Diffusion and electrophoretic mobility of single-stranded RNA from molecular dynamics simulations

Hydrodynamic properties of small single-stranded RNA homopolymers with three and six nucleotides in free solution are determined from molecular dynamics simulations in explicit solvent. We find that the electrophoretic mobility increases with increasing RNA length, consistent with experiment. Diffusion coefficients of RNA, corrected for finite-size effects and solvent viscosity, agree well with those estimated from experiments and hydrodynamic calculations. The diffusion coefficients and electrophoretic mobilities satisfy a Nernst-Einstein relation in which the effective charge of RNA is reduced by the charge of transiently bound counterions. Fluctuations in the counterion atmosphere are shown to enhance the diffusive spread of RNA molecules drifting along the direction of the external electric field. As a consequence, apparent diffusion coefficients measured by capillary zone electrophoresis can be significantly larger than the actual values at certain experimental conditions.     

Integrating experimental data with molecular simulations to investigate RNA structural dynamics

Conformational dynamics is crucial for ribonucleic acid (RNA) function. Techniques such as nuclear magnetic resonance, cryo-electron microscopy, small- and wide-angle X-ray scattering, chemical probing, single-molecule Förster resonance energy transfer, or even thermal or mechanical denaturation experiments probe RNA dynamics at different time and space resolutions. Their combination with accurate atomistic molecular dynamics (MD) simulations paves the way for quantitative and detailed studies of RNA dynamics. First, experiments provide a quantitative validation tool for MD simulations. Second, available data can be used to refine simulated structural ensembles to match experiments. Finally, comparison with experiments allows for improving MD force fields that are transferable to new systems for which data is not available. Here we review the recent literature and provide our perspective on this field.     

A short guide to long non-coding RNA gene nomenclature

The HUGO Gene Nomenclature Committee (HGNC) is the only organisation authorised to assign standardised nomenclature to human genes. Of the 38,000 approved gene symbols in our database (http://www.genenames.org), the majority represent protein-coding (pc) genes; however, we also name pseudogenes, phenotypic loci, some genomic features, and to date have named more than 8,500 human non-protein coding RNA (ncRNA) genes and ncRNA pseudogenes. We have already established unique names for most of the small ncRNA genes by working with experts for each class. Small ncRNAs can be defined into their respective classes by their shared homology and common function. In contrast, long non-coding RNA (lncRNA) genes represent a disparate set of loci related only by their size, more than 200 bases in length, share no conserved sequence homology, and have variable functions. As with pc genes, wherever possible, lncRNAs are named based on the known function of their product; a short guide is presented herein to help authors when developing novel gene symbols for lncRNAs with characterised function. Researchers must contact the HGNC with their suggestions prior to publication, to check whether the proposed gene symbol can be approved. Although thousands of lncRNAs have been predicted in the human genome, for the vast majority their function remains unresolved. lncRNA genes with no known function are named based on their genomic context. Working with lncRNA researchers, the HGNC aims to provide unique and, wherever possible, meaningful gene symbols to all lncRNA genes.     

Investigating biological systems using first principles Car-Parrinello molecular dynamics simulations

Density functional theory (DFT)-based Car-Parrinello molecular dynamics (CPMD) simulations describe the time evolution of molecular systems without resorting to a predefined potential energy surface. CPMD and hybrid molecular mechanics/CPMD schemes have recently enabled the calculation of redox properties of electron transfer proteins in their complex biological environment. They provided structural and spectroscopic information on novel platinum-based anticancer drugs that target DNA, also setting the basis for the construction of force fields for the metal lesion. Molecular mechanics/CPMD also lead to mechanistic hypotheses for a variety of metalloenzymes. Recent advances that increase the accuracy of DFT and the efficiency of investigating rare events are further expanding the domain of CPMD applications to biomolecules.     

Validation of the mechanism of cholesterol binding by StAR using short molecular dynamics simulations

We previously proposed an original two-state cholesterol binding mechanism by StAR, in which the C-terminal α-helix of StAR gates the access of cholesterol to its binding site cavity. This cavity, which can accommodate one cholesterol molecule, was proposed to promote the reversible unfolding of the C-terminal α-helix and allow for the entry and dissociation of cholesterol. In our molecular model of the cholesterol–StAR complex, the hydrophobic moiety of cholesterol interacts with hydrophobic amino acid side-chains located in the C-terminal α-helix and at the bottom of the cavity. In this study, we present a structural in silico analysis of StAR. Molecular dynamics simulations showed that point mutations of Phe²⁶⁷, Leu²⁷¹ or Leu²⁷⁵ at the α-helix 4 increased the gyration radius (more flexibility) of the protein's structure, whereas the salt bridge double mutant E169M/R188M showed a decrease in flexibility (more compactness). Also, in the latter case, an interaction between Met¹⁶⁹ and Phe²⁶⁷ disrupted the hydrophobic cavity, rendering it impervious to ligand binding. These obtained results are in agreement with previous in vitro experiments, and provide further validation of the two-state binding mode of action.     

Creating realistic silica nanopores for molecular dynamics simulations

We present a detailed approach to create realistic silica pores for computer simulations especially molecular dynamics (MD) simulations. These pores are essential for all different kinds of simulations with liquids in silica confinements. Despite wide use of silica pores in simulations, a detailed documentation how to create these pores for simulations still lacks. This issue is of high significance because with the help of this paper every researcher can build own silica pores with desired geometries and is not stick to already existing pores. We discuss problems that might occur during the whole process and how to solve these problems. So far more than 3 different silica pores have been created with this method and used successfully as confinement material in MD simulations.     

Higher order multipole moments for molecular dynamics simulations

In conventional force fields, the electrostatic potential is represented by atom-centred point charges. This choice is in principle arbitrary, but technically convenient. Point charges can be understood as the first term of multipole expansions, which converge with an increasing number of terms towards the accurate representation of the molecular potential given by the electron density distribution. The use of multipole expansions can therefore improve the force field accuracy. Technically, the implementation of atomic multipoles is more involved than the use of point charges. Important points to consider are the orientation of the multipole moments during the trajectory, conformational dependence of the atomic moments and stability of the simulations which are discussed here.

Implementation of force distribution analysis for molecular dynamics simulations

Background  

The way mechanical stress is distributed inside and propagated by proteins and other biopolymers largely defines their function. Yet, determining the network of interactions propagating internal strain remains a challenge for both, experiment and theory. Based on molecular dynamics simulations, we developed force distribution analysis (FDA), a method that allows visualizing strain propagation in macromolecules.     

Membrane proteins: molecular dynamics simulations

Molecular dynamics simulations of membrane proteins are making rapid progress, because of new high-resolution structures, advances in computer hardware and atomistic simulation algorithms, and the recent introduction of coarse-grained models for membranes and proteins. In addition to several large ion channel simulations, recent studies have explored how individual amino acids interact with the bilayer or snorkel/anchor to the headgroup region, and it has been possible to calculate water/membrane partition free energies. This has resulted in a view of bilayers as being adaptive rather than purely hydrophobic solvents, with important implications, for example, for interaction between lipids and arginines in the charged S4 helix of voltage-gated ion channels. However, several studies indicate that the typical current simulations fall short of exhaustive sampling, and that even simple protein-membrane interactions require at least ca. 1mus to fully sample their dynamics. One new way this is being addressed is coarse-grained models that enable mesoscopic simulations on multi-mus scale. These have been used to model interactions, self-assembly and membrane perturbations induced by proteins. While they cannot replace all-atom simulations, they are a potentially useful technique for initial insertion, placement, and low-resolution refinement.     

A continuum-atomistic method for incorporating Joule heating into classical molecular dynamics simulations

A hybrid atomistic-continuum method is presented for incorporating Joule heating into large-scale molecular dynamics (MD) simulations. When coupled to a continuum thermostat, the method allows resistive heating and heat transport in metals to be modeled without explicitly including electronic degrees of freedom. Atomic kinetic energies in a MD simulation are coupled via an ad hoc feedback loop to continuum current and heat transfer equations that are solved numerically on a finite difference grid (FDG). For resistive heating, the resistance in each region of the FDG is calculated from the experimental resistivity, atomic density, and average kinetic energy in the MD simulation. A network of resistors is established from which the potential at every FDG region is calculated given an applied voltage. The potential differences and the resistance between connected FDG regions are used to calculate the current between the two points and the heat generated from that current. This information is then added back into the atomic simulation. The method is demonstrated by simulating Joule heating and melting, along with associated changes in current, of single and bundles of metal nanowires, as well as a “pinched” wire under applied strain.     

The conformation of dehydroalanine in short homopeptides: molecular dynamics simulations of a 6-residue chain

A molecular dynamics study about the conformational preferences in a chloroform solution of a homo-oligomer constituted by six residues of dehydroalanine is presented. For this purpose, two sets of force-field parameters and explicit solvent molecules have been used. Furthermore, ab initio calculations have been performed in order to estimate 1[H]-NMR chemical shifts. Results have been compared with experimental data.     

Effects of base substitutions in an RNA hairpin from molecular dynamics and free energy simulations

Contributions of individual interactions in the GGCGCAAGCC hairpin containing a GCAA tetraloop were studied by computer simulations using base substitutions. The G in the first tetraloop position was replaced by inosine (I) or adenosine (A), and the G in the C-G basepair closing the tetraloop was replaced by I. These substitutions eliminate particular hydrogen bonds proposed in the nuclear magnetic resonance model of the GCAA tetraloop. Molecular dynamics simulations of the GCAA tetraloop in aqueous solvent displayed a well-defined hydrogen pattern between the first and last loop nucleotides (G and A) stabilized by a bridging water molecule. Substitution of G-->I in the basepair closing the tetraloop did not significantly influence the loop structure and dynamics. The ICAA loop maintained the overall structure, but displayed variation in the hydrogen-bond network within the tetraloop itself. Molecular dynamics simulations of the ACAA loop led to conformational heterogeneity of the resulting structures. Changes of hairpin formation free energy associated with substitutions of individual bases were calculated by the free energy perturbation method. The calculated decrease of the hairpin stability upon G-->I substitution in the C-G basepair closing the tetraloop was in good agreement with experimental thermodynamic data. Our theoretical estimates for G-->I and G-->A mutations located in the tetraloop suggest larger loop destabilization than corresponding experimental results. The extent of conformational sampling of the structures resulting from base substitutions and its impact on the calculated free energy was discussed.     

OmpA: Gating and dynamics via molecular dynamics simulations

Outer membrane proteins (OMPs) of Gram-negative bacteria have a variety of functions including passive transport, active transport, catalysis, pathogenesis and signal transduction. Whilst the structures of ∼ 25 OMPs are currently known, there is relatively little known about their dynamics in different environments. The outer membrane protein, OmpA from Escherichia coli has been studied extensively in different environments both experimentally and computationally, and thus provides an ideal test case for the study of the dynamics and environmental interactions of outer membrane proteins. We review molecular dynamics simulations of OmpA and its homologues in a variety of different environments and discuss possible mechanisms of pore gating. The transmembrane domain of E. coli OmpA shows subtle differences in dynamics and interactions between a detergent micelle and a lipid bilayer environment. Simulations of the crystallographic unit cell reveal a micelle-like network of detergent molecules interacting with the protein monomers. Simulation and modelling studies emphasise the role of an electrostatic-switch mechanism in the pore-gating mechanism. Simulation studies have been extended to comparative models of OmpA homologues from Pseudomonas aeruginosa (OprF) and Pasteurella multocida (PmOmpA), the latter model including the periplasmic C-terminal domain.     

A statistical approach to the interpretation of molecular dynamics simulations of calmodulin equilibrium dynamics

A sample of 35 independent molecular dynamics (MD) simulations of calmodulin (CaM) equilibrium dynamics was prepared from different but equally plausible initial conditions (20 simulations of the wild-type protein and 15 simulations of the D129N mutant). CaM's radius of gyration and backbone mean-square fluctuations were analyzed for the effect of the D129N mutation, and simulations were compared with experiments. Statistical tests were employed for quantitative comparisons at the desired error level. The computational model predicted statistically significant compaction of CaM relative to the crystal structure, consistent with the results of small-angle X-ray scattering (SAXS) experiments. This effect was not observed in several previously reported studies of (Ca2+)(4)-CaM, which relied on a single MD run. In contrast to radius of gyration, backbone mean-square fluctuations showed a distinctly non-normal and positively skewed distribution for nearly all residues. Furthermore, the D129N mutation affected the backbone dynamics in a complex manner and reduced the mobility of Glu123, Met124, Ile125, Arg126, and Glu127 located in the adjacent alpha-helix G. The implications of these observations for the comparisons of MD simulations with experiments are discussed. The proposed approach may be useful in studies of protein equilibrium dynamics where MD simulations fall short of properly sampling the conformational space, and when the comparison with experiments is affected by the reproducibility of the computational model.     

The implementation of slab geometry for membrane-channel molecular dynamics simulations

Slab geometric boundary conditions are applied in the molecular dynamics simulation of a simple membrane-channel system. The results of the simulation were compared to those of an analogous system using normal three-dimensional periodic boundary conditions. Analysis of the dynamics and electrostatics of the system show that slab geometric periodicity eliminates the artificial bulk water orientational polarization that is present while using normal three-dimensional periodicity. Furthermore, even though the water occupancy and volume of our simple channel is the same when using either method, the electrostatic properties are considerably different when using slab geometry. In particular, the orientational polarization of water is seen to be different in the interior of the channel. This gives rise to a markedly different electric field within the channel. We discuss the implications of slab geometry for the future simulation of this type of system and for the study of channel transport properties.     

Improved molecular dynamics simulations for the determination of peptide structures

In this article a few methods or modifications proven to be useful in the conformational examination of peptides and related molecules by molecular dynamics are illustrated. The first is the explicit use of organic solvents in the simulations. For many cases such solvents are appropriate since the nmr measurements (or other experimental observations) were carried out in the same solvent. Here, the use of dimethylsulfoxide and chloroform in molecular dynamics is described, with some advantages of the use of these solvents highlighted. A constant allowing for the scaling of the nonbonded interactions of the force field, an idea previously employed in distance geometry and simulated annealing, has been implemented. The usefulness of this method is that when the nonbonded term is turned to zero, atoms can pass through each other, while the connectivity of the molecule is maintained. It will be shown that such simulations, if a sufficient driving force is present (i.e., nuclear Overhauser effects restraints), can produce the correct stereoconfiguration (i.e., chiral center) as well as configurational isomer (i.e., cis/trans isomers). Lastly, a penalty term for coupling constants directly related to the Karplus curve has been plemented into the potential energy force field. The advantages of this method over the commonly used dihedral angle restraining are discussed. In particular, it is shown that with more than one coupling constant about a dihedral angle a great reduction of the allowed conformational space is obtained. © 1993 John Wiley & Sons, Inc.     

Protocol for MM/PBSA molecular dynamics simulations of proteins

Continuum solvent models have been employed in past years for understanding processes such as protein folding or biomolecular association. In the last decade, several attempts have been made to merge atomic detail molecular dynamics simulations with solvent continuum models. Among continuum models, the Poisson-Boltzmann solvent accessible surface area model is one of the oldest and most fundamental. Notwithstanding its wide usage for simulation of biomolecular electrostatic potential, the Poisson-Boltzmann equation has been very seldom used to obtain solvation forces for molecular dynamics simulation. We propose here a fast and reliable methodology to implement continuum forces in standard molecular mechanics and dynamics algorithms. Results for a totally unrestrained 1 ns molecular dynamics simulation of a small protein are quantitatively similar to results obtained by explicit solvent molecular dynamics simulations.     

E. coli RNase HI and the phosphonate-DNA/RNA hybrid: molecular dynamics simulations

A model for the complex between E. coli RNase HI and the DNA/RNA hybrid (previously refined by molecular dynamics simulations) was used to determine the impact of the internucleotide linkage modifications (either 3-O-CH2-P-O-5' or 3-O-P-CH2-O-5) on the ability of the modified-DNA/RNA hybrid to create a complex with the protein. Modified internucleotide linkages were incorporated systematically at different positions close to the 3-end of the DNA strand to interfere with the DNA binding site of RNase H. Altogether, six trajectories were produced (length 1.5ns). Mutual hydrogen bonds connecting both strands of the nucleic acids hybrid, DNA with RNase H, RNA with RNase H, and the scissile bond with the Mg++. 4H2O chelate complex (bound in the active site) were analyzed in detaiL Many residues were involved in binding of the DNA (Arg88, Asn84, Trp85, Trp104, Tyr73, Lys99, Asn100, Thr43, and Asn 16) and RNA (Gln76, Gln72, Tyr73, Lys122, Glu48, Asn44, and Cys13) strand to the substrate-binding site of the RNase H enzyme. The most remarkable disturbance of the hydrogen bonding net was observed for structures with modified internucleotide linkages positioned in a way to interact with the Trp104, Tyr73, Lys99, and Asn100 residues (situated in the middle of the DNA binding site, where a cluster of Trp residues forms a rigid core of the protein structure).