Dimer Fraction Research Articles

New structural forms of a mycobacterial adenylyl cyclase Rv1625c.

Rv1625c is one of 16 adenylyl cyclases encoded in the genome of Mycobacterium tuberculosis. In solution Rv1625c exists predominantly as a monomer, with a small amount of dimer. It has been shown previously that the monomer is active and the dimeric fraction is inactive. Both fractions of wild-type Rv1625c crystallized as head-to-head inactive domain-swapped dimers as opposed to the head-to-tail dimer seen in other functional adenylyl cyclases. About half of the molecule is involved in extensive domain swapping. The strain created by a serine residue located on a hinge loop and the crystallization condition might have led to this unusual domain swapping. The inactivity of the dimeric form of Rv1625c could be explained by the absence of the required catalytic site in the swapped dimer. A single mutant of the enzyme was also generated by changing a phenylalanine predicted to occur at the functional dimer interface to an arginine. This single mutant exists as a dimer in solution but crystallized as a monomer. Analysis of the structure showed that a salt bridge formed between a glutamate residue in the N-terminal segment and the mutated arginine residue hinders dimer formation by pulling the N-terminal region towards the dimer interface. Both structures reported here show a change in the dimerization-arm region which is involved in formation of the functional dimer. It is concluded that the dimerization arm along with other structural elements such as the N-terminal region and certain loops are vital for determining the oligomeric nature of the enzyme, which in turn dictates its activity.

Plasma Species Measurements in the Plume of an Iodine Fueled Hall Thruster

The plasma plume from a low-power Hall-effect thruster fueled by iodine vapor was characterized with several wetted plasma probes. The plasma source included a laboratory propellant feed system and a laboratory model Hall thruster powered by a breadboard power processing unit. The hollow cathode was fed with xenon. Iodine and xenon ion velocity and energy distributions were measured with an ExB probe, an electrostatic analyzer, and a combined electrostatic analyzer/ExB probe. Multiply charged species were detected in the plume when the thruster was operated with iodine and xenon. Diatomic iodine ions were also detected. The dimer fraction was found to vary with thruster operating conditions and probe location in degrees from the center of the beam. The dimer fraction was also found to decrease as discharge current and voltage increased. With the thruster operating at a discharge potential of 250 V, the fraction of the high-energy ion flux comprised of dimers exceeded 10% over a broad region of the plume at low power. Significant dimer fractions could increase thrust to power by useful amounts at the cost of some overall efficiency.

A DNA sequence recognition loop on APOBEC3A controls substrate specificity.

APOBEC3A (A3A), one of the seven-member APOBEC3 family of cytidine deaminases, lacks strong antiviral activity against lentiviruses but is a potent inhibitor of adeno-associated virus and endogenous retroelements. In this report, we characterize the biochemical properties of mammalian cell-produced and catalytically active E. coli-produced A3A. The enzyme binds to single-stranded DNA with a Kd of 150 nM and forms dimeric and monomeric fractions. A3A, unlike APOBEC3G (A3G), deaminates DNA substrates nonprocessively. Using a panel of oligonucleotides that contained all possible trinucleotide contexts, we identified the preferred target sequence as TC (A/G). Based on a three-dimensional model of A3A, we identified a putative binding groove that contains residues with the potential to bind substrate DNA and to influence target sequence specificity. Taking advantage of the sequence similarity to the catalytic domain of A3G, we generated A3A/A3G chimeric proteins and analyzed their target site preference. We identified a recognition loop that altered A3A sequence specificity, broadening its target sequence preference. Mutation of amino acids in the predicted DNA binding groove prevented substrate binding, confirming the role of this groove in substrate binding. These findings shed light on how APOBEC3 proteins bind their substrate and determine which sites to deaminate.

English

This study investigates the effect of different concentrations (0.50 to 1.50%) of thermo-chemically modified lysozyme on the growth of selected bacteria, that is, Gram-negative Pseudomonas fragi, Pseudomonas fluorescens, Escherichia coli, Proteus mirabilis and Gram-positive Listeria innocua, Leuconostoc mesenteroides in nutrient broth. The content of lysozyme polymeric forms after modification was evaluated by sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). The dimer and trimer fractions of the modified lysozyme were 35.9 and 33.1%, respectively. The modified lysozyme exhibited higher surface hydrophobicity and lower hydrolytic activity than monomer. No inhibitory action of lysozyme monomer was observed in relation to the tested Gram-negative bacterial strains. The application of modified lysozyme at a concentration of 0.75% and higher, caused a complete reduction of bacterial counts as early as after 1 h incubation in case of P. fragi, P. fluorescens and E. coli or after 6 h incubation in case of P. mirabilis. After 6 h incubation both in samples with an addition of monomer and those with modified lysozyme a complete growth inhibition of L. mesenteroides was recorded. Modified lysozyme showed a stronger antibacterial activity against L. innocua than native lysozyme. Conducted analyses indicate that changes in the structure of enzyme molecules leads to an increase in the antibacterial action mainly towards Gram-negative bacteria. Key words: Antibacterial activity, modified lysozyme.

Катализаторы на основе высокомодульного цеолита типа MOR

This paper is dedicated to synthesis of MOR-type zeolite specimens in Na- and H-form of high phase purity and having crystallinity degree of nearly 100% with molar ratio of SiO2/Al2O3 = 15-24 in deep-decationated form, and investigation of their properties. To obtain chemically homogeneous silica-alumina hydrogel, mixing of crystal-forming components was implemented with reaction mass being continuously stirred. The stirring was still continuing during the process of crystallization. Crystallization was conducted at a temperature of 150-160 ºС. The compositions of oligomers were quantified by high-performance liquid chromatography. The conversion of the initial olefin, its composition and amount of its isomers, the amount of low molecular weight oligomers (with a molecular weight less than the dimers) as well as the number of dimer were measured by gas-liquid chromatography. The dimer fraction was extracted by reaction mixture distillation under reduced pressure and analyzed by nuclear magnetic resonance (NMR) ¹H, ¹³C, IR- and mass spectrometry methods. By method of temperature-programmed ammonia desorption it was found that with zeolite module increasing there was an increase in strength of zeolite acid sites. For evaluation of catalytic properties of the derived H-form of MOR-type zeolite specimens with various modules, model reaction of a-methylstyrene dimerization was used. It was found that with zeolite module increasing there was an increase in cyclic dimers formation selectivity from 20 to 52%. The α-methylstyrene conversions on MOR-type zeolite specimens were investigated at a temperature of 80 ºС with a 10% catalyst in nitrogen atmosphere. The catalyst was calcinated prior to the reaction at a temperature of 300 ºС during 2-3 hours with rare gas current in the reaction flask. Implementation of the method developed makes it possible to expand the raw materials base, simplify the synthesis and reduce the cost of powdery mordenite type zeolite.

Ring-Opening Reactions of α- and β-Pinenes in Pressurized Hot Water in the Absence of Any Additive

Reactions of α- and β-pinenes in pressurized hot water were examined in a batch reactor made of a SS316 1/2-in. tube at temperatures of 250–400 °C, pressures of 4–30 MPa, and reaction times of 1–30 min in the absence of any additive under an argon atmosphere. The maximum yields of limonene from α-pinene were ca. 70% in 20 min at 300 °C or 1 min at 400 °C. Limonene was obtained from β-pinene in ca. 16% yield for 30 min at 300 °C and 1 min at 400 °C. Reversible production of myrcene in 14% yield and formation of unidentified C20 dimer fractions were noted for 1 min at 370 °C from β-pinene. The conversion of α-pinene to limonene took place under anhydrous conditions, albeit at slightly lower yield of 65% compared to processes conducted in the presence of water, where increased limonene yield of 70% was observed for 1 min at 400 °C. The conversion of β-pinene to limonene under anhydrous conditions was limited to 6.1% in contrast to 11.9% in the presence of water for 7 min at 370 °C. In the presence of oxygen, p-cymene was formed in 23% and 24% yield at the expense of limonene from α- and β-pinenes, respectively, for 30 min at 400 °C.

Intermolecular interactions and conformation of antibody dimers present in IgG1 biopharmaceuticals

Intermolecular interactions and conformation in dimer species of Palivizumab, a monoclonal antibody (IgG1), were investigated to elucidate the physical and chemical properties of the dimerized antibody. Palivizumab solution contains ∼1% dimer and 99% monomer. The dimer species was isolated by size-exclusion chromatography and analysed by a number of methods including analytical ultracentrifugation-sedimantetion velocity (AUC-SV). AUC-SV in the presence of sodium dodecyl sulphate indicated that approximately half of the dimer fraction was non-covalently associated, whereas the other half was dimerized by covalent bond. Disulphide bond and dityrosine formation were likely to be involved in the covalent dimerization. Limited proteolysis of the isolated dimer by Lys-C and mass spectrometry for the resultant products indicated that the dimer species were formed by Fab-Fc or Fab-Fab interactions, whereas Fc-Fc interactions were not found. It is thus likely that the dimerization occurs mainly via the Fab region. With regard to the conformation of the dimer species, the secondary and tertiary structures were shown to be almost identical to those of the monomer. Furthermore, the thermal stability turned out also to be very similar between the dimer and monomer.

Effect of Modified Lysozyme on the Microflora and Sensory Attributes of Ground Pork

The effects of lysozyme monomer and thermochemically modified lysozyme on the microflora and sensory attributes of heated and unheated ground pork were investigated in this study. The dimer and trimer fractions of the modified lysozyme were 36.1 and 33.5%, respectively. The modified lysozyme exhibited higher hydrophobicity (40,600 U/mg of protein) and lower enzymatic activity (1,020 U/mg of protein) than the hydrophobicity (890 U/mg of protein) and activity (17,950 U/mg of protein) of the lysozyme monomer. Portions of ground pork (150 g) without lysozyme or supplemented with 5 mg/g lysozyme or modified lysozyme and either not heated or heated at 60°C for 10 min were stored at 4 ± 1°C and sampled at various times between 1 and 144 h. Meat color was not affected by either additive. After storage for 72 h, the mean odor score for meat supplemented with modified lysozyme and heated decreased from 5.0 at 1 h to 4.1, while the scores for all the other preparations were ≤3.2. After 144 h, the counts of Pseudomonas and Enterobacteriaceae in meat that was supplemented with modified lysozyme and not heated were, respectively, 1.1 and 0.9 log less than in the controls, and the numbers in such meat that was heated were, respectively, 1.2 and 2.4 log less than the numbers in the controls. The counts in meat supplemented with lysozyme and the controls were comparable. Heat treatment increased the bacteriostatic effect of modified lysozyme on gram-negative bacteria.

Characterisation of non-polar dimers formed during thermo-oxidative degradation of β-sitosterol

Thermo-oxidative degradation of sterols at temperature typical for frying leads to the formation of oxidised derivatives, fragmented sterols and oligomers. Recent research on sterol oxidation focuses mainly on the oxysterol derivatives formation to the exclusion of compounds with high molecular mass. The aim of this work was to decipher the chemical structure of non-polar dimers formed during β-sitosterol oxidation at 180°C in the presence of oxygen. The dimer fraction was separated by size-exclusion chromatography (SEC) after pre-fractionation on silica gel. The chemical structure of the dimers was assessed by 1D and 2D NMR, IR, Raman and MS spectroscopies. NMR data confirmed that the predominant non-polar dimer formed during β-sitosterol oxidative degradation has a configuration of 3β,3β′-disitosteryl ether. Data from IR and Raman spectroscopies further proved it chemical structure. Applied analytical techniques also confirmed presence of dimers with different configuration than disteryl ethers.

On the Relative Stability of Dimeric Interfaces of the Mu-Opioid Receptor Inferred from Recent Crystallographic Studies

The mu-opioid (MOP) receptor is the predominant target for most clinically used analgesics. In response to demonstrated correlations between opioid dependence and the MOP receptor, researchers have focused on alternative targets. Opioid receptor oligomers are among those that have been suggested to mediate analgesia without the common opioid-related adverse effects. Thus, obtaining a molecular-level understanding of the nature of receptor-receptor interactions in the membrane, either within, or between receptor subtypes, can both create new opportunities for drug discovery, and address the role of oligomerization in receptor function. The recent MOP receptor crystal structure has inspired hypotheses of dimerization contacts, specifically: a closely packed interface involving transmembrane (TM) helices TM5 and TM6, and a less compact one involving TM1, TM2, and helix 8 (H8). These interfaces exhibit similar arrangements to those found in crystals of the chemokine receptor CXCR4 and the kappa-opioid receptor, respectively. While it is tempting to speculate that the tighter TM5/TM6 arrangement has physiological relevance, additional studies are necessary to a) understand the relative dimer stability at TM5/TM6, TM1/TM2/H8, and other interfaces in the membrane, b) evaluate the contribution of the engineered T4 lysozyme (T4L) to the TM5/TM6 association, and c) investigate possible variability across different receptors. To begin to address these questions we have performed umbrella sampling molecular dynamics simulations of coarse-grained representations of the MOP receptor interacting at the TM5/TM6 (with and without T4L) or TM1/TM2/H8 interfaces in an explicit lipid-water environment. We have derived relative estimates of the dimerization free energy at the two specific interfaces and from these we suggest relative dimer lifetimes and dimeric fractions in a lipid mimetic environment. This information can help design future experiments aimed at understanding the role of dimerization in receptor function.

Trimeric Tau Is Toxic to Human Neuronal Cells at Low Nanomolar Concentrations

In Alzheimer's disease (AD), tau aggregates into fibrils and higher order neurofibrillary tangles, a key histopathological feature of AD. However, soluble oligomeric tau species may play a more critical role in AD progression since these tau species correlate better with neuronal loss and cognitive dysfunction. Recent studies show that extracellular oligomeric tau can inhibit memory formation and synaptic function and also transmit pathology to neighboring neurons. However, the specific forms of oligomeric tau involved in toxicity are still unknown. Here, we used two splice variants of recombinant human tau and generated monomeric, dimeric, and trimeric fractions of each isoform. The composition of each fraction was verified chromatographically and also by atomic force microscopy. The toxicity of each fraction toward both human neuroblastoma cells and cholinergic-like neurons was assessed. Trimeric, but not monomeric or dimeric, tau oligomers of both splice variants were neurotoxic at low nanomolar concentrations. Further characterization of tau oligomer species with disease-specific modifications and morphologies is necessary to identify the best targets for the development of biomarker and therapeutic development for AD and related tauopathies.

Potential dependence of ferritin monomer, dimer and oligomer adsorption on a polycrystalline Au surface

An electrochemical quartz crystal microbalance (EQCM) was used to measure the effects of in situ potential changes on the adsorption and desorption of ferritin on polycrystalline Au. As-received horse-spleen ferritin, and purified ferritin monomer, dimer and higher oligomer fractions separated by gel chromatography were compared. Experiments were carried out at pH 5 where the ferritin is expected to have a net negative charge. Despite electrostatic repulsion, significant adsorption was observed on Au even at the most negative potentials studied, though it could be suppressed by using a low ionic strength electrolyte. This adsorption was attributed to hydrophobic interactions caused by partial denaturation of the protein, and it was greater for as-received ferritin containing higher oligomers than for monomeric ferritin. The reversibility of the adsorption was also lower when higher oligomers were present. Potential dependent adsorption/desorption could be useful as a diagnostic method for protein solutions.

Progress in demonstrating homochiral selection in prebiotic RNA synthesis

The dual properties of RNA as an enzyme catalyst (ribozyme) and its ability to store genetic information suggest that early life could have been based on RNA. We have synthesized RNA oligomers up to 50-mer chain lengths by Na+-montmorillonite catalyzed reactions of 5′-activated mononucleotides. For studying chiral selectivity, the reactions of racemic mixtures of D, L-ImpA and D, L-ImpU were carried out on Na+-montmorillonite. The dimer, trimer, tetramer and pentamer fractions (yields 43.3%, 14.5%, 5.8% and 3.0%, respectively) were investigated for homochiral selection. These products were collected via ion exchange HPLC, their terminal 5′-phosphate was cleaved by alkaline phosphatase and further analyzed by reverse phase HPLC. Twelve linear and three cyclic dimers were isolated and characterized. The homochirality of dimers was 63.5±0.8%. Out of the sixteen trimers isolated, ten were homochiral with an overall homochirality of 74.2±1.6%. The tetramers and pentamers were separated into 24 and 20 isomers, respectively. Their co-elution with those formed in the binary reactions of D-ImpA with D-ImpU on Na+-montmorillonite revealed 92.7±2.0% and 97.2±0.5% homochirality, respectively. These results suggest that Na+-montmorillonite not only catalyzes the prebiotic synthesis of RNA but it also facilitates homochiral selection. Work is in progress to determine chiral selectivity in the reaction mixtures of activated nucleotides of racemic A, U, G and C on Na+-montmorillonite.

Sputtering of dimers off a silicon surface

We present experimental and molecular-dynamics simulation results of the sputtering of a Si surface by 2keV Ar ions. Results on both the monomer and dimer distributions are presented. In simulation, these distributions follow a generalized Thompson law with power exponent n=2 and n=3, respectively. The experimental data, obtained via plasma post-ionization in an SNMS (secondary neutral mass spectrometry) apparatus, show good agreement with respect to the dimer fraction, and the relative energy distributions of dimers and monomers. The consequences for the dimer sputtering mechanism are discussed.

Characterization of the hemoglobin of the backswimmer Anisops deanei (Hemiptera)

While O2-binding hemoglobin-like proteins are present in many insects, prominent amounts of hemoglobin have only been found in a few species. Backswimmers of the genera Anisops and Buenoa (Notonectidae) have high concentrations of hemoglobin in the large tracheal cells of the abdomen. Oxygen from the hemoglobin is delivered to a gas bubble and controls the buoyant density, which enables the bugs to maintain their position without swimming and to remain stationary in the mid-water zone where they hunt for prey. We have obtained the cDNA sequences of three Anisops deanei hemoglobin chains by RT-PCR and RACE techniques. The deduced amino acid sequences show an unusual insertion of a single amino acid in the conserved helix E, but this does not affect protein stability or ligand binding kinetics. Recombinant A. deanei hemoglobin has an oxygen affinity of P50 = 2.4 kPa (18 torr) and reveals the presence of a dimeric fraction or two different conformations. The absorption spectra demonstrate that the Anisops hemoglobin is a typical pentacoordinate globin. Phylogenetic analyses show that the backswimmer hemoglobins evolved within Heteroptera and most likely originated from an intracellular hemoglobin with divergent function.

Quantifying the Dimerization Energy of a CLC Transporter in Lipid Bilayers

Recently, a structurally stable and functional monomeric form of the normally homodimeric Cl−/H+ antiporter CLC-ec1 was designed by introducing two tryptophan mutations at the dimer interface, I201W and I422W. Several single tryptophan mutant constructs show intermediate stability between monomer and dimer state, as observed by size exclusion chromatography. In addition, the monomer and dimer populations can be shifted by adding lipids to the purified protein in detergent micelles, indicating that the system is undergoing reversible dimerization. We are now developing CLC-ec1 into a model for measuring dimerization energetics in a lipid bilayer. To measure the free energy, as well as enthalpic and entropic contributions, we must determine the fraction of monomer and dimer in the total protein population at different temperatures. To do this, we introduce cysteine residues into the extracellular loop adjacent to the dimerization interface, and fluorescently label the protein with tetramethylrhodamine- (TMR) or fluorescein-maleimide. The fluorescent protein is then reconstituted into liposomes at known concentration, and the fraction of dimer is measured in one of two ways. In the first method, protein is labeled with TMR at a position that allows the rhodamine molecules to undergo self-quenching by forming non-fluorescent pairs when in the dimer state. Addition of 0.5% SDS dissociates the dimer, as confirmed by glutaraldehyde cross-linking, leading to an increase in fluorescence at the rhodamine peak emission wavelength, and a measurement of the dimer population. In the second method, protein is co-labeled with fluorescein and TMR, and the Forster resonance energy transfer signal is measured from the dimer complexes. These studies introduce CLC-ec1 reversible dimerization as a simplified model for the thermodynamics of membrane protein folding and TM helix recognition in the membrane environment.

Yielding and shear induced melting of 2D mixed crystals of spheres and dimers

We performed constant stress and constant shear rate non-equilibrium molecular dynamics simulations to study the effects on mechanical properties of introducing varying amounts of dimers (dumbbells) in two-dimensional mixed crystals of monomers (spheres) and dimers. We observe that dislocation cages formed by blocking orientations of dimer particles introduce two opposing effects on the mechanical strength and rheological properties of such systems. Upon increasing the dimer concentration, the number of dislocations and vacancies increases, ‘weakening’ the crystal, but also the size of dislocation cages reduces, reinforcing the crystal by an effect similar to ‘grain-boundary strengthening’. In our constant stress simulations, these competing effects lead to a non-monotonic dependence of yield stress on the dimer fraction wherein it first drops and then increases. In our constant shear simulations, the crystals exhibit a plateau region in the stress vs. strain rate curves right around the yield stress values, confirming a change in dynamical and structural behavior at these points. Further, we find evidence of an unusual transitional phase before the crystal melts as shear increases: for any composition of the crystal an intermediate state is reached where a dynamic balance occurs between the rate of crystal growth (driven by thermodynamics) and the rate of melting (driven by shear). Such an intermediate state (whose onset agrees with the plateau region in the stress vs. strain curves) exhibits a heterogeneous structure and shear banding at a local level, while exhibiting a hexatic-like behavior at the macroscopic level. We also find differences in the solid and hexatic-like phases with respect to their stress relaxation mechanism, with the latter exhibiting a phase lag between the instantaneous stress and the ‘micro-structural yielding’ event.

Isolating Petroleum Pitch Oligomers via Semi-continuous Supercritical Extraction

Dense-gas/supercritical extraction (DGE/SCE) was used for the fractionation of a representative petroleum pitch, M-50, into its oligomeric constituents using toluene as the extractive solvent. A small, pilot-scale column was operated in the semi-continuous/semi-batch mode under a linear positive temperature gradient of 380–330 °C from the top to the bottom of the column and over a pressure range of 15–75 bar. This DGE column was used to produce high-purity monomer, dimer, and trimer fractions of M-50 suitable for use as molecular standards for petroleum pitches and for polycyclic aromatic hydrocarbon (PAH) oligomers. During the experimental runs, matrix-assisted laser desorption/ionization (MALDI) mass spectrometry (MS) was used to monitor in real time the progress of the separation (and oligomeric purity) by performing rapid analyses of the molecular-weight distributions (MWDs) of the overhead fractions being collected. These real-time analyses provided us with the ability to fine-tune in situ the operat...

Assembly and Separation of Semiconductor Quantum Dot Dimers and Trimers

Repeated precipitation of colloidal semiconductor quantum dots (QD) from a good solvent by adding a poor solvent leads to an increasing number of QD oligomers after redispersion in the good solvent. By using density gradient ultracentrifugation we have been able to separate QD monomer, dimer, and trimer fractions from higher oligomers in such solutions. In the corresponding fractions QD dimers and trimers have been enriched up to 90% and 64%, respectively. Besides directly coupled oligomers, QD dimers and trimers were also assembled by linkage with a rigid terrylene diimide dye (TDI) and separated again by ultracentrifugation. High-resolution transmission electron micrographs show that the interparticle distances are clearly larger than those for directly coupled dots proving that the QDs indeed are cross-linked by the dye. Moreover, energy transfer from the QDs to the TDI "bridge" has been observed. Individual oligomers (directly coupled or dye-linked) can be readily deposited on a substrate and studied simultaneously by scanning force and optical microscopy. Our simple and effective scheme is applicable to a wide range of ligand stabilized colloidal nanoparticles and opens the way to a detailed study of electronic coupling in, e.g., QD molecules.

Progress in demonstrating total homochiral selection in montmorillonite-catalyzed RNA synthesis

The Na+-montmorillonite-catalyzed reactions of 5′-phosphorimidazolides of nucleosides generates RNA oligomers. The question arises as to how chiral selectivity was introduced into this biopolymer from a simple chemical system. We have demonstrated homochiral selection in quaternary reactions of a racemic mixture of d,l-ImpA and d,l-ImpU on Na+-montmorillonite. The dimer, trimer, tetramer and pentamer fractions were investigated for homochiral selection. The products were collected via ion exchange HPLC and their terminal 5′-phosphate was cleaved by alkaline phosphatase. These fractions were analyzed by reverse phase HPLC for the identification of homochiral and heterochiral isomers. Encouraged by favorable homochiral excesses of dimer (63.5±0.8%) and trimer (74.3±1.7%), the study was extended to the analysis of higher oligomers. The tetramer and pentamer of the quaternary reaction were separated into 26 and 22 isomers, respectively, on a reverse phase column. Their co-elution with those formed in the binary reactions of d-ImpA and d-ImpU on Na+-montmorillonite revealed 92.7±2.0% and 97.2±0.5% homochirality of the tetramer and pentamer, respectively. These results suggest that Na+-montmorillonite not only catalyzes the prebiotic synthesis of RNA but it also facilitates homochiral selection.