No abstract

Despite the fact that lead poisoning is the most common disease of environmental origin in the United States, the spectroscopic properties of aqueous Pb(II) coordination compounds have not been extensively investigated. Spectroscopic techniques that can be used to probe the fundamental coordination chemistry of Pb(II) will aid in both the development of water-soluble ligands that bind lead both tightly and selectively and the characterization of potential biological targets. Here, we report the preparation and characterization of a series of Pb(II) complexes of amido- derivatives of EDTA. The 207Pb chemical shift observed in these complexes (2441, 2189, and 1764 ppm for [Pb(EDTA)]2-, Pb(EDTA-N2), and [Pb(EDTA-N4)]2+, respectively) provides an extremely sensitive measure of the local environment and the charge on each complex. These shifts help to map out the lead chemical shift range that can be expected for biologically relevant sites. In addition, we report the first two-dimensional 207Pb-1H heteronuclear multiple-quantum correlation (HMQC) nuclear magnetic resonance spectra and demonstrate that this experiment can provide useful information about the lead coordination environment in aqueous Pb(II) complexes. Because this technique allows 207Pb-1H couplings through three bonds to be identified readily, 207Pb-1H NMR spectroscopy should prove useful for the investigation of Pb(II) in more complex systems (e.g., biological and environmental samples).

Random copolymers of ethylene and butene contain crystallizable sequences of different length l. Isothermal crystallization of model ethylene−butene random copolymers having 21 and 73 branches per 1000 backbone carbon atoms was studied by differential scanning calorimetry (DSC). Two melting peaks, which correspond to two crystal populations, are seen after isothermal crystallization. High-temperature melting is attributed to lamellar crystals formed from the longest sequences in the copolymer. These thicken during crystallization and melt at temperatures as large as 22 °C above the crystallization temperature T c. A second crystal population melts around T c + 5 °C. It is concluded that these crystals are dominated by shorter sequences that solidify with little or no folding and hence resemble fringed micelles. Thickening is suppressed in this set of crystals, so they melt near T c. Low melting crystals are shown to grow more slowly than the lamellar structures; the difference in transformation rates is attributed to lower undercooling and restricted mass transport for crystallization of short sequences. No comparable evidence for two crystal populations is seen when unbranched polyethylene or a heterogeneous copolymer (19 branches/1000 C) is crystallized isothermally under similar conditions.

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