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An NMR-guided procedure for refining crystal structures has recently been introduced and shown to produce unusually high resolution structures. Herein, this procedure, is modified to include 15N shift tensors instead of the 13C values employed previously. This refinement involves six benchmark structures and 45 15N tensors. All refined structures show a statistically significant improvement in NMR fit over energy based refinements. Metrics other than NMR agreement indicate that NMR refinement does not introduce errors with no significant changes observed in atom positions or diffraction patterns. However, refinement does change bond lengths by more than experimental uncertainty with most bond types become shorter than diffraction values. Although this decrease is small (1–4 pm), it significantly alters computed 15N tensors. The NMR refinement was further evaluated by refining two tripeptides. These structures rapidly converged and achieved an NMR agreement equivalent to benchmark values. To ensure accurate comparisons, a complete atomic structure of the tripeptide AGG was determined by single crystal neutron diffraction at 0.58 Å resolution, allowing unambiguous determination of all hydrogen positions. To verify that all NMR refinements represent genuine improvements rather than artifacts of DFT methods, an independent approach was included to evaluate the final NMR refined coordinates. This analysis employs cluster methods and the PBE0 functional. The unusually small 15N NMR root-mean-square error of the final refined structures (3.6 ppm) supports the conclusion that the changes made represent improvements over both diffraction coordinates and lattice-including DFT energy refined coordinates. [ABSTRACT FROM AUTHOR]

Since 1993, it has been known that 13C chemical shift tensor (i.e., δ11, δ22, and δ33) provides information sufficient to distinguish between COOH and COO− sites. Herein, four previously unreported metrics are proposed for differentiating COOH/COO− moieties. A new relationship is also introduced that correlates the asymmetry (i.e., δ11–δ22) of COOH sites to the proximity of hydrogen bond donating partners within 2.6 Å with high accuracy (±0.05 Å). Conversely, a limitation to all proposed metrics is that they fail to distinguish between COO− and hydrogen disordered COOH sites. To reconcile this omission, a new approach is proposed based on T1 measurements of both 1H and 13C. The 13C T1 values are particularly sensitive with the T1 for hydrogen disordered COOH moieties found to be nearly six times smaller than T1's from COO− sites. [ABSTRACT FROM AUTHOR]

The connectivity, conformation, tautomeric form, and dynamics of a new depsidone (perisalazinic acid) were characterized using one‐bond 13C13C NMR scalar couplings (1JCC) obtained from the INADEQUATE experiment. Characterization of perisalazinic acid using more conventional NMR techniques is problematic due to the extremely limited number of CH protons present. In the present study, 81 candidate structures were considered and a best fit structure was selected by comparing computed 1JCC values for each candidate to 15 experimental values. Of the six flexible moieties in perisalazinic acid, three are adequately represented by a single orientation stabilized by intramolecular hydrogen bonding. The three remaining groups are present as mixtures of conformers with two sites consisting of a pair of conformations and another disordered over six orientations. This study demonstrates the feasibility of complete three‐dimensional structural characterization of an unknown using only theoretical and experimental 1JCC values. [ABSTRACT FROM AUTHOR]

The article explains the behavior of the functional neutral and charged brushes, which are obtained from the self-assembled and surface-initiated polymers. These brushes are shown to be made of single component and are also easily soluble in water.

We examined associations of perceived ambivalent sexism with women’s outcomes in university Science, Technology, Engineering, and Mathematics (STEM) courses as a function of their STEM identity. Women (N = 592) who varied in STEM identification reported on their personal experiences with benevolent and hostile sexism and indicated their STEM major intentions, STEM self-efficacy, and STEM grade point average (GPA). Women perceived more benevolent sexism (i.e., protective paternalism and complementary gender differentiation) than hostile sexism in STEM courses, and their STEM identity moderated the associations between sexism and STEM outcomes. Among weakly-identified (but not strongly-identified) women, protective paternalism predicted lower STEM major intentions, STEM self-efficacy, and STEM GPA; hostile sexism predicted lower STEM GPA. Male STEM students (N = 163) reported more protective paternalism attitudes than hostile sexism attitudes, suggesting that women’s perceptions were not without warrant. We discuss implications of these results for understanding women’s underrepresentation in STEM and advise STEM educators to avoid well-intended, but paternalistic, messages that convey negative stereotypes about women’s STEM competence. [ABSTRACT FROM AUTHOR]