actinomycin D (ActD) treatment. In contrast, the ultra-structure of the nucleolus was not significantly changed by ATP depletion but was changed by ActD treatment. These results suggest that the nucleolus behaves similarly to an open aqueous-phase medium with an increased molecular crowding effect that depends on both energy and transcription.

Atomic mobilities, interdiffusivities and their related diffusional behaviors in fcc Co–Cr–Ni alloys

Diffusion couples for fcc Co–Cr–Ni ternary system are prepared, all of which are annealed at 1573K for 50h. Interdiffusion coefficients are evaluated at the intersection points of two independent diffusion paths sharing a common composition. Such main and cross interdiffusion coefficients are then used to inversely retrieve atomic mobilities of Co, Cr and Ni in fcc Co–Cr–Ni alloys, with the aid of available thermodynamic description. Further validation concerning the quality of atomic mobilities is conducted with computational simulations for diffusion couples annealed at 1573K for 50h, the results of which are compared with experimental data. This work contributes to the establishment of a general Co- or Ni-based kinetic database for computational design of superalloys.

Analytical and Numerical Calculations of Diffusion Effects on the Intermolecular Multiple Quantum Coherences in Solution NMR

We calculated the analytical solutions to evaluate the effects of diffusion on the iMQCs in CRAZED pulse sequences. The obtained solution has been verified using the results of numerical simulation. The diffusional behaviors of the iMQCs during both the evolution and detection time periods could be examined by the analytical solution, which was quite different from those of the conventional NMR experiments. Understanding the difference in diffusional behaviors could lead to useful applications such as diffusion MRI based on the detection of iMQCs. The intermolecular multiple quantum coherences (iMQCs) that are generated by intermolecular dipolar interactions between distant spins on different molecules, have recently gained considerable attention because their properties are intrinsically different from those of conventional single quantum coherences (SQCs) in solution NMR. This feature allows for a wider range of applications in NMR and MR imaging.

Tuning Gas‐Diffusion through Dynameric Membranes: Toward Rubbery Organic Frameworks (ROFs)

AbstractObtaining high permeability whilst keeping a reasonable selectivity is the most important challenge in the development of membrane systems for gas separation. Satisfactory performance is usually obtained with polymeric membranes through which gas transport is controlled by gas‐diffusivity in glassy polymers and by gas‐solubility in rubbery polymers. During the last decade, important advances in this field have been made possible by molecular control of gas separation properties. The combination or replacement of classical glassy polymers with metal‐organic crystalline frameworks (crystalline MOFs), such as zeolitic imidazolate frameworks (ZIFs) or other zeolites, provides reasonable permeability through the porous networks formed, and high selectivity, due to so‐called ‘selectivity centers’, which interact specifically with the gas molecules. Despite impressive progress, difficulties in obtaining homogeneous, mechanically stable, thin layer MOFs on various supports have been encountered. Given the observed high permeabilities of rubbery polymers and their flexible casting properties, it should be very interesting to build rubbery organic frameworks (ROFs), as alternative materials for gas membrane separation systems. Here we use low macromolecular constituents and dialdehyde core connectors, in order to constitutionally generate ROFs. Distinct from rubbery polymeric membranes, the performance of the ROFs depends univocally on diffusional behaviors of gas molecules through the network. For all gases, a precise molecular composition of linear and star‐type macromonomers generates an optimal free volume for a maximal diffusion through the matrix. These results should initiate new interdisciplinary discussions about highly competitive systems for gas separation, which are constitutionally controlled on a molecular scale.

Diffusion in macromolecular crowded media: Monte Carlo simulation of obstructed diffusion vs. FRAP experiments

The diffusion of tracer particles in 3D macromolecular crowded media has been studied using two methodologies, simulation and experimental, with the aim of comparing their results. First, the diffusion of a tracer in an obstructed 3D lattice with mobile and big size obstacles has been analyzed through a Monte Carlo (MC) simulation procedure. Secondly, fluorescence recovery after photobleaching (FRAP) experiments have been carried out to study the diffusion of a model protein (alpha-chymotrypsin) in in vitro crowded solution where two type of Dextran molecules are used as crowder agents. To facilitate the comparison, the relative size between the tracer and the crowder is the same in both studies. The results indicate a qualitative agreement between the diffusional behaviors observed in the two studies. The dependence of the anomalous diffusion exponent and the limiting diffusion coefficient on the obstacle size and excluded volume shows, in both cases, a similar tendency. The introduction of a reduced mobility parameter in the simulation model accounting for the short-range tracer–obstacle interactions allows obtaining a quantitative agreement between the limiting diffusion coefficient values yielded by both procedures. The simulation–experiment quantitative agreement for the anomalous diffusion exponent requires further improvements. As far as we know, this is the first reported work where both techniques are used in parallel to study the diffusion in macromolecular crowded media.

Diffusional Behaviors of Polystyrenes with Different Molecular Weights in the Same PMMA Gel Network Elucidated by Time-Dependent Diffusion NMR Spectroscopy

Diffusional behaviors of probe polystyrenes (PSs) with Mw = 4K, 19K, 29K, and 400K and chloroform as solvent were observed in the same PMMA gel matrix with the diffusing time varied (4−500 ms) by diffusing-time-dependent 1H NMR in order to elucidate how the same gel network structure is differently detected by the probes with different sizes. The diffusion of the solvent and the smallest PS with Mw = 4K proved to be of a single mode, which is safely ascribed to the sufficiently small size of the probes compared with the network mesh (∼1.5 nm). The other PS molecules that should be larger than or comparable to the mesh size showed a multimode diffusional behavior, which was analyzed as a dual mode diffusion composed of a fast component and a slow one. Thus, those larger PS molecules were capable to “probe” the inhomogeneity for the mesh size distribution of the network. On the other hand, the so-called restricted diffusion or diffusing-time dependence for the diffusion coefficient D was observed only for t...

Diffusional Behavior of Poly(β-benzyl l-aspartate) in the Rodlike, Random-Coil, and Intermediate Forms as Studied by High Field-Gradient 1H NMR Spectroscopy

The diffusion coefficients of poly(beta-benzyl L-aspartate)(PBLA) with the alpha-helix (rodlike) form in chloroform and the random-coil form in a mixture of chloroform and trifluoroacetic acid have been measured as a function of the PBLA concentration (C(PBLA)) by using high field-gradient 1H NMR. The PBLA concentration range for the former is from 0.11 to 9.20 wt % and for the latter it is from 0.20 to 25.2 wt %. From these experimental results, it is found that the diffusion coefficient of PBLA in the rodlike form is much smaller than that of PBLA in the random-coil form at the same PBLA concentration. This implies that the small diffusion coefficients of PBLA in the rodlike form as compared with those in the random-coil form come from the relatively large radius of gyration Rg. Further, it is found that the diffusional behavior of PBLA in the rodlike form is different in the three PBLA concentration regions, that is, region 1 (C(PBLA) = 0.11 approximately 0.39 wt %), region 2 (C(PBLA) = 0.39 approximately 0.86 wt %), and region 3 (C(PBLA) = 0.86 approximately 9.20 wt %). In the random-coil form, the diffusion is different in the two PBLA concentration regions, that is, region I (C(PBLA) = 0.20 approximately 1.03 wt %) and region II (C(PBLA) = 1.03 approximately 25.2 wt %). These diffusional behaviors in the rodlike form and in the random-coil form can be reasonably explained by Tinland-Maret-Rinaudo theory and de Gennes theory, respectively. Further, transitional change from the rodlike form to the random-coil form is discussed on the basis of the diffusional behavior.

High- and low-mobility populations of HP1 in heterochromatin of mammalian cells.

Heterochromatin protein 1 (HP1) is a conserved nonhistone chromosomal protein with functions in euchromatin and heterochromatin. Here we investigated the diffusional behaviors of HP1 isoforms in mammalian cells. Using fluorescence correlation spectroscopy (FCS) and fluorescence recovery after photobleaching (FRAP) we found that in interphase cells most HP1 molecules (50-80%) are highly mobile (recovery halftime: t(1/2) approximately 0.9 s; diffusion coefficient: D approximately 0.6-0.7 microm(2) s(-1)). Twenty to 40% of HP1 molecules appear to be incorporated into stable, slow-moving oligomeric complexes (t(1/2) approximately 10 s), and constitutive heterochromatin of all mammalian cell types analyzed contain 5-7% of very slow HP1 molecules. The amount of very slow HP1 molecules correlated with the chromatin condensation state, mounting to more than 44% in condensed chromatin of transcriptionally silent cells. During mitosis 8-14% of GFP-HP1alpha, but not the other isoforms, are very slow within pericentromeric heterochromatin, indicating an isoform-specific function of HP1alpha in heterochromatin of mitotic chromosomes. These data suggest that mobile as well as very slow populations of HP1 may function in concert to maintain a stable conformation of constitutive heterochromatin throughout the cell cycle.