Simulations of ion current in realistic models of ion channels: the KcsA potassium channel.

Realistic studies of ion current in biologic channels present a major challenge for computer simulation approaches. All-atom molecular dynamics simulations involve serious time limitations that prevent their use in direct evaluation of ion current in channels with significant barriers. The alternative use of Brownian dynamics (BD) simulations can provide the current for simplified macroscopic models. However, the time needed for accurate calculations of electrostatic energies can make BD simulations of ion current expensive. The present work develops an approach that overcomes some of the above challenges and allows one to simulate ion currents in models of biologic channels. Our method provides a fast and reliable estimate of the energetics of the system by combining semimacroscopic calculations of the self-energy of each ion and an implicit treatment of the interactions between the ions, as well as the interactions between the ions and the protein-ionizable groups. This treatment involves the use of the semimacroscopic version of the protein dipole Langevin dipole (PDLD/S) model in its linear response approximation (LRA) implementation, which reduces the uncertainties about the value of the protein "dielectric constant." The resulting free energy surface is used to generate the forces for on-the-fly BD simulations of the corresponding ion currents. Our model is examined in a preliminary simulation of the ion current in the KcsA potassium channel. The complete free energy profile for a single ion transport reflects reasonable energetics and captures the effect of the protein-ionized groups. This calculated profile indicates that we are dealing with the channel in its closed state. Reducing the barrier at the gate region allows us to simulate the ion current in a reasonable computational time. Several limiting cases are examined, including those that reproduce the observed current, and the nature of the productive trajectories is considered. The ability to simulate the current in realistic models of ion channels should provide a powerful tool for studies of the biologic function of such systems, including the analysis of the effect of mutations, pH, and electric potentials.

[Evaluation of a drying procedure for dairy products coming in direct contact with flue gases from the oncologic viewpoint]. / Onkogigienicheskaia otsenka tekhnologii sushki molochnykh produktov pri priamom ikh kontakte s dymovymi gazami.

The study was concerned with ambient air and heat-transfer medium levels of benzo(a)-pyrene (BP) and N-nitrosoamines (NA) measured at the input and output of the drying chamber of two milk dryers using different systems of heat generation. Said concentrations were ascertained in the dried product (casein), too. One installation used electricity-heated ambient air, while the other--air-diluted products of natural gas combustion. No significant difference was registered for both dryers between ambient-air and heat-transfer medium concentrations of BP and two NAs detected--NDMA and NDEA. There was no significant difference between BP levels in the casein produced on those installations, the raw material coming from the same lot. The study registered reverse changes in NDMA and NDEA concentrations, with ambient air being heated in the heat generator and heat-transfer agent cooled in the drying chamber.

[Benz(a)pyrenes and N-nitrosamines in dry dairy products]. / Izuchenie soderzhaniia benz(a)pyrena i N-nitrozoaminov v sukhikh molochnykh produktakh.

Results of works induding study of effect of different factors (environment, type of equipment, technology) on content of cancerogenic substances in dry milk products were summarized. Possible reasons of appearance of cancerogens in milk in process of drying were determined, dataes of concentration of benz(a)pyren and N-nytrozamens in milk and warmth carrier before and after drying were established. Explanation of separate appearances which connect with change of quantity of cancerogens in product on different stages of drying was suggested.