An Extended Model for the Evolution of Prebiotic Homochirality: A Bottom-Up Approach to the Origin of Life

A generalized autocatalytic model for chiral polymerization is investigated in detail. Apart from enantiomeric cross-inhibition, the model allows for the autogenic (non-catalytic) formation of left and right-handed monomers from a substrate with reaction rates $\epsilon_L$ and $\epsilon_R$, respectively. The spatiotemporal evolution of the net chiral asymmetry is studied for models with several values of the maximum polymer length, N. For N=2, we study the validity of the adiabatic approximation often cited in the literature. We show that the approximation obtains the correct equilibrium values of the net chirality, but fails to reproduce the short time behavior. We show also that the autogenic term in the full N=2 model behaves as a control parameter in a chiral symmetry- breaking phase transition leading to full homochirality from racemic initial conditions. We study the dynamics of the N -> infinity model with symmetric ($\epsilon_L = \epsilon_R$) autogenic formation, showing that it only achieves homochirality for $\epsilon < \epsilon_c$, where $\epsilon_c$ is an N-dependent critical value. For $\epsilon \leq \epsilon_c$ we investigate the behavior of models with several values of N, showing that the net chiral asymmetry grows as tanh(N). We show that for a given symmetric autogenic reaction rate, the net chirality and the concentrations of chirally pure polymers increase with the maximum polymer length in the model. We briefly discuss the consequences of our results for the development of homochirality in prebiotic Earth and possible experimental verification of our findings

Punctuated Chirality

Most biomolecules occur in mirror, or chiral, images of each other. However, life is homochiral: proteins contain almost exclusively levorotatory (L) amino acids, while only dextrorotatory (R) sugars appear in RNA and DNA. The mechanism behind this fundamental asymmetry of life remains an open problem. Coupling the spatiotemporal evolution of a general autocatalytic polymerization reaction network to external environmental effects, we show through a detailed statistical analysis that high intensity and long duration events may drive achiral initial conditions towards chirality. We argue that life's homochirality resulted from sequential chiral symmetry breaking triggered by environmental events, thus extending the theory of punctuated equilibrium to the prebiotic realm. Applying our arguments to other potentially life-bearing planetary platforms, we predict that a statistically representative sampling will be racemic on average.Comment: 13 pages, 4 color figures. Final version published in Origins of Life and Evolution of Biospheres. Typos corrected, figures improved, and a few definitions and word usage clarifie

Knowledge systems, health care teams, and clinical practice: a study of successful change

Clinical teams are of growing importance to healthcare delivery, but little is known about how teams learn and change their clinical practice. We examined how teams in three US hospitals succeeded in making significant practice improvements in the area of antimicrobial resistance. This was a qualitative cross-case study employing Soft Knowledge Systems as a conceptual framework. The purpose was to describe how teams produced, obtained, and used knowledge and information to bring about successful change. A purposeful sampling strategy was used to maximize variation between cases. Data were collected through interviews, archival document review, and direct observation. Individual case data were analyzed through a two-phase coding process followed by the cross-case analysis. Project teams varied in size and were multidisciplinary. Each project had more than one champion, only some of whom were physicians. Team members obtained relevant knowledge and information from multiple sources including the scientific literature, experts, external organizations, and their own experience. The success of these projects hinged on the teams' ability to blend scientific evidence, practical knowledge, and clinical data. Practice change was a longitudinal, iterative learning process during which teams continued to acquire, produce, and synthesize relevant knowledge and information and test different strategies until they found a workable solution to their problem. This study adds to our understanding of how teams learn and change, showing that innovation can take the form of an iterative, ongoing process in which bits of K&I are assembled from multiple sources into potential solutions that are then tested. It suggests that existing approaches to assessing the impact of continuing education activities may overlook significant contributions and more attention should be given to the role that practical knowledge plays in the change process in addition to scientific knowledge

QCD and strongly coupled gauge theories : challenges and perspectives

We highlight the progress, current status, and open challenges of QCD-driven physics, in theory and in experiment. We discuss how the strong interaction is intimately connected to a broad sweep of physical problems, in settings ranging from astrophysics and cosmology to strongly coupled, complex systems in particle and condensed-matter physics, as well as to searches for physics beyond the Standard Model. We also discuss how success in describing the strong interaction impacts other fields, and, in turn, how such subjects can impact studies of the strong interaction. In the course of the work we offer a perspective on the many research streams which flow into and out of QCD, as well as a vision for future developments.Peer reviewe