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Michael Bachmann

Publications and source records attributed to Michael Bachmann.

At least 73 records · Page 4Linked to original sources

Thermodynamics and Kinetics of a Go Proteinlike Heteropolymer Model with Two-State Folding Characteristics

We present results of Monte Carlo computer simulations of a coarse-grained hydrophobic-polar Go-like heteropolymer model and discuss thermodynamic properties and kinetics of an exemplified heteropolymer, exhibiting two-state folding behavior. It turns out that general, characteristic folding features of realistic proteins with a single free-energy barrier can also be observed in this simplified model, where the folding transition is primarily driven by the hydrophobic force.

cond-mat.soft↗

Thermodynamics of Peptide Aggregation Processes. An Analysis from Perspectives of Three Statistical Ensembles

We employ a mesoscopic model for studying aggregation processes of protein-like hydrophobic-polar heteropolymers. By means of multicanonical Monte Carlo computer simulations, we find strong indications that peptide aggregation is a phase separation process, in which the microcanonical entropy exhibits a convex intruder due to nonnegligible surface effects of the small systems. We analyze thermodynamic properties of the conformational transitions accompanying the aggregation process from the multicanonical, canonical, and microcanonical perspective. It turns out that the microcanonical description is particularly advantageous as it allows for unraveling details of the phase-separation transition in the thermodynamic region, where the temperature is not a suitable external control parameter anymore.

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Differences in Solution Behavior among Four Semiconductor-Binding Peptides

Recent experiments have identified peptides with adhesion affinity for GaAs and Si surfaces. Here we use all-atom Monte Carlo (MC) simulations with implicit solvent to investigate the behavior in aqueous solution of four such peptides, all with 12 residues. At room temperature, we find that all the four peptides are largely unstructured, which is consistent with experimental data. At the same time, we find that one of the peptides is structurally different and more flexible, compared to the others. This finding points at structural differences as a possible explanation for differences in adhesion properties between these peptides. By also analyzing designed mutants of two of the peptides, an experimental test of this hypothesis is proposed.

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Phase Separation in Peptide Aggregation Processes - Multicanonical Study of a Mesoscopic Model

We have performed multicanonical computer simulations of a small system of short protein-like heteropolymers and found that their aggregation transition possesses similarities to first-order phase separation processes. Not being a phase transition in the thermodynamic sense, the observed folding-binding behavior exhibits fascinating features leading to the conclusion that the temperature is no suitable control parameter in the transition region. More formally, for such small systems the microcanonical interpretation is more favorable than the typically used canonical picture.

cond-mat.soft↗

Thermodynamics of Protein Folding from Coarse-Grained Models' Perspectives

Folding and aggregation of proteins, the interaction between proteins and membranes, as well as the adsorption of organic soft matter to inorganic solid substrates belong to the most interesting challenges in understanding structure and function of complex macromolecules. This is reasoned by the interdisciplinary character of the associated questions ranging from the molecular origin of the loss of biological functionality as, for example, in Alzheimer's disease to the development of organic circuits for biosensory applications. In this lecture, we focus on the analysis of mesoscopic models for protein folding, aggregation, and hybrid systems of soft and solid condensed matter. The simplicity of the coarse-grained models allows for a more universal description of the notoriously difficult problem of protein folding. In this approach, classifications of structure formation processes with respect to the conformational pseudophases are possible. This is similar in aggregation and adsorption processes, where the individual folding propensity is influenced by external forces. The main problem in studies of conformational transitions is that the sequences of amino acids proteins are built up of are necessarily of finite length and, therefore, a thermodynamic limit does not exist. Thus, structural transitions are not phase transitions in the strict thermodynamic sense and the analysis of pseudouniversal aspects is intricate, as apparently small-system effects accompany all conformational transitions and cannot be neglected.

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Identification of Characteristic Protein Folding Channels in a Coarse-Grained Hydrophobic-Polar Peptide Model

Folding channels and free-energy landscapes of hydrophobic-polar heteropolymers are discussed on the basis of a minimalistic off-lattice coarse-grained model. We investigate how rearrangements of hydrophobic and polar monomers in a heteropolymer sequence lead to completely different folding behaviors. Studying three exemplified sequences with the same content of hydrophobic and polar residues, we can reproduce within this simple model two-state folding, folding through intermediates, as well as metastability.

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Freezing and Collapse of Flexible Polymers on Regular Lattices in Three Dimensions

We analyze the crystallization and collapse transition of a simple model for flexible polymer chains on simple cubic and face-centered cubic lattices by means of sophisticated chain-growth methods. In contrast to bond-fluctuation polymer models in certain parameter ranges, where these two conformational transitions were found to merge in the thermodynamic limit, we conclude from our results that the two transitions remain well-separated in the limit of infinite chain lengths. The reason for this qualitatively distinct behavior is presumably due to the ultrashort attractive interaction range in the lattice models considered here.

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Specific Adhesion of Peptides on Semiconductor Surfaces in Experiment and Simulation

We report on self-assembly, clustering, and conformational phases of peptides on inorganic semiconductor surfaces. The peptide-covered surface fraction can differ by a factor of 25, depending mainly on surface and peptide polarity. Low adhesion induces large and soft clusters, which also have high contact angles to the surface. Direct surface adhesion of a peptide molecule competes with forming molecular aggregates which offer an overall reduced surface contact. Simulating a simple hybrid model yields a pseudophase diagram with a rich, temperature and solvent-quality dependent variety of subphases which are specific to the hydrophobicity and polarity of the considered substrates.

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Structural Properties of Small Semiconductor-Binding Synthetic Peptides

We have performed exhaustive multicanonical Monte Carlo simulations of three 12-residue synthetic peptides in order to investigate the thermodynamic and structural properties as well as the characteristic helix-coil transitions. In these studies, we employ a realistic model where the interactions between all atoms are taken into account. Effects of solvation are also simulated by using an implicit solvent model.

cond-mat.soft↗

Microcanonical Analyses of Peptide Aggregation Processes

We propose the use of microcanonical analyses for numerical studies of peptide aggregation transitions. Performing multicanonical Monte Carlo simulations of a simple hydrophobic-polar continuum model for interacting heteropolymers of finite length, we find that the microcanonical entropy behaves convex in the transition region, leading to a negative microcanonical specific heat. As this effect is also seen in first-order-like transitions of other finite systems, our results provide clear evidence for recent hints that the characterisation of phase separation in first-order-like transitions of finite systems profits from this microcanonical view.

cond-mat.soft↗

Two-State Folding, Folding through Intermediates, and Metastability in a Minimalistic Hydrophobic-Polar Model for Proteins

Within the frame of an effective, coarse-grained hydrophobic-polar protein model, we employ multicanonical Monte Carlo simulations to investigate free-energy landscapes and folding channels of exemplified heteropolymer sequences, which are permutations of each other. Despite the simplicity of the model, the knowledge of the free-energy landscape in dependence of a suitable system order parameter enables us to reveal complex folding characteristics known from real bioproteins and synthetic peptides, such as two-state folding, folding through weakly stable intermediates, and glassy metastability.

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Minimalistic Hybrid Models for the Adsorption of Polymers and Peptides to Solid Substrates

We have performed chain-growth simulations of minimalistic hybrid lattice models for polymers interacting with interfaces of attractive solid substrates in order to gain insights into the conformational transitions of the polymers in the adsorption process. Primarily focusing on the dependence of the conformational behavior on temperature and solubility we obtained pseudophase diagrams with a detailed structure of conformational subphases. In the study of hydrophobic-polar peptides in the vicinity of different types of substrates, we found a noticeable substrate specificity of the assembly of hydrophobic domains in the conformations dominating the adsorption subphases.

cond-mat.soft↗

Chain-Growth Simulations of Lattice-Peptide Adsorption to Attractive Substrates

Based on a newly developed contact-density chain-growth algorithm, we have simulated a nongrafted peptide in the vicinity of different attractive substrates. We analyzed the specificity of the peptide adsorption by focussing on the conformational transitions the peptide experiences in the binding/unbinding processes. In a single simulation run, we obtained the contact density, i.e., the distribution of intrinsic monomer-monomer contacts and monomer-substrate nearest-neighbor contacts. This allows a systematic reweighting to all values of external control parameters such as temperature and solvent quality after the simulation. The main result is the complete solubility-temperature pseudo-phase diagram which is based on the corresponding specific-heat profile. We find a surprisingly rich structure of pseudo-phases that can roughly be classified into compact and expanded conformations in both regimes, adsorption and desorption. Furthermore, underlying subphases were identified, which, in particular, appear noticeably in the compact pseudo-phases.

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Reduction to the Simplest - The Complexity of Minimalistic Heteropolymer Models

Simple coarse-grained hydrophobic-polar models for heteropolymers as the lattice HP and the off-lattice AB model allow a general classification of characteristic behaviors for hydrophobic-core based tertiary folding. The strongly reduced computational efforts enable one to reveal systematically the thermodynamic properties of comparatively long sequences in a wide temperature range of conformational activity. Based on a suitable cooperativity parameter, characteristic folding channels and free-energy landscapes, which have strong similarities with realistic folding paths, can be analysed.

cond-mat.soft↗

Adsorption Phenomena at Organic-Inorganic Interfaces

The qualitative solvent- and temperature-dependent conformational behavior of a peptide in the proximity of solid substrates with different adsorption properties is investigated by means of a simple lattice model. The resulting pseudophase diagrams exhibit a complex structure, which can be understood by analysing the minima of the free-energy landscape in dependence of appropriate system parameters.

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Substrate Adhesion of a Nongrafted Flexible Polymer in a Cavity

In a contact density chain-growth study we investigate the solubility-temperature pseudo-phase diagram of a lattice polymer in a cavity with an attractive surface. In addition to the main phases of adsorbed and desorbed conformations we find numerous subphases of collapsed and expanded structures.

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Density of States for HP Lattice Proteins

The density of states contains all informations on energetic quantities of a statistical system, such as the mean energy, free energy, entropy, and specific heat. As a specific application, we consider in this work a simple lattice model for heteropolymers that is widely used for studying statistical properties of proteins. For short chains, we have derived exact results from conformational enumeration, while for longer ones we developed a multicanonical Monte Carlo variant of the nPERM-based chain growth method in order to directly simulate the density of states. For simplification, only two types of monomers with respective hydrophobic (H) and polar (P) residues are regarded and only the next-neighbour interaction between hydrophobic monomers, being nonadjacent along the chain, is taken into account. This is known as the HP model for the folding of lattice proteins.

cond-mat.soft↗

Conformational Transitions of Heteropolymers

We study conformational transitions of simple coarse-grained models for protein-like heteropolymers on the simple cubic lattice and off-lattice, respectively, by means of multicanonical sampling algorithms. The effective hydrophobic/polar models do not require the knowledge of the native topology for a given sequence of residues as input. Therefore these models are eligible to investigate general properties of the tertiary folding behaviour of such protein-like heteropolymers.

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