Biocatalytic Synthesis of Stereospecific Triketide Lactones using Polyketide Synthases

Polyketide synthases are modular enzymes that create and modify large acyl chains. The domains and modules of polyketide synthases allow us to create molecules that resemble naturally occurring products by applying a biocatalytic in vitro in vivo approach to a diketide acyl chain. We showed that a triketide lactone of desired stereochemistry could be made using a domain and module from the polyketide synthase found in Saccharopolyspora erythraea, 6-Deoxyerythronolide B Synthase. Future projects will explore this approach using different domains and modules.Biochemistr

Simplicial Multivalued Maps and the Witness Complex for Dynamical Analysis of Time Series

Topology based analysis of time-series data from dynamical systems is powerful: it potentially allows for computer-based proofs of the existence of various classes of regular and chaotic invariant sets for high-dimensional dynamics. Standard methods are based on a cubical discretization of the dynamics and use the time series to construct an outer approximation of the underlying dynamical system. The resulting multivalued map can be used to compute the Conley index of isolated invariant sets of cubes. In this paper we introduce a discretization that uses instead a simplicial complex constructed from a witness-landmark relationship. The goal is to obtain a natural discretization that is more tightly connected with the invariant density of the time series itself. The time-ordering of the data also directly leads to a map on this simplicial complex that we call the witness map. We obtain conditions under which this witness map gives an outer approximation of the dynamics, and thus can be used to compute the Conley index of isolated invariant sets. The method is illustrated by a simple example using data from the classical H\'enon map.Comment: laTeX, 9 figures, 32 page

Wettability-independent droplet transport by \emph{Bendotaxis}

We demonstrate \textit{bendotaxis}, a novel mechanism for droplet self-transport at small scales. A combination of bending and capillarity in a thin channel causes a pressure gradient that, in turn, results in the spontaneous movement of a liquid droplet. Surprisingly, the direction of this motion is always the same, regardless of the wettability of the channel. We use a combination of experiments at a macroscopic scale and a simple mathematical model to study this motion, focussing in particular on the time scale associated with the motion. We suggest that \emph{bendotaxis} may be a useful means of transporting droplets in technological applications, for example in developing self-cleaning surfaces, and discuss the implications of our results for such applications.Comment: 5 pages, 4 figures. Supplementary Information available on reques

Evaluating the share performance of socially responsible investment on the Johannesburg Stock Exchange

Socially responsible investing (SRI) integrates environmental, social and governance (ESG) issues into the investment decision-making process. Growing ESG concerns and the uncovering of corporate scandals have catalysed the substantial growth in SRI portfolios worldwide. Notwithstanding its increasing popularity, barriers to further SRI growth have been identified. Traditional investing practices suggest that theoretically, SRI may underperform conventional investment strategies. However, despite the vast amount of literature on SRI, empirical studies have yielded a mixture of results regarding fund performance. The JSE SRI Index was launched in 2004 to promote transparent business practices. It was discontinued at the end of 2015 succeeded by a new Responsible Investment Index established by the JSE in association with FTSE Russell. The aim of the research was to evaluate the share performance of the JSE SRI Index from 2004-2015. Additionally, the indices were categorised by environmental impact to further analyse disparity among share returns. The study was also divided into two sub-periods, 2004-2009 and 2010-2015, with the latter following the endorsement of integrated reporting by the King III Code as a listing requirement in 2010. A single-factor Capital Asset Pricing Model (CAPM) was used to assess differences in risk-adjusted returns. Engle-Granger and Johansen tests were employed to explore the possibility of a cointegrating relationship between the indices. No significant difference between returns was observed for 2004-2009, with the SRI Index exhibiting statistically significant inferior risk-adjusted returns for the latter half of the study. Overall, a significant difference between share returns was found, with CAPM results suggesting that the JSE SRI Index underperformed the All Share Index by -2.33% per annum throughout the time span of the study. Engle-Granger and Johansen test results indicated the existence of a cointegrating relationship over the first half of the study. However, there was no cointegration between the two indices for 2004-2015, which may be attributed to no significant relationship found for the latter years. Results support the notion that investors pay the price to invest ethically on the JSE. Inferior risk-adjusted returns associated with SRI may be a major barrier to its development in South African markets

Species interactions in ephemeral patch systems: spatial, temporal, and spatio-temporal influences

This dissertation demonstrates the importance of spatial, temporal, and spatio-temporal mechanisms in influencing species interactions within ephemeral patch systems. This work extends prior theory using individual-based modeling to show that the autocorrelation of habitat can have a strong influence on plant-pollinator communities. Pollinators in plant-pollinator communities must regularly disperse to receptive plants, and in model systems often feed on ephemerally available seeds. Because the probability of a seed being eaten by a pollinator will be directly affected by the number of pollinators that visit a plant flower, the frequency at which seed-eating pollinators visit flowers has a major impact on the costs and benefits associated with the mutualism. High pollinator visitation increases per capita pollination, but also the number of seeds consumed. The number of pollinators visiting a plant is expected to increase with increasing plant density. Where plant habitat is especially dense, plant density will be high. If pollinator dispersal is limited, plants in these regions of high density will support high densities of pollinators that are able to move efficiently between plants. As a result of frequent pollinator visitation, plants with many nearby conspecific neighbors will produce disproportionately more pollinators than seeds. Because pollinators compete with non-pollinating exploiters of mutualisms, plants in these locations will also have lowered rates of exploitation. This is the case for the plant-pollinator interaction between the Sonoran Desert rock fig (Ficus petiolaris) and its wasp pollinator (Pegoscapus), wherein fig trees with a higher number of conspecific neighboring trees are visited by more foundress pollinators, produce fewer seeds, and are less likely to be exploited by non-pollinators. Because the costs, benefits, and stability of mutualisms with seed-eating pollinators are all strongly influenced by pollinator and exploiter birth rates, and because figs in particular are often a keystone resource for local frugivores, the work in this dissertation is of broad interest to ecology, evolution, and conservation. This dissertation also extends prior theory on competitors in ephemeral patch systems through the introduction of lottery models, which are used demonstrate how a temporal storage effect facilitates competitor coexistence when competitors develop within patches in overlapping generations. Conceptually unique to this work, the mechanism generating the temporal environmental variance on which coexistence relies is space itself, in the form of varying minimum between patch dispersal distance. When the minimum dispersal distance required to move from ephemeral patch to ephemeral patch changes over time, the superiority of competitors using patch resources can also change over time if they invest differently in fecundity versus dispersal ability. Given a trade-off in fecundity and dispersal ability, coexistence is predicted over a wide range of biologically realistic parameters for non-pollinating competitor fig wasps. An empirical prediction of this modeling work is that the fecundities of competitors in fig wasp communities will be negatively correlated with wasp dispersal abilities. Egg load estimates and two independent metrics of wasp dispersal ability show that non-pollinating competitors associated with F. petiolaris have negatively correlated fecundities and dispersal abilities. These results demonstrate that wasp population storage and a fecundity-dispersal ability trade-off are critical mechanisms for maintaining fig wasp diversity in at least one fig wasp community, and likely many others. And they more broadly demonstrate the importance of a fluctuating environment on species interactions and coexistence in ephemeral patch communities

Novel pericyclic routes to strained organic molecules: Theoretical and experimental studies

The chemistry of cyclobutyne (20) and 1,2-cyclobutadiene (21) has been investigated. Research efforts were focused on attempting to independently generate and trap these highly reactive species. The trapping experiments were inconclusive in each case. Calculations using MCSCF theory indicate that perfluorination of cyclobutyne will have a profound effect that stabilizes the cyclobutyne relative to rearrangement. However, experiments designed to generate and trap perfluorocyclobutyne (73) did not provide evidence for this compound. The development of new routes to strained organic compounds through the extension of known pericyclic reactions is described. These novel pericyclic reactions have been investigated by ab initio calculations and flash vacuum pyrolysis (FVP) experiments. MP4/6-31G*//MP2/6-31G* calculations support the possibility that strained six-membered rings can be prepared by replacement of a double bond with a triple bond in the prototypical Diels/Alder cycloaddition eq. (1) or electrocyclic reaction eq. (7). The novel cycloadditions explored involve the replacement of the traditional 1,3-butadiene component of (4+2) cycloadditions with vinylacetylene (19), 1,3-butadiyne (160), and strans-butadiene (233). Electrocyclic reactions of 1,2,3-cyclohexatriene (167), 1,2,4-cyclohexatriene (166), cyclohexen-3-yne (247), and 1,2,3,4-cyclohexatetraene (243) have been investigated. Experimental evidence is presented for the first example of intramolecular diyne + alkene and diyne + alkyne cycloadditions. The latter process is believed to give an o-benzyne intermediate, which is subsequently reduced to indan. This cycloaddition is unique in that it may be considered a cycloaromatization because an aromatic ring is directly generated. The mechanism and generality of diyne cycloadditions is examined