New nanoparticles based on well-defined dextran esters were prepared by a dialysis process. Dextran was converted into a propionate with a degree of substitution of 1.70 and, subsequently, acylated under homogeneous reaction conditions with a pyroglutamic acid imidazolide, which is prepared in situ by conversion of pyroglutamic acid with N,N-carbonyldiimidazole. The synthesis path allows perfect control of the amphiphilicity and solubility. The highly functionalized polysaccharide derivative avoids the collapse of the nanostructure due to the prevention of hydrogen bond formation. The major fraction of the dextran propionate pyroglutamate nanoparticles investigated by SEM and AFM exhibits narrow size distribution with 370 nm as mean diameter and uniform spherical shape. The SEM image verifies that polymeric nanoparticles in the suspensions did not undergo any morphological changes within 3 weeks.
For the modification of medically useful biomaterials from bacterially synthesized cellulose, fleeces of Acetobacter xylinum have been produced in the presence of 0.5, 1.0, and 2.0% (m/v) carboxymethylcellulose (CMC), methylcellulose (MC), and poly(vinyl alcohol) (PVA), respectively, in the Hestrin‐Schramm culture medium. The incorporation of the water‐soluble polymers into cellulose and their influence on the structure, crystal modifications, and material properties are described. With IR and solid‐state 13C NMR spectroscopy of the fleeces, the presence of the cellulose ethers and an increase in the amorphous parts of the cellulose modifications (NMR results) have been detected. The incorporation is represented by a higher product yield, too. As demonstrated by scanning electron microscopy, a porelike cellulose network structure forms in the presence of CMC and MC. This modified structure increases the water retention ability (expressed as the water content), the ion absorption capacity, and the remaining nitrogen‐containing residues from the culture medium or bacteria cells. The water content of bacterial cellulose (BC) in the never dried state and the freeze‐dried, reswollen state can be controlled by the CMC concentration in the culture solution. The freeze‐dried, reswollen BC‐CMC (2.0%) contains 96% water after centrifugation, whereas standard BC has only 73%. About 98% water is included in a BC‐MC composite in the wet state, and about 93% is included in the reswollen state synthesized in the presence of 0.5, 1.0, or 2.0% MC. These biomaterial composites can be stored in the dried state and reswollen before use, reaching a higher water absorption than pure, never dried BC. The copper ion capacity of BC‐CMC composites increases proportionally with the added amount of CMC. BC‐CMC (0.5%) can absorb 3 times more copper ions than original BC. In the case of 0.5 and 1.0% PVA additions to the culture solution, this polymer cannot be detected in the cellulose fleeces after they are washed. Nevertheless the presence of PVA in the culture medium effects a decreased product yield, a retention of nitrogen‐containing residues in the material during purification, a reduced water absorption ability, and a slightly higher copper ion capacity in comparison with original BC. The water content of freeze‐dried, reswollen BC‐PVA (0.5%) is only 62%. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 463–470, 2004
The solid-state NMR assignments of the 13C resonances of bacterial cellulose Iα were reinvestigated by INADEQUATE experiments on uniformly 13C-enriched samples from Acetobacter xylinum. Additionally, we determined the principal chemical shift tensor components of each 13C labeled site from a 2D iso-aniso RAI (recoupling of anisotropy information) spectrum acquired at magic angle spinning speed of 10 kHz. On the basis of these NMR data, the crystal structure of cellulose Iα was refined using the 13C chemical shifts for target functions. Starting off with coordinates derived from neutron scattering, our molecular dynamics simulations yielded four ensembles of 200 structures, two ensembles for hydrogen bond scheme A and B and two ensembles for different chemical shift assignments I and II, giving 800 structures in total. These were subsequently geometry-optimized with the given isotropic chemical shift constraints applying crystallographic boundary conditions, to identify a structure for every ensemble that fit best to the experimental NMR data. The resulting four model structures were then assessed by simulating the chemical shift tensors (using the bond polarization theory) and comparing these values with the experimental chemical shift anisotropy information (obtained by RAI). The earlier neutron diffraction study had reported two possible occupation schemes for the hydrogen-bonded hydroxyl-groups (A, B) which connect the cellulose chains. From these two possibilities, our NMR results single out pattern A as the most probable structure. In this work, the first time crystallographic boundary conditions were applied for 13C chemical shift structure refinement for molecular dynamics simulations and Newton−Raphson geometry optimization.
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