NMR study of aqueous tetraalkylammonium silicates

Silicon-29 nuclear magnetic resonance (NMR) spectroscopy has been used to investigate the equilibrium distribution and kinetics of silicate oligomers in aqueous tetraalkylammonium silicate solutions. Certain tetraalkylammonium cations (particularly tetramethylammonium cation) stabilize specific silicate oligomers, such as the cubic octamer and prismatic hexamer, by forming a protective shell that shields these oligomers from interaction with paramagnetic ions and impedes Si-Si chemical exchange. For tetramethylammonium silicate solutions, the dependence of the initial rate of cubic octamer formation on alkalinity, silica concentration and tetramethylammonium cation concentration has been investigated and a mechanism for the formation of cubic octamer proposed. For tetraalkylammonium silicate solutions with OH~:Si < 1:1 which favour the cubic octamer, at least four resonances which occur down-frequency of the cubic octamer resonance have been attributed to stable protonated states of the cubic octamer. Alkali-metal cations added to tetramethylammonium silicate solutions destabilize the cubic octamer probably by displacing tetramethylammonium cations which surround the oligomer

A Method for Structure–Activity Analysis of Quorum-Sensing Signaling Peptides from Naturally Transformable Streptococci

Many species of streptococci secrete and use a competence-stimulating peptide (CSP) to initiate quorum sensing for induction of genetic competence, bacteriocin production, and other activities. These signaling molecules are small, unmodified peptides that induce powerful strain-specific activity at nano-molar concentrations. This feature has provided an excellent opportunity to explore their structure–function relationships. However, CSP variants have also been identified in many species, and each specifically activates its cognate receptor. How such minor changes dramatically affect the specificity of these peptides remains unclear. Structure–activity analysis of these peptides may provide clues for understanding the specificity of signaling peptide–receptor interactions. Here, we use the Streptococcus mutans CSP as an example to describe methods of analyzing its structure–activity relationship. The methods described here may provide a platform for studying quorum-sensing signaling peptides of other naturally transformable streptococci

Lateral Marsh Edge Erosion as a Source of Sediments for Vertical Marsh Accretion

With sea level rise accelerating and sediment inputs to the coast declining worldwide, there is concern that tidal wetlands will drown. To better understand this concern, sources of sediment contributing to marsh elevation gain were computed for Plum Island Sound estuary, MA, USA. We quantified input of sediment from rivers and erosion of marsh edges. Maintaining elevation relative to the recent sea level rise rate of 2.8 mm yr−1 requires input of 32,299 MT yr−1 of sediment. The input from watersheds is only 3,210 MT yr−1. Marsh edge erosion, based on a comparison of 2005 and 2011 LiDAR data, provides 10,032 MT yr−1. This level of erosion is met by \u3c0.1% of total marsh area eroded annually. Mass balance suggests that 19,070 MT yr−1 should be of tidal flat or oceanic origin. The estuarine distribution of 14C and 13C isotopes of suspended particulate organic carbon confirms the resuspension of ancient marsh peat from marsh edge erosion, and the vertical distribution of 14C‐humin material in marsh sediment is indicative of the deposition of ancient organic carbon on the marsh platform. High resuspension rates in the estuarine water column are sufficient to meet marsh accretionary needs. Marsh edge erosion provides an important fraction of the material needed for marsh accretion. Because of limited sediment supply and sea level rise, the marsh platform maintains elevation at the expense of total marsh area

The changing carbon cycle of the coastal ocean

The carbon cycle of the coastal ocean is a dynamic component of the global carbon budget. But the diverse sources and sinks of carbon and their complex interactions in these waters remain poorly understood. Here we discuss the sources, exchanges and fates of carbon in the coastal ocean and how anthropogenic activities have altered the carbon cycle. Recent evidence suggests that the coastal ocean may have become a net sink for atmospheric carbon dioxide during post-industrial times. Continued human pressures in coastal zones will probably have an important impact on the future evolution of the coastal ocean's carbon budget

Early anthropogenic transformation of the Danube-Black Sea system

© The Author(s), 2012. This article is distributed under the terms of the Creative Commons Attribution License. The definitive version was published in Scientific Reports 2 (2012): 582, doi:10.1038/srep00582.Over the last century humans have altered the export of fluvial materials leading to significant changes in morphology, chemistry, and biology of the coastal ocean. Here we present sedimentary, paleoenvironmental and paleogenetic evidence to show that the Black Sea, a nearly enclosed marine basin, was affected by land use long before the changes of the Industrial Era. Although watershed hydroclimate was spatially and temporally variable over the last ~3000 years, surface salinity dropped systematically in the Black Sea. Sediment loads delivered by Danube River, the main tributary of the Black Sea, significantly increased as land use intensified in the last two millennia, which led to a rapid expansion of its delta. Lastly, proliferation of diatoms and dinoflagellates over the last five to six centuries, when intensive deforestation occurred in Eastern Europe, points to an anthropogenic pulse of river-borne nutrients that radically transformed the food web structure in the Black Sea.This study was supported by grants OISE 0637108, EAR 0952146, OCE 0602423 and OCE 0825020 from the National Science Foundation and grants from the Woods Hole Oceanographic Institution

Probing anisotropic intermolecular forces in nematic liquid crystals using NMR and computer simulations

Molecules of similar size and shape, but with different electrostatic properties are used to investigate the effects of molecular dipoles, quadrupoles and polarizabilities on the orientational ordering of several solutes co-dissolved in nematic liquid crystals. Permanent dipoles have a negligible influence on solute orientational order and effects from molecular polarizability interactions could not be separated from short-range interactions. Order parameters predicted from strong, short-range repulsive forces coupled with interactions between the solute quadrupole and the average electric field gradient felt by the solute (EFG) are consistent with experimental values. For liquid crystals utilized in this study, the calculated values of the (EFGys are the same sign and of similar magnitude to the (_FG)'s determined previously from experiments on D₂ and HD. However, in contradiction to these experimental results, the (EFG)'s determined from computer simulations of hard particles with embedded point quadrupoles is found to be very dependent on the properties of the particle. For a particular nematic liquid crystal (55 wt% ZLI 1132 in EBBA), the contribution to solute ordering from long-range electrostatic interactions is found to be negligible. This conclusion is supported by computer simulation studies of hard particles; models for short-range interactions which best fit the NMR experimental solute order parameters also best fit the simulation results. Experimentally determined second rank orientational order parameters and structural parameters of solutes are calculated from vibrationally and non-vibrationally corrected nuclear dipolar coupling constants; accurate dipolar couplings are obtained from analysis of the high-resolution nuclear magnetic resonance (NMR) spectra. For the more complicated molecules spectral parameters are first estimated from analysis of multiple quantum NMR spectra. In some cases, a modified version of a least-squares routine which independently adjusts chemical shifts, order parameters, structural parameters and/or dipolar couplings is used.Science, Faculty ofChemistry, Department ofGraduat