Influence of the Dielectric Constant on the Ionic Current Rectification of Bipolar Nanopores.

In this paper, we investigate how the dielectric constant, ϵ, of an electrolyte solvent influences the current rectification characteristics of bipolar nanopores. It is well recognized that bipolar nanopores with two oppositely charged regions rectify current when exposed to an alternating electric potential difference. Here, we consider dilute electrolytes with NaCl only and with a mixture of NaCl and charged nanoparticles. These systems are studied using two levels of description, all-atom explicit water molecular dynamics (MD) simulations and coarse-grained implicit solvent MD simulations. The charge density and electric potential profiles and current-voltage relationship predicted by the implicit solvent simulations with ϵ = 11.3 show good agreement with the predictions from the explicit water simulations. Under nonequilibrium conditions, the predictions of the implicit solvent simulations with a dielectric constant closer to the one of bulk water are significantly different from the predictions obtained with the explicit water model. These findings are closely aligned with experimental data on the dielectric constant of water when confined to nanometric spaces, which suggests that ϵ decreases significantly compared to its value in the bulk. Moreover, the largest electric current rectification is observed in systems containing nanoparticles when ϵ = 78.8. Using enhanced sampling, we have shown that this larger rectification arises from the presence of a significantly deeper minimum in the free energy of the system with a larger ϵ, and when a negative voltage bias is applied. Since implicit solvent models and mean-field continuum theories are often used to design Janus membranes based on bipolar nanopores, this work highlights the importance of properly accounting for the effects of confinement on the dielectric constant of the electrolyte solvent. The results presented here indicate that the dielectric constant in implicit solvent simulations may be used as an adjustable parameter to approximately account for the effects of nanometric confinement on aqueous electrolyte solvents.

United Network for Organ Sharing (UNOS) Database Analysis of Factors Associated With Kidney Transplant Time on Waiting List.

INTRODUCTION:  In the United States (U.S.), African Americans and other minority groups have longer wait times for kidney transplantation than Caucasians. To date, many studies analyzing time spent on the waitlist for each race/ethnicity have been done. However, there are few to no studies examining waitlist time after the 2019 policy changes to the geographic distribution of donated kidneys. METHODS: Data collected from the National Organ Procurement and Transplantation Network database were used to analyze associations between race and time spent on the waitlist for a kidney transplant in the U.S. Additional sub-categorical data were analyzed to determine further associations and potential covariates, such as gender, age, citizenship, primary source of payment, region of transplant center, BMI, Kidney Donor Profile Index (KDPI), renal diagnosis, and presence/type of diabetes. Data were analyzed using odds ratios and validated by Bonferroni-Holm's corrected chi-square tests at confidence intervals of 95% to determine if there are statistically significant differences between transplant time spent on the waitlist and ethnicity, as well as age, diagnosis category, region of transplant center, and KDPI. RESULTS: Statistically significant increased odds of remaining on the transplant list at two years existed for all non-white races/ethnicities, except those identifying as multiracial. Asian American candidates had the greatest odds of remaining on the waitlist greater than two years in comparison to white candidates: 1.51 times that of a patient categorized as white (odds ratio [OR] 1.51, confidence interval [CI] 1.44-1.57). African American/Black, (OR 1.38, CI 1.34-1.43) Pacific Islander (OR 1.38, CI 1.17-1.63), Hispanic candidates (OR 1.37, CI 1.32-1.41), and American Indian or Native Alaskan candidates (OR 1.23, CI 1.12-1.46) also had increased odds of remaining on the transplant waitlist greater than two years compared to white candidates. DISCUSSION: In this study, ethnic disparities persisted as a barrier for non-white individuals receiving treatment for end-stage kidney disease, specifically in the context of time spent on the waitlist for a kidney transplant. Further research is needed regarding the causes of these disparities in time spent on the waitlist, such as cultural restrictions in organ donation, racial differences in parameters for organ match, and institutionalized racism in health care practitioners.

Laparoscopic urinary undiversion due to recurrent neobladder vaginal fistula: from orthotopic neobladder to ileal conduit / Desderivación urinaria laparoscópica por fístula neo-vesico vaginal recurrente: de neovejiga ortotópica a conducto ileal

Objective: Describe our experience in laparoscopic urinary undiversion due to recurrent neobladder vaginal fistula (NBVF). Methods: Retrospective review of patients who underwent laparoscopic urinary undiversion. Complications were characterized according to the Clavien classification. The Patient Global Impression of Improvement (PGII) questionnaire was used at one year of follow-up to assess the quality of life (QOL). Results: Three laparoscopic urinary undiversions, from orthotopic neobladder to ileal conduit. Two patients underwent laparoscopic cystectomy due to bladder cancer, and one open pelvic exenteration due to cervical cancer. All patients had received previous pelvic radiotherapy. Complications during the first 2 months were Clavien II (two patients). According to the PGII score, two patients felt "much better" and one felt "very much better." Conclusion: Urinary undiversion is a last resort, complex procedure. Even though, it may be the only chance to improve QOL in patients with recurrent or unrepairable NBVF. A laparoscopic approach with neobladder resection, fistulectomy, and intracorporeal ileal conduit is feasible. Further studies are required to assess the best approach in the management of NBVF.

Objetivo: Describir nuestra experiencia en desderivación urinaria laparoscópica por fístula neo-vesico vaginal recurrente (NBVF). Métodos: Revisión retrospectiva de pacientes a los que se les realizó desderivación urinaria laparoscópica. Las complicaciones se caracterizaron según la clasificación de Clavien. El cuestionario de Impresión Global de Mejora del Paciente (PGII) se utilizó al año de seguimiento para evaluar la calidad de vida (QoL). Resultados: 3 desderivaciones urinarias laparoscópicas, de neovejiga ortotópica a conducto ileal. Dos pacientes se sometieron a cistectomía laparoscópica por cáncer de vejiga y una exenteración pélvica abierta por cáncer de cuello uterino. Todos los pacientes habían recibido radioterapia pélvica previa. Las complicaciones durante los primeros 2 meses fueron Clavien II (2 pacientes). Según la puntuación PGII, dos pacientes se sintieron «Mucho mejor¼ y uno se sintió «Muchísimo mejor.¼ Conclusión: La desderivación urinaria laparoscopica es un procedimiento complejo y de último recurso. Sin embargo, puede ser la única oportunidad para mejorar la calidad de vida en pacientes con NBVF recurrentes o irreparables. El abordaje laparoscópico con resección de neovejiga, fistulectomía y conducto ileal intracorpóreo es factible. Se requieren más estudios para evaluar el mejor enfoque en el manejo de NBVF.

Ionic Transport in Electrostatic Janus Membranes. An Explicit Solvent Molecular Dynamic Simulation.

Janus, or two-sided, charged membranes offer promise as ionic current rectifiers. In such systems, pores consisting of two regions of opposite charge can be used to generate a current from a gradient in salinity. The efficiency of nanoscale Janus pores increases dramatically as their diameter becomes smaller. However, little is known about the underlying transport processes, particularly under experimentally accessible conditions. In this work, we examine the molecular basis for rectification in Janus nanopores using an applied electric field. Molecular simulations with explicit water and ions are used to examine the structure and dynamics of all molecular species in aqueous electrolyte solutions. For several macroscopic observables, the results of such simulations are consistent with experimental observations on asymmetric membranes. Our analysis reveals a number of previously unknown features, including a pronounced local reorientation of water molecules in the pores, and a segregation of ionic species that had not been anticipated by previously reported continuum analyses of Janus pores. Using these insights, a model is proposed for ionic current rectification in which electric leakage at the pore entrance controls net transport.

Effect of charge inversion on nanoconfined flow of multivalent ionic solutions.

A comprehensive understanding of fluid dynamics of dilute electrolyte solutions in nanoconfinement is essential to develop more efficient nanofluidic devices. In nanoconduits, the electrical double layer can occupy a considerable part of the channel cross-section, therefore the transport properties of a nanoconfined electrolyte solution can be altered by interfacial phenomena such as the charge inversion (CI). CI is an electrokinetic effect that has been associated with the presence of hydrated multivalent cations in nanoconfinement. Here, all-atom molecular dynamics simulations are employed to study the structure and dynamics of aqueous multivalent electrolyte solutions within slit-shaped silica channels. All simulations are conducted for more than 100 ns to capture the equilibrium ion distribution, the interfacial hydrodynamic properties, and to reveal the influence of CI on nanoconfined fluid transport. The electrolyte solutions consist of water as solvent, chloride as co-ion and different counter-ions, i.e., sodium, magnesium and aluminum. We find that the interfacial viscosity is related to the concentration and valence of the counter-ions in the solution. Our results suggest that higher CI is correlated to the presence of a layer of fluid with augmented viscosity adjacent to the channel wall. As the thickness of this interfacial high-viscosity fluid increases, lower flow rates are measured whereas higher interfacial viscosities and friction coefficients are computed.

[Pneumatic lithotripsy vs Holmium: YAG Laser lithotripsy for the treatment of ureteral stones.] / Litotricia neumática vs. litotricia láser Ho: YAG en el tratamiento de la litiasis ureteral.

OBJECTIVE: Pneumatic lithotripsy (PL) and Ho: YAG laser lithotripsy (LL) are the most widely accepted methods in the endoscopic treatment of ureteral lithiasis. The objective is to compare efficacy and safety of pneumatic lithotripsy vs. Ho: YAG laser lithotripsy in the treatment of ureteral lithiasis. MATERIAL AND METHODS: Prospective, single-blind, multicenter study. Adult patients were recruited from August 2017 to March 2019, in 23 institutions throughout Argentina. Patient demographics, stone characteristics, presence of double J stent prior to the intervention, stonefree rate (SF) and postoperative complications were evaluated and analyzed. RESULTS: A total of 366 patients with ureteral lithiasis were included, 204 in the PL group and 162 in the LL group. The SF rate was significantly higher in the LL group (77% vs. 92%), OR 3 .43 (1.76 to 6.70). The complication rate was significantly lower in the LL group (9.8% vs. 2.5%), OR 0.23 (0.07 to 0.71). In the multivariate analysis, the use of Ho: YAG energy, the location of the lithiasis in the distal ureter, and the preoperative placement of double J stent, were found to be predictors of SF status. CONCLUSIONS: Ho: YAG laser lithotripsy has a higher stone-free rate and a lower complication rate compared to pneumatic lithotripsy.

OBJETIVO: La litotricia neumática (LN) y láser Ho: YAG (LH) son los métodos más aceptados en el tratamiento endoscópico de la litiasis ureteral. El objetivo es comparar eficacia y seguridad de la litotricia neumática vs. litotricia láser Ho: YAG en el tratamiento de la litiasis ureteral.MATERIAL Y MÉTODOS: Estudio prospectivo, simple ciego, multicéntrico. Se incluyeron pacientes adultos que se sometieron a tratamiento de litiasis ureteral, desde agosto de 2017 a marzo de 2019, en 23 instituciones nacionales. Las variables analizadas incluyeron: datos demográficos, tamaño y ubicación del lito, presencia de catéter doble J previo al procedimiento, tasa libre de litiasis (LL) y de complicaciones. RESULTADOS: Se incluyeron un total de 366 pacientes con litiasis ureteral, 204 en el grupo LN y 162 en el grupo LH. La tasa libre de litiasis fue significativamente superior en el grupo LH (77% vs. 92%), OR 3,43 (1,76 a 6,70) y la tasa de complicaciones fue significativamente menor en el grupo LH (9,8% vs. 2,5%), OR 0,23 (0,07 a 0,71). En el análisis multivariado, la utilización de la energía Ho: YAG, la ubicación de la litiasis en uréter distal y la colocación preoperatoria de catéter doble J, resultaron ser factores predictores del estado LL.CONCLUSIÓN: La litotricia láser Ho: YAG presenta una mayor tasa libre de litiasis y una menor tasa de complicaciones, en comparación con la litotricia neumática.

Club Drugs and Psychiatric Sequelae: An Issue of Vulnerability and Previous Psychiatric History.

The pursuit of pleasure among clubbers and disco-goers often involves drug use. However, whether substance use may represent a relevant risk factor contributing to the development of psychiatric symptoms and of mental illness remains debated. The purposes of this study were to evaluate the percentage of subjects who developed long-lasting psychiatric symptoms in a sample of subjects reporting use of substances in nightclubs, and to evaluate the role of a previous psychiatric diagnosis in these subjects. Data were collected during three consecutive years in dedicated nursing units inside all the nightclubs of Ibiza, in emergency hospital rooms at the Can Misses Hospital and inside the psychiatric ward. A total of 10,163 subjects required medical assistance inside discos in the medical-nursing units, of which 223 required transfers to hospital emergency rooms. Of these, 110 required subsequent psychiatric hospitalization. Ninety-one (82.7 %) of these patients had a positive psychiatric history, which was also found in thirty-one of the 113 subjects (27.4%) not requiring psychiatric hospitalization. Negative psychiatric history was negatively associated with hospitalization (Coefficient = -2.574; p = 0.000) and for subjects with a negative psychiatric history the odds to be hospitalized changed by a factor of 0.076. Gender, age, civil status and nationality were not significant predictors of hospitalization. Overall, the number of subjects who developed major psychiatric disorders appeared to be limited. However, the presence of a psychiatric history here played a crucial role. Club drugs are therefore able to induce psychiatric sequelae requiring hospitalization mainly in subjects who are already vulnerable from a psychopathological point of view.

A simple microswimmer model inspired by the general equation for nonequilibrium reversible-irreversible coupling.

A simple mean-field microswimmer model is presented. The model is inspired by the nonequilibrium thermodynamics of multi-component fluids that undergo chemical reactions. These thermodynamics can be rigorously described in the context of the GENERIC (general equation for the nonequilibrium reversible-irreversible coupling) framework. More specifically, this approach was recently applied to non-ideal polymer solutions [T. Indei and J. D. Schieber, J. Chem. Phys. 146, 184902 (2017)]. One of the species of the solution is an unreactive polymer chain represented by the bead-spring model. Using this detailed description as inspiration, we then make several simplifying assumptions to obtain a mean-field model for a Janus microswimmer. The swimmer model considered here consists of a polymer dumbbell in a sea of reactants. One of the beads of the dumbbell is allowed to act as a catalyst for a chemical reaction between the reactants. We show that the mean-squared displacement (MSD) of the center of mass of this Janus dumbbell exhibits ballistic behavior at time scales at which the concentration of the reactant is large. The time scales at which the ballistic behavior is observed in the MSD coincide with the time scales at which the cross-correlation between the swimmer's orientation and the direction of its displacement exhibits a maximum. Since the swimmer model was inspired by the GENERIC framework, it is possible to ensure that the entropy generation is always positive, and therefore, the second law of thermodynamics is obeyed.

1CPN: A coarse-grained multi-scale model of chromatin.

A central question in epigenetics is how histone modifications influence the 3D structure of eukaryotic genomes and, ultimately, how this 3D structure is manifested in gene expression. The wide range of length scales that influence the 3D genome structure presents important challenges; epigenetic modifications to histones occur on scales of angstroms, yet the resulting effects of these modifications on genome structure can span micrometers. There is a scarcity of computational tools capable of providing a mechanistic picture of how molecular information from individual histones is propagated up to large regions of the genome. In this work, a new molecular model of chromatin is presented that provides such a picture. This new model, referred to as 1CPN, is structured around a rigorous multiscale approach, whereby free energies from an established and extensively validated model of the nucleosome are mapped onto a reduced coarse-grained topology. As such, 1CPN incorporates detailed physics from the nucleosome, such as histone modifications and DNA sequence, while maintaining the computational efficiency that is required to permit kilobase-scale simulations of genomic DNA. The 1CPN model reproduces the free energies and dynamics of both single nucleosomes and short chromatin fibers, and it is shown to be compatible with recently developed models of the linker histone. It is applied here to examine the effects of the linker DNA on the free energies of chromatin assembly and to demonstrate that these free energies are strongly dependent on the linker DNA length, pitch, and even DNA sequence. The 1CPN model is implemented in the LAMMPS simulation package and is distributed freely for public use.

The Effects of the Interplay between Motor and Brownian Forces on the Rheology of Active Gels.

Active gels perform key mechanical roles inside the cell, such as cell division, motion, and force sensing. The unique mechanical properties required to perform such functions arise from the interactions between molecular motors and semiflexible polymeric filaments. Molecular motors can convert the energy released in the hydrolysis of ATP into forces of up to piconewton magnitudes. Moreover, the polymeric filaments that form active gels are flexible enough to respond to Brownian forces but also stiff enough to support the large tensions induced by the motor-generated forces. Brownian forces are expected to have a significant effect especially at motor activities at which stable noncontractile in vitro active gels are prepared for rheological measurements. Here, a microscopic mean-field theory of active gels originally formulated in the limit of motor-dominated dynamics is extended to include Brownian forces. In the model presented here, Brownian forces are included accurately, at real room temperature, even in systems with high motor activity. It is shown that a subtle interplay, or competition, between motor-generated forces and Brownian forces has an important impact on the mass transport and rheological properties of active gels. The model predictions show that at low frequencies the dynamic modulus of active gels is determined mostly by motor protein dynamics. However, Brownian forces significantly increase the breadth of the relaxation spectrum and can affect the shape of the dynamic modulus over a wide frequency range even for ratios of motor to Brownian forces of more than a hundred. Since the ratio between motor and Brownian forces is sensitive to ATP concentration, the results presented here shed some light on how the transient mechanical response of active gels changes with varying ATP concentration.

A Molecular View of the Dynamics of dsDNA Packing Inside Viral Capsids in the Presence of Ions.

Genome packing in viruses and prokaryotes relies on positively charged ions to reduce electrostatic repulsions, and induce attractions that can facilitate DNA condensation. Here we present molecular dynamics simulations spanning several microseconds of dsDNA packing inside nanometer-sized viral capsids. We use a detailed molecular model of DNA that accounts for molecular structure, basepairing, and explicit counterions. The size and shape of the capsids studied here are based on the 30-nanometer-diameter gene transfer agents of bacterium Rhodobacter capsulatus that transfer random 4.5-kbp (1.5 µm) DNA segments between bacterial cells. Multivalent cations such as spermidine and magnesium induce attraction between packaged DNA sites that can lead to DNA condensation. At high concentrations of spermidine, this condensation significantly increases the shear stresses on the packaged DNA while also reducing the pressure inside the capsid. These effects result in an increase in the packing velocity and the total amount of DNA that can be packaged inside the nanometer-sized capsids. In the simulation results presented here, high concentrations of spermidine3+ did not produce the premature stalling observed in experiments. However, a small increase in the heterogeneity of packing velocities was observed in the systems with magnesium and spermidine ions compared to the system with only salt. The results presented here indicate that the effect of multivalent cations and of spermidine, in particular, on the dynamics of DNA packing, increases with decreasing packing velocities.

Mechanical Response of DNA-Nanoparticle Crystals to Controlled Deformation.

The self-assembly of DNA-conjugated nanoparticles represents a promising avenue toward the design of engineered hierarchical materials. By using DNA to encode nanoscale interactions, macroscale crystals can be formed with mechanical properties that can, at least in principle, be tuned. Here we present in silico evidence that the mechanical response of these assemblies can indeed be controlled, and that subtle modifications of the linking DNA sequences can change the Young's modulus from 97 kPa to 2.1 MPa. We rely on a detailed molecular model to quantify the energetics of DNA-nanoparticle assembly and demonstrate that the mechanical response is governed by entropic, rather than enthalpic, contributions and that the response of the entire network can be estimated from the elastic properties of an individual nanoparticle. The results here provide a first step toward the mechanical characterization of DNA-nanoparticle assemblies, and suggest the possibility of mechanical metamaterials constructed using DNA.

Tension-Dependent Free Energies of Nucleosome Unwrapping.

Nucleosomes form the basic unit of compaction within eukaryotic genomes, and their locations represent an important, yet poorly understood, mechanism of genetic regulation. Quantifying the strength of interactions within the nucleosome is a central problem in biophysics and is critical to understanding how nucleosome positions influence gene expression. By comparing to single-molecule experiments, we demonstrate that a coarse-grained molecular model of the nucleosome can reproduce key aspects of nucleosome unwrapping. Using detailed simulations of DNA and histone proteins, we calculate the tension-dependent free energy surface corresponding to the unwrapping process. The model reproduces quantitatively the forces required to unwrap the nucleosome and reveals the role played by electrostatic interactions during this process. We then demonstrate that histone modifications and DNA sequence can have significant effects on the energies of nucleosome formation. Most notably, we show that histone tails contribute asymmetrically to the stability of the outer and inner turn of nucleosomal DNA and that depending on which histone tails are modified, the tension-dependent response is modulated differently.

The role of filament length, finite-extensibility and motor force dispersity in stress relaxation and buckling mechanisms in non-sarcomeric active gels.

After relaxing some assumptions we apply a single-chain mean-field mathematical model recently introduced [RSC Adv. (2014)] to describe the role of molecular motors in the mechanical properties of active gels. The model allows physics that are not available in models postulated on coarser levels of description. Moreover it proposes a level of description that allows the prediction of observables at time scales too difficult to achieve in multi-chain simulations for realistic filament lengths and densities. We model the semiflexible filaments that compose the active gel as bead-spring chains; molecular motors are accounted for by using a mean-field approach, in which filaments undergo transitions of one motor attachment state depending on the state of the probe filament. The level of description includes the end-to-end distance and attachment state of the filaments, and the motor-generated forces, as stochastic state variables which evolve according to a proposed differential Chapman-Kolmogorov equation. The motor-generated forces are drawn from a stationary distribution of motor stall forces. We consider bead-spring chains with multiple beads, explore the effect of finite-extensibility of the strands and incorporate into the model motor force distributions that have been measured experimentally. The model can no longer be solved analytically but is amenable to numerical simulation. This version of the model allows a more quantitative description of buckling dynamics [Lenz et. al. PRL, 2012, 108, 238107] and the dynamic modulus of active gels. The effect of finite extensibility of the filament strands on the dynamic modulus was also found to be in agreement with the microrheology experiments of Mizuno et. al., [Science, 2007, 315, 370-373].

Competing effects of particle and medium inertia on particle diffusion in viscoelastic materials, and their ramifications for passive microrheology.

We analyze the appropriate form for the generalized Stokes-Einstein relation (GSER) for viscoelastic solids and fluids when bead inertia and medium inertia are taken into account, which we call the inertial GSER. It was previously shown for Maxwell fluids that the Basset (or Boussinesq) force arising from medium inertia can act purely dissipatively at high frequencies, where elasticity of the medium is dominant. In order to elucidate the cause of this counterintuitive result, we consider Brownian motion in a purely elastic solid where ordinary Stokes-type dissipation is not possible. The fluctuation-dissipation theorem requires the presence of a dissipative mechanism for the particle to experience fluctuating Brownian forces in a purely elastic solid. We show that the mechanism for such dissipation arises from the radiation of elastic waves toward the system boundaries. The frictional force associated with this mechanism is the Basset force, and it exists only when medium inertia is taken into consideration in the analysis of such a system. We consider first a one-dimensional harmonic lattice where all terms in the generalized Langevin equation--i.e., the elastic term, the memory kernel, and Brownian forces-can be found analytically from projection-operator methods. We show that the dissipation is purely from radiation of elastic waves. A similar analysis is made on a particle in a continuum, three-dimensional purely elastic solid, where the memory kernel is determined from continuum mechanics. Again, dissipation arises only from radiation of elastic shear waves toward infinite boundaries when medium inertia is taken into account. If the medium is a viscoelastic solid, Stokes-type dissipation is possible in addition to radiational dissipation so that the wave decays at the penetration depth. Inertial motion of the bead couples with the elasticity of the viscoelastic material, resulting in a possible resonant oscillation of the mean-square displacement (MSD) of the bead. On the other hand, medium inertia (the Basset force) tends to attenuate the oscillations by the dissipation mechanism described above. Thus competition between bead inertia and medium inertia determines whether or not the MSD oscillates. We find that, if the medium density is larger than 4/7 of the bead density, the Basset damping will suppress oscillations in the MSD; this criterion is sufficient but not necessary to present oscillations.

Treating inertia in passive microbead rheology.

The dynamic modulus G(*) of a viscoelastic medium is often measured by following the trajectory of a small bead subject to Brownian motion in a method called "passive microbead rheology." This equivalence between the positional autocorrelation function of the tracer bead and G(*) is assumed via the generalized Stokes-Einstein relation (GSER). However, inertia of both bead and medium are neglected in the GSER so that the analysis based on the GSER is not valid at high frequency where inertia is important. In this paper we show how to treat both contributions to inertia properly in one-bead passive microrheological analysis. A Maxwell fluid is studied as the simplest example of a viscoelastic fluid to resolve some apparent paradoxes of eliminating inertia. In the original GSER, the mean-square displacement (MSD) of the tracer bead does not satisfy the correct initial condition. If bead inertia is considered, the proper initial condition is realized, thereby indicating an importance of including inertia, but the MSD oscillates at a time regime smaller than the relaxation time of the fluid. This behavior is rather different from the original result of the GSER and what is observed. What is more, the discrepancy from the GSER result becomes worse with decreasing bead mass, and there is an anomalous gap between the MSD derived by naïvely taking the zero-mass limit in the equation of motion and the MSD for finite bead mass as indicated by McKinley et al. [J. Rheol. 53, 1487 (2009)]. In this paper we show what is necessary to take the zero-mass limit of the bead safely and correctly without causing either the inertial oscillation or the anomalous gap, while obtaining the proper initial condition. The presence of a very small purely viscous element can be used to eliminate bead inertia safely once included in the GSER. We also show that if the medium contains relaxation times outside the window where the single-mode Maxwell behavior is observed, the oscillation can be attenuated inside the window. This attenuation is realized even in the absence of a purely viscous element. Finally, fluid inertia also affects the bead autocorrelation through the Basset force and the fluid dragged around with the bead. We show that the Basset force plays the same role as the purely viscous element in high-frequency regime, and the oscillation of MSD is suppressed if fluid density and bead density are comparable.

Chromate reduction by Arthrobacter CR47 in biofilm packed bed reactors.

Bacterial strain Cr47 was isolated from a landfarming process soil sample. It was identified, by 16s rDNA sequencing, as Arthrobacter sp. The time course of the Cr(VI) reduction was monitored in batch operated packed bed biofilm reactors (12 mL void volume) and in recirculating packed bed biofilm reactors (100 mL void volume) inoculated with bacterial strain Cr47. The reduction was evaluated with, 30 mg L(-1) Cr(VI) laboratory solutions prepared with K2Cr2O7 and enriched with glucose-minimal medium, and with 30 mg L(-1) Cr(VI) industrial model solutions prepared with chrome plating waste waters enriched with sucrose-minimal medium. Under batch mode the reduction reaction by the biofilm seemed to fit well an exponential-decay model with a first order kinetic parameter of 0.071 mg(L h)(-1) Cr(VI). In the recirculating reactor, monitored after 4 weeks from inoculation and fed with laboratory solutions the removal rate was 0.79 mg(L h)(-1). In the reactor fed with the industrial model solutions the maximum Cr(VI) removal rate attained was 0.49 mg(L h)(-1). Artrobacter sp. packed bed biofilm reactors achieved Cr(VI) reduction rates comparable to other aerobic and anaerobic fixed film bioreactors previously reported.