Configurational Fluctuations Research Articles

Native and nonnative reactions in ethanolamine ammonia-lyase are actuated by different dynamics

We address the contribution of select classes of solvent-coupled configurational fluctuations to the complex choreography involved in configurational and chemical steps in an enzyme by comparing native and nonnative reactions conducted at different protein internal sites. The low temperature, first-order kinetics of covalent bond rearrangement of the cryotrapped substrate radical in coenzyme B12-dependent ethanolamine ammonia-lyase (EAL) from Salmonella enterica display a kink, or increase in slope, of the Arrhenius plot with decreasing temperature. The event is associated with quenching of a select class of reaction-actuating collective fluctuations in the protein hydration layer. For comparison, a nonnative, radical reaction of the protein interior cysteine sulfhydryl group with hydrogen peroxide (H2O2) is introduced by cryotrapping EAL in an aqueous H2O2 eutectic system. The low-temperature aqueous H2O2 protein hydration and mesodomain solvent phases surrounding cryotrapped EAL are characterized by using TEMPOL spin probe electron paramagnetic resonance spectroscopy, including a freezing transition of the eutectic phase that orders the protein hydration layer. Kinetics of the cysteine-H2O2 reaction in the EAL protein interior are monitored by DEPMPO spin trapping of hydroxyl radical product. In contrast to the native reaction, the linear Arrhenius relation for the nonnative cysteine-H2O2 reaction is maintained through the solvent-protein ordering transition. The nonnative reaction is coupled to the generic local, incremental fluctuations that are intrinsic to globular proteins. The comparative approach supports the proposal that select coupled solvent-protein configurational fluctuations actuate the native reaction, and suggests that select dynamical coupling contributes to the degree of catalysis in enzymes.

Dissecting the Molecular Origin of g-Tensor Heterogeneity and Strain in Nitroxide Radicals in Water: Electron Paramagnetic Resonance Experiment versus Theory.

Nitroxides are common EPR sensors of microenvironmental properties such as polarity, numbers of H-bonds, pH, and so forth. Their solvation in an aqueous environment is facilitated by their high propensity to form H-bonds with the surrounding water molecules. Their g- and A-tensor elements are key parameters to extracting the properties of their microenvironment. In particular, the gxx value of nitroxides is rich in information. It is known to be characterized by discrete values representing nitroxide populations previously assigned to have different H-bonds with the surrounding waters. Additionally, there is a large g-strain, that is, a broadening of g-values associated with it, which is generally correlated with environmental and structural micro-heterogeneities. The g-strain is responsible for the frequency dependence of the apparent line width of the EPR spectra, which becomes evident at high field/frequency. Here, we address the molecular origin of the gxx heterogeneity and of the g-strain of a nitroxide moiety (HMI: 2,2,3,4,5,5-hexamethylimidazolidin-1-oxyl, C9H19N2O) in water. To treat the solvation effect on the g-strain, we combined a multi-frequency experimental approach with ab initio molecular dynamics simulations for structural sampling and quantum chemical EPR property calculations at the highest realistically affordable level, including an explicitly micro-solvated HMI ensemble and the embedded cluster reference interaction site model. We could clearly identify the distinct populations of the H-bonded nitroxides responsible for the gxx heterogeneity experimentally observed, and we dissected the role of the solvation shell, H-bond formation, and structural deformation of the nitroxide in the creation of the g-strain associated with each nitroxide subensemble. Two contributions to the g-strain were identified in this study. The first contribution depends on the number of hydrogen bonds formed between the nitroxide and the solvent because this has a large and well-understood effect on the gxx-shift. This contribution can only be resolved at high resonance frequencies, where it leads to distinct peaks in the gxx region. The second contribution arises from configurational fluctuations of the nitroxide that necessarily lead to g-shift heterogeneity. These contributions cannot be resolved experimentally as distinct resonances but add to the line broadening. They can be quantitatively analyzed by studying the apparent line width as a function of microwave frequency. Interestingly, both theory and experiment confirm that this contribution is independent of the number of H-bonds. Perhaps even more surprisingly, the theoretical analysis suggests that the configurational fluctuation broadening is not induced by the solvent but is inherently present even in the gas phase. Moreover, the calculations predict that this broadening decreases upon solvation of the nitroxide.

Host Cell Membrane Capture by the SARS-CoV-2 Spike Protein Fusion Intermediate.

Cell entry by SARS-CoV-2 is accomplished by the S2 subunit of the spike S protein on the virion surface by capture of the host cell membrane and fusion with the viral envelope. Capture and fusion require the prefusion S2 to transit to its potent fusogenic form, the fusion intermediate (FI). However, the FI structure is unknown, detailed computational models of the FI are unavailable, and the mechanisms and timing of membrane capture and fusion are not established. Here, we constructed a full-length model of the SARS-CoV-2 FI by extrapolating from known SARS-CoV-2 pre- and postfusion structures. In atomistic and coarse-grained molecular dynamics simulations the FI was remarkably flexible and executed giant bending and extensional fluctuations due to three hinges in the C-terminal base. The simulated configurations and their giant fluctuations are quantitatively consistent with SARS-CoV-2 FI configurations measured recently using cryo-electron tomography. Simulations suggested a host cell membrane capture time of ∼2 ms. Isolated fusion peptide simulations identified an N-terminal helix that directed and maintained binding to the membrane but grossly underestimated the binding time, showing that the fusion peptide environment is radically altered when attached to its host fusion protein. The large configurational fluctuations of the FI generated a substantial exploration volume that aided capture of the target membrane, and may set the waiting time for fluctuation-triggered refolding of the FI that draws the viral envelope and host cell membrane together for fusion. These results describe the FI as machinery that uses massive configurational fluctuations for efficient membrane capture and suggest novel potential drug targets.

A Step toward Amino Acid-Labeled DNA Sequencing: Boosting Transmission Sensitivity of Graphene Nanogap.

Existing obstacles in next-generation DNA sequencing techniques, for instance, high noise, high translocation speed, and configurational fluctuations, call for approaches capable of reaching the goal and accelerating the process of personalized medicine development. The labeling nucleotide approach has the potential to overcome these barriers and boost the recognition sensitivity of a solid-state nanodevice. In this theoretical report, the first-principles density functional theory calculations have been employed to study the role of three different labels, tyrosine (Tyr), aspartic acid (Asp), and arginine (Arg), for labeling DNA nucleotides and study their effect in rapid and controlled DNA sequencing at atomic resolution. Remarkable differences in interaction energy values are noticed in all three cases of differently labeled nucleotides. The zero-bias transmission spectra confirm that proposed labels have the ability to detect the individual nucleotide, amplifying the tunneling current sensitivity by several orders of magnitude. The current-voltage characteristics of Arg-labeled nucleotides are found to be promising for single nucleotide recognition even at a very low bias voltage of 0.1 V.

Revisiting DNA Sequence-Dependent Deformability in High-Resolution Structures: Effects of Flanking Base Pairs on Dinucleotide Morphology and Global Chain Configuration.

DNA carries more than the list of biochemical ingredients that drive the basic functions of living systems. The sequence of base pairs includes a multitude of structural and energetic signals, which determine the degree to which the long, threadlike molecule moves and how it responds to proteins and other molecules that control its processing and govern its packaging. The chemical composition of base pairs directs the spatial disposition and fluctuations of successive residues. The observed arrangements of these moieties in high-resolution protein–DNA crystal structures provide one of the best available estimates of the natural, sequence-dependent structure and deformability of the double-helical molecule. Here, we update the set of knowledge-based elastic potentials designed to describe the observed equilibrium structures and configurational fluctuations of the ten unique base-pair steps. The large number of currently available structures makes it possible to characterize the configurational preferences of the DNA base-pair steps within the context of their immediate neighbors, i.e., tetrameric context. Use of these knowledge-based potentials shows promise in accounting for known effects of sequence in long chain molecules, e.g., the degree of curvature reported in classic gel mobility studies and the recently reported sequence-dependent responses of supercoiled minicircles to nuclease cleavage.

Elucidating the role of network topology dynamics on the coil-stretch transition hysteresis in extensional flow of entangled polymer melts

Dissipative particle dynamics (DPD) simulations are performed on coarse-grained replicas of linear, monodisperse entangled polyethylene melts C1000H2002 and C3000H6002 undergoing both steady-state and transient planar elongational flow (PEF). The fidelity of the DPD simulations is verified by direct comparison of flow topological and rheological properties of a 334-particle chain liquid against the united-atom C1000H2002 liquid, simulated using nonequilibrium molecular dynamics (NEMD). These DPD simulations demonstrate that a flow-induced coil-stretch transition (CST) and its associated hysteresis caused by configurational microphase separation, as observed in previous NEMD simulations of PEF, can be replicated using a more computationally efficient coarse-grained system. Results indicate that the breadth of the CST hysteresis loop is enlarged for the longer molecule liquid relative to the shorter one. Furthermore, relaxation simulations reveal that reducing the applied flow Deborah number (De) from a high value corresponding to a homogeneous phase of highly stretched molecules to a De within the biphasic region results in a two-stage relaxation process. There is a fast initial stratification of the kinetically trapped highly stretched chains into regions of highly extended and less extended chains, which displays similar behavior to a system undergoing a spinodal decomposition caused by spatial configurational free energy fluctuations. After a short induction period of apparently random duration, the less extended chain regions experience a stochastic nucleation event that induces configurational relaxation to domains composed of coiled molecules over a much longer time scale, leaving the more highly extended chains in surrounding sheetlike domains. The time scales of these two relaxation processes are of the same order of magnitude as the Rouse and disengagement times of the equilibrium liquids.

Resolution and characterization of contributions of select protein and coupled solvent configurational fluctuations to radical rearrangement catalysis in coenzyme B12-dependent ethanolamine ammonia-lyase.

Coenzyme B12 (adenosylcobalamin) -dependent ethanolamine ammonia-lyase (EAL) is the signature enzyme in ethanolamine utilization metabolism associated with microbiome homeostasis and disease conditions in the human gut. The enzyme conducts a complex choreography of bond-making/bond-breaking steps that rearrange substrate to products through a radical mechanism, with themes common to other coenzyme B12-dependent and radical enzymes. The methods presented are targeted to test the hypothesis that particular, select protein and coupled solvent configurational fluctuations contribute to enzyme function. The general approach is to correlate enzyme function with an introduced perturbation that alters the properties (for example, degree of concertedness, or collectiveness) of protein and coupled solvent dynamics. Methods for sample preparation and low-temperature kinetic measurements by using temperature-step reaction initiation and time-resolved, full-spectrum electron paramagnetic resonance spectroscopy are detailed. A framework for interpretation of results obtained in ensemble systems under conditions of statistical equilibrium within the reacting, globally unstable state is presented. The temperature-dependence of the first-order rate constants for decay of the cryotrapped paramagnetic substrate radical state in EAL, through the chemical step of radical rearrangement, displays a piecewise-continuous Arrhenius dependence from 203 to 295K, punctuated by a kinetic bifurcation over 219-220K. The results reveal the obligatory contribution of a class of select collective protein and coupled solvent fluctuations to the interconversion of two resolved, sequential configurational substates, on the decay time scale. The select class of collective fluctuations also contributes to the chemical step. The methods and analysis are generally applicable to other coenzyme B12-dependent and related radical enzymes.

Towards Measuring the Maxwell–Boltzmann Distribution of a Single Heated Particle

The Maxwell–Boltzmann distribution is a hallmark of statistical physics in thermodynamic equilibrium linking the probability density of a particle’s kinetic energies to the temperature of the system that also determines its configurational fluctuations. This unique relation is lost for Hot Brownian Motion, e.g., when the Brownian particle is constantly heated to create an inhomogeneous temperature in the surrounding liquid. While the fluctuations of the particle in this case can be described with an effective temperature, it is not unique for all degrees of freedom and suggested to be different at different timescales. In this work, we report on our progress to measure the effective temperature of Hot Brownian Motion in the ballistic regime. We have constructed an optical setup to measure the displacement of a heated Brownian particle with a temporal resolution of 10 ns giving a corresponding spatial resolution of about 23 pm for a 0.92 μm PMMA particle in water. Using a gold-coated polystyrene (AuPS) particle of 2.15 μm diameter we determine the mean squared displacement of the particle over more than six orders of magnitude in time. Our data recovers the trends for the effective temperature at long timescales, yet shows also clear effects in the region of hydrodynamic long time tails.

Taking Water into Account with the Fragment Molecular Orbital Method.

This chapter describes the current status of development of the fragment molecular orbital (FMO) method for analyzing the electronic state and intermolecular interactions of biomolecular systems in solvent. The orbital energies and the inter-fragment interaction energies (IFIEs) for a specific molecular structure can be obtained directly by performing FMO calculations by exposing water molecules and counterions around biomolecular systems. Then, it is necessary to pay attention to the thickness of the water shell surrounding the biomolecules. The single-point calculation for snapshots from MD trajectory does not incorporate the effects of temperature and configurational fluctuation, but the SCIFIE (statistically corrected IFIE) method is proposed as a many-body correlated method that partially compensates for this deficiency. Furthermore, implicit continuous dielectric models have been developed as effective approaches to incorporating the screening effect of the solvent in thermal equilibrium, and we illustrate their usefulness for theoretical evaluation of IFIEs and ligand-binding free energy on the basis of the FMO-PBSA (Poisson-Boltzmann surface area) method and other computational methods.

Towards the theoretical limitations of X-ray nanocrystallography at high intensity: the validity of the effective-form-factor description.

X-ray free-electron lasers (XFELs) broaden horizons in X-ray crystallography. Facilitated by the unprecedented high intensity and ultrashort duration of the XFEL pulses, they enable us to investigate the structure and dynamics of macromolecules with nano-sized crystals. A limitation is the extent of radiation damage in the nanocrystal target. A large degree of ionization initiated by the incident high-intensity XFEL pulse alters the scattering properties of the atoms leading to perturbed measured patterns. In this article, the effective-form-factor approximation applied to capture this phenomenon is discussed. Additionally, the importance of temporal configurational fluctuations at high intensities, shaping these quantities besides the average electron loss, is shown. An analysis regarding the applicability of the approach to targets consisting of several atomic species is made, both theoretically and via realistic radiation-damage simulations. It is concluded that, up to intensities relevant for XFEL-based nanocrystallography, the effective-form-factor description is sufficiently accurate. This work justifies treating measured scattering patterns using conventional structure-reconstruction algorithms.

Two Dynamical Regimes of the Substrate Radical Rearrangement Reaction in B12-Dependent Ethanolamine Ammonia-Lyase Resolve Contributions of Native Protein Configurations and Collective Configurational Fluctuations to Catalysis.

The kinetics of the substrate radical rearrangement reaction step in B12-dependent ethanolamine ammonia-lyase (EAL) from Salmonella typhimurium are measured over a 92 K temperature range. The observed first-order rate constants display a piecewise-continuous Arrhenius dependence, with linear regions over 295 → 220 K (monoexponential) and 214 → 203 K (biexponential) that are delineated by a kinetic bifurcation and kinks at 219 and 217 K, respectively. The results are interpreted by using a free energy landscape model and derived microscopic kinetic mechanism. The bifurcation and kink transitions correspond to the effective quenching of two distinct sets of native collective protein configurational fluctuations that (1) reconfigure the protein within the substrate radical free energy minimum, in a reaction-enabling step, and (2) create the protein configurations associated with the chemical step. Below 217 K, the substrate radical decay reaction persists. Increases in activation enthalpy and entropy of both the microscopic enabling and reaction steps indicate that this non-native reaction coordinate is conducted by local, incremental fluctuations. Continuity in the Arrhenius relations indicates that the same sets of protein groups and interactions mediate the rearrangement over the 295 to 203 K range, but with a repertoire of configurations below 217 K that is restricted, relative to the native configurations accessible above 219 K. The experimental features of a culled reaction step, first-order kinetic measurements, and wide room-to-cryogenic temperature range, allow the direct demonstration and kinetic characterization of protein dynamical contributions to the core adiabatic, bond-making/bond-breaking reaction in EAL.

Cluster Variation Method as a Theoretical Tool for the Study of Phase Transformation

Cluster variation method (CVM) has been widely employed to calculate alloy phase diagrams. The atomistic feature of the CVM is consistent with first-principles electronic structure calculations, and the combination of CVM with electronic structure calculation enables one to formulate free energy from the first-principles. CVM free energy conveys affluent information of a given system, and the second-order derivative traces the stability locus against configurational fluctuation. The kinetic extension of the CVM is the path probability method (PPM) which is utilized to calculate transformation and relaxation kinetics associated with the temperature change. Hence, the CVM and PPM are coherent methods to perform a synthetic study from initial non-equilibrium to final equilibrium states. By utilizing CVM free energy as a homogeneous free energy density term, one can calculate the time evolution of ordered domains within the phase field method. Finally, continuous displacement cluster variation method (CDCVM) is discussed as the recent development of CVM. CDCVM is capable of introducing the local lattice displacement into the free energy. Moreover, it is shown that CDCVM can be extended to study collective atomic displacements leading to displacive phase transformation.

Effects of configurational changes on electrical resistivity during glass-liquid transition of two bulk metal-alloy glasses.

Consequences of increase in structural fluctuations on heating Pd40Ni10Cu30P20 and Zr46.75Ti8.25Cu7.5Ni10Be27.5 through their glass to liquid transition range were investigated by measuring the electrical resistivity, ρ, an electron scattering property. The temperature coefficient of resistivity (TCR = (1/ρ) dρ/dT) of the liquid and glassy states is negative. The plots of their ρ against T in the Tg (glass to liquid transition) range show a gradual change in the slope similar to the change observed generally for the plots of the density, elastic modulus, and refractive index. As fluctuations in the melt structure involve fewer configurations on cooling, ρ increases. In the energy landscape description, the melt's structure explores fewer minima with decrease in T, vibrational frequencies increase, and electron scattering and ρ increase. Plots of (-dρ/dT) against T resemble the plot of the specific heat of other glasses and show a sub-Tg feature and a rapid rise at T near Tg. Analysis shows that the magnitude of negative TCR is dominated by change in the phonon characteristics, and configurational fluctuations make it more negative. The TCR of the liquid and glassy states seems qualitatively consistent with the variation in the structure factor in Ziman's model for pure liquid metals as extended by Nagel to metal alloys and used to explain the negative TCR of a two-component metal glass.

Arginine-phosphate salt bridges between histones and DNA: intermolecular actuators that control nucleosome architecture.

Structural bioinformatics and van der Waals density functional theory are combined to investigate the mechanochemical impact of a major class of histone-DNA interactions, namely, the formation of salt bridges between arginine residues in histones and phosphate groups on the DNA backbone. Principal component analysis reveals that the configurational fluctuations of the sugar-phosphate backbone display sequence-specific directionality and variability, and clustering of nucleosome crystal structures identifies two major salt-bridge configurations: a monodentate form in which the arginine end-group guanidinium only forms one hydrogen bond with the phosphate, and a bidentate form in which it forms two. Density functional theory calculations highlight that the combination of sequence, denticity, and salt-bridge positioning enables the histones to apply a tunable mechanochemical stress to the DNA via precise and specific activation of backbone deformations. The results suggest that selection for specific placements of van der Waals contacts, with high-precision control of the spatial distribution of intermolecular forces, may serve as an underlying evolutionary design principle for the structure and function of nucleosomes, a conjecture that is corroborated by previous experimental studies.

Post-Markovian quantum master equations from classical environment fluctuations

In this paper we demonstrate that two commonly used phenomenological post-Markovian quantum master equations can be derived without using any perturbative approximation. A system coupled to an environment characterized by self-classical configurational fluctuations, the latter obeying a Markovian dynamics, defines the underlying physical model. Both Shabani-Lidar equation [A. Shabani and D. A. Lidar, Phys. Rev. A 71, 020101(R) (2005)] and its associated approximated integrodifferential kernel master equation are obtained by tracing out two different bipartite Markovian Lindblad dynamics where the environment fluctuations are taken into account by an ancilla system. Furthermore, conditions under which the non-Markovian system dynamics can be unraveled in terms of an ensemble of measurement trajectories are found. In addition, a non-Markovian quantum jump approach is formulated. Contrary to recent analysis [L. Mazzola, E. M. Laine, H. P. Breuer, S. Maniscalco, and J. Piilo, Phys. Rev. A 81, 062120 (2010)], we also demonstrate that these master equations, even with exponential memory functions, may lead to non-Markovian effects such as an environment-to-system backflow of information if the Hamiltonian system does not commutate with the dissipative dynamics.

Nanometer-scale water droplet free from the constraint of reverse micelles at low temperatures

Temperature dependence of the configurational fluctuation of water confined in a reverse micellar solution has been studied by absorption spectroscopy of a probe molecule. We have found that the configurational fluctuation is liquidlike below the homogeneous nucleation temperature. This is proposed to be due to a large reduction in the confinement of water, and is explained in terms of water shedding from the reverse micelle. Further, the configurational fluctuation is frozen at ~210 K. A reverse micellar solution is considered to be a promising candidate for studies of supercooled water.

Determination of multiwavelength anomalous diffraction coefficients at high x-ray intensity

The high-intensity version of multiwavelength anomalous diffraction (MAD) has a potential for solving the phase problem in femtosecond crystallography with x-ray free-electron lasers (XFELs). For MAD phasing, it is required to calculate or measure the MAD coefficients involved in the key equation, which depend on XFEL pulse parameters. In this work, we revisit the generalized Karle–Hendrickson equation to clarify the importance of configurational fluctuations of heavy atoms induced by intense x-ray pulses, and investigate the high-intensity cases of transmission and fluorescence measurements of samples containing heavy atoms. Based on transmission/fluorescence and diffraction experiments with crystalline samples of known structures, we propose an experimental procedure to determine all MAD coefficients at high x-ray intensity, which can be used in ab initio phasing for unknown structures.

Non-exponential nature of calorimetric and other relaxations: Effects of 2 nm-size solutes, loss of translational diffusion, isomer specificity, and sample size

Certain distributions of relaxation times can be described in terms of a non-exponential response parameter, β, of value between 0 and 1. Both β and the relaxation time, τ0, of a material depend upon the probe used for studying its dynamics and the value of β is qualitatively related to the non-Arrhenius variation of viscosity and τ0. A solute adds to the diversity of an intermolecular environment and is therefore expected to reduce β, i.e., to increase the distribution and to change τ0. We argue that the calorimetric value β(cal) determined from the specific heat [Cp = T(dS∕dT)p] data is a more appropriate measure of the distribution of relaxation times arising from configurational fluctuations than β determined from other properties, and report a study of β(cal) of two sets of binary mixtures, each containing a different molecule of ∼2 nm size. We find that β(cal) changes monotonically with the composition, i.e., solute molecules modify the nano-scale composition and may increase or decrease τ0, but do not always decrease β(cal). (Plots of β(cal) against the composition do not show a minimum.) We also analyze the data from the literature, and find that (i) β(cal) of an orientationally disordered crystal is less than that of its liquid, (ii) β(cal) varies with the isomer's nature, and chiral centers in a molecule decrease β(cal), and (iii) β(cal) decreases when a sample's thickness is decreased to the nm-scale. After examining the difference between β(cal) and β determined from other properties we discuss the consequences of our findings for theories of non-exponential response, and suggest that studies of β(cal) may be more revealing of structure-freezing than studies of the non-Arrhenius behavior. On the basis of previous reports that β → 1 for dielectric relaxation of liquids of centiPoise viscosity observed at GHz frequencies, we argue that its molecular mechanism is the same as that of the Johari-Goldstein (JG) relaxation. Its spectrum becomes broader on cooling and its unimodal distribution reversibly changes to a bimodal distribution, each of β < 1. Kinetic freezing of the slower modes of the bimodal distribution produces a glass. After this bifurcation, the faster, original relaxation persists as a weak JG relaxation at T → T(g), and in the glassy state.

Configurational fluctuations and flavin-substrate interactions in the flavoenzyme ThyX studied by time- and spectrally resolved fluorescence

Femtosecond-resolved fluorescence of bacterial thymidilate synthase using a Kerr-gate based setup identifies a close-by tyrosine involved in flavin fluorescence quenching, shows that the substrate dUMP acts as a strong quencher itself and highlights functional configurational flexibility

Polymers Pushing Polymers: Polymer Mixtures in Thermodynamic Equilibrium with a Pore

Studies on the ejection of viral genomes from capsids use osmotic stress to push a DNA molecule into the viral capsid by the action of PEG in solution. Identities of the polymer forced into the nano-cavity (DNA) and the polymer pushing it (PEG) differ. We analyze a variation on the problem with a simple model of a binary polymer mixture and find that polymer size dependent partitioning into a pore also has a complex dependence on the composition of the polymer mixture and the pore-penetration penalty. We analyze a polydisperse polymer solution of big and small polymer chains (bPEG and sPEG, respectively) of which only the sPEG is allowed to pay an energy penalty and enter the pore. We introduce an ansatz for the free energy of the polymer mixture that is consistent with our previous phenomenological fit to the equation of state of bulk uncharged polymers. We then analyze the osmotic pressure of this solution in equilibrium with a pore for various amounts of sPEG. Calculating the corresponding partition coefficient allows us to assess the pushing forces exerted by the external polymer solution via its osmotic pressure and ascertain that some polymers can push others to enter the pore in a kind of osmotic tug-of-war. We find that pore penetration is governed by two different penetration free energy scales. One scale represents the osmotic pushing strength of the external polymer mixture. The other scale comprises the interplay between the pore energy penalty and the translational mixing entropy (van't Hoft) and configurational (des Cloizeaux) fluctuations of the external solution. This principle should have applications in many areas of nano-science dealing with partitioning of polymer chains into small enclosures.