Fundamental Limits on Sensing Chemical Concentrations with Linear Biochemical Networks

Living cells often need to extract information from biochemical signals that are noisy. We study how accurately cells can measure chemical concentrations with signaling networks that are linear. For stationary signals of long duration, they can reach, but not beat, the Berg-Purcell limit, which relies on uniformly averaging in time the fluctuations in the input signal. For short times or nonstationary signals, however, they can beat the Berg-Purcell limit, by non-uniformly time-averaging the input. We derive the optimal weighting function for time averaging and use it to provide the fundamental limit of measuring chemical concentrations with linear signaling networks.Comment: To appear in Physical Review Letters, 7 pages, 4 figure

The thermodynamics of computational copying in biochemical systems

Living cells use readout molecules to record the state of receptor proteins, similar to measurements or copies in typical computational devices. But is this analogy rigorous? Can cells be optimally efficient, and if not, why? We show that, as in computation, a canonical biochemical readout network generates correlations; extracting no work from these correlations sets a lower bound on dissipation. For general input, the biochemical network cannot reach this bound, even with arbitrarily slow reactions or weak thermodynamic driving. It faces an accuracy-dissipation trade-off that is qualitatively distinct from and worse than implied by the bound, and more complex steady-state copy processes cannot perform better. Nonetheless, the cost remains close to the thermodynamic bound unless accuracy is extremely high. Additionally, we show that biomolecular reactions could be used in thermodynamically optimal devices under exogenous manipulation of chemical fuels, suggesting an experimental system for testing computational thermodynamics.Comment: Accepted versio

OSIRIS Payload for DLR's BiROS Satellite

Direct optical communication links might offer a solution for the increasing demand of transmission capacity in satellite missions. Although direct space-to-ground links suffer from limited availability due to cloud coverage, the achievable data rates can be higher by orders of magnitude compared to traditional RF communication systems. DLR’s Institute for Communication and Navigation is currently developing an experimental communication payload for DLR’s BiROS satellite. The OSIRIS payload consists of a tracking sensor for a precise alignment between satellite and groundstation, an optical uplink channel, the two different and independent laser sources and the optical bench with the transmission optics. This paper will give an overview about the BiROS satellite, the OSIRIS payload and the performance of the system, including space-qualification of the hardware and transmission tests

Using experimental and computational energy equilibration to understand hierarchical self-assembly of Fmoc-dipeptide amphiphiles

Despite progress, a fundamental understanding of the relationships between the molecular structure and self-assembly configuration of Fmoc-dipeptides is still in its infancy. In this work, we provide a combined experimental and computational approach that makes use of free energy equilibration of a number of related Fmoc-dipeptides to arrive at an atomistic model of Fmoc-threonine-phenylalanine-amide (Fmoc-TF-NH2) which forms twisted fibres. By using dynamic peptide libraries where closely related dipeptide sequences are dynamically exchanged to eventually favour the formation of the thermodynamically most stable configuration, the relative importance of C-terminus modifications (amide versus methyl ester) and contributions of aliphatic versus aromatic amino acids (phenylalanine F vs. leucine L) is determined (F > L and NH2 > OMe). The approach enables a comparative interpretation of spectroscopic data, which can then be used to aid the construction of the atomistic model of the most stable structure (Fmoc-TF-NH2). The comparison of the relative stabilities of the models using molecular dynamic simulations and the correlation with experimental data using dynamic peptide libraries and a range of spectroscopy methods (FTIR, CD, fluorescence) allow for the determination of the nanostructure with atomistic resolution. The final model obtained through this process is able to reproduce the experimentally observed formation of intertwining fibres for Fmoc-TF-NH2, providing information of the interactions involved in the hierarchical supramolecular self-assembly. The developed methodology and approach should be of general use for the characterization of supramolecular structures

Trans-Mississippi Southerners in the Union Army, 1862-1865

Men from throughout the Trans-Mississippi South enlisted in the Union army during the Civil War both in existing northern regiments and in units raised specifically for the purpose of enlisting southerners. The men who joined and fought represented almost every social and ethnic division within the region and contributed substantially to the success of Union arms during the war. Examining a single regiment from each state or territory in the region (except Louisiana, where one white and one black unit were chosen due to segregation) reveals similarities of background, experience and purpose. Louisiana\u27s contributions to the Union army were primarily black soldiers, although a smaller number of white immigrants and freeholders also served. Texas\u27 contribution was equally divided between native-born southerners and Hispanics, while the Indian Territory contributed Native Americans from several southern tribes. Arkansas\u27 Union soldiers were split equally between white farmers from the northwestern corner of the state and freed slaves from the southeast. Service varied among the several regiments, but included active campaigning, anti-guerrilla operations and the far more mundane garrison duty. Men succumbed to disease in extraordinary numbers due in part to their position at the end of an extended logistic system in an ignored backwater of the war. These southerners represent the staunchest internal opposition to the Confederacy and contributed significantly to the restoration of Federal authority. Whatever their background these soldiers possessed a strong ideological attachment to the Union and endured severe hardships and oppression in order to vindicate a cause many valued more than their own lives

The Chicago Board of Trade Battery in the Civil War

In The Chicago Board of Trade Battery in the Civil War, Dennis W. Belcher not only illuminates the experiences of the battery’s men, but “also sheds new light on artillery units” which reviewer Christopher M. Reign writes is “a topic of recent interest among military historians of the war.” The book is an “essential building block” from which future researchers will “benefit,” Reign concludes

Leadership Lessons: The Campaigns for Vicksburg, 1862-1863

Taking Civil War Leaders to Task Kevin Dougherty’s Leadership Lessons: The Campaigns for Vicksburg, 1862-1863 offers a concise summary of the events that led to the city’s capture in July, 1863, but focuses on the campaign’s utility as a primer for modern leaders in “war, busines...

Quantitative phase mixing for Hamiltonians with trapping

We prove quantitative decay estimates of macroscopic quantities generated by the solutions to linear transport equations driven by a general family of Hamiltonians. The associated particle trajectories are all trapped in a compact region of phase-space and feature a non-degenerate elliptic stagnation point. The analysis covers a large class of Hamiltonians generated by the radially symmetric compactly supported equilibria of the gravitational Vlasov-Poisson system. Working in radial symmetry, our analysis features both the 1+2-dimensional case and the harder 1+1-dimensional case, where all the particles have the same value of the modulus of angular momentum. The latter case is also of importance in both the plasma physics case and two dimensional incompressible fluid flows.Comment: 60 pages, added the decay of the macroscopic density to Theorem 1.