From chaos to order: Chain-length dependence of the free energy of formation of meso-tetraalkylporphyrin self-assembled monolayer polymorphs

© 2016 American Chemical Society. We demonstrate that systematic errors can be reduced and physical insight gained through investigation of the dependence of free energies for meso-tetraalkylporphyrin self-assembled monolayers (SAMs) polymorphism on the alkyl chain length m. These SAMs form on highly ordered pyrolytic graphite (HOPG) from organic solution, displaying manifold densities and atomic structures. SAMs with m = 11-19 are investigated experimentally while those with m = 6-28 are simulated using density-functional theory (DFT). It is shown that, for m = 15 or more, the alkyl chains crystallize to dominate SAM structure. Meso-tetraalkylporphyrin SAMs of length less than 11 have never been observed, a presumed effect of inadequate surface attraction. Instead, we show that free energies of SAM formation actually enhance as the chain length decreases. The inability to image regular SAMs stems from the appearance of many polymorphic forms of similar free energy, preventing SAM ordering. We also demonstrate a significant odd/even effect in SAM structure arising from packing anomalies. Comparison of the chain-length dependence of formation free energies allows the critical dispersion interactions between molecules, solvent, and substrate to be directly examined. Interpretation of the STM data combined with measured enthalpies indicates that Grimme's D3 explicit-dispersion correction and the implicit solvent correction of Floris, Tomasi and Pascual Ahuir are both quantitatively accurate and very well balanced to each other

Evolutionary-inspired probabilistic search for enhancing sampling of local minima in the protein energy surface

<p>Abstract</p> <p>Background</p> <p>Despite computational challenges, elucidating conformations that a protein system assumes under physiologic conditions for the purpose of biological activity is a central problem in computational structural biology. While these conformations are associated with low energies in the energy surface that underlies the protein conformational space, few existing conformational search algorithms focus on explicitly sampling low-energy local minima in the protein energy surface.</p> <p>Methods</p> <p>This work proposes a novel probabilistic search framework, PLOW, that explicitly samples low-energy local minima in the protein energy surface. The framework combines algorithmic ingredients from evolutionary computation and computational structural biology to effectively explore the subspace of local minima. A greedy local search maps a conformation sampled in conformational space to a nearby local minimum. A perturbation move jumps out of a local minimum to obtain a new starting conformation for the greedy local search. The process repeats in an iterative fashion, resulting in a trajectory-based exploration of the subspace of local minima.</p> <p>Results and conclusions</p> <p>The analysis of PLOW's performance shows that, by navigating only the subspace of local minima, PLOW is able to sample conformations near a protein's native structure, either more effectively or as well as state-of-the-art methods that focus on reproducing the native structure for a protein system. Analysis of the actual subspace of local minima shows that PLOW samples this subspace more effectively that a naive sampling approach. Additional theoretical analysis reveals that the perturbation function employed by PLOW is key to its ability to sample a diverse set of low-energy conformations. This analysis also suggests directions for further research and novel applications for the proposed framework.</p

Ethical behavior, Leadership, and Decision Making

Ethical principles applied in organizations can broaden individual and corporate priorities beyond profit and shareholder enrichment. Ethical factors may influence leaders to make sound decisions to protect the organization from unethical behavior. The purpose of this study was to examine if a correlation exists between the independent variables of leadership and decision making, and the dependent variable of ethical behavior. Drucker\u27s theory of management served as the theoretical framework. Data collection involved 2 survey instruments, the Authentic Leadership Questionnaire and the Moral Potency Questionnaire from 98 participants from retail businesses in the southwestern United States. The model as a whole was able to significantly predict ethical behavior (F(2, 95) = 12.79, p \u3c .01), R-² = .21. However, none of the individual predictors was significant. The existence of multicollinearity between the 2-predictor variables offers a plausible explanation for this phenomenon. Therefore, these results should be viewed with caution. Implications for positive social change include examining behaviors and leadership capabilities in individuals, communities, organizations, and institutions. The findings may contribute to social change by providing leaders with information to improve strategies when making decisions involving ethical behavior in the workplace