527 research outputs found
External control strategies for self-propelled particles: optimizing navigational efficiency in the presence of limited resources
We experimentally and numerically study the dependence of different
navigation strategies regarding the effectivity of an active particle to reach
a predefined target area. As the only control parameter, we vary the particle's
propulsion velocity depending on its position and orientation relative to the
target site. By introducing different figures of merit, e.g. the time to target
or the total consumed propulsion energy, we are able to quantify and compare
the efficiency of different strategies. Our results suggest, that each strategy
to navigate towards a target, has its strengths and weaknesses and none of them
outperforms the other in all regards. Accordingly, the choice of an ideal
navigation strategy will strongly depend on the specific conditions and the
figure of merit which should be optimized
Noninvasive Measurement of Dissipation in Colloidal Systems
According to Harada and Sasa [Phys. Rev. Lett. 95, 130602 (2005)], heat
production generated in a non-equilibrium steady state can be inferred from
measuring response and correlation functions. In many colloidal systems,
however, it is a nontrivial task to determine response functions, whereas
details about spatial steady state trajectories are easily accessible. Using a
simple conditional averaging procedure, we show how this fact can be exploited
to reliably evaluate average heat production. We test this method using
Brownian dynamics simulations, and apply it to experimental data of an
interacting driven colloidal system
Critical Casimir forces in colloidal suspensions on chemically patterned surfaces
We investigate the behavior of colloidal particles immersed in a binary
liquid mixture of water and 2,6-lutidine in the presence of a chemically
patterned substrate. Close to the critical point of the mixture, the particles
are subjected to critical Casimir interactions with force components normal and
parallel to the surface. Because the strength and sign of these interactions
can be tuned by variations in the surface properties and the mixtures
temperature, critical Casimir forces allow the formation of highly ordered
monolayers but also extend the use of colloids as model systems.Comment: 4 papges, 4 figures, accepted at Phys. Rev. Let
The SMART model: Soft Membranes Adapt and Respond, also Transiently, in the presence of antimicrobial peptides:
Biophysical and structural studies of peptide-lipid interactions, peptide topology and dynamics have changed our view on how antimicrobial peptides insert and interact with membranes. Clearly, both the peptides and the lipids are highly dynamic, change and mutually adapt their conformation, membrane penetration and detailed morphology on a local and a global level. As a consequence, the peptides and lipids can form a wide variety of supramolecular assemblies in which the more hydrophobic sequences preferentially, but not exclusively, adopt transmembrane alignments and have the potential to form oligomeric structures similar to those suggested by the transmembrane helical bundle model. In contrast, charged amphipathic sequences tend to stay intercalated at the membrane interface where they cause pronounced disruptions of the phospholipid fatty acyl packing. At increasing local or global concentrations, the peptides result in transient membrane openings, rupture and ultimately lysis. Depending on peptide-to-lipid ratio, lipid composition and environmental factors (temperature, buffer composition, ionic strength, etc.), the same peptide sequence can result in a variety of those responses. Therefore, the SMART model has been introduced to cover the full range of possibilities. With such a view in mind, novel antimicrobial compounds have been designed from amphipathic polymers, peptide mimetics, combinations of ultra-short polypeptides with hydrophobic anchors or small designer molecules
Highly synergistic antimicrobial activity of magainin 2 and PGLa peptides is rooted in the formation of supramolecular complexes with lipids
Magainin 2 and PGLa are cationic, amphipathic antimicrobial peptides which when added as equimolar mixture exhibit a pronounced synergism in both their antibacterial and pore-forming activities. Here we show for the first time that the peptides assemble into defined supramolecular structures along the membrane interface. The resulting mesophases are quantitatively described by state-of-the art fluorescence self-quenching and correlation spectroscopies. Notably, the synergistic behavior of magainin 2 and PGLa correlates with the formation of hetero-domains and an order-of-magnitude increased membrane affinity of both peptides. Enhanced membrane association of the peptide mixture is only observed in the presence of phophatidylethanolamines but not of phosphatidylcholines, lipids that dominate bacterial and eukaryotic membranes, respectively. Thereby the increased membrane-affinity of the peptide mixtures not only explains their synergistic antimicrobial activity, but at the same time provides a new concept to increase the therapeutic window of combinatorial drugs
Theory of orientational ordering in colloidal molecular crystals
Freezing of charged colloids on square or triangular two-dimensional periodic
substrates has been recently shown to realize a rich variety of orientational
orders. We propose a theoretical framework to analyze the corresponding
structures. A fundamental ingredient is that a non spherical charged object in
an electrolyte creates a screened electrostatic potential that is anisotropic
at any distance. Our approach is in excellent agreement with the known
experimental and numerical results, and explains in simple terms the reentrant
orientational melting observed in these so called colloidal molecular crystals.
We also investigate the case of a rectangular periodic substrate and predict an
unusual phase transition between orientationnaly ordered states, as the aspect
ratio of the unit cell is changed.Comment: 4 pages, to appear in Phys. Rev. Let
Noise enhanced performance of ratchet cellular automata
We present the first experimental realization of a ratchet cellular automaton
(RCA) which has been recently suggested as an alternative approach for
performing logical operations with interacting (quasi) particles. Our study was
performed with interacting colloidal particles which serve as a model system
for other dissipative systems i.e. magnetic vortices on a superconductor or
ions in dissipative optical arrays. We demonstrate that noise can enhance the
efficiency of information transport in RCA and consequently enables their
optimal operation at finite temperatures.Comment: accepted for publication at Phys. Rew. Let
Molecular packing of amphipathic peptides on the surface of lipid membranes
When polypeptides bind to the membrane surface, they become confined to a restricted quasi-two-dimensional space where peptide-peptide interactions become highly relevant, and the concept of a crowded medium is appropriate. Within this crowded environment interesting effects like clustering, separation of phases, cooperative alignment, and common movements occur. Here we investigated such effects by measuring distances between fluorophore-labeled peptides in the range </=1 nm by fluorescence self-quenching. For helical peptides with dimensions of approximately 1 x 3 nm such a small "ruler" is sensitive to the packing of the labeled peptides and thereby to their molecular arrangement. A novel approach to characterize peptide-peptide interactions within membranes is presented using the designer peptide LAH4. This sequence changes membrane topology in a controlled manner being transmembrane at neutral conditions but oriented parallel to the surface at low pH. Experimental measurements of the fluorescence self-quenching of close-by chromophores and the changes that occur upon dilution with unlabeled peptides are used to analyze the peptide distribution within the membrane surface. The data show a strong effect of electrostatic interactions and under some experimental conditions clustering of the peptides. Furthermore, the results suggest that at pH 4 the peptides arrange along the membrane surface in an ordered mesophase-like arrangement
Biophysical Investigations Elucidating the Mechanisms of Action of Antimicrobial Peptides and Their Synergism
Biophysical and structural investigations are presented with a focus on the membrane lipid interactions of cationic linear antibiotic peptides such as magainin, PGLa, LL37, and melittin. Observations made with these peptides are distinct as seen from data obtained with the hydrophobic peptide alamethicin. The cationic amphipathic peptides predominantly adopt membrane alignments parallel to the bilayer surface; thus the distribution of polar and non-polar side chains of the amphipathic helices mirror the environmental changes at the membrane interface. Such a membrane partitioning of an amphipathic helix has been shown to cause considerable disruptions in the lipid packing arrangements, transient openings at low peptide concentration, and membrane disintegration at higher peptide-to-lipid ratios. The manifold supramolecular arrangements adopted by lipids and peptides are represented by the 'soft membranes adapt and respond, also transiently' (SMART) model. Whereas molecular dynamics simulations provide atomistic views on lipid membranes in the presence of antimicrobial peptides, the biophysical investigations reveal interesting details on a molecular and supramolecular level, and recent microscopic imaging experiments delineate interesting sequences of events when bacterial cells are exposed to such peptides. Finally, biophysical studies that aim to reveal the mechanisms of synergistic interactions of magainin 2 and PGLa are presented, including unpublished isothermal titration calorimetry (ITC), circular dichroism (CD) and dynamic light scattering (DLS) measurements that suggest that the peptides are involved in liposome agglutination by mediating intermembrane interactions. A number of structural events are presented in schematic models that relate to the antimicrobial and synergistic mechanism of amphipathic peptides when they are aligned parallel to the membrane surface.PMC602300
The histidine-rich peptide LAH4-L1 strongly promotes PAMAM-mediated transfection at low nitrogen to phosphorus ratios in the presence of serum
Non-viral vectors are widely used and investigated for the delivery of genetic material into cells. However, gene delivery barriers like lysosomal degradation, serum inhibition and transient gene expression so far still limit their clinical applications. Aiming to overcome these limitations, a pH-sensitive hybrid gene vector (PSL complex) was designed by self-assembly of poly(amidoamine) (PAMAM) dendrimers, the histidine-rich peptide LAH4-L1 and the sleeping beauty transposon system (SB transposon system, a plasmid system capable of efficient and precise genomic insertion). Transfection studies revealed that PSL complexes achieved excellent efficiency in all investigated cell lines (higher than 90% in HeLa cells and over 30% in MDCK cells, a difficult-to-transfect cell line). Additionally, the PSL complexes showed high serum tolerance and exhibited outstanding transfection efficiency even in medium containing 50% serum (higher than 90% in HeLa cells). Moreover, a high level of long-term gene expression (over 30% in HeLa cells) was observed. Furthermore, PSL complexes not only resulted in high endocytosis, but also showed enhanced ability of endosomal escape compared to PAMAM/DNA complexes. These results demonstrate that simple association of PAMAM dendrimers, LAH4-L1 peptides and the SB transposon system by self-assembly is a general and promising strategy for efficient and safe gene delivery.PMC557505
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