Direct knock-on of desolvated ions governs strict ion selectivity in K+ channels

The seeming contradiction that K+ channels conduct K+ ions at maximal throughput rates while not permeating slightly smaller Na+ ions has perplexed scientists for decades. Although numerous models have addressed selective permeation in K+ channels, the combination of conduction efficiency and ion selectivity has not yet been linked through a unified functional model. Here, we investigate the mechanism of ion selectivity through atomistic simulations totalling more than 400 μs in length, which include over 7,000 permeation events. Together with free-energy calculations, our simulations show that both rapid permeation of K+ and ion selectivity are ultimately based on a single principle: the direct knock-on of completely desolvated ions in the channels' selectivity filter. Herein, the strong interactions between multiple 'naked' ions in the four filter binding sites give rise to a natural exclusion of any competing ions. Our results are in excellent agreement with experimental selectivity data, measured ion interaction energies and recent two-dimensional infrared spectra of filter ion configurations

Integration of Social Concerns in Collaborative Logistics and Transportation Networks

Part 21: Mobility and LogisticsInternational audienceFreight transport optimization, long based solely on an economic approach, happen today through the integration of environmental and/or social concerns, in line with the objectives of sustainable development. In the case of Small and Very Small Enterprises, these objectives cannot be reached individually, and these companies have to join their efforts to find collective solutions. Therefore, the Fleet Size and Mix Vehicle Routing Problem (FSMVRP) was adapted to take into account social objectives, and results are compared to Vehicle Routing Problem with homogeneous fleet. An exact mathematical formulation of the extended problem was developed. Computational experiments for the problem formulation are performed using CPLEX and give a solution of a small instance to illustrate the problem. The model is tested on a case study of optimal parcel pickup, from many manufacturers to a common depot in the agri-food sector

A Comparison of Homogeneous and Heterogeneous Vehicle Fleet Size in Green Vehicle Routing Problem

Part 1: Knowledge-Based SustainabilityInternational audienceTo balance a fragmented logistics organization, Small and Medium Enterprises have to find collective solutions to decrease their environmental impact. Especially when the demand at each producer takes the form of small packages and low quantities this paper examines the effect of the introduction of a consolidation center on the environmental issue. Therefore, the Fleet Size and Mix Vehicle Routing Problem (FSMVRP) was adapted in order to minimize CO2 emission. An exact mathematical formulation of the extended problem was developed to investigate the difference between homogeneous and heterogeneous fleet size on the environmental issue. Computational experiments for the problem formulation are performed using CPLEX and give a solution of a small instance to illustrate the problem. A case study focuses on optimal parcel picking up, from many producers to a common depot in the agri-food sector