254 research outputs found
The influence of mesh structure on simulated water levels and flow velocities in meander rivers
Stochastic Approach to Enantiomeric Excess Amplification and Chiral Symmetry Breaking
Stochastic aspects of chemical reaction models related to the Soai reactions
as well as to the homochirality in life are studied analytically and
numerically by the use of the master equation and random walk model. For
systems with a recycling process, a unique final probability distribution is
obtained by means of detailed balance conditions. With a nonlinear
autocatalysis the distribution has a double-peak structure, indicating the
chiral symmetry breaking. This problem is further analyzed by examining
eigenvalues and eigenfunctions of the master equation. In the case without
recycling process, final probability distributions depend on the initial
conditions. In the nonlinear autocatalytic case, time-evolution starting from a
complete achiral state leads to a final distribution which differs from that
deduced from the nonzero recycling result. This is due to the absence of the
detailed balance, and a directed random walk model is shown to give the correct
final profile. When the nonlinear autocatalysis is sufficiently strong and the
initial state is achiral, the final probability distribution has a double-peak
structure, related to the enantiomeric excess amplification. It is argued that
with autocatalyses and a very small but nonzero spontaneous production, a
single mother scenario could be a main mechanism to produce the homochirality.Comment: 25 pages, 6 figure
Fluctuation Induced Homochirality
We propose a new mechanism for the achievment of homochirality in life
without any autocatalytic production process. Our model consists of a
spontaneous production together with a recycling cross inhibition in a closed
system.
It is shown that although the rate equations for this system predict no
chiral symmetry breaking, the stochastic master equation predicts complete
homochirality. This is because the fluctuation induced by the discreteness of
population numbers of participating molecules plays essential roles. This
fluctuation conspires with the recyling cross inhibition to realize the
homochirality.Comment: 13 pages, 6 figure
Kinetics of the urea–urease clock reaction with urease immobilized in hydrogel beads
Feedback driven by enzyme catalyzed reactions occurs widely in biology and has been well characterized in single celled organisms such as yeast. There are still few examples of robust enzyme oscillators in vitro that might be used to study nonlinear dynamical behavior. One of the simplest is the urea–urease reaction that displays autocatalysis driven by the increase in pH accompanying the production of ammonia. A clock reaction was obtained from low to high pH in batch reactor and bistability and oscillations were reported in a continuous flow rector. However, the oscillations were found to be irreproducible and one contributing factor may be the lack of stability of the enzyme in solution at room temperature. Here, we investigated the effect of immobilizing urease in thiol-poly(ethylene glycol) acrylate (PEGDA) hydrogel beads, prepared using emulsion polymerization, on the urea–urease reaction. The resultant mm-sized beads were found to reproduce the pH clock and, under the conditions employed here, the stability of the enzyme was increased from hours to days
Homochirality and the need of energy
The mechanisms for explaining how a stable asymmetric chemical system can be
formed from a symmetric chemical system, in the absence of any asymmetric
influence other than statistical fluctuations, have been developed during the
last decades, focusing on the non-linear kinetic aspects. Besides the absolute
necessity of self-amplification processes, the importance of energetic aspects
is often underestimated. Going down to the most fundamental aspects, the
distinction between a single object -- that can be intrinsically asymmetric --
and a collection of objects -- whose racemic state is the more stable one --
must be emphasized. A system of strongly interacting objects can be described
as one single object retaining its individuality and a single asymmetry; weakly
or non-interacting objects keep their own individuality, and are prone to
racemize towards the equilibrium state. In the presence of energy fluxes,
systems can be maintained in an asymmetric non-equilibrium steady-state. Such
dynamical systems can retain their asymmetry for times longer than their
racemization time.Comment: 8 pages, 7 figures, submitted to Origins of Life and Evolution of
Biosphere
Ester hydrolysis: Conditions for acid autocatalysis and a kinetic switch
Autocatalysis can be used to obtain a sharp switch in state after a programmable time lag. Here, autocatalysis driven by acid concentration during the dissolution and hydrolysis of solid esters was investigated. In a generic model of the process, conditions were identified for observation of a kinetic switch with the introduction of an inhibitor species, bicarbonate, to delay the onset of autocatalysis. The kinetic profiles from the hydrolysis of two esters, d-gluconic acid δ-lactone and dl-lactide, were examined and evidence for dissolution-limited acid autocatalysis was obtained with lactide
Social media and disasters: human security, environmental racism, and crisis communication in Hurricane Irma response
Social media has been widely recognized as key communication channel in disaster situations. However, there is limited empirical investigation on how the intersecting issues of social order, environmental impacts, and crisis communication unfold from the perspective of a social media user. This study examines 60,449 tweets to and from the news media in Florida during and immediately after Hurricane Irma in September, 2017. Based on a critical review of the literature coupled with an eight-category coding scheme (including second-hand reporting, reporting on self-experience, requesting help, coordinating relief efforts, and expressing well wishes), the article assesses the content and timing of tweets before, during, and after the storm. It finds that thematically, twitter coverage not only covers the storm itself but pressing social issues such as looting, price gouging, the privileging of elites in rebuilding efforts, environmental vulnerability, and abandoning pets. Temporally, the volume of different tweets peaked and dropped at different stages; for example, tweets about personal experience peaked when the hurricane hit the ground while requests for help peaked in the days after the hurricane. The study allows for a better understanding of the sociological, environmental, and even social justice impacts and related disaster response through the use of social media
Innovation, low energy buildings and intermediaries in Europe: systematic case study review
As buildings throughout their lifecycle account for circa 40% of total energy use in Europe, reducing energy use of the building stock is a key task. This task is, however, complicated by a range of factors, including slow renewal and renovation rates of buildings, multiple non- coordinated actors, conservative building practices, and limited competence to innovate. Drawing from academic literature published during 2005-2015, this article carries out a systematic review of case studies on low energy innovations in the European residential building sector, analysing their drivers. Specific attention is paid to intermediary actors in facilitating innovation processes and creating new opportunities. The study finds that qualitative case study literature on low energy building innovation has been limited, particularly regarding the existing building stock. Environmental concerns, EU, national and local policies have been the key drivers; financial, knowledge and social sustainability and equity drivers have been of modest importance; while design, health and comfort, and market drivers have played a minor role. Intermediary organisations and individuals have been important through five processes: (1) facilitating individual building projects, (2) creating niche markets, (3) implementing new practices in social housing stock, (4) supporting new business model creation, and (5) facilitating building use post construction. The intermediaries have included both public and private actors, while local authority agents have acted as intermediaries in several cases
Perceptions of involvement in advance care planning and emotional functioning in patients with advanced cancer
Purpose: Advance Care Planning (ACP) is positively associated with the quality of care, but its impact on emotional functioning is ambiguous. This study investigated the association between perceptions of ACP involvement and emotional functioning in patients with advanced cancer. Methods: This study analyzed baseline data of 1,001 patients of the eQuiPe study, a prospective, longitudinal, multicenter, observational study on quality of care and quality of life in patients with advanced cancer in the Netherlands. Patients with metastatic solid cancer were asked to participate between November 2017 and January 2020. Patients’ perceptions of ACP involvement were measured by three self-administered statements. Emotional functioning was measured by the EORTC-QLQ-C30. A linear multivariable regression analysis was performed while taking gender, age, migrant background, education, marital status, and symptom burden into account. Results: The majority of patients (87%) reported that they were as much involved as they wanted to be in decisions about their future medical treatment and care. Most patients felt that their relatives (81%) and physicians (75%) were familiar with their preferences for future medical treatment and care. A positive association was found between patients’ perceptions of ACP involvement and their emotional functioning (b=0.162, p<0.001, 95%CI[0.095;0.229]) while controlling for relevant confounders. Conclusions: Perceptions of involvement in ACP are positively associated with emotional functioning in patients with advanced cancer. Future studies are needed to further investigate the effect of ACP on emotional functioning. Trial registration number: NTR6584 Date of registration: 30 June 2017 Implications for Cancer Survivors: Patients’ emotional functioning might improve from routine discussions regarding goals of future care. Therefore, integration of ACP into palliative might be promising
Practical Applications as a Source of Credibility: A Comparison of Three Fields of Dutch Academic Chemistry
In many Western science systems, funding structures increasingly stimulate academic research to contribute to practical applications, but at the same time the rise of bibliometric performance assessments have strengthened the pressure on academics to conduct excellent basic research that can be published in scholarly literature. We analyze the interplay between these two developments in a set of three case studies of fields of chemistry in the Netherlands. First, we describe how the conditions under which academic chemists work have changed since 1975. Second, we investigate whether practical applications have become a source of credibility for individual researchers. Indeed, this turns out to be the case in catalysis, where connecting with industrial applications helps in many steps of the credibility cycle. Practical applications yield much less credibility in environmental chemistry, where application-oriented research agendas help to acquire funding, but not to publish prestigious papers or to earn peer recognition. In biochemistry practical applications hardly help in gaining credibility, as this field is still strongly oriented at fundamental questions. The differences between the fields can be explained by the presence or absence of powerful upstream end-users, who can afford to invest in academic research with promising long term benefits
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