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Detailed description
Contact Information
Principal Investigators:
Prof. Bjørn Torger Stokke
Department of Physics
Høgskoleringen 5
N-7491 Trondheim, Norway
Email: bjorn.stokke@ntnu.no
tel. +47 73593434
Assoc. Prof. Marit Sletmoen
Department of Physics
Høgskoleringen 5
N-7491 Trondheim, Norway
Email: marit.sletmoen@ntnu.no
tel. +47 73593463
Assoc. Prof. Rita Dias
Department of Physics
Høgskoleringen 5
N-7491 Trondheim, Norway
Email: rita.dias@ntnu.no
tel. +47 73593422
Biological polymers: Research Areas
Bioresponsive gels as signal transducers in biosensors
Various hydrogel materials designed to adopt an equilibrium swelling state selectively depending on a biological relevant molecule can be utilized for label-free biosensing. This requires sensitive readout-technology for the monitoring of changes in the hydrogel swelling. We are employing an interferometric readout platform with resolution of 2 nanometer in changes of the optical length within a hydrogel. This set-up allows us to explore biospecific responsive hydrogels as signal transducers for label-free biosensing. The interferometric readout platform is fiber-optic based and the hemispherical 50-60 μm radius hydrogel is covalently attached at the end of the fiber. We have realized a glucose sensing hydrogel material on this platform, where tuning of glucose selectivity is achieved by incorporating a postive charge moiety in addition to the PBA based recognition. We have also realized an oligonucleotide sensing material based on competitive displacement hybridization, which can be designed with a large number of sequence recognition capabilities.
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Single-molecular pair interactions
Single-molecular interaction studies were initially undertaken to elucidate pairwise interactions of selected sets of biololgical macromolecules. One example is related to determination of the mode of action of the mannuronan C-5 epimerase when catalyzing the conversion of the mannuronic acid to its C-5 epimer, guluronic acid, at the polymer level. Such single-molecule interactions are currently adopted for characterization of other molecular pairs, e.g., selected part of immunlogical signalling cascades (toll-like receptor 9 (TLR9)), mucin - lectin interactions, DNA-repair enzymes and selected high molecular weight polysaccharides forming physical gels.
Back to topPolyelectrolyte complexes
Electrostatic interactions are important driving forces for numerous biological processes, e.g. organisation of DNA to packed chromosones, or interactions between enzymes and charged ligands. This is currently receiving interest, as polycation-induced condensation of DNA is a possible first step in preparing a therapeutic gene delivery vector. The research is two-fold, investigating the use of chitosan (and modified chitosans) for compaction of DNA for gene delivery, and in general investigating the influence of macromolecular properties and preparation conditions on the structure formation of condensing semiflexible biopolymers.
Back to topStructure and properties of (1,3)-β-D-glucans and interactions with polynucleotides.
(1,3)-β-D-glucans form a group of biologically active biopolymers that exist in different structural organisations depending on the environmental conditions. The biological effects of (1,3)-β-D-glucans is a core issue stimulating large research efforts of the molecular properties and their consequences for action as biological response modifiers. Our laboratory was in 1991 the first to report that (1,3)-β-D-glucans are able to form a topological cyclic macromolecule. We have performed studies to elucidate both the stability, structure and biological activity of (1,3)-β-D-glucans exposed to different pre-treatments. The fascination for these molecules increased further following the finding of their ability to form complexes of defined geometry with a number of structures.
Back to topBiopolymer multilayers
Multilayers are formed by the alternate deposition of polyanionic and polycationic polymers, and have potential applications within surface modifications, optical devices and separation membranes. Surface topography might influence the biocompatibility, adhesive and optical properties of these systems. We are therefore looking at how molecular properties of biopolymers and preparation conditions are influencing the surface topography of biopolymer multilayers. Part of the work includes developing routines for user-independent, quantitative extraction of parameters for analysis of the surface topography.
Back to topStructure of polysaccharide gels
Gels can be formed from a range of biopolymers, based on various types of physical or chemical crosslinks. Alginates form gels in aqueous Ca2+-containing solutions by lateral association of chain segments. Lately, the effect of adding free guluronic acid blocks on the gelation kinetics, swelling response and gel strength has been studied. Using rheology, scattering techniques and ultramicroscopy the fractal dimension and junction zone multiplicity of the gels are investigated. The same experimental procedures are used to study gels made from other biopolymers, i.e. (1,3)-β-D-glucans, chitosan and xanthan.
Back to topToll-like receptors of the immune system studied by atomic force microscopy and confocal microscopy
Toll-like receptors (TLR) are responsible for an immediate response of the innate immune system against invading pathogens. TLR recognize evolutionary conserved microbial patterns such as glycolipids and bacterial DNA. While many of the trafficking and signaling processes that play a role in TLR mediated immune response have been characterized, the exact mechanisms of interaction between TLR and various ligands and other interacting partners during activation are not known yet. The goal of this project is to study the molecular details of the first step in the TLR signaling cascade; the activation of the receptors.
Back to topPhysics of enzymatic mode of action
Polysaccharide modifying enzymes may generate specific sequence patterns in polymers (e.g. epimerisation), or produce oligomers of a certain length as a result of depolymerisation. We are interested in the kinetics, specificity and mode of action of alginate epimerases and lyases. We also study the sequence specificities of lysozyme depolymerisation of partially N-acetylated chitosans and the mode of action of chitin deacetylases. The systems are studied using NMR, dynamic force spectroscopy and simulations. The alginate epimerase AlgE4 produces an alginate with long stretches of alternating MG sequences. The kinetics of formation of this sequence pattern could not be accounted for by a random attack model. Results obtained from dynamic force spectroscopy indicated a processive mode of action. This enzyme might thus be the first known polysaccharide modifying processive enzyme
Back to topStudies of weak intermolecular interactions using optical tweezers
Optical tweezers is a powerful technique for studies of weak intermolecular interactions. We are studying weak carbohydrate – carbohydrate interactions at the single molecular pair level using dual beam optical tweezers. Preliminary data obtained for carbohydrate self-interactions reveal the existence of multiple interactions; each withstanding forces < 20 pN. The experimental data also reveal the physical distance separating the interactions sites along the polymeric molecule. The resolution of the data, both with respect to force and spacial resolution, are considerably improved compared to what can be obtained when performing force measurements using AFM, and illustrates the strength of the optical tweezers approach for the study of weak and polyvalent interactions.
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Amyloid structures studied by atomic force microscopy (AFM) and total internal reflection fluorescence microscopy (TIRFM)
Due to abnormal alterations of its secondary structure, a protein can form aggregated insoluble fibrils that are deposited extracellulary leading to amyloidosis. Abnormal accumulation of amyloids is associated with many neurodegenerative diseases: systemic amyloidosis, Alzheimer's disease, maturity onset diabetes, and the prion-related transmissible spongiform encephalopathies. We study insulin amyloids to resolve the structure and organization at the level of single aggregates. Amyloids used in this study are labeled with newly designed fluorescent probes that can be potentially used as biomarkers to identify amyloid aggregates in histopathological studies. AFM offers a possibility to visualize individual amyloid structures revealing the detailed ultrastructure at nanoscale. When combined with the TIRFM, a very sensitive tool to visualize sinlge fluorophores, the ultrastructural information can be colocalized with the fluorescent signal originated from the bound probes.
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Monte Carlo studies on polyelectrolyte complexes
The interactions between oppositely charged polyelectrolytes and the formation of polyelectrolyte complexes (PECs) has attracted much interest, partially due to fact that PECs prepared using DNA (polyplexes) are considered promising gene delivery vehicles in gene transfection. Also, films and capsules containing oppositely charged polyelectrolytes method have potential applications in biosensoring, catalysis, optical devises and drug delivery. We use Monte Carlo simulations of coarse-grained models to study the structural properties and the topology of polyelectrolyte complexes formed by a long polyion (that mimics DNA, for example) and shorter and oppositely charged polyions (condensing agents). We have studied variations in the number, chain length and charge of the condensing agents, as well as mixtures of different condensing agents.
Back to topAdsorption of macromolecules onto responsive surfaces. Monte Carlo simulations
Protein and polymer adsorption onto lipid bilayers and monolayers is fundamentally interesting and has great importance in medicine, biology and technological applications such as biosensors, implants, food and pharmaceutical formulations. These systems have thus been vastly studied using experimental, theoretical and computer simulation approaches using simple or more complicated model systems. One of the most striking characteristics of surfactants and lipid bilayers is the ability of the individual molecules to react to an adsorbing object as opposed to hard and homogeneous surfaces. The dynamic modes of the individual amphiphilic molecules include lateral diffusion and vertical excursions out of the bilayer and both modes have been shown to influence the adsorption degree and conformation of interacting macromolecules.
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