Core 2A Research Articles

Wide & deep learning for predicting relative mineral compositions of sediment cores solely based on XRF scans, a case study from Pleistocene Paleolake Olduvai, Tanzania

This study develops a method to use deep learning models to predict the mineral assemblages and their relative abundances in paleolake cores using high-resolution XRF core scan elemental data and X-ray diffraction (XRD) mineralogical results from the same core taken at coarser resolution. It uses the XRF core scan data along with published mineralogical information from the Olduvai Gorge Coring Project (OGCP) 2014 sediment cores 1A, 2A, and 3A from Paleolake Olduvai, Tanzania. Both regression and classification models were developed using a Keras deep learning framework to assess the predictability of mineral assemblages with their relative abundances (in regression models) or at least the mineral assemblages (in classification models) using XRF core scan data. Models were created using the Sequential class and Functional API with different model architectures. The correlation matrix of element ratios calculated from XRF element intensity records from the cores and XRD-derived mineralogical information was used to select the most useful features to train the models. 1057 training data records were used for the models. Lithological classes were also used for some models using Wide & Deep neural networks since those combine the benefits of memorization and generalization for mineral prediction. The results were validated using 265 validation data records unseen by the model and discuss the accuracy of models using six test records. The optimized Deep Neural Network (DNN) classification model achieved over 86% binary accuracy while the regression models were also able to predict the relative mineral abundances of samples with high accuracies. Overall, the study shows the efficacy of a carefully crafted Deep Learning (DL) model for predicting mineral assemblages and abundances using high-resolution XRF core scan data.

Millennial-scale variability of the Antarctic ice sheet during the early Miocene

Millennial-scale ice sheet variability (1-15 kyr periods) is well documented in the Quaternary, providing insight into critical atmosphere-ocean-cryosphere interactions that can inform the mechanism and pace of future climate change. Ice sheet variability at similar frequencies is comparatively less known and understood prior to the Quaternary during times, where higher atmospheric pCO2 and warmer climates prevailed, and continental-scale ice sheets were largely restricted to Antarctica. In this study, we evaluate a high-resolution clast abundance dataset (ice-rafted debris) that captures East Antarctic ice sheet variability in the western Ross Sea during the early Miocene. This dataset is derived from a 100 m-thick mudstone interval in the ANtarctic DRILLing (ANDRILL or AND) core 2A, which preserves a record of precession and eccentricity variability. The sedimentation rates are of appropriate resolution to also characterize the signature of robust, subprecession cyclicity. Strong sub-precession (~10 kyr) cyclicity is observed, with an amplitude modulation in lockstep with eccentricity, indicating a relationship between high-frequency Antarctic ice sheet dynamics and astronomical forcing. Bicoherence analysis indicates that many of the observed millennial-scale cycles (as short as 1.2 kyr) are associated with nonlinear interactions (combination or difference tones) between each other and the Milankovitch cycles. The presence of these cycles during the Miocene reveals the ubiquity of millennial-scale ice sheet variability and sheds light on the interactions between Earth's atmosphere, ocean, and ice in climates warmer than the Quaternary.

Alkenones in Pleistocene Upper Bed I (1.803–1.900 Ma) sediments from Paleolake Olduvai, Tanzania

The Olduvai Gorge Coring Project (OGCP) recovered sequences of Pleistocene lake sediments that correlate with those known from outcrops directly associated with hominin fossil horizons. The sedimentary succession from Core 2A (02° 58′ 43′' S, 035° 19′ 25.5′' E), which targeted the ancient lake-basinal depocenter, includes stratigraphic intervals (∼1.86 Ma) sandwiched between the Bed I basalt and Tuff IF dated horizons, where remains of two hominin holotypes have been recovered. The lower part of this sequence (72.3–86.9 mbs) consists of lacustrine claystones interbedded with sandy claystones characterized by high Corg (av. 2.5%) and includes several laminated intervals. It displays shifts in terrestrial vegetation recorded by values for δ13CTOC and δ2HnC31 that reflect changes in the proportions of C3 vs C4 plants and precipitation, respectively. These temporal profiles represent higher-resolution signatures of the precession-scale wet/dry cycles previously recognized in the outcropping sedimentary succession of Olduvai Gorge. Several samples from this interval contain alkenones comprising C37 and C39 alkadien-2-ones and alkatrien-2-ones, and C38 and C40 alkadien-3-ones and alkatrien-3-ones. They are distinguished from the vast majority of alkenone distributions reported in lake sediments from polar and temperate latitudes (e.g., Greenland, N. and S. America, Europe, Russia, China, S. Africa, Antarctica) by the absence of tetraunsaturated alkenones (e.g., C37:4). The alkenone distributions vary stratigraphically with higher concentrations in the drier intervals of the precession-scale cycles that exhibit characteristics resembling those of Group II haptophytes found in contemporary saline/alkaline environments rather than those of Group I haptophytes from freshwater ecosystems. Paleolake Olduvai represents the first reported occurrence of alkenones in a low-latitude Pleistocene lake, where the absence of alkatetraenones may reflect contributions from strains of Group II haptophytes that are favored by warmer temperatures or saline conditions. Temporal variations in unsaturation indices (U37K', U38K') and the proportions of alken-2-ones and alken-3-ones likely reflect influences on alkenone production by haptophytes that are associated with changes in the lake environment primarily driven by hydrologic cycles (e.g., salinity and alkalinity) rather than lake temperatures.

Biased preservation of Pleistocene climate variability proxies at Olduvai Gorge, Tanzania

Olduvai Gorge in northern Tanzania contains a fossiliferous, well-characterized Pleistocene sedimentary record and provides the opportunity to study the relationships between a changing climate, ecology, and hominin evolution. The Olduvai Gorge Coring Project drilled four cores (1A, 2A, 3A, and 3B) into the depocenter of Paleolake Olduvai in 2014 to achieve increased temporal resolution of local climate and ecological data, and investigate the influence and timing of regional climate and tectonics on local signals. We present high-resolution records of bulk organic carbon isotopes (δ13Corg, ‰) from Cores 2A and 3A, total organic carbon (wt%) from Cores 2A and 3A, and organic carbon‑nitrogen ratios (C:N) from Core 2A. Previous work at Olduvai linked % TOC and δ13Corg to orbitally paced variations in lake depth and ecosystem dynamics from Upper Bed I and Lower Bed II (1.9–1.7 Ma), associated with eccentricity maxima and the presence of a perennial saline-alkaline lake in the basin. Bulk organic geochemical properties in both cores exhibit marked shifts in variance and magnitude at 1.9 and 1.7 Ma. Low % TOC values prior to and following 1.9–1.7 Ma implicate low productivity and/or increased degradation of organic matter, while C:N ratios from Core 2A reflect increased aquatic or bacterial input. Within the 1.9 to 1.7 Ma interval, high % TOC is dominated by terrestrial inputs as evidenced by high C:N ratios, and bulk δ13Corg captures high variability C3–C4 ecosystem dynamics. Climate variability is highest from 1.9–1.7 Ma, but the δ13Corg records are not consistent between Core 2A and Core 3A. From 1.9–1.7 Ma, Core 3A has increased indicators of erosion relative to Core 2A, suggesting a sedimentary aliasing of the δ13Corg record in Core 3A. Outside of the 1.9–1.7 Ma interval, both changes in organic carbon source and preferential preservation of proxies for wet conditions may obscure true climate variability, inviting further investigation into the ability of the Olduvai Gorge sedimentary record to test climate variability hypotheses for hominin evolutionary events.

Lake conditions and detrital sources of Paleolake Olduvai, Tanzania, reconstructed using X-ray Diffraction analysis of cores

The Olduvai Basin of northern Tanzania contains well-known Pleistocene archaeological sites in a palaeoenvironmental setting of interfingering paleolake and volcanic fan deposits. This study documents the mineral assemblages, as determined qualitatively through bulk X-ray Diffraction (XRD) of systematically sampled lacustrine sediments from Olduvai Gorge Coring Project (OGCP) Cores 1A, 2A, and 3A, from about 45 m below the surface (mbs) to the base of all three cores. This interval includes fluvio-lacustrine sediments below and between two major progradations of Ngorongoro volcanic deposits recorded in Core 2A, and all of Olduvai Bed I and the lower part of Bed II in all three cores. Changes in mineral assemblage track changes in sediment sources and lake water chemistry. Abundant quartz indicates sediment sources derived from eroded Precambrian basement quartzite and gneiss exposures to the west, southwest, and north. In contrast, the presence of anorthoclase feldspar (sometimes associated with minor augite) indicates southerly and easterly sources, derived from the then active volcanoes (Ngorongoro, Olmoti) of the Ngorongoro Volcanic Highlands. The presence and relative abundance of specific zeolites provides information about Paleolake Olduvai's pH conditions and the relative activities of various dissolved cations (K+, Na+, Ca2+, Si) at the time of alteration, and the presence and abundance of authigenic K-feldspar indicates post-depositional exposure to saline-alkaline pore fluids.The mineral assemblages recorded in the cores reveal both large-scale trends in sediment composition over time, and smaller-scale changes related to fluctuating lake levels. In the lacustrine deposits near the base of Core 2A (interval NSF-1), analcime dominates the zeolite assemblage (with minor clinoptilolite), whereas carbonates and authigenic K-feldspar are rare. This interval is followed by more lacustrine deposits in which phillipsite and chabazite are the dominant zeolites (and analcime is absent). Lacustrine deposition was interrupted by the first of two pulses of coarse Ngorongoro volcaniclastic material (NVF-1). In the lacustrine interval between the two Ngorongoro volcanic pulses (NSF-2, recovered only in Core 2A), chabazite and erionite dominate the zeolite assemblage, carbonate and K-feldspar are rare, and analcime is absent, indicating less extreme saline-alkaline conditions and greater Ca2+ availability for zeolite formation. Volcanic glass is preserved, along with abundant zeolites, including phillipsite, chabazite, and erionite, in altered volcanic materials from the upper Ngorongoro volcanic pulse (NVF-2).In the lacustrine deposits of Lower Bed I (below the Bed I basalts but above NVF-2, only recovered in Core 3A), zeolites and K-feldspar are rare to absent, consistent with wetter conditions and fresher lake waters. Carbonates are ubiquitous throughout the lacustrine intervals of Beds I and II. In lacustrine deposits above the Bed I basalts (in Upper Bed I), phillipsite is the dominant zeolite, and quartz is rare. Alternating intervals of zeolite and K-feldspar-rich and poor sediments suggest variations between wetter and drier conditions throughout Upper Bed I and into Lower Bed II. Aragonite and associated dolomite in Bed I during the deposition of a claystone Mg-anomaly points to high magnesium concentrations and elevated Mg2+/Ca2+ ratios during that time. A major change between lowermost Bed II and overlying Bed II lacustrine sediments includes an abrupt increase in quartz coupled with an increase in the zeolite analcime, consistent with the onset of more saline-alkaline conditions, contemporaneous with overall drier conditions.

Biogeochemical evidence from OGCP Core 2A sediments for environmental changes preceding deposition of Tuff IB and climatic transitions in Upper Bed I of the Olduvai Basin

Sediment cores recovered by the Olduvai Gorge Coring Project (OGCP) in 2014 afford the opportunity to examine the coupling of biogeochemical evidence for climatic and environmental change within the context of hominin evolution at this renowned East African locality. Investigations of elemental (total organic carbon – TOC or Corg wt%, carbon to nitrogen ratio - C/N), molecular (source-specific biomarkers), and isotopic (δ13Corg, δ2HnC31) compositions of organic matter provide evidence for temporal changes in sedimentary materials derived from terrestrial plants (C3, C4) and aquatic producers (algae, sponges, cyanobacteria), and in precipitation. The 13 kyr record of Upper Bed I immediately preceding Tuff IB extends high-resolution stratigraphic profiles of precession-scale alternations of wetter and drier conditions to encompass the entire interval from the Bed I Basalt to Tuff IB. The second wetter interval, designated W2, records local influences on climate and water supply consistent with evidence for discrete sediment sources based its physical properties (e.g., gamma radiation, magnetic susceptibility) compared with the overall sequence. The δ13CTOC values for wetter interval W2 reveal two millennial-scale (~2.5 kyr) drier episodes followed by a shift in the dominance of C3 over C4 plants accompanying the transition to a drier climate (D3). Moreover, biogeochemical data for Upper Bed I show that changes from drier to wetter conditions occur more rapidly (~900 yr) than wetter to drier transitions (>2.6 kyr), based on interpolated ages. In addition, biomarker profiles indicate that aquatic plants, primarily algae and macrophytes, may have been subject to more profound and faster fluctuations than variations in terrestrial vegetation expressed in terms of the relative proportions of woodland and grassland settings. Thus, environmental and climatic changes not only influenced the availability of resources of food and shelter for hominins within the Olduvai region but also led to their variation on centennial to millennial to precessional timescales.

New Olduvai Basin stratigraphy and stratigraphic concepts revealed by OGCP cores into the Palaeolake Olduvai depocentre, Tanzania

The Olduvai Gorge Coring Project (OGCP) drilled four boreholes 1A, 2A, 3A, 3B at three sites into the central palaeolake of the Olduvai Basin depocentre. Previously known Beds I, II, III, IV, Masek Beds and Ndutu are identifiable in the upper part of the cores, and from these formations predominantly lacustrine facies associations are recognised. These are described and then interpreted in terms of their palaeoenvironmental significance. However, stratigraphy known from natural outcrop exposures is more than doubled by strata intersected below. For example, an extra 135 m of pre-Bed I stratigraphy in Core 2A records the entire volcanic history of a fan-delta sourced from Ngorongoro Volcano. Beneath that fan sequence is an interval of fluvio-lacustrine non-volcaniclastic sediments. For the latter the new stratigraphic term Naibor Soit Formation is introduced, because these strata were discovered for the first time in Boreholes 2A and 3A, to either side of Naibor Soit Inselberg, where they thus lap onto its basement topography. The volcanically sourced unit above is named Ngorongoro Formation, including Marker Tuffs CFCT (Coarse Feldspar Crystal Tuff) and the Naabi Ignimbrite within its topmost portion. Only the dominantly lacustrine sediments above the main Ngorongoro sourced fan body are now attributed to Lower Bed I. The Ngorongoro Formation defined here contains primary Tuff Markers NgA to NgQ, and volcaniclastic fan-delta deposits that interfinger with and overlie claystones and interbedded sandstones of the fluvio-lacustrine Naibor Soit Formation. Boreholes 1A and 2A intersect the Bed I Basalt, but not Borehole 3A, where the position of the lava is marked by three separated volcaniclastic sandstone layers, containing pebble-sized basalt scoriae, likely correlating to three complex basalt flows at Locality 9A and Orkeri and allowing assessment of syn-basalt sedimentation. Disconformities bounding Beds I, II, III, IV, Masek Beds and Ndutu Beds provide important stratigraphic markers for the novel application of sequence stratigraphic concepts, as they mark lowstands of Palaeolake Olduvai, guiding outcrop to core correlations. Major disconformities are identified in: (1) Core 1A, where the Lower Bed II Crocodile Valley Incision Surface cuts out Tuff IF; (2) Core 1A, where disconformities and facies within Beds II, III, IV, and Masek Beds are identified; (3) Cores 2A and 3A, where Tuff IF is identified below the Lower to Middle Bed II disconformity; and (4) Lower Bed I claystones and Tuff IA in Core 3A that correlate to similar sedimentary units from Locality 66e in the Western Gorge, but which are absent from Core 2A because of a newly identified major disconformity, which causes the Bed I lava to sit directly on top of the CFCT marker.

Palaeosalinity and palaeoclimatic geochemical proxies (elements Ti, Mg, Al) vary with Milankovitch cyclicity (1.3 to 2.0 Ma), OGCP cores, Palaeolake Olduvai, Tanzania

The analysis of geochemical palaeoclimate and palaeosalinity proxy elements Ti, Mg, and Al, derived from X-ray fluorescence (XRF) scans of Olduvai Beds I and II from Olduvai Gorge Coring Project (OGCP) borehole Cores 2A and 3A, provides a record of cyclic variation between ~1.3 Ma and ~2.0 Ma. The boreholes were drilled into the depocentre of the Olduvai Basin between Fifth and FLK Faults, where Palaeolake Olduvai was most persistent and deepest. During most of Bed I the lake was particularly deep and probably meromictic, preserving high TOC contents and commonly preserving fine lamination due to lack of bioturbation. Accretion rates were also high during Bed I, when rates of basinal subsidence were maximal due to crustal stretching, associated with basaltic volcanism, towards the end of bimodal Ngorongoro volcanism. Basaltic magma effusive activity is manifested as tuffs, scoriaceous layers and the Bed I Basalt complex lava flows. A magnesium anomaly is recorded in the claystone geochemistry at this time and deposition of dolomite and limestone beds are restricted to this syn-volcanic phase of basin history. During Bed I deposition, accretion rates (0.23 mm/yr) were high enough to permit recognition of cycles with an average periodicity of 22.3 kyr corresponding to the Earth's precession. Only the high values during the Mg anomaly are adequate for the application of the palaeosalinity proxy element ratio Mg/Al. But Ti counts provide a cyclic record in both Bed I and Bed II. During Bed II deposition, the accretion rate was much slower (0.058 mm/yr) and cyclicity averaged 40.4 kyr, corresponding to Earth's orbital obliquity. The Bed II interval corresponds to MIS Stages 40 to 64. Coincidence of precessional and obliquity minima at 1.8 Ma explains the superdrought that affected the basin at the time of emplacement of Tuff IF, when the lake was dried out. The aridity of the sequence containing Tuff IA is also associated with a precessional minimum. The cyclic record suggests that three Bed I Basalt flows were extruded at ~1.94 Ma during a time span lasting between 6 kyr and 15 kyr.

Biogeochemical evidence for environmental changes of Pleistocene Lake Olduvai during the transitional sequence of OGCP core 2A that encompasses Tuff IB (~1.848 Ma)

Olduvai Gorge is renowned for discoveries of hominin fossils and tools in a well-resolved sedimentary context, representing one of the foremost sites in East Africa that has afforded critical evidence of hominin evolution. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered the first deep sediment cores from this location. These cores provide the first opportunity to examine an extensive stratigraphic record from one location and independent of the weathering and related degradation typical of outcrop samples. Samples from these cores have been correlated to Bed I tuffs and basalt marker beds that are well characterized from outcrops within the gorge. This study focuses on the biogeochemical investigation of a ~10 m segment from OGCP sediment core 2A, where age constraints indicate deposition occurred over ~50,000 years (from 1.8055 ± 0.003 Ma to 1.8551 ± 0.013 Ma). The segment includes Tuff IB and IF, which were deposited at a time that prior research indicates represents marked changes in the paleoenvironment at Olduvai, with indications that Paleolake Olduvai may have been completely desiccated by the time Tuff IF was deposited. This environmental change is recorded by the composition of sedimentary organic matter (OM) in terms of bulk organic geochemical properties observed as a decrease in % TOC and δ13Corg. In addition, temporal variations in source-specific biomarkers (n-alkanes, phytane, steradienes, A-norsteranes and alkenones) reflect a shift from a deeper lake setting with a prevalence of aquatic-sourced OM combined with input from terrestrial plant waxes to a more shallow, perennial lake where evidence of burrows and erosional surfaces is associated with microbially degraded OM and intermittent aquatic biomarkers. Thus, variations in biogeochemical proxies complement the sedimentological evidence confirming increased periods of prolonged desiccation of Paleolake Olduvai beginning around the time of Tuff IB deposition.

Aquatic biomarkers record Pleistocene environmental changes at Paleolake Olduvai, Tanzania

Several hypotheses invoke climatic variability as a driving force for hominin evolution. Thus, high-resolution records of climate and environmental variability from anthropologically significant locations can help test these hypotheses. Sedimentary sequences recovered by the Olduvai Gorge Coring Project (OGCP) help evaluate climatic and environmental changes at Olduvai Gorge, Tanzania through the analyses of various biogeochemical proxies. The stratigraphic sequence of OGCP Core 2A can be correlated with horizons associated with hominins and is chronologically constrained by distinctive dated horizons, such as the Bed I Basalt and Tuff IB. The lacustrine interval from 76.6 to 86.9 m depth is ideally suited for high-resolution analyses of biogeochemical proxies as it is rich in organic carbon (>1%TOC). The hydrogen isotopic composition of nC31 in this interval of OGCP Core 2A records the effects of precession-driven wet-dry cycles on the terrestrial environment that led to alternations between woodland and grassland ecosystems, comparable to those documented by previous investigations of outcrop analogues at Olduvai. Here, we examine stratigraphic variations in the abundance of biomarkers (C28 steradienes, C17 and C23n-alkanes, C27 and C28 A-norsteranes, fern-8-ene, and chromans) derived from aquatic organisms (algae, cyanobacteria, sponges, macrophytes, etc.) to determine the response of Paleolake Olduvai to climate variability. In general, these aquatic biomarkers reflect the productivity of the lake environment and exhibit the same precession-driven wet-dry cycles recorded by terrestrial biogeochemical signatures. However, they also provide evidence of abrupt (<~300 yr) changes in lake level and corresponding aquatic communities superimposed on the longer-term Milankovitch cycles. Thus, evidence for climatic variability is manifested through the pacing and intensity of changes in both terrestrial and aquatic ecosystems, but the different rates of the responses on land and in the aquatic environment potentially had a complex influence on water and food resources that were important factors for hominin habitation and evolution.

Sub-Milankovitch paleoclimatic and paleoenvironmental variability in East Africa recorded by Pleistocene lacustrine sediments from Olduvai Gorge, Tanzania

Over the past century, numerous discoveries throughout East Africa have advanced our understanding of hominin evolution and provided substantive evidence that climatic and environmental variability played a critical role in evolutionary developments. Stratigraphic records with high temporal resolution aid in testing evolutionary hypotheses that invoke changes in climate and environment at various timescales as drivers of hominin evolution. Olduvai Gorge, Tanzania has a rich history of hominin fossil discoveries from its ~2.0 Ma sedimentary sequence, which includes multiple deep lake intervals. In 2014, the Olduvai Gorge Coring Project (OGCP) recovered a sequence of sediment cores that provide an extensive record of the Pleistocene sedimentary history, including paleolacustrine systems in this area. With >94% core recovery, these cores are ideally suited for high-resolution analyses of organic geochemical proxies and provide an exceptional opportunity to build on previously published outcrop-based paleoenvironmental data from Olduvai. The OGCP core 2A section between 76.6 and 86.9 m depth is considered as a time-stratigraphic equivalent of strata that host the hominin fossils OH24 and OH56 at the Olduvai DK archaeological site. This depositional interval age-bracketed between ~1.88 Ma (Bed I Basalt) and ~1.85 Ma (Tuff IB) was sampled for organic geochemical analyses. The carbon isotopic composition of organic matter (δ13CTOC) from this section varies between values representative of more forested and open grassland ecosystems over ~21 kyr. This observation is consistent with the Milankovitch precession cycle driving the availability of water, which was previously observed in lower resolution studies of outcrop samples. Complementary organic geochemical proxies provide further evidence of these shifts in environmental conditions and record sub-Milankovitch scale changes superimposed on the precession cycle. This suggests the occurrence of short-term fluctuations in the environments inhabited by hominins, which opens new lines of investigation on how environmental scenarios may affect particular evolutionary mechanisms that drove various evolutionary responses.

Bis-indole derivatives with antitumor activity turn out to be specific ligands of human telomeric G-quadruplex.

Bis-indolinone derivatives having either 2,6-disubstituted pyridine core (1a and 1b) or 1,10-disubstituted phenanthroline core (2a and 2b), already known to have antitumor activity, have been tested as potential G-quadruplex binders. Compounds 2a and 2b are able to selectively stabilize G-quadruplex over duplex DNA, and also to discriminate among different G-quadruplex structures, having a particular affinity for the parallel form of the human telomeric G-quadruplex. Both compounds are also able to induce telomeric DNA damage that may explain the activity of these compounds.

Glycoconjugated Polymer. 3. Synthesis and Amphiphilic Property of Core-Glycoconjugated Star-Shaped Polystyrene

Coupling reactions of 2,2,6,6-tetramethylpiperidinyloxy-terminated polystyrene (PS-TEMPO) using divinylbenzene (DVB) as a linking agent were performed in the presence of 4-vinylbenzyl glucoside peracetate (1a) and 4-vinylbenzyl maltohexaoside peracetate (1b). To prepare core-glycoconjugated star-shaped polymers with various amphiphilic properties, the molar ratio of 1 and DVB in the feed (F1, [1]/[DVB]) was changed from 0.13 to 0.38 for the coupling reaction. Star-shaped polystyrenes having 1a and 1b units in their cores (2a and 2b, respectively) were obtained by purification using preparative SEC. The weight-average molecular weights determined by static laser light scattering measurements (Mw,SLS), the polydispersity indices determined by size exclusion chromatography (Mw/Mn), the number of polystyrene arms (Narm), and the core-composition ratio (f1, [1 unit]/[DVB unit]) were 2.89 × 105−3.31 × 105, 1.36−1.38, 18, and 0.09−0.23 for 2a and 2.28 × 105−2.60 × 105, 1.29−1.35, 11−14, and 0.05−0.17 for 2b, respectively. Amphiphilic star-shaped polystyrenes with cores containing glucose and maltohexaose as hydrophilic segments (3a and 3b, respectively), which were obtained by the deacetylation of 2a and 2b, exhibited an encapsulation ability toward water-soluble molecules, such as methyl orange (MO), in chloroform. The amounts of encapsulated MO remarkably increased with the increasing hydrophilic property of 3. A positive Cotton effect was observed in the circular dichroism spectrum for the MO/3 system in the adsorption area of MO, indicating that MO existed in a chiral segment, i.e., the glycoconjugated core.

Supramolecular staircase via self-assembly of disklike molecules at the solid-liquid interface.

A series of soluble hexabenzocoronene (HBC) derivatives with pendant optically active (S)-3,7-dimethyloctanyl and (R,S)-3,7-dimethyloctanyl (mixture of stereoisomers) hydrocarbon side chains with and without a phenylene spacer were assembled into differently ordered arrays at the interface between a solution and the basal plane of highly oriented pyrolytic graphite (HOPG). Molecularly resolved scanning tunneling microscopy (STM) images revealed that all derivatives self-assemble into oriented crystals in quasi-two dimensions. However, while for the alkyl-substituted HBCs (1,4) all of the single aromatic cores within a monolayer exhibit the same contrast in the STM, the single aromatic cores with a phenylene group between the alkyl side chains and the aromatic core (2a,2b,3) exhibit different contrasts within a monolayer. For the disks carrying racemic branched or n-alkyl side chains (2b,3) a random distribution of the two different contrasts within the 2D-crystal is observed, while the optically active phenylene-alkyl-substituted HBC (2a) exhibits a periodical distribution of three contrasts within the monolayer. We attribute the different contrasts of the aromatic cores in the presence of the phenylene groups to a loss of the planarity of the whole molecule and different conformations, which allow the conjugated disks to attain different equilibrium positions above the surface of HOPG. In the case of the optically active side chains a regular superstructure with three distinctly different positions such as in a staircase is attained. The self-assembly processes are governed by the interplay of intramolecular as well as intermolecular and interfacial interactions. In the present case, the interactions may induce both the molecules to acquire well distinct positions along the z axis and to adopt different conformations. The reported results open new avenues of exploration. For instance, the different couplings of conjugated molecules with the substrate at different separations can be investigated by means of scanning tunneling spectroscopy (STS). Furthermore, experiments on the STM tip-induced switching of single molecules embedded in a monolayer appear feasible.

Aminoxyl radicals bearing a mesogenic core

A series of aminoxyl radicals (nitronyl nitroxide or TEMPO) bearing biphenylcarbonitrile as a mesogenic core ( 2a– 2c and 3a– 3c ) was prepared. Among them, unusual magnetic transition from the original Curie–Weiss phase to another magnetic phase well expressed by a singlet–triplet model was found in the 4′-undecyloxy-4-biphenylcarbonitrile derivative with oxocarbonyl-TEMPO through the thermal transition.

Analogs of (A2'p)nA. Correlation of structure of analogs of ppp(A2'p)2A and (A2'p)2A with stability and biological activity.

Analogs of (A2'p)2A core and ppp(A2'p)2A were chemically synthesized and their susceptibility to phosphodiesterase degradation and ability to either activate an endonuclease or to inhibit cell growth were determined. The absence of the internal 3'-OH groups ((3'dA2'p)2A) resulted in a 5-fold increase in stability, but also in a 10-fold decrease in activity, as measured by (a) activation of an endonuclease in cell-free extracts and inhibition of protein synthesis in intact cells by the 5'-triphosphate species and (b) inhibition of DNA synthesis in synchronized cells by the core analogs. An uncharged derivative of this analog containing two methylphosphotriesters, although significantly more stable, was even less active. Additional deletion of the terminal 3'-OH ((3'd A2'p)23'dA) resulted in a further 6-fold increase in stability (30-fold overall increase in stability), as well as approximately a 2-fold increase in ability to inhibit cell growth, as compared to the natural 2'5' A core. The analog lacking a terminal 2'-OH as well as lacking the internal 3'-OH group ((3'dA2'p)22'dA) showed an overall 15-fold increased stability, yet showed very little activity in inhibiting cell growth. The most stable (120-fold increased overall stability) as well as most active analog was a xyloadenosine analog of 2'5' A core, (xyloA2'p)2xyloA. These results show that modification of the 3'-terminal OH appears to be most important in increasing 2'-5' A core stability as well as biological activity. However, the mechanism of cell growth inhibition by these 2'-5' A core analogs may involve pathways different from those utilized by the 2'-5' A-dependent endonuclease.

Zero total dispersion in single‐mode optical fibers over an extended spectral range

The waveguide and material dispersion are investigated for various silica‐based single‐mode optical fibers with arbitrary refractive‐index profiles. It is shown that in the case of the doubly clad optical fiber with an index difference between the core and the outer cladding Δ1 = 1.0% and that between the inner and the outer cladding Δ2 = −1.0%, where the diameter of the core 2a = 7.2 μm and the thickness of the inner cladding t = 1.1 μm, the total dispersion is reduced to within ±1 ps km−1 nm−1 over a wide spectral range of λ = 1.35–1.67 μm.

Dispersion minimisation in single-mode fibres over a wide spectral range

The waveguide and material dispersion are investigated for various silica-based single-mode optical fibres with arbitrary refractive-index profiles. It is shown that in the case of the doubly clad optical fibre with an index difference between the core and the outer cladding Δ1=1% and that between the inner cladding and the outer Δ2=−1%, the diameter of the core 2a=7.2 μm, and the thickness of the inner cladding t=0.3a, the total dispersion is reduced within ± 1 ps/km/nm over a wide spectral range of λ=1.35–1.67 μm.