A complementary approach that combines NMR measurements, analysis of X-ray and neutron powder diffraction data and advanced quantum mechanical calculations was employed to study the α-polymorph of L-polylactide. Such a strategy, which is known as NMR crystallography, to the best of our knowledge, is used here for the first time for the fine refinement of the crystal structure of a synthetic polymer. The GIPAW method was used to compute the NMR shielding parameters for the different models, which included the α-PLLA structure obtained by 2-dimensional wide-angle X-ray diffraction (WAXD) at -150 °C (model M1) and at 25 °C (model M2), neutron diffraction (WAND) measurements (model M3) and the fully optimized geometry of the PLLA chains in the unit cell with defined size (model M4). The influence of changes in the chain conformation on the (13)C σ(ii) NMR shielding parameters is shown. The correlation between the σ(ii) and δ(ii) values for the M1-M4 models revealed that the M4 model provided the best fit. Moreover, a comparison of the experimental (13)C NMR spectra with the spectra calculated using the M1-M4 models strongly supports the data for the M4 model. The GIPAW method, via verification using NMR measurements, was shown to be capable of the fine refinement of the crystal structures of polymers when coarse X-ray diffraction data for powdered samples are available.

Three linear dipeptides Phe-Phe (FF), Tyr-Ala (YA) and Asp-Phe(OMe) (DF-OMe, also known as aspartame) were investigated via solid state (SS) NMR spectroscopy, differential scanning calorimetry (DSC), mass spectrometry, and scanning electron microscopy (SEM). Both 1D and 2D SS NMR techniques (1H MAS, 13C CP/MAS, and 1H–13C inverse HETCOR ultrafast MAS) were used to study the thermal stability and chemical processes of the self-assembled structures: peptide nanotubes (PNTs) and peptide nanowires (PNWs). Each of the investigated dipeptides underwent thermal rearrangement to cyclic dipeptides, also known as diketopiperazines (DKP). Employment of variable temperature (VT) 13C NMR measurements revealed that the cyclization of Phe-Phe (FF) PNT began at a temperature of 373 K, which is lower than the temperature reported previously. The process to form FF DKP would be anticipated to occur because of the removal of water from the hydrophilic channel of the PNTs. When FF PNT is thermally treated carefully and the subtle nanostructure is not damaged, the empty channel can be refilled with water during the diffusion process. An analysis of the thermal stability of YA dipeptide revealed that, as in case of FF, a synthesis of YA DKP is a facile process and can be performed in NMR rotor. YA DKP forms PNTs, which are more thermally stable than FF PNTs. Finally, aspartame forms fibrils and peptide nanowires, which is particulary important because it is commonly applied in the food industry.

The crystal structure of (1R,2S,3R,5R)-3-amino-6,6-dimethyl-2-hydroxybicyclo[3.1.1]heptane 1 was determined and it is presented in reference to the structure of (1R,2S,3R,5R)-3-(p-tosylamino)-6,6-dimethyl-2-hydroxybicyclo[3.1.1]heptane 2. 1H and 13C chemical shifts parameters for both structures and for whole unit cells were calculated by using the GIPAW (gauge including projector augmented waves) method. Theoretically calculated chemical shift tensor parameters were verified by 13C CP MAS, 2D PASS, and 13C–1H FSLG HETCOR results to obtain a full structural assignment for 13C and 1H resonances in the solid-state. PISEMA MAS experiment was performed to determine the molecular dynamics of aminoalcohol 1. The comparison of two structures, obtained after all-atom positions optimization after the GIPAW calculations, revealed small conformational differences consistent with the single-crystal X-ray diffaction results.

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Two analogous Schiff bases, (S,E)-2-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylimino)methyl)phenol (1) and (S,Z)-2-hydroxy-6-((1-hydroxy-3-methyl-1,1-diphenylbutan-2-ylamino)methylene)cyclohexa-2,4-dienone (2), exist in the solid state as phenol-imine and keto-amine tautomers, respectively. Their crystal structures were solved using the X-ray diffraction method. Sample 1 forms orthorhombic crystals of space group P2(1)2(1)2(1), while 2 forms monoclinic crystals of space group P2(1). In each sample, one molecule is in the asymmetric unit of the crystal structure. One-dimensional and two-dimensional solid state NMR techniques were used for structure assignment and for inspection of the (13)C and (15)N δ(ii) of the chemical shift tensor (CST) values. NMR study indicates that the span (Ω = δ(11)-δ(33)) and the skew (κ = 3(δ(22)-δ(iso)/Ω) are extremely sensitive to change in the tautomeric form of the Schiff bases. Theoretical calculations of NMR shielding parameters for 1 and 2 and a model compound with reduced aliphatic residue were performed using the GIAO method with B3LYP functional and 6-311++g(d,p) basis sets. From comparative analysis of the experimental and theoretical parameters, it was concluded that the position of hydrogen in the intramolecular bridge has tremendous influence on (13)C and (15)N CST parameters. Inspection of Ω and κ parameters allowed for the establishment of the nature of the hydrogen bonding and the assignment of the equilibrium proton position in the intramolecular bridges in the solid state.

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Background Data on the factors that influence mortality after surgery in South Africa are scarce, and neither these data nor data on risk‐adjusted in‐hospital mortality after surgery are routinely collected. Predictors related to the context or setting of surgical care delivery may also provide insight into variation in practice. Variation must be addressed when planning for improvement of risk‐adjusted outcomes. Our objective was to identify the factors predicting in‐hospital mortality after surgery in South Africa from available data. Methods A multivariable logistic regression model was developed to identify predictors of 30‐day in‐hospital mortality in surgical patients in South Africa. Data from the South African contribution to the African Surgical Outcomes Study were used and included 3800 cases from 51 hospitals. A forward stepwise regression technique was then employed to select for possible predictors prior to model specification. Model performance was evaluated by assessing calibration and discrimination. The South African Surgical Outcomes Study cohort was used to validate the model. Results Variables found to predict 30‐day in‐hospital mortality were age, American Society of Anesthesiologists Physical Status category, urgent or emergent surgery, major surgery, and gastrointestinal‐, head and neck‐, thoracic‐ and neurosurgery. The area under the receiver operating curve or c‐statistic was 0.859 (95% confidence interval: 0.827–0.892) for the full model. Calibration, as assessed using a calibration plot, was acceptable. Performance was similar in the validation cohort as compared to the derivation cohort. Conclusion The prediction model did not include factors that can explain how the context of care influences post‐operative mortality in South Africa. It does, however, provide a basis for reporting risk‐adjusted perioperative mortality rate in the future, and identifies the types of surgery to be prioritised in quality improvement projects at a local or national level.

A group of bidentate Schiff bases derived from enantiomerically pure (S)--methylbenzylamine was synthesized. Crystal structure was determined for three compounds. Schiff bases were used as chiral ligands in the asymmetric addition of Et2Zn to aldehydes. The obtained enantioselectivity was e.e.=8-94% depending on the substrate and the best was observed for (S,E)-2-(1-(1-phenylethylimino)-ethyl)phenol. The enantioselectivity increase was connected with the substituent-induced electronic effects in the substrate molecules. Molecular modeling resulted in the models of the 3D structures of Zn-Zn complex catalysts containing investigated Schiff bases, which were consistent with the reported chirality of the addition product and explained observed e.e. The presented transition state models allow explaining the change of the absolute configuration of diethylzinc addition product in the case of using ortho-substituted aldehydes.