Computational study of the thermal conductivity in defective carbon nanostructures

We use non-equilibrium molecular dynamics simulations to study the adverse role of defects including isotopic impurities on the thermal conductivity of carbon nanotubes, graphene and graphene nanoribbons. We find that even in structurally perfect nanotubes and graphene, isotopic impurities reduce thermal conductivity by up to one half by decreasing the phonon mean free path. An even larger thermal conductivity reduction, with the same physical origin, occurs in presence of structural defects including vacancies and edges in narrow graphene nanoribbons. Our calculations reconcile results of former studies, which differed by up to an order of magnitude, by identifying limitations of various computational approaches

Non-equilibrium two-phase coexistence in a confined granular layer

We report the observation of the homogenous nucleation of crystals in a dense layer of steel spheres confined between two horizontal plates vibrated vertically. Above a critical vibration amplitude, two-layer crystals with square symmetry were found to coexist in steady state with a surrounding granular liquid. By analogy to equilibrium hard sphere systems, the phase behavior can be explained through entropy maximization. However, dramatic non-equilibrium effects are present, including a significant difference in the granular temperatures of the two phases.Comment: 4 pages, 3 figures, RevTex4 forma

Dynamic Pathways for Viral Capsid Assembly

We develop a class of models with which we simulate the assembly of particles into T1 capsid-like objects using Newtonian dynamics. By simulating assembly for many different values of system parameters, we vary the forces that drive assembly. For some ranges of parameters, assembly is facile, while for others, assembly is dynamically frustrated by kinetic traps corresponding to malformed or incompletely formed capsids. Our simulations sample many independent trajectories at various capsomer concentrations, allowing for statistically meaningful conclusions. Depending on subunit (i.e., capsomer) geometries, successful assembly proceeds by several mechanisms involving binding of intermediates of various sizes. We discuss the relationship between these mechanisms and experimental evaluations of capsid assembly processes.Comment: 13 pages, 13 figures. Submitted to Biophys.

Effects of an Unusual Poison Identify a Lifespan Role for Topoisomerase 2 in Saccharomyces Cerevisiae

A progressive loss of genome maintenance has been implicated as both a cause and consequence of aging. Here we present evidence supporting the hypothesis that an age-associated decay in genome maintenance promotes aging in Saccharomyces cerevisiae (yeast) due to an inability to sense or repair DNA damage by topoisomerase 2 (yTop2). We describe the characterization of LS1, identified in a high throughput screen for small molecules that shorten the replicative lifespan of yeast. LS1 accelerates aging without affecting proliferative growth or viability. Genetic and biochemical criteria reveal LS1 to be a weak Top2 poison. Top2 poisons induce the accumulation of covalent Top2-linked DNA double strand breaks that, if left unrepaired, lead to genome instability and death. LS1 is toxic to cells deficient in homologous recombination, suggesting that the damage it induces is normally mitigated by genome maintenance systems. The essential roles of yTop2 in proliferating cells may come with a fitness trade-off in older cells that are less able to sense or repair yTop2-mediated DNA damage. Consistent with this idea, cells live longer when yTop2 expression levels are reduced. These results identify intrinsic yTop2-mediated DNA damage as a potentially manageable cause of aging

A Novel Caloric Restriction-Like Mimetic Affects Longevity in Yeast by Reprogramming Core Metabolic Pathways

Glucose limitation is a simple intervention that extends yeast replicative lifespan (RLS) via the same pathway(s) thought to mediate the benefits of caloric restriction (CR) in mammals. Here we report on “C1”, a small molecule that mimics key aspects of CR. C1 was identified in a high throughput screen for drug-like molecules that reverse the RLS shortening effect of the sirtuin inhibitor and NAD+ precursor nicotinamide. C1 reduces the cellular dependence on glycolysis and the pentose phosphate pathway, even in the presence of glucose, and compensates by elevating fatty acid -oxidation to maintain acetyl-CoA levels. C1 acts either downstream of Sir2 or in an independent CR pathway. In this regard, chemical-genetic interactions indicate that C1 influences Tor2 signaling via effects on phosphoinositide pools. Key activities of C1 extend to mammals. C1 stimulates -oxidation in mammalian cells, and in mice, reduces levels of triacylglycerides and cholesterol in livers of lean and obese mice. C1 confers oxidative resistance to diamide in both yeast and mammalian cells. In conclusion, C1 induces global changes in metabolism in yeast and mammalian cells that mimic aspects of CR. Future work will be aimed at identifying the cellular target of C1

HLA and cross-reactive antigen group matching for cadaver kidney allocation

Background. Allocation of cadaver kidneys by graded human leukocyte antigen (HLA) compatibility scoring arguably has had little effect on overall survival while prejudicing the transplant candidacy of African-American and other hard to match populations. Consequently, matching has been proposed of deduced amino acid residues of the individual HLA molecules shared by cross- reactive antigen groups (CREGs). We have examined the circumstances under which compatibility with either method impacted graft survival. Methods. Using Cox proportional hazards regression modeling, we studied the relationship between levels of conventional HLA mismatch and other donor and recipient factors on primary cadaver kidney survival between 1981 and 1995 at the University of Pittsburgh (n=1,780) and in the United Network for Organ Sharing (UNOS) Scientific Registry during 1991-1995 (n=31,291). The results were compared with those obtained by the matching of amino acid residues that identified CREG-compatible cases with as many as four (but not five and six) HLA mismatches. Results. With more than one HLA mismatch (>85% of patients in both series), most of the survival advantage of a zero mismatch was lost. None of the HLA loci were 'weak.' In the UNOS (but not Pittsburgh) category of one-HLA mismatch (n=1334), a subgroup of CREG-matched recipients (35.3%) had better graft survival than the remaining 64.7%, who were CREG-mismatched. There was no advantage of a CREG match in the two- to four-HLA incompatibility tiers. Better graft survival with tacrolimus was observed in both the Pittsburgh and UNOS series. Conclusions. Obligatory national sharing of cadaver kidneys is justifiable only for zero-HLA-mismatched kidneys. The potential value of CREG matching observed in the one-HLA-mismatched recipients of the UNOS (but not the Pittsburgh) experience deserves further study

Sum rules for isospin centroids in pick-up reactions on general multishell target states

Sum Rules equations for pick-up reactions are presented for the first time for the energy centroids of states both for the isospin T_< (\equiv T_0 - 1 \over 2) and T_> (\equiv T_0 + {1 \over 2}) of the final nucleus when a nucleon is picked up from a general multishell target state with isospin T_0. These equations contain two-body correlation terms, , which, at the present moment, are difficult to handle analytically. These terms are managed by combining these equations with the known stripping reactions equations. Sample applications of these equations to experimental data are presented.Comment: 11 pages, LaTe

Unusually High Thermal Conductivity of Carbon Nanotubes

Combining equilibrium and non-equilibrium molecular dynamics simulations with accurate carbon potentials, we determine the thermal conductivity $\lambda$ of carbon nanotubes and its dependence on temperature. Our results suggest an unusually high value ${\lambda}{\approx}6,600$~W/m$\cdot$K for an isolated (10,10) nanotube at room temperature, comparable to the thermal conductivity of a hypothetical isolated graphene monolayer or diamond. Our results suggest that these high values of $\lambda$ are associated with the large phonon mean free paths in these systems; substantially lower values are predicted and observed for the basal plane of bulk graphite.Comment: 4 pages 3 figures (5 postscript files), submitted for publicatio

Autistic young people adaptively use gaze to facilitate joint attention during multi-gestural dyadic interactions

Autistic people often experience difficulties navigating face-to-face social interactions. Historically, the empirical literature has characterised these difficulties as cognitive ‘deficits’ in social information processing. However, the empirical basis for such claims is lacking, with most studies failing to capture the complexity of social interactions, often distilling them into singular communicative modalities (e.g. gaze-based communication) that are rarely used in isolation in daily interactions. The current study examined how gaze was used in concert with communicative hand gestures during joint attention interactions. We employed an immersive virtual reality paradigm, where autistic (n = 22) and non-autistic (n = 22) young people completed a collaborative task with a non-autistic confederate. Integrated eye-, head- and hand-motion-tracking enabled dyads to communicate naturally with each other while offering objective measures of attention and behaviour. Autistic people in our sample were similarly, if not more, effective in responding to hand-cued joint attention bids compared with non-autistic people. Moreover, both autistic and non-autistic people demonstrated an ability to adaptively use gaze information to aid coordination. Our findings suggest that the intersecting fields of autism and social neuroscience research may have overstated the role of eye gaze during coordinated social interactions. Lay abstract: Autistic people have been said to have ‘problems’ with joint attention, that is, looking where someone else is looking. Past studies of joint attention have used tasks that require autistic people to continuously look at and respond to eye-gaze cues. But joint attention can also be done using other social cues, like pointing. This study looked at whether autistic and non-autistic young people use another person’s eye gaze during joint attention in a task that did not require them to look at their partner’s face. In the task, each participant worked together with their partner to find a computer-generated object in virtual reality. Sometimes the participant had to help guide their partner to the object, and other times, they followed their partner’s lead. Participants were told to point to guide one another but were not told to use eye gaze. Both autistic and non-autistic participants often looked at their partner’s face during joint attention interactions and were faster to respond to their partner’s hand-pointing when the partner also looked at the object before pointing. This shows that autistic people can and do use information from another person’s eyes, even when they don’t have to. It is possible that, by not forcing autistic young people to look at their partner’s face and eyes, they were better able to gather information from their partner’s face when needed, without being overwhelmed. This shows how important it is to design tasks that provide autistic people with opportunities to show what they can do