Steric Interactions Research Articles

Dynamic Domain Links Substrate Binding and Catalysis in the Factor-Inhibiting-HIF-1.

The interplay between active-site chemistry and functionally relevant enzyme motions can provide useful insights into selective enzyme modulation. Modulation of the hypoxia-sensing function of factor-inhibiting-HIF-1 (FIH) enzyme is a potential therapeutic strategy in disease states such as ischemia and cancer. The hypoxia-sensing function of FIH relies in major part on the tight coupling of the first half of the catalytic mechanism which involves O2 activation and eventual succinate production to the second half which involves HIF-1α/CTAD substrate hydroxylation. In this study, we demonstrate the role of a loop hinge domain in FIH (FIH102-118) called the 100s loop in maintaining this particular tight coupling. Molecular dynamics patterns from Gaussian Network Model (iGNM) database analysis of FIH identified the 100s loop as one dynamic domain containing a hinge residue (Tyr102) with a potential substrate positioning role. Enzymological and biophysical studies of the 100s loop point mutants revealed altered enzyme kinetics with the exception of the conservative FIH mutant Y102F, which suggests a sterics-related role for this residue. Removal of the bulk of Tyr102 (Y102A) resulted in succinate production, autohydroxylation, and an O2 binding environment comparable to wild-type FIH. However, the HIF-1α/CTAD substrate hydroxylation of this mutant was significantly reduced which implies that (1) the FIH loop hinge residue Tyr102 does not affect O2 activation, (2) the stacking steric interaction of Tyr102 is important in substrate positioning for productive hydroxylation, and (3) Tyr102 is important for the synchronization of O2 activation and substrate hydroxylation.

Single-Particle Tracking in Poly(Ethylene Glycol) Diacrylate: Probe Size Effect on the Diffusion Behaviors of Nanoparticles in Unentangled Polymer Solutions.

A thorough understanding of the relevant factors governing the transport of nanoparticles in poly(ethylene glycol) diacrylate (PEGDA) is crucial for many applications utilizing this polymer. Here, single-particle tracking (SPT) was used to systematically investigate the role of the probe size (3-200 nm) on the diffusion behaviors of individual fluorescent nanoparticles in semidilute and unentangled PEGDA solutions. The quantitative assessment of the SPT data via the recorded single-particle trajectories and diffusion coefficients (D) not only showed that the observed probe dynamics in PEGDA were temporally and spatially heterogeneous, but more importantly that the measured D were observed to be significantly reduced (vs in solvent) and strongly size-dependent. We explained these results based on a modified multiscale model for particle diffusion, built upon well-established hydrodynamics and obstruction theories. We furthermore showed that the presence of steric interactions and probe confinement effects in highly crowded, unentangled PEGDA microstructures can lead to deviations in the single-particle displacements from the expected Gaussian behavior, as revealed by the van Hove displacement distributions and the associated non-Gaussian parameters. This study has demonstrated the power of SPT methods in offering an advanced characterization of the transport characteristics in complex polymer structures, overcoming challenges posed by traditional characterization techniques.

Controlling Electronic Coupling of Acene Chromophores on Quantum Dot Surfaces through Variable-Concentration Ligand Exchange.

Controlling the binding of functional organic molecules on quantum dot (QD) surfaces and the resulting ligand/QD interfacial structure determines the resulting organic-inorganic hybrid behavior. In this study, we vary the binding of tetracenedicarboxylate ligands bound to PbS QDs cast in thin films by performing solid-state ligand exchange of as-produced bound oleate ligands. We employ comprehensive Fourier-transform infrared (FTIR) analysis coupled with ultraviolet-visible (UV-vis) spectrophotometric measurements, transient absorption, and Density Functional Theory (DFT) simulations to study the QD/ligand surface structure and resulting optoelectronic properties. We find that there are three primary QD/diacid structures, each with a distinct binding mode dictated by the QD-ligand and ligand-ligand intermolecular and steric interactions. They can be accessed nearly independently of one another via different input ligand concentrations. Low concentrations produce mixed oleate/tetracene ligand structures where the tetracene carboxylates tilt toward QD surfaces. Intermediate concentrations produce mixed oleate/tetracene ligand structures with ligand-ligand interactions through intramolecular hydrogen bonding with the ligands perpendicular to the QD surface and weaker QD/ligand electronic interactions. High concentrations result in full ligand exchange, and the ligands tilt toward the surface while the QD film compacts. When the tetracene ligands tilt or lie flat on the QD surface, the benzene ring π-system interacts strongly with the p-orbitals at the PbS surface and produces strong QD-ligand interactions evidenced through QD/ligand state mixing, with a coupling energy of ≈700 meV.

Slippery rheotaxis: new regimes for guiding wall-bound microswimmers

The near-surface locomotion of microswimmers under the action of background flows has been studied extensively, whereas the intervening effects of complex surface properties remain hitherto unknown. Intending to delineate the shear-driven dynamics near a planar slippery wall, we adopt the squirmer model of microswimmers and employ a three-dimensional analytical-numerical framework in bispherical coordinates. It is interpreted that both the self-propulsion and the external shear flow are redistributed due to hydrodynamic slippage, followed by modulations in the thrust torque on the microswimmer. Phase portraits of the quasi-steady dynamics indicate that the stable upstream swimming states, known as ‘rheotaxis’, are significantly modulated by the slip length compared with the no-slip case. For puller swimmers, an intricate interplay among the modulated interfacial friction near a slippery surface, velocity gradients of the shear flow and the strength of the squirmer parameter promotes a critical shear rate beyond which a wide range of new rheotactic states exist. Consequently, an escaping microswimmer may exhibit rheotaxis or an existing rheotactic state annihilates due to crashing. Although stable states are absent for pushers without steric interactions, transitions from escaping and undamped oscillations to ‘rheotaxis’ occur in the presence of wall repulsion, but only until the other characteristics are overwhelmed by escape due to enhanced shear. Disclosing the ability of hydrodynamic slippage in broadening the scope of migration against a background flow for a wide range of parameters, the present work paves the way for investigations on the entrapment of microswimmers near complex pathways or sorting using selective rheotaxis.

Studies on thiophene-thiazole-carbohydrazides as methicillin-resistant staphylococcus aureus (MRSA) agents: Insight from DFT and Molecular docking simulation

Methicillin-resistant Staphylococcus aureus (MRSA) is an increasingly common bacterial infection that poses a serious threat to human health. MRSA is resistant to many common antibiotics, making it particularly difficult to treat. To address this issue, the density functional theory (DFT) approach was employed in this study to examine thiophene- and thiazole-containing carbohydrazides for their biological activities. Quantitative experimental and spectroscopic results were obtained and compared to the theoretical. The FMO reactivity parameters revealed superior biological activity and reactivity of compound 6 G in gas phase, polar and non-polar solvents according to the data. Additionally, molecular electrostatic potential (MESP) analysis and non-covalent interaction (NCI) indicated that compound 6F contained more weak and steric repulsion interactions, while compound 6 G featured minimal van der Waals forces of attraction. Also, compound 6 G had the highest stabilization energy in the NBO analysis which show a better donor - acceptor charge transfer and electron delocalization. Molecular docking analyses with standard drugs showed higher binding affinities for compound 6 G with 3VSL than with CIPRO. The in-silico analysis suggests that 6F and 6 G have great bioactivity for tackling MRSA.

Theory of electrolyte solutions in a slit charged pore: Effects of structural interactions and specific adsorption of ions.

In this paper, we present a continuation of our research on modeling electrolyte solutions within charged pores. We make use of the model developed by Blossey et al. [Phys. Rev. E 95, 060602 (2017)], which takes into account the structural interactions between ions through a bilinear form over the gradients of local ionic concentrations in the grand thermodynamic potential, as well as their steric interactions through the lattice gas model. The structural interactions may describe the effects of the molecular structure of ions at a phenomenological level. For example, these effects include steric effects due to non-spherical shapes of ions, their conformational lability, and solvent effects. In addition, we explore their specific interactions with the pore walls by incorporating external attractive potentials. Our primary focus is on observing the behavior of ionic concentration profiles and the disjoining pressure as the pore width changes. By starting with the local mechanical equilibrium condition, we derive a general expression for the disjoining pressure. Our findings indicate that considering the structural interactions of ions leads to a pronounced minimum on the disjoining pressure profiles at small pore widths. We attribute this minimum to the formation of electric double layers on the electrified surfaces of the pore. In addition, our results demonstrate that the inclusion of the attractive interactions of ions with the pore walls enhances this minimum and shifts it to smaller pore thicknesses. Our theoretical discoveries may be useful for those involved in supercapacitor electrochemical engineering, particularly when working with porous electrodes that have been infused with concentrated electrolyte solutions.

X-ray and Electron Diffraction Observations of Steric Zipper Interactions in Metal-Induced Peptide Cross-β Nanostructures.

The steric zipper is a common hydrophobic packing structure of peptide side chains that forms between two adjacent β-sheet layers in amyloid and related fibrils. Although previous studies have revealed that peptide fragments derived from native protein sequences exhibit steric zipper structures, their de novo designs have rarely been studied. Herein, steric zipper structures were artificially constructed in the crystalline state by metal-induced folding and assembly of tetrapeptide fragments Boc-3pa-X1-3pa-X2-OMe (3pa: β-(3-pyridyl)-l-alanine; X1 and X2: hydrophobic amino acids). Crystallographic studies revealed two types of packing structures, interdigitation and hydrophobic contact, that result in a class 1 steric zipper geometry when the X1 and X2 residues contain alkyl side chains. Furthermore, a class 3 steric zipper geometry was also observed for the first time among any reported steric zippers when using tetrapeptide fragments with (X1, X2) = (Thr, Thr) and (Phe, Leu). The system could also be extended to a knob-hole-type zipper using a pentapeptide sequence.

Experimental measurements and numerical simulations of the transport and retention of nanocrystal CdSe/ZnS quantum dots in saturated porous media: Effects of electrolytes, organic ligand, and natural organic matter

This study explores the transport and retention of CdSe/ZnS quantum dot (QD) nanoparticles in water-saturated sand columns as a function of electrolytes (Na+ and Ca2+), ionic strength, organic ligand citrate, and Suwannee River natural organic matter (SRNOM). Numerical simulations were carried out to understand the mechanisms that govern the transport and interactions of QDs in porous media and to assess how environmental parameters impact these mechanisms. An increase in the ionic strength of NaCl and CaCl2 increased QDs retention in porous media. The reduction of the electrostatic interactions screened by dissolved electrolyte ions and the increase of divalent bridging effect are the causes for this enhanced retention behavior. Citrate or SRNOM enhanced QDs transport in NaCl and CaCl2 systems by either increasing the repulsion energy barrier or inducing the steric interactions between QDs and the quartz sand collectors. A non-exponential decay characterized the retention profiles of QDs along the distance to the inlet. The modeling results indicated the four models containing the attachment, detachment, and straining terms – Model 1: M1-attachment, Model 2: M2-attachment and detachment, Model 3: M3-straining, and Model 4: M4-attachment, detachment, and straining – closely simulated the observed breakthrough curves (BTCs) but inadequately described the retention profiles.

Identified Isosteric Replacements of Ligands' Glycosyl Domain by Data Mining.

Biologically equivalent replacements of key moieties in molecules rationalize scaffold hopping, patent busting, or R-group enumeration. Yet, this information may depend upon the expert-defined space, and might be subjective and biased toward the chemistries they get used to. Most importantly, these practices are often informatively incomplete since they are often compromised by a try-and-error cycle, and although they depict what kind of substructures are suitable for the replacement occurrence, they fail to explain the driving forces to support such interchanges. The protein data bank (PDB) encodes a receptor-ligand interaction pattern and could be an optional source to mine structural surrogates. However, manual decoding of PDB has become almost impossible and redundant to excavate the bioisosteric know-how. Therefore, a text parsing workflow has been developed to automatically extract the local structural replacement of a specific structure from PDB by finding spatial and steric interaction overlaps between the fragments in endogenous ligands and particular ligand fragments. Taking the glycosyl domain for instance, a total of 49 520 replacements that overlap on nucleotide ribose were identified and categorized based on their SMILE codes. A predominately ring system, such as aliphatic and aromatic rings, was observed; yet, amide and sulfonamide replacements also occur. We believe these findings may enlighten medicinal chemists on the structure design and optimization of ligands using the bioisosteric replacement strategy.

NMR and FT-IR Investigation Study and Topological Analysis of Various Conformations of 2,5-dimethyl-2,5-dihydroxyl-1,4-dithian using Density Functional Theory Method

Abstract: 1,4-dithian-2,5-dihydroxyl is the major source for the synthesis of sulfur-containing heterocyclic compounds such as thiophene and 1,3-thiazole derivatives, which these compounds are widely used in pharmaceuticals and agricultural industries. Sulfur-containing heterocycles, such as 2-aminothiophene, thiazolidine, oxothiazolidine, and thiazoles, can be comprehensively synthesized from dimerization of the derivatives of 1,4-dithian-2,5- dihydroxyl. FT-IR and NMR spectra of axial and equatorial conformations of 2,5-dimethyl-2,5-dihydroxyl-1,4- dithian compound were investigated by density functional theory (DFT) using B3LYP/6-311++G** and M06- 2X/aug-cc-pVDZ computational methods. The topological properties of the electron charge density were calculated by the quantum theory of atoms in molecules (QTAIM) and non-covalent interactions/reduced electron density gradient (NCI-RDG) theoretical methods. The obtained results of the analyses were also discussed in more detail. The results showed that the steric interactions are more prominent than the van der Waals interactions, and in both conformations, steric interactions are included in the middle of the 1,4-dithian ring and there is hydrogen bonding between hydroxyl groups and 1,4-dithian ring structure heteroatoms in the axial conformation while this interaction is not observed in the case of equatorial conformation. The presence of these interactions was confirmed using NBO analysis.

Spontaneous velocity alignment of Brownian particles with feedback-induced propulsion (a)

Based on Brownian dynamics simulations we study the collective behavior of a two-dimensional system of repulsively interacting colloidal particles, where each particle is propelled by a repulsive feedback force with time delay τ. Although the pair interactions are purely isotropic we observe a spontaneous, large-scale alignment of the velocity vectors. This phenomenon persists for long times and occurs in the absence of steady-state clustering. We explain our observations by a combination of the effect of steric interactions yielding local velocity ordering, and the effect of time delay, that generates cluster dissolution, velocity persistence and velocity alignment over large distances. Overall, the behavior reveals intriguing similarities, but also differences, with that observed in models of active matter, such as active Brownian particles and the Vicsek model.

Spectroscopic characterization, polar solvation effects, DFT studies, and the antiviral inhibitory potency of a novel terpolymer based on p-Phenylenediamine – Guanidine – Formaldehyde (PGF) ligand

This study articulates the synthesis, spectroscopic characterization, solvation effects, theoretical calculations, and molecular docking analysis of a novel p-Phenylenediamine – Guanidine – Formaldehyde (PGF) terpolymer ligand as potential antiviral agents. The terpolymer's structure was established using FT-IR, UV-visible, 1H, and 13C NMR spectroscopy. Its thermal degradation was also examined using thermogravimetric analysis. Theoretical studies based on density functional theory (DFT) at the B3LYP-D3(BJ)/6-311G++G (2d, 2p) level of theory was utilized to investigate the influence of solvation (in DMSO, water, ethanol and methanol) on the structural, electronic, and spectroscopic properties of the studied polymer. Results of the visual studies suggest that the complexes displayed an enormously weaker interactions or Van der Walls interactions with sparsely strong hydrogen bonding, along with minimally strong steric interactions or repulsion. Therefore, it can be surmised that the iso-surface present in PG_DMSO_OPT, PG_ETHANOL_OPT, PG_METHANOL_OPT and PG_WATER_OPT are prominent for van der walls interactions with strong hydrogen bonding contacts which makes the complexes very promising for drug interactions and/or Protein-Ligand Interaction. Summarily, molecular docking analysis indicated that the PGF terpolymer ligand has potent inhibitory effects on HIV target receptors, particularly 3LAK HIV protein; due to the proteins’ high negative binding affinity, and greater number of conventional hydrogen bonds compared to 3LAN. Summarily, this study presents a new terpolymer ligand with potential biological applications, particularly in the development of HIV inhibitors.

Energy-Dependent Charge Separation in Conjugated Polymer Electrolyte Complexes

Supramolecular complexes have great potential as light-harvesting materials, as intermolecular structural organization can be manipulated through steric or electrostatic interactions to impact electronic coupling between energy and charge donors and acceptors. Here, we examine the relative rates and efficiencies of charge transfer in conjugated polymer electrolyte complexes (CPECs) based on a polythiophene electron donor (PTAK) and polyfluorene electron acceptor (PFPI). These CPECs are characterized by ordered polymer microstructures, as evident from spectral signatures of strong excitonic coupling within the PTAK component from steady-state UV–vis absorption spectra. We find that PTAK polarons are generated within tens of picoseconds through a combination of prompt and delayed charge separation following direct photoexcitation or energy transfer from PFPI. Further, we find that decreasing the length of charged PTAK side chains or increasing excitation energy increases the driving force for electron transfer to increase charge separation rates and yields for polarons, with the greatest relative yields observed at excitation energies that initiate PFPI-to-PTAK energy transfer. Charge separation between components can be rationalized from a canonical Marcus picture, whereby excess vibrational energy effectively lowers the barrier for PTAK-to-PFPI charge separation. This contrasts with the recently reported ultrafast (<100 fs) charge separation in small-molecule/polythiophene electrolyte complexes that is attributed to strong orbital mixing that gives rise to charge generation via CT exciton states. These results provide insights into conditions for realizing charge separation in concert with energy transfer in CPECs as light-harvesting materials.

Biodegradation of polyurethanes by Serratia liquefaciens L135 and its polyurethanase: In silico and in vitro analyses

Polyurethanes (PUs) are found in many everyday products and their disposal leads to environmental accumulation. Therefore, there is an urgent need to develop ecologically sustainable techniques to biodegrade and recycle this recalcitrant polymer and replace traditional methods that form harmful by-products. Serratia liquefaciens L135 secretes a polyurethanase with lipase activity, and this study explores the biodegradation of PUs by this bacterium and its enzyme through in silico and in vitro analyses. PUs monomers and tetramers were constructed in silico and tested with modeled and validated structure of the polyurethanase from S. liquefaciens. The molecular docking showed that all PUs monomers presented favorable interactions with polyurethanase (values of binding energy between −84.75 and −121.71 kcal mol−1), including PU poly[4,4′-methylenebis (phenyl isocyanate)-alt-1,4-butanediol/di (propylene glycol)/polycaprolactone] (PCLMDI). Due to repulsive steric interactions, tetramers showed less favorable interactions (values between 24.26 and −45.50 kcal mol−1). In vitro analyses evaluated the biodegradation of PUs: Impranil® and PCLMDI; this latter showed high binding energy with this polyurethanase in silico. The biodegradation of Impranil® by S. liquefaciens and its partially purified polyurethanase was confirmed in agar by forming a transparent halo. Impranil® disks inoculated with S. liquefaciens and incubated at 30 °C for six days showed rupture of the PU structure, possibly due to the formation of cracks visualized by scanning electron microscopy (SEM). PCLMDI films were also biodegraded by S. liquefaciens after 60 days of incubation, with the formation of pores and cracks visualized by SEM. The biodegradation may have occurred due to the action of polyurethanase produced by this bacterium. This work provides essential information on the potential of S. liquefaciens to biodegrade PUs through in silico analyses combined with in vitro analyses.

Atropselective Organocatalytic Synthesis of Chiral Compounds Containing Nitrogen along the Axis of Chirality

Atropisomers, i.e., stereoisomers that are distinct because their free rotation about a single bond is hindered by steric interactions between nearby bulky groups or by electrostatics, may interact with their surroundings in different ways, and may also exhibit different properties. They may be found as natural products, as pharmaceutical or agricultural active ingredients, as chiral ligands and organocatalysts, and in functional materials. Our ability to synthesize them stereoselectively and in a sustainable way, using achiral materials and simply with the aid of an organocatalyst and mild conditions, has become a hot topic in research. This review provides an overview of recent achievements in the synthesis of atropisomers containing C-N and N-N axes of chirality.

Effect of reduced inherent organic matter on stability and transport behaviors of black soil colloids

Soil organic matter plays an important role in the stability, transport, and fate of soil colloids. At present, studies have mostly focused on the effects of adding exogenous organic matter on soil colloidal properties, while there is very limited research on the effect of reduced inherent soil organic matter on the environmental behavior of soil colloids. This study investigated the stability and transport behavior of black soil colloids (BSC) and black soil colloids with reduced inherent organic matter (BSC-ROM) under different ionic strength (5, 50 mM) and background solution pH (4.0, 7.0, and 9.0) conditions. Meanwhile, the release behavior of two soil colloids in the saturated sand column under transient ionic strength conditions was also studied. The results showed that both ionic strength reduction and pH increase increased the negative charges of BSC and BSC-ROM, and improved the electrostatic repulsion between soil colloids and grain surface, thereby promoting the stability and mobility of soil colloids. The decrease in inherent organic matter had little effect on the surface charge of soil colloids, suggesting that the electrostatic repulsion was not the main force affecting the stability and mobility of BSC and BSC-ROM, and reducing inherent organic matter might significantly reduce the stability and mobility of soil colloids by weakening the steric hindrance interaction. The decrease of transient ionic strength reduced the depth of the energy minimum and activated the soil colloids retained on the surface of the grain at three pH conditions. This study is helpful to predict the potential impact of soil organic matter degradation on the fate of BSC in natural environment system.

Inflationary theory of branching morphogenesis in the mouse salivary gland

The mechanisms that regulate the patterning of branched epithelia remain a subject of long-standing debate. Recently, it has been proposed that the statistical organization of multiple ductal tissues can be explained through a local self-organizing principle based on the branching-annihilating random walk (BARW) in which proliferating tips drive a process of ductal elongation and stochastic bifurcation that terminates when tips encounter maturing ducts. Here, applied to mouse salivary gland, we show the BARW model struggles to explain the large-scale organization of tissue. Instead, we propose that the gland develops as a tip-driven branching-delayed random walk (BDRW). In this framework, a generalization of the BARW, tips inhibited through steric interaction with proximate ducts may continue their branching program as constraints become alleviated through the persistent expansion of the surrounding tissue. This inflationary BDRW model presents a general paradigm for branching morphogenesis when the ductal epithelium grows cooperatively with the domain into which it expands.

Using pressure to unravel the structure–dynamic-disorder relationship in metal halide perovskites

The exceptional optoelectronic properties of metal halide perovskites (MHPs) are presumed to arise, at least in part, from the peculiar interplay between the inorganic metal-halide sublattice and the atomic or molecular cations enclosed in the cage voids. The latter can exhibit a roto-translative dynamics, which is shown here to be at the origin of the structural behavior of MHPs as a function of temperature, pressure and composition. The application of high hydrostatic pressure allows for unraveling the nature of the interaction between both sublattices, characterized by the simultaneous action of hydrogen bonding and steric hindrance. In particular, we find that under the conditions of unleashed cation dynamics, the key factor that determines the structural stability of MHPs is the repulsive steric interaction rather than hydrogen bonding. Taking as example the results from pressure and temperature-dependent photoluminescence and Raman experiments on MAPbBr_3 but also considering the pertinent MHP literature, we provide a general picture about the relationship between the crystal structure and the presence or absence of cationic dynamic disorder. The reason for the structural sequences observed in MHPs with increasing temperature, pressure, A-site cation size or decreasing halide ionic radius is found principally in the strengthening of the dynamic steric interaction with the increase of the dynamic disorder. In this way, we have deepened our fundamental understanding of MHPs; knowledge that could be coined to improve performance in future optoelectronic devices based on this promising class of semiconductors.

Reversible Modulation of Conductivity in Azobenzene Polyelectrolytes Using Light

Incorporating light-responsive azobenzene into polyelectrolytes couples photoinduced changes in steric interactions and polarity to ionic conductivity. The reversible isomerization of an azobenzene moiety yields a 2–7 times change in the ion conductivity (σtrans > σcis) depending on the polymer composition. These trends cannot be explained by differences in the glass-transition temperatures of the polymers. Instead, UV–vis spectroscopy reveals a bathochromic shift in the π → π* transition of cis-poly(azobenzene) upon adding lithium bis(trifluoromethane)sulfonimide (LiTFSI) salt, suggesting that coordination of the cis isomer with Li+ is responsible for its lower conductivity. In summary, azobenzene is a simple and convenient functional unit to control the conductivity of polymeric materials using light.

Derivation of a macroscopic model for Brownian hard needles

We study the role of anisotropic steric interactions in a system of hard Brownian needles in two dimensions. Despite having no volume, non-overlapping needles exclude a volume in configuration space that influences the macroscopic evolution of the system. Starting from the stochastic particle system, we use the method of matched asymptotic expansions and conformal mapping to systematically derive a nonlinear non-local partial differential equation for the evolution of the population density in position and orientation. We consider the regime of high rotational diffusion, resulting in an equation for the spatial density that allows us to compare the effective excluded volume of a hard-needle system with that of a hard-sphere system. We further consider spatially homogeneous solutions and find an isotropic to nematic transition as density increases, consistent with Onsager’s theory.