Biomedical subjects

Ferdinand Kühner

Publications and source records attributed to Ferdinand Kühner.

4 recordsLinked to original sources

PubMed2007

Force-induced DNA slippage.

DNA containing repetitive sequences displays richer dynamics than heterogeneous sequences. In the genome the number of repeat units of repetitive sequences, known as microsatellites, may vary during replication by DNA slippage and their expansion gives rise to serious disorders. We studied the mechanical properties of repetitive DNA using dynamic force spectroscopy and found striking differences compared with ordinary heterogeneous sequences. Repetitive sequences dissociate at lower forces and elongate above a certain threshold force. This yield force was found to be rate dependent. Following the rapid stretching of the DNA duplex, the applied force relaxes by stepwise elongation of this duplex. Conversely, contraction of the DNA duplex can be observed at low forces. The stepwise elongation and shortening is initiated by single slippage events, and single-molecule experiments might help to explain the molecular mechanisms of microsatellites formation. In addition to the biological importance, the remarkable properties of repetitive DNA can be useful for different nanomechanical applications.

DNA↗

PubMed2006

Friction of single polymers at surfaces.

Atomic force microscope (AFM) single molecule force spectroscopy has been used to investigate the friction coefficient of individual polymers adsorbed onto a solid support. The polymer chains were covalently attached to an AFM tip and were allowed to adsorb on a mica surface. Different polymers (ssDNA, polyallylamine) were chosen to cover a range of friction coefficients. During the experiment, the AFM tip was retracted in- and off-plane which results, depending on the chosen conditions, in a desorption of the polymer from the surface, a sliding across the surface, or a combination of both. Thus, the obtained force-extension spectra reveal detailed information on the mobility of a polymer chain on a surface under experimentally accessible conditions. This study demonstrates that absorbed polymers with comparable desorption forces may exhibit drastically different in plane mobility.

Adenylosuccinate Synthase↗

PubMed2006

Scaling exponent and Kuhn length of pinned polymers by single molecule force spectroscopy.

The end-to-end distance and the contour length of single polymers in dynamic adsorbate layers were measured with a mechanical approach. Individual polysaccharide chains were covalently pinned to the surface with one segment and picked up randomly with an atomic force microscope tip. The polymer section between pinpoint and the pickup point was stretched by retracting the tip from the surface. The pinpoint was derived by measuring the normal force while laterally scanning the surface at constant height. For carboxy-methyl-amylose, a Kuhn length of 0.44 nm and a scaling exponent of 0.74 were found.

Amylose↗

PubMed2006

Weakly bound water molecules shorten single-stranded DNA.

In this paper, we measure the single chain elasticity of an oligomer single-stranded DNA (ssDNA) in both aqueous and nonaqueous, apolar liquid environments by AFM-based single molecule force spectroscopy. We find a marked deviation between the force-extension relations recorded for the two conditions. This difference is attributed to the additional energy required to break the H-bond-directed water bridges around the ssDNA chain in aqueous solutions, which are nonexistent in organic solvents. The results obtained in 8 M guanidine-HCl solution provide more evidence that water bridges around ssDNA originate the observed deviation. On the basis of the results obtained by an ab initio quantum mechanics calculation, a parameter-free freely rotating chain model is proposed. We find that this model is in perfect agreement with the experimental force-extension curve obtained in organic solvents, which further corroborates our assumption. On the basis of the experimental results, it is suggested that the weak H-bonding between ssDNA and water molecules may be a precondition for stable double-stranded DNA to exist in water.

1-Propanol↗