Multiscale Computational Study of Cellulose Acetate-Water Miscibility: Insights from Molecularly Informed Field-Theoretic Modeling.

Cellulose acetate (CA), a prominent water-soluble derivative of cellulose, is a promising biodegradable ingredient that has applications in films, membranes, fibers, drug delivery, and more. In this work, we present a molecularly informed field-theoretic model for CA to explore its phase behavior in aqueous solutions. By integrating atomistic details into large-scale field-theoretic simulations via the relative entropy coarse-graining framework, our approach enables efficient calculations of CA's miscibility window as a function of the degree of substitution (DS) of cellulose hydroxyl groups with acetate side chains. This allows us to capture the intricate phase behavior of CA, particularly its unique miscibility at intermediate substitution, without relying on experimental input. Additionally, the model directly probes CA solution behavior specific to the relative DS at C2, C3, and C6 alcohol sites, providing insights for the rational design of water-soluble CA for diverse applications. This work demonstrates a promising integration of molecularly informed field theories, complementing wet-lab experimentation, for engineering the next-generation polymeric materials with precisely tailored properties.

Structural insights into cognate versus near-cognate discrimination during decoding.

The structural basis of the tRNA selection process is investigated by cryo-electron microscopy of ribosomes programmed with UGA codons and incubated with ternary complex (TC) containing the near-cognate Trp-tRNA(Trp) in the presence of kirromycin. Going through more than 350 000 images and employing image classification procedures, we find ∼8% in which the TC is bound to the ribosome. The reconstructed 3D map provides a means to characterize the arrangement of the near-cognate aa-tRNA with respect to elongation factor Tu (EF-Tu) and the ribosome, as well as the domain movements of the ribosome. One of the interesting findings is that near-cognate tRNA's acceptor stem region is flexible and CCA end becomes disordered. The data bring direct structural insights into the induced-fit mechanism of decoding by the ribosome, as the analysis of the interactions between small and large ribosomal subunit, aa-tRNA and EF-Tu and comparison with the cognate case (UGG codon) offers clues on how the conformational signals conveyed to the GTPase differ in the two cases.

Structural characterization of mRNA-tRNA translocation intermediates.

Cryo-EM analysis of a wild-type Escherichia coli pretranslocational sample has revealed the presence of previously unseen intermediate substates of the bacterial ribosome during the first phase of translocation, characterized by intermediate intersubunit rotations, L1 stalk positions, and tRNA configurations. Furthermore, we describe the domain rearrangements in quantitative terms, which has allowed us to characterize the processivity and coordination of the conformational reorganization of the ribosome, along with the associated changes in tRNA ribosome-binding configuration. The results are consistent with the view of the ribosome as a molecular machine employing Brownian motion to reach a functionally productive state via a series of substates with incremental changes in conformation.

Ordered liquids and hydrogels from alkenyl succinic ester terminated bola-amphiphiles for large-scale applications.

The present study describes an economic and scalable approach to aqueous mesophases from bola-amphiphiles (BA) obtained via nucleophilic addition of dimer fatty acid based α,ω-polyesterdiols (PES) on cyclic acid anhydrides and conversion of the carboxylic end groups into ammonium salts. Novel bola-amphiphilic head groups are introduced using alkenyl succinic anhydrides (ASA). The additional terminal hydrophobic side chains favour the self-assembly of polymeric BA of different molecular weights into nanoscale anisotropic objects, their shape and ordering into nematic or lamellar-like phases being dependent on the length and structural uniformity of the ASA chains. Corresponding diester based on C15 (hydrogenated bisphenol-A, HBA) and C8 (1,4-cyclohexanedimethanol, CHDM) spacers have been prepared and the self-assembly of the resulting BA in water has been studied using SAXS, (2)H-NMR and optical polarization microscopy. While the rigid C8 spacer impedes any ordering, ASA capped C15 tends to form ordered hydrogels over extended regions of the phase diagram that resemble mesh phases and L(α)/L(3) polymorphism. Rheological and simulation results confirm the presence of elastically responding bicontinuous morphologies and biased porous assemblies resembling interconnected mesh phases. Both the use of the dimer fatty acid based spacer as well as of ASA head groups open up large-scale applications of ordered liquids (or hydrogels) as a formulation basis for e.g. films, coatings and adhesives.

Phage Display Screening as a Rational Approach to Design Additives for Selective Crystallization Control in Construction Systems.

The design of additives showing strong and selective interactions with certain target surfaces is key to crystallization control in applied reactive multicomponent systems. While suitable chemical motifs can be found through semi-empirical trial-and-error procedures, bioinspired selection techniques offer a more rationally driven approach and explore a much larger space of possible combinations in a single assay. Here, phage display screening is used to characterize the surfaces of crystalline gypsum, a mineral of broad relevance for construction applications. Based on next-generation sequencing of phages enriched during the screening process, a triplet of amino acids, DYH, is identified as the main driver for adsorption on the mineral substrate. Furthermore, oligopeptides containing this motif prove to exert their influence in a strictly selective manner during the hydration of cement, where the sulfate reaction (initial setting) is strongly retarded while the silicate reaction (final hardening) remains unaffected. In the final step, these desired additive characteristics are successfully translated from the level of peptides to that of scalable synthetic copolymers. The approach described in this work demonstrates how modern biotechnological methods can be leveraged for the systematic development of efficient crystallization additives for materials science.

Mechanisms of SecM-mediated stalling in the ribosome.

Nascent-peptide modulation of translation is a common regulatory mechanism of gene expression. In this mechanism, while the nascent peptide is still in the exit tunnel of the ribosome, it induces translational pausing, thereby controlling the expression of downstream genes. One example is SecM, which inhibits peptide-bond formation in the ribosome's peptidyl transferase center (PTC) during its own translation, upregulating the expression of the protein translocase SecA. Although biochemical experiments and cryo-electron microscopy data have led to the identification of some residues involved in SecM recognition, the full pathway of interacting residues that connect SecM to the PTC through the ribosome has not yet been conclusively established. Here, using the cryo-electron microscopy data, we derived the first (to our knowledge) atomic model of the SecM-stalled ribosome via molecular-dynamics flexible fitting, complete with P- and A-site tRNAs. Subsequently, we carried out simulations of native and mutated SecM-stalled ribosomes to investigate possible interaction pathways between a critical SecM residue, R163, and the PTC. In particular, the simulations reveal the role of SecM in altering the position of the tRNAs in the ribosome, and thus demonstrate how the presence of SecM in the exit tunnel induces stalling. Finally, steered molecular-dynamics simulations in which SecM was pulled toward the tunnel exit suggest how SecA interacting with SecM from outside the ribosome relieves stalling.

Stereochemical errors and their implications for molecular dynamics simulations.

BACKGROUND: Biological molecules are often asymmetric with respect to stereochemistry, and correct stereochemistry is essential to their function. Molecular dynamics simulations of biomolecules have increasingly become an integral part of biophysical research. However, stereochemical errors in biomolecular structures can have a dramatic impact on the results of simulations. RESULTS: Here we illustrate the effects that chirality and peptide bond configuration flips may have on the secondary structure of proteins throughout a simulation. We also analyze the most common sources of stereochemical errors in biomolecular structures and present software tools to identify, correct, and prevent stereochemical errors in molecular dynamics simulations of biomolecules. CONCLUSIONS: Use of the tools presented here should become a standard step in the preparation of biomolecular simulations and in the generation of predicted structural models for proteins and nucleic acids.

Applications of the molecular dynamics flexible fitting method.

In recent years, cryo-electron microscopy (cryo-EM) has established itself as a key method in structural biology, permitting the structural characterization of large biomolecular complexes in various functional states. The data obtained through single-particle cryo-EM has recently seen a leap in resolution thanks to landmark advances in experimental and computational techniques, resulting in sub-nanometer resolution structures being obtained routinely. The remaining gap between these data and revealing the mechanisms of molecular function can be closed through hybrid modeling tools that incorporate known atomic structures into the cryo-EM data. One such tool, molecular dynamics flexible fitting (MDFF), uses molecular dynamics simulations to combine structures from X-ray crystallography with cryo-EM density maps to derive atomic models of large biomolecular complexes. The structures furnished by MDFF can be used subsequently in computational investigations aimed at revealing the dynamics of the complexes under study. In the present work, recent applications of MDFF are presented, including the interpretation of cryo-EM data of the ribosome at different stages of translation and the structure of a membrane-curvature-inducing photosynthetic complex.

Peptide synthesis in aqueous environments: the role of extreme conditions and pyrite mineral surfaces on formation and hydrolysis of peptides.

A comprehensive study of free energy landscapes and mechanisms of COS-mediated polymerization of glycine via N-carboxy anhydrides (NCAs, "Leuchs anhydrides") and peptide hydrolysis at the water-pyrite interface at extreme thermodynamic conditions is presented. Particular emphasis is set on the catalytic effects of the mineral surface including the putative role of the ubiquitous sulfur vacancy defects. It is found that the mere presence of a surface is able to change the free energetics of the elementary reaction steps. This effect can be understood in terms of a reduction of entropic contributions to the reactant state by immobilizing the reactants and/or screening them from bulk water in a purely geometric ("steric") sense. Additionally, the pyrite directly participates chemically in some of the reaction steps, thus changing the reaction mechanism qualitatively compared to the situation in bulk water. First, the adsorption of reactants on the surface can preform a product-like structure due to immobilizing and scaffolding them appropriately. Second, pyrite can act as a proton acceptor, thus replacing water in this role. Third, sulfur vacancies are found to increase the reactivity of the surface. The finding that the presence of pyrite speeds up the rate-determining step in the formation of peptides with respect to the situation in bulk solvent while stabilizing the produced peptide against hydrolysis is of particular interest to the hypothesis of prebiotic peptide formation at hydrothermal aqueous conditions. Apart from these implications, the generality of the studied organic reactions are of immediate relevance to many fields such as (bio)geochemistry, biomineralization, and environmental chemistry.

Closed-Form Coexistence Equation for Phase Separation of Polymeric Mixtures in Dissipative Particle Dynamics.

To date, no extensive study of the phase diagram for binary fluid mixtures in dissipative particle dynamics (DPD) has been published. This is especially pertinent for newer parameterization schemes where the self-self interaction, or the effective volume, of different particle types is varied. This work presents an exhaustive study of the parameter space concerning DPD particles with soft interaction potentials. Moreover, we propose a closed-form coexistence equation or binodal curve that is inspired by the Flory-Huggins model. This equation describes the phase diagram of all binary mixtures made up out of monomers, homopolymers, and the mixtures thereof when self-self interactions are varied. The mean absolute percentage error (MAPE) of the equation on simulated data, including validation simulations, is 1.02%. The equation can a priori predict the phase separation of mixtures using only DPD interaction parameters. The proposed coexistence equation can therefore be used to directly validate interaction parameters resulting from novel parameterization schemes, including coarse graining and equations of state, without the need for additional simulations. Finally, it is shown that the choice of bond potential markedly influences phase behavior.

Molecular dynamics flexible fitting: a practical guide to combine cryo-electron microscopy and X-ray crystallography.

Hybrid computational methods for combining structural data from different sources and resolutions are becoming an essential part of structural biology, especially as the field moves toward the study of large macromolecular assemblies. We have developed the molecular dynamics flexible fitting (MDFF) method for combining high-resolution atomic structures with cryo-electron microscopy (cryo-EM) maps, that results in atomic models representing the conformational state captured by cryo-EM. The method has been applied successfully to the ribosome, a ribonucleoprotein complex responsible for protein synthesis. MDFF involves a molecular dynamics simulation in which a guiding potential, based on the cryo-EM map, is added to the standard force field. Forces proportional to the gradient of the density map guide an atomic structure, available from X-ray crystallography, into high-density regions of a cryo-EM map. In this paper we describe the necessary steps to set up, run, and analyze MDFF simulations and the software packages that implement the corresponding functionalities.

Dynamical magnetostructural properties of Anabaena ferredoxin.

A mixed quantum/classical investigation of the dynamical magnetostructural properties, that is, "magnetodynamics," of oxidized Anabaena PCC7119 ferredoxin is carried out at room temperature in two distinct conformational states. This protein hosts a [2Fe-2S] cluster in which two iron centers are antiferromagnetically coupled to an overall low-spin electronic ground state that has a genuine multireference character. To study the magnetodynamics of this prosthetic group, an approximate spin projection method is formulated in the framework of density functional theory that allows for multideterminant ab initio molecular dynamics simulations to be carried out efficiently. By using this scheme, the influence of both thermal fluctuations and conformational motion on the structure of the [2Fe-2S] cluster and on the dynamics of the antiferromagnetic coupling constant, J(t), has been investigated. In addition to demonstrating how sensitively the shape of the [2Fe-2S] core itself is affected by hydrogen bonding, the analyses reveal a complex dynamical coupling of J to both local vibrations and large-amplitude motion. It is shown that this interplay can be understood in terms of specific vibrational modes and distinct hydrogen-bonding patterns between the iron-sulfur cluster and the protein backbone, respectively. This implies going beyond the Goodenough-Kanamori rules for angular magnetostructural correlations of oxidized iron-sulfur prosthetic groups.

Peptide synthesis in aqueous environments: the role of extreme conditions on peptide bond formation and peptide hydrolysis.

The mechanisms and free energetics underlying the formation of peptides from alpha-amino acids and alpha-amino acid N-carboxyanhydrides (NCAs) in bulk water at both ambient and extreme temperature and pressure conditions were investigated using accelerated ab initio molecular dynamics. In particular, peptide bond formation using an activated amino acid in form of its NCA, subsequent decarboxylation, as well as hydrolysis of the formed peptide were studied using glycine. It is shown to what extent thermodynamic conditions affect the reaction mechanisms qualitatively and the energetics quantitatively in solution. In particular, the zwitterionic intermediate in the peptidization step found in ambient water degenerates into a transient species in hot-pressurized water, whereas the hydrolysis reaction is found to follow qualitatively different pathways at ambient and extreme conditions. The work also quantifies the impact of extreme solvent conditions on both peptide bond formation and peptide hydrolysis in aqueous media. Beyond the specific case, the results provide important insights into how elevated temperatures and increased pressures affect organic reactions in aqueous solutions.

Peptide synthesis in aqueous environments: the role of extreme conditions on amino acid activation.

The free energy surfaces and reaction mechanisms underlying the activation of amino acids by COS in bulk water at ambient conditions as well as extreme temperature-pressure thermodynamic conditions were studied using accelerated ab initio molecular dynamics. The results for the reaction sequence leading from glycine to its activated form, a so-called Leuchs anhydride or alpha-amino acid N-carboxyanhydride (NCA), suggest that extreme conditions not far from the critical point of water may favor the formation of this activated species. This is traced back to appropriately affecting relative stabilities of neutral versus charged or zwitterionic molecular species which shifts equilibria, affects relative barriers, and thus modifies reaction rates. Furthermore, it is shown that the N-carboxyanhydride of glycine is not formed from N-thiocarboxyl glycine by its direct cyclization, but instead an indirect mechanism, the so-called isocyanate route, is clearly preferred at both conditions. The work quantitatively underpins the impact of extreme solvent conditions on the investigated organic reactions in aqueous media which implies that the presented results are of relevance to fields such as prebiotic chemistry and green chemistry.

Influence of extreme thermodynamic conditions and pyrite surfaces on peptide synthesis in aqueous media.

Free energy landscapes and reaction mechanisms underlying the synthesis of diglycine in water were studied computationally. It was found that amino acid activation by carbonyl sulfide, leading to the formation of a cyclic alpha-amino acid N-carboxyanhydride (NCA, or Leuchs anhydride), preferentially follows an indirect pathway that involves an isocyanate intermediate. Extreme temperature and pressure conditions accelerate peptidization greatly compared to the ambient bulk water environment and are shown to favor, in general, concerted versus stepwise mechanisms. Finally, a pyrite surface, FeS2 (001), is found to lower reaction barriers further by decreasing fluctuations and by assisting the preformation of the cyclic five-membered NCA ring due to scaffolding.

Conformational dynamics of minimal elastin-like polypeptides: the role of proline revealed by molecular dynamics and nuclear magnetic resonance.

Previous molecular dynamics studies of the elastin-like peptide (ELP) GVG(VPGVG) predict that this ELP undergoes a conformational transition from an open to a more compact closed state upon an increase in temperature. These structural changes occurring in this minimal elastin model at the so-called inverse temperature transition (ITT), which takes place when elastin is heated to temperatures of about 20-40 (omicron)C, are investigated further in this work by means of a combined theoretical and experimental approach. To do this, additional extensive classical molecular dynamics (MD) simulations of the capped octapeptide are carried out, analyzed, and compared to data obtained from homonuclear magnetic resonance (NMR) spectroscopy of the same octapeptide. Moreover, in the previous simulations, the proline residue in the ELP is found to act as a hinge, thereby allowing for the large-amplitude opening and closing conformational motion of the ITT. To explore the role of proline in such elastin repeating units, a point mutant (P5I), which replaces the proline residue with an isoleucine residue, is also investigated using the aforementioned theoretical and experimental techniques. The results show that the site-directed mutation completely alters the properties of this ELP, thus confirming the importance of the highly conserved proline residue in the ITT. Furthermore, a correlation between the two different methods employed is seen. Both methods predict the mutant ELP to be present in an unstructured form and the wild type ELP to have a beta-turn-like structure. Finally, the role of the peptidyl cis to trans isomerization of the proline hinge is assessed in detail.

Inverse temperature transition of a biomimetic elastin model: reactive flux analysis of folding/unfolding and its coupling to solvent dielectric relaxation.

The inverse temperature transition (ITT) of a biomimetic model for elastin, capped GVG(VPGVG) in liquid water, is investigated by a comprehensive classical molecular dynamics study. The temperature dependence of the solvation structure and dynamics of the octapeptide are compared using three common force fields, CHARMM, GROMOS, and OPLS. While these force fields differ in quantitative detail, they all predict this octapeptide to undergo a "folding transition" to closed conformations upon heating and a subsequent "unfolding transition" to open conformations at still higher temperatures, thus reproducing the ITT scenario. The peptide kinetics is analyzed within the reactive flux formalism applied to the largest-amplitude mode extracted from principal component analysis, and the solvent's dielectric fluctuations are obtained from the total water dipole autocorrelations. Most importantly, preliminary evidence for an intimate coupling of peptide folding/unfolding dynamics, and thus the ITT, and dielectric relaxation of bulk water is given, possibly being consistent with a "slave mode" picture.

Multiscale Materials Modeling in an Industrial Environment.

In this review, we sketch the materials modeling process in industry. We show that predictive and fast modeling is a prerequisite for successful participation in research and development processes in the chemical industry. Stable and highly automated workflows suitable for handling complex systems are a must. In particular, we review approaches to build and parameterize soft matter systems. By satisfying these prerequisites, efficiency for the development of new materials can be significantly improved, as exemplified here for formulation polymer development. This is in fact in line with recent Materials Genome Initiative efforts sponsored by the US government. Valuable contributions to product development are possible today by combining existing modeling techniques in an intelligent fashion, provided modeling and experiment work hand in hand.

Cryo-electron microscopy modeling by the molecular dynamics flexible fitting method.

The increasing power and popularity of cryo-electron microscopy (cryo-EM) in structural biology brought about the development of so-called hybrid methods, which permit the interpretation of cryo-EM density maps beyond their nominal resolution in terms of atomic models. The Cryo-EM Modeling Challenge 2010 is the first community effort to bring together developers of hybrid methods as well as cryo-EM experimentalists. Participating in the challenge, the molecular dynamics flexible fitting (MDFF) method was applied to a number of cryo-EM density maps. The results are described here with special emphasis on the use of symmetry-based restraints to improve the quality of atomic models derived from density maps of symmetric complexes; on a comparison of the stereochemical quality of atomic models resulting from different hybrid methods; and on application of MDFF to electron crystallography data.

Recognition of the regulatory nascent chain TnaC by the ribosome.

Regulatory nascent chains interact with the ribosomal exit tunnel and modulate their own translation. To characterize nascent chain recognition by the ribosome at the atomic level, extensive molecular dynamics simulations of TnaC, the leader peptide of the tryptophanase operon, inside the exit tunnel were performed for an aggregate time of 2.1 mus. The simulations, complemented by quantum chemistry calculations, suggest that the critical TnaC residue W12 is recognized by the ribosome via a cation-pi interaction, whereas TnaC's D16 forms salt bridges with ribosomal proteins. The simulations also show that TnaC-mediated translational arrest does not involve a swinging of ribosomal protein L22, as previously proposed. Furthermore, bioinformatic analyses and simulations suggest nascent chain elements that may prevent translational arrest in various organisms. Altogether, the current study unveils atomic-detail interactions that explain the role of elements of TnaC and the ribosome essential for translational arrest.