Stability of racemic and chiral steady states in open and closed chemical systems

The stability properties of models of spontaneous mirror symmetry breaking in chemistry are characterized algebraically. The models considered here all derive either from the Frank model or from autocatalysis with limited enantioselectivity. Emphasis is given to identifying the critical parameter controlling the chiral symmetry breaking transition from racemic to chiral steady-state solutions. This parameter is identified in each case, and the constraints on the chemical rate constants determined from dynamic stability are derived.Comment: 25 pages, 1 figure. To appear in Physics Letters A (2008

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

Chiral Crystal Growth under Grinding

To study the establishment of homochirality observed in the crystal growth experiment of chiral molecules from a solution under grinding, we extend the lattice gas model of crystal growth as follows. A lattice site can be occupied by a chiral molecule in R or S form, or can be empty. Molecules form homoclusters by nearest neighbor bonds. They change their chirality if they are isolated monomers in the solution. Grinding is incorporated by cutting and shafling the system randomly. It is shown that Ostwald ripening without grinding is extremely slow to select chirality, if possible. Grinding alone also cannot achieve chirality selection. For the accomplishment of homochirality, we need an enhanced chirality change on crystalline surface. With this "autocatalytic effect" and the recycling of monomers due to rinding, an exponential increase of crystal enantiomeric excess to homochiral state is realized.Comment: 10 pages, 5 figure

Generalist care managers for the treatment of depressed medicaid patients in North Carolina: A pilot study

BACKGROUND: In most states, mental illness costs are an increasing share of Medicaid expenditures. Specialized depression care managers (CM) have consistently demonstrated improvements in patient outcomes relative to usual primary care (UC), but are costly and may not be fully utilized in smaller practices. A generalist care manager (GCM) could manage multiple chronic conditions and be more accepted and cost-effective than the specialist depression CM. We designed a pilot program to demonstrate the feasibility of training/deploying GCMs into primary care settings. METHODS: We randomized depressed adult Medicaid patients in 2 primary care practices in Western North Carolina to a GCM intervention or to UC. GCMs, already providing services in diabetes and asthma in both study arms, were further trained to provide depression services including self-management, decision support, use of information systems, and care management. The following data were analyzed: baseline, 3- and 6-month Patient Health Questionnaire (PHQ9) scores; baseline and 6-month Short Form (SF) 12 scores; Medicaid claims data; questionnaire on patients' perceptions of treatment; GCM case notes; physician and office staff time study; and physician and office staff focus group discussions. RESULTS: Forty-five patients were enrolled, the majority with preexisting depression. Both groups improved; the GCM group did not demonstrate better clinical and functional outcomes than the UC group. Patients in the GCM group were more likely to have prescriptions of correct dosing by chart data. GCMs most often addressed comorbid conditions (36%), then social issues (27%) and appointment reminders (14%). GCMs recorded an average of 46 interactions per patient in the GCM arm. Focus group data demonstrated that physicians valued using GCMs. A time study documented that staff required no more time interacting with GCMs, whereas physicians spent an average of 4 minutes more per week. CONCLUSION: GCMs can be trained in care of depression and other chronic illnesses, are acceptable to practices and patients, and result in physicians prescribing guideline concordant care. GCMs appear to be a feasible intervention for community medical practices and to warrant a larger scale trial to test their appropriateness for Medicaid programs nationally

The influence of multi-morbidity and self-reported socio-economic standing on the prevalence of depression in an elderly Hong Kong population

<b>Background</b> There has been an increasing prevalence of both depression and chronic medical conditions globally but the relationship between depression and multi-morbidity is not well understood. The aim of the present study was to investigate the relationship between depression, multi-morbidity (number of chronic medical conditions, and measures of socioeconomic standing (SES) in an elderly Hong Kong population.<p></p> <b>Methods</b> Cross sectional study. Information on clinically relevant depressive symptoms, measured by the Geriatric Depression Scale (GDS), and demographic and chronic medical conditions were collected using standardized questionnaires. Information collected on SES included educational status (ES), maximum ever income (MEI), and self-perceived social standing in local community (SES-COM) and in Hong Kong generally (SES-HK). Analysis was conducted using multiple logistic regression.<p></p> <b>Results</b> Depression rates were similar in men and women (GDS caseness 8.1% vs 8.4%). Multi-morbidity of chronic medical conditions was common (40% of men and 46% of women had three or more). In the overall sample, the prevalence of depression was associated with the number of chronic medical conditions (OR 1.27; CI: 1.16–1.39). In addition, SES-HK and SES-COM were significant independent variables.<p></p> <b>Conclusion</b> In this elderly Hong Kong population, depression prevalence rose markedly with number of chronic medical conditions and SES-HK and SES-COM

Speeding up Viedma Deracemization through Water catalyzed and Reactant Self‐catalyzed Racemization

Viedma deracemization is based on solution phase racemization, dissolution of racemic or scalemic conglomerates and crystal growth through autocatalytic cluster formation. With rate limiting racemization, its acceleration by appropriate catalysts may result in speeding up deracemization. A conglomerate‐forming chiral compound may principally racemize directly, or via reverse of its formation reaction. For a hydrazine derivative, we investigated available racemization pathways in presence of pyrrolidine or thiourea amine as base catalysts: via Mannich or aza‐Michael reaction steps and their reverse, or by enolization. Racemization by enolization was computationally found to dominate, both under water‐free conditions and in presence of water, involving a multitude of different pathways. Faster racemization in presence of water resulted indeed in more rapid deracemization, when the base was pyrrolidine. Under water‐free conditions, the role of water as enolization catalyst is assumed by chiral hydrazine itself – in autocatalytic racemization and in which both reactant and product are catalysts