A new procedure for the site-selective measurement of heteronuclear magnetic dipole-dipole interactions involving quadrupolar nuclei has been developed. Particular attention is given to the case of S{I} rotational echo double resonance (REDOR) with S ) 1 / 2 and I ) 3 / 2 , where the nuclear electric quadrupole splitting affects the efficiency of the π(I) pulses in recoupling the S-I dipolar interactions. Detailed two-and threespin simulations using the SIMPSON code indicate that the REDOR response is sensitively affected by the quotient of the nutation frequency and the nuclear electric quadrupole frequency (ν 1 /ν q ). Multispin systems can be conveniently analyzed using a curvature determination of REDOR data in the limit of short dipolar evolution times (∆S/S 0 < 0.2), to yield dipolar second moment (M 2 ) values. The apparent M 2 values measured in this fashion include a scaling factor f 1 , the magnitude of which can be predicted if ν 1 /ν q is known. Based on this finding, a robust method has been developed to analyze experimental REDOR data in multispin systems, in terms of average dipolar M 2 values, which are shown to be affected by distributions of magnetic dipolar and nuclear electric quadrupolar coupling constants only to a minor extent. Validation experiments on suitable model compounds indicate that dipolar M 2 values can be measured within 10% accuracy in this fashion for crystalline compounds. For glasses, the error may be higher, because of possible cumulative errors that may occur from unknown geometries and distribution effects. Experiments conducted on glassy sodium phosphates indicate that the 31 P-23 Na dipolar interaction strengths in these systems are differentiable for the various Q (n) sites and increase as the charge of the phosphate moiety increases.

Modern solid state nuclear magnetic resonance presents new powerful opportunities for the elucidation of medium range order in glasses in the sub-nanometer region. In contrast to standard chemical shift spectroscopy, the strategy presented here is based on the precise measurement and quantitative analysis of internuclear magnetic dipole-dipole interactions, which can be related to distance information in a straightforward manner. The review discusses the most commonly employed experimental techniques, producing dipolar coupling information in both homo- and heteronuclear spin systems. The approach is particularly powerful in combination with magic-angle sample spinning, producing site-resolved dipolar coupling information. We present new applications to oxide-based network glasses, permitting network connectivities and spatial cation distributions to be elucidated.

The development of non-petrochemical sources for the plastics industry continues to progress as large multinationals focus on renewable resources to replace fossil carbon. Many bacteria are known to accumulate polyoxoesters as water-insoluble granules in the cytoplasm. The thermoplastic and/or elastomeric behaviour of these biodegradable polymers holds promise for the development of various technological applications. Here, we report the synthesis and characterization of microbial polythioesters (PTEs), a novel class of biopolymers of general technological relevance. Biosynthesis of PTE homopolymers was achieved using a recombinant strain of Escherichia coli that expressed a non-natural pathway consisting of a butyrate kinase, a phosphotransbutyrylase, and a PHA synthase. Different homopolymers were produced, consisting of either 3-mercaptopropionate, 3-mercaptobutyrate, or 3-mercaptovalerate repeating units, if the respective mercaptoalkanoic acids were provided as precursor substrates to the fermentative process. The PTEs contributed up to 30% (w/w) of the cellular dry weight and were identified as hydrophobic inclusions in the cytoplasm. The chemical and stereochemical homogeneity of the purified PTEs were identified by different methods, and the estimated physical properties were compared to the oxypolyester equivalents, revealing low crystalline order and, for the poly(3-mercaptopropionate) improved thermal stability. The ability to produce PTEs through a biosynthetic route opens up new avenues in the field of biomaterials.

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