Comparative study of wall shear stress at the ascending aorta for different mechanical heart valve prostheses

An experimental study is reported which investigates the wall shear stress (WSS) distribution in a transparent model of the human aorta comparing a bileaflet mechanical heart valve (BMHV) with a trileaflet mechanical heart valve (TMHV) in physiological pulsatile flow. Elastic micro-pillar WSS sensors, calibrated by micro-Particle-Image-Velocimetry measurement, are applied to the wall along the ascending aorta. Peak WSS values are observed almost twice in BMHV compared to TMHV. Flow field analyses illuminate that these peaks are linked to the jet-like flows generated in the valves interacting with the aortic wall. Not only the magnitude but also the impact regions are specific for the different valve designs. The side-orifice jets generated by BMHV travel along the aortic wall in the ascending aorta and cause a whole range impact, while the jets generated by TMHV impact further downstream in the ascending aortic generating less severe WSS

Phylogenetic structure and formation mechanism of shrub communities in arid and semiarid areas of the Mongolian Plateau

The mechanisms of species coexistence within a community have always been the focus in ecological research. Community phylogenetic structure reflects the relationship of historical processes, regional environments, and interactions between species, and studying it is imperative to understand the formation and maintenance mechanisms of community composition and biodiversity. We studied the phylogenetic structure of the shrub communities in arid and semiarid areas of the Mongolian Plateau. First, the phylogenetic signals of four plant traits (height, canopy, leaf length, and leaf width) of shrubs and subshrubs were measured to determine the phylogenetic conservation of these traits. Then, the net relatedness index (NRI) of shrub communities was calculated to characterize their phylogenetic structure. Finally, the relationship between the NRI and current climate and paleoclimate (since the Last Glacial Maximum, LGM) factors was analyzed to understand the formation and maintenance mechanisms of these plant communities. We found that desert shrub communities showed a trend toward phylogenetic overdispersion; that is, limiting similarity was predominant in arid and semiarid areas of the Mongolian Plateau despite the phylogenetic structure and formation mechanisms differing across habitats. The typical desert and sandy shrub communities showed a significant phylogenetic overdispersion, while the steppified desert shrub communities showed a weak phylogenetic clustering. It was found that mean winter temperature (i.e., in the driest quarter) was the major factor limiting steppified desert shrub phylogeny distribution. Both cold and drought (despite having opposite consequences) differentiated the typical desert to steppified desert shrub communities. The increase in temperature since the LGM is conducive to the invasion of shrub plants into steppe grassland, and this process may be intensified by global warming

A simple theory of protein folding kinetics

We present a simple model of protein folding dynamics that captures key qualitative elements recently seen in all-atom simulations. The goals of this theory are to serve as a simple formalism for gaining deeper insight into the physical properties seen in detailed simulations as well as to serve as a model to easily compare why these simulations suggest a different kinetic mechanism than previous simple models. Specifically, we find that non-native contacts play a key role in determining the mechanism, which can shift dramatically as the energetic strength of non-native interactions is changed. For protein-like non-native interactions, our model finds that the native state is a kinetic hub, connecting the strength of relevant interactions directly to the nature of folding kinetics

Assessing the preservation of cytosine methylation in ancient DNA from five prehistoric Native American populations

textCytosine methylation of CpG dinucleotides is an important epigenetic mark that regulates gene expression in humans. While methylation patterns in extant populations have been widely studied, few studies have attempted to analyze methylation in ancient DNA. Indeed, it was only recently shown that methyl groups can be preserved in ancient DNA. However, it is unknown how often methylation patterns can be recovered from ancient samples with preserved nuclear DNA. If they are frequently preserved, it may ultimately be possible to infer patterns of gene activity at the population level in ancient times. In this study, I assessed the preservation of cytosine methylation in ancient DNA from the remains of 30 prehistoric Native Americans from California, Illinois, Kentucky, and Mexico. These samples were previously shown to contain endogenous mitochondrial and nuclear DNA. I analyzed the cytosine methylation states of CpG-rich retrotransposons, which are epigenetically inactivated by cytosine methylation in humans. Unmethylated cytosines were converted to uracils by treatment with sodium bisulfite. Bisulfite products were pyrosequenced, and C-to-T conversions at potentially methylated CpG dinucleotides were quantified. I found that cytosine methylation is readily recoverable from human remains with preserved nuclear DNA from various localities over the time depth tested (~6000 years). This study presents the first direct evidence of cytosine methylation in ancient human remains, and suggests that it may be possible to analyze patterns of gene activity in ancient populations.Anthropolog

Metrics for measuring distances in configuration spaces

In order to characterize molecular structures we introduce configurational fingerprint vectors which are counterparts of quantities used experimentally to identify structures. The Euclidean distance between the configurational fingerprint vectors satisfies the properties of a metric and can therefore safely be used to measure dissimilarities between configurations in the high dimensional configuration space. We show that these metrics correlate well with the RMSD between two configurations if this RMSD is obtained from a global minimization over all translations, rotations and permutations of atomic indices. We introduce a Monte Carlo approach to obtain this global minimum of the RMSD between configurations