13C-13C coupling constants
Dotaz
Zobrazit nápovědu
Conformational preferences of two C-glycosyl analogues of Manp-(1 → 3)-Manp, were studied using a combined method of theoretical and experimental chemistry. Molecular dynamics was utilized to provide conformational behavior along C-glycosidic bonds of methyl 3-deoxy-3-C-[(α-d-mannopyranosyl)methyl]-α-d- and l-mannopyranosides. The OPLS2005 and Glycam06 force fields were used. Simulations were performed with explicit water (TIP3P) and methanol. Results were compared with a complete conformational scan at the MM4 level with the dielectric constant corresponding to methanol. In order to verify predicted conformational preferences, vicinal 3JHH NMR coupling constants were calculated by the Karplus equation on simulated potential energy surfaces (PES). A set of new parameters for the Karplus equation was also designed. Predicted 3JHH were compared with experimental data. We also used reverse methodology, in which the 3JHH coupling constants were calculated at the DFT level for each family of (ϕ, ψ)-conformers separately and then experimental values were decomposed onto calculated 3JHH couplings in order to obtain experimentally derived populations of conformers. As an alternative method of evaluation of preferred conformers, analysis of sensitive 13C chemical shifts was introduced. We were able to thoroughly discuss several fundamental issues in predictions of preferred conformers of C-saccharides, such as the solvent effect, reliability of the force field, character of empirical Karplus equation or applicability of NMR parameters in predictions of conformational preferences in general.
Alginate gels are an outstanding biomaterial widely applicable in tissue engineering, medicine, and pharmacy for cell transplantation, wound healing and efficient bioactive agent delivery, respectively. This contribution provides new and comprehensive insight into the atomic-resolution structure and dynamics of polyvalent ion-cross-linked alginate gels in microbead formulations. By applying various advanced solid-state NMR (ssNMR) spectroscopy techniques, we verified the homogeneous distribution of the cross-linking ions in the alginate gels and the high degree of ion exchange. We also established that the two-component character of the alginate gels arises from the concentration fluctuations of residual water molecules that are preferentially localized along polymer chains containing abundant mannuronic acid (M) residues. These hydrated M-rich blocks tend to self-aggregate into subnanometer domains. The resulting coexistence of two types of alginate chains differing in segmental dynamics was revealed by 1H-13C dipolar profile analysis, which indicated that the average fluctuation angles of the stiff and mobile alginate segments were about 5-9° or 30°, respectively. Next, the 13C CP/MAS NMR spectra indicated that the alginate polymer microstructure was strongly dependent on the type of cross-linking ion. The polymer chain regularity was determined to systematically decrease as the cross-linking ion radius decreased. Consistent with the 1H-1H correlation spectra, regular structures were found for the gels cross-linked by relatively large alkaline earth cations (Ba2+, Sr2+, or Ca2+), whereas the alginate chains cross-linked by bivalent transition metal ions (Zn2+) and trivalent metal cations (Al3+) exhibited significant irregularities. Notably, however, the observed disordering of the alginate chains was exclusively attributed to the M residues, whereas the structurally well-defined gels all contained guluronic acid (G) residues. Therefore, a key role of the units in M-rich blocks as mediators promoting the self-assembly of alginate chains was experimentally confirmed. Finally, combining 2D 27Al 3Q/MAS NMR spectroscopy with density functional theory (DFT) calculations provided previously unreported insight into the structure of the Al3+ cross-linking centers. Notably, even with a low residual amount of water, these cross-linking units adopt exclusively 6-fold octahedral coordination and exhibit significant motion, which considerably reduces quadrupolar coupling constants. Thus, the experimental strategy presented in this study provides a new perspective on cross-linked alginate structure and dynamics for which high-quality diffraction data at the atomic resolution level are inherently unavailable.
... determined by high-resolution mass spectrometry 50 -- Nuclear magnetic resonance 52 -- Regions of the 13C ... ... NMR spectrum 56 -- Different ways of describing chemical shift 57 -- A guided tour of the 13C NMR spectra ... ... 246 -- Equilibrium constants vary with temperature 248 -- Introducing kinetics: how to make reactions ... ... constants 415 -- Identifying products spectroscopically 418 -- Tables 422 -- Shifts in proton NMR are ... ... 78 -- The Heck reaction couples together an organic halide or triflate and an alkene 1079 -- Cross-coupling ...
Second edition xxv, 1234 stran : ilustrace ; 28 cm
- Konspekt
- Organická chemie
- NLK Obory
- chemie, klinická chemie
- NLK Publikační typ
- učebnice vysokých škol
