Biomedical subjects

Andreas Schüring

Publications and source records attributed to Andreas Schüring.

3 recordsLinked to original sources

PubMed2006

Single-file diffusion near channel boundaries.

Molecular transport under the conditions of single-file diffusion was investigated near the channel boundaries by using dynamic Monte Carlo and molecular dynamics simulations of tracer exchange between single-file channels and their surroundings. The boundary effect reported in our recent papers (Vasenkov S.; Kärger, J. Phys. Rev. E 2002, 66, 052601. Schüring, A.; Vasenkov S.; Fritzsche, S. J. Phys. Chem. B 2005, 109, 16711) was studied in detail. This boundary effect is characterized by deviations of the intrachannel concentration profiles of tracer molecules observed in the case of single-file diffusion near the channel boundaries from the corresponding profiles typical for normal diffusion. It has been shown in our previous studies that these deviations occur under the conditions when the potential-energy difference inside and outside of single-file channels was both comparable and much larger than the activation energy of intrachannel diffusion. Here, we report a quantitative model describing the boundary effect. According to this model, an occurrence of the boundary effect is related to a complex character of diffusion in finite single-file systems. Such diffusion can be described by the following two types of movements occurring in parallel: (i) correlated displacements of all molecules in any particular channel and (ii) fast displacements of single molecules, which are uncorrelated with the displacements of all other molecules in the same channel. The latter displacements are restricted to a certain length interval that depends on the channel length and the channel occupancy. This length interval is shown to determine the extensions of the channel margins where the boundary effect is observed.

Diffusion↗

PubMed2006

The androgen receptor CAG repeat modifies the impact of testosterone on insulin resistance in women with polycystic ovary syndrome.

OBJECTIVE: Hyperandrogenism is a central feature of the polycystic ovary syndrome (PCOS) and might worsen insulin resistance (IR) often seen in PCOS. Androgens act through the androgen receptor (AR). A polymorphic CAG repeat sequence within the AR gene was reported to modulate its transactivation activity. Therefore, we investigated a putative interaction between testosterone and the CAG repeat length polymorphism with respect to IR. DESIGN: In 63 PCOS women with normal glucose tolerance free testosterone, the biallelic CAG repeat length and a multiplicative interaction term were investigated by multiple linear regression analysis for an association with IR as indicated by the homeostasis model assessment of IR (HOMA-IR). RESULTS: Free testosterone was correlated with HOMA-IR. The impact of testosterone on HOMA-IR was modified by the AR CAG length as indicated by an interaction term. This interaction remained significant after adjustment for smoking, age and body mass index. While there was a positive association of free testosterone with HOMA-IR, the interaction term was inversely associated. The model, which explained 42.5% of the variation of HOMA-IR predicted that in carriers of short CAG lengths, an increase in testosterone increased IR. This effect attenuated with rising biallelic CAG length until it turns into the opposite at a CAG length longer than 23. The results were confirmed by using CIGMA as another measure of IR. CONCLUSIONS: The association between testosterone and IR is modified by the CAG repeat polymorphism within the AR. Therefore, the evaluation of testosterone effects on IR seems to require consideration of the AR CAG repeat polymorphism in PCOS women.

Adult↗

PubMed2005

Influence of boundaries of nanoporous crystals on molecular exchange under the conditions of single-file diffusion.

We study the tracer exchange of molecules between the phase adsorbed in one-dimensional channels and the surrounding gas phase by molecular dynamics simulations. Under the conditions of single-file diffusion, a novel boundary effect is observed. The shape of the tracer-exchange concentration profiles deviates from those obtained under the conditions of normal diffusion. Compared to the profiles for normal diffusion, which correspond to the same degree of exchange, the equilibrium concentration is reached faster at the boundaries and slower in the middle part of the channel in the case of single-file diffusion. This boundary effect is observed for the system neopentane in AlPO4-5 (which was chosen as a reference system), as well as for modified systems. The effect can be understood considering two diffusion mechanisms which occur in parallel. First, the diffusion of the whole chain of particles, that is, the center-of-mass diffusion, obeying the laws of normal diffusion. Second, the individual movement of the particles relative to the center of mass of the chain. The second mechanism admits additional displacements which, on average, lead to an accelerated exchange of the marginal particles.

Journal Article↗