Biomedical subjects

S Obukhov

Publications and source records attributed to S Obukhov.

5 recordsLinked to original sources

PubMed2004

Long range bond-bond correlations in dense polymer solutions.

The scaling of the bond-bond correlation function P1(s) along linear polymer chains is investigated with respect to the curvilinear distance s along the flexible chain and the monomer density rho via Monte Carlo and molecular dynamics simulations. Surprisingly, the correlations in dense three-dimensional solutions are found to decay with a power law P1(s) approximately s(-omega) with omega=3/2 and the exponential behavior commonly assumed is clearly ruled out for long chains. In semidilute solutions, the density dependent scaling of P1(s) approximately g(-omega(0))(s/g)(-omega) with omega(0)=2-2nu=0.824 (nu=0.588 being Flory's exponent) is set by the number of monomers g(rho) in an excluded volume blob. Our computational findings compare well with simple scaling arguments and perturbation calculation. The power-law behavior is due to self-interactions of chains caused by the chain connectivity and the incompressibility of the melt.

Journal Article↗

PubMed2003

Arrested swelling of highly entangled polymer globules.

Upon aging, a collapsed long chain evolves from a crumpled state to a self-entangled globule which can be thought of as a large knot. Swelling of an equilibrium globule in good solvent is a two-step process: (i) fast swelling into an arrested stretched structure with conserved entanglement topology followed by (ii) slow disentanglement. Using computer simulation, we found both mass-mass (m-m) and entanglement-entanglement (e-e) power law correlations inside the swollen globule. The m-m correlations are characterized by a set of two exponents in agreement with a Flory-type argument. The e-e correlations are also characterized by two exponents, both of them larger (by approximately 0.3) than the related m-m exponents. We interpret this difference as evidence of distance-dependent repulsion E=-0.3ln((rho)k(B)T between entanglements sliding along the polymer chain.

Models, Chemical↗

PubMed2001

Counterion phase transitions in dilute polyelectrolyte solutions.

In dilute solutions of rodlike polyelectrolytes some counterions are distributed far from polyions while others are located in their vicinity in the regions of cylindrical symmetry of the electrostatic potential. For these cylindrical regions around rodlike polyelectrolytes we find an exact solution of the nonlinear Poisson-Boltzmann equation for the case of nonzero net charge in these regions. This exact solution implies three qualitatively different phases of counterion distribution around the polyions with second order phase transitions between these phases.

Journal Article↗

PubMed1996

Deterministic model of DNA gel electrophoresis in strong electric fields.

We present a new model for the motion of a megabase-long DNA molecule undergoing gel electrophoresis. We assume that the dynamics of large segments of DNA is almost deterministic and can be described by a set of simple mechanical equations. This allows the numerical study of gel electrophoresis of ultra-high molecular weight DNA. A strong electric field forces DNA in a gel into a tree-like structure with branches-loops of different sizes. We determined the loop-size distribution function. This distribution has a power law form, confirming the hypothesis of the statistical self-similarity of a moving polymer. We find periodic configuration changes in the motion of a circular polymer, with the average period proportional to the molecular weight. During the period, a polymer goes through three distinct phases: a simple V-shape configuration, a growing tree, and a decaying tree. For a linear polymer this periodicity is much less pronounced because of additional perturbations to the dynamics caused by free ends. A circular polymer stays in a simple V-shaped configuration about 30% of the time, independent of molecular weight (10% for a linear polymer).

Computer Simulation↗

PubMed1995

Successful removal of large pineal region meningiomas: two case reports.

BACKGROUND: Pineal region meningiomas are extremely rare tumors and comprise about 8% of tumors of this region. Two cases of large pineal region meningiomas in young males are presented. METHODS: Computed tomography (CT) scan and cerebral angiography were used to evaluate the patients preoperatively. Both patients were operated on through an occipital transtentorial approach either as a single- or two-stage procedure. RESULTS: Preoperative work-up revealed that both tumors received their main blood supply from the posterior lateral choroidal arteries and were therefore originating from the velum interpositum cerebri (superior tela choroidea). Complete tumor removal was effected in the first patient using a right occipital transtentorial approach. In the second patient, after partial removal using the same approach, complete resection was achieved at a second stage via the left occipital transtentorial approach 3 months later. CONCLUSION: Cerebral angiography, which must include vertebral angiography, is important in the preoperative surgical planning of these tumors. The occipital transtentorial approach provides good access. However, a two-stage procedure using right and left sides may be necessary for large meningiomas receiving a bilateral blood supply from both posterior lateral choroidal arteries. Diagnosis, the surgical approach, and methods of tumor excision are discussed.

Adult↗