A Modular Approach to Bistable Magnets: From Extended Architectures to Molecules

This dissertation describes structural and magnetic studies of three-dimensional Prussian Blue analogs (PBAs), one-dimensional single chain compounds, and discrete “zero-dimensional” molecules. All compounds that were studied are members of a large class of materials known as molecular nanomagnets which exhibit a barrier to spin reversal due to magnetic anisotropy which can surpass the superparamagnetic limit of classical materials. Single-molecule magnets (SMMs) which can exhibit long-lived relaxation times for their magnetic states are the prototype for the field and are poised for implementation in next generation nanoelectronic devices. The goal of this dissertation research was to determine sources of anisotropy in the quest for interesting magnetic properties. Prussian Blue, FeIII4[FeII(CN)6]3·16H2O, exhibits ferromagnetic coupling with a Curie temperature of approximately 5 K. Different synthetic attempts historically yield slightly different formulations of the compound. We have revisited the structural analysis of this compound and have obtained the first crystal structure of Turnbull’s Blue, KFeIII[FeII(CN)6]. Capping ligands can reduce the dimension of PBAs while retaining their interesting properties. This strategy is presented by the reactions of tptz (tptz = 2,4,6-tris(2-pyridyl)-1,3,5-triazine) complexes of Cu(II), Co(II), and Fe(III) with [MV(CN)8]3- (M = Mo, W) anions which led to three new heterobimetallic chains based on ladder- or square structural motifs. The compound {[FeIII(tptz)WV(CN)8]·2CH3OH}∞ exhibits weak out-of-phase AC signals at low temperature, which indicates single-chain magnet behavior. The blocking of all open coordination sites results in discrete molecules with the potential for SMM behavior. Underexplored transition and lanthanide metals with specific ligand fields afford single-ion anisotropies providing an easy-axis for the magnetization. In this regard, the new trigonally capped titanium (III) precursor, [Et4N][Tp*Ti(CN)3] (Tp* = tris(3,5-dimethylpyrazolyl)borohydride), was prepared. Concomitant hydrolysis, oxidation, and coordination of this titanium precursor affords a family of trinuclear carboxylate compounds, {(Tp*TiIV)(μ2-OAc)2(μ2-O)M(μ2-O)(μ2-OAc)2(TiIVTp*)} (M = CrIII, MnII, CoII), with interesting magnetic properties. The use of the equatorially coordinating bis(trimethylsilyl)amide ligand, [N(SiMe3)2]-, enforces a trigonal pyramidal framework for the stabilization of prolate lanthanide SMMs. The compounds, [Li(THF)4]{LnIII[N(SiMe3)2]3Cl} (Ln = Er, Tm), exhibit single-molecule magnet behavior with the erbium analog showing magnetic hysteresis up to 3 K

The chronology and tectonic style of landscape evolution along the elevated Atlantic continental margin of South Africa resolved by joint apatite fission track and (U-Th-Sm)/He thermochronology

Atlantic-type continental margins have long been considered “passive” tectonic settings throughout the entire postrift phase. Recent studies question the long-term stability of these margins and have shown that postrift uplift and reactivation of preexisting structures may be a common feature of a continental margin's evolution. The Namaqualand sector of the western continental margin of South Africa is characterized by a ubiquitously faulted basement but lacks preservation of younger geological strata to constrain postrift tectonic fault activity. Here we present the first systematic study using joint apatite fission track and apatite (U-Th-Sm)/He thermochronology to achieve a better understanding on the chronology and tectonic style of landscape evolution across this region. Apatite fission track ages range from 58.3 ± 2.6 to 132.2 ± 3.6 Ma, with mean track lengths between 10.9 ± 0.19 and 14.35 ± 0.22 µm, and mean (U-Th-Sm)/He sample ages range from 55.8 ± 31.3 to 120.6 ± 31.4 Ma. Joint inverse modeling of these data reveals two distinct episodes of cooling at approximately 150–130 Ma and 110–90 Ma with limited cooling during the Cenozoic. Estimates of denudation based on these thermal histories predict approximately 1–3 km of denudation coinciding with two major tectonic events. The first event, during the Early Cretaceous, was driven by continental rifting and the development and removal of synrift topography. The second event, during the Late Cretaceous, includes localized reactivation of basement structures as well as regional mantle-driven uplift. Relative tectonic stability prevailed during the Cenozoic, and regional denudation over this time is constrained to be less than 1 km

Characterization of Intrinsic Properties of Promoters.

Accurate characterization of promoter behavior is essential for the rational design of functional synthetic transcription networks such as logic gates and oscillators. However, transcription rates observed from promoters can vary significantly depending on the growth rate of host cells and the experimental and genetic contexts of the measurement. Furthermore, in vivo measurement methods must accommodate variation in translation, protein folding, and maturation rates of reporter proteins, as well as metabolic load. The external factors affecting transcription activity may be considered to be extrinsic, and the goal of characterization should be to obtain quantitative measures of the intrinsic characteristics of promoters. We have developed a promoter characterization method that is based on a mathematical model for cell growth and reporter gene expression and exploits multiple in vivo measurements to compensate for variation due to extrinsic factors. First, we used optical density and fluorescent reporter gene measurements to account for the effect of differing cell growth rates. Second, we compared the output of reporter genes to that of a control promoter using concurrent dual-channel fluorescence measurements. This allowed us to derive a quantitative promoter characteristic (ρ) that provides a robust measure of the intrinsic properties of a promoter, relative to the control. We imposed different extrinsic factors on growing cells, altering carbon source and adding bacteriostatic agents, and demonstrated that the use of ρ values reduced the fraction of variance due to extrinsic factors from 78% to less than 4%. This is a simple and reliable method to quantitatively describe promoter properties.TJR was supported by a Microsoft Research studentship and EC FP7 Project No. 612146 (PLASWIRES) awarded to JH, JRB by a Microsoft Research studentship and internship, and FF by CONICYT-PAI/Concurso Nacional de Apoyo al Retorno de Investigadores/as desde el Extranjero Folio 8213002 7, and EPSRC grant EP/H019162/1 awarded to JH. JWA acknowledges the EPSRC and the Wellcome Trust for support.This is the author accepted manuscript. The final version is available from ACS via http://dx.doi.org/10.1021/acssynbio.5b0011

Developmental Regulation of an Adhesin Gene during Cellular Morphogenesis in the Fungal Pathogen Candida albicans

Peer reviewedPublisher PD