Double‐network (DN) hydrogels with high mechanical strength have been synthesized using the natural polymers bacterial cellulose (BC) and gelatin. As‐prepared BC contains 90 % water that can easily be squeezed out, with no more recovery in its swelling property. Gelatin gel is brittle and is easily broken into fragments under a modest compression. In contrast, the fracture strength and elastic modulus of a BC–gelatin DN gel under compressive stress are on the order of megapascals, which are several orders of magnitude higher than those of gelatin gel, and almost equivalent to those of articular cartilage. A similar enhancement in the mechanical strength was also observed for the combination of BC with polysaccharides, such as sodium alginate, gellan gum, and ι‐carrageenan.

Endohedral metallofullerenes have attracted special interest as spherical molecules with novel properties that are not expected from empty fullerenes.['] The higher fullerenes can even encapsulate two metal atoms inside the carbon cages to form soluble and relatively air-stable endohedral dimetallofullerenes. Because of the difficulty in producing pure samples in large quantities, experimental characterization of these species has been hindered. Important progress was marked by the successful isolation and purification of endohedral dimetallofullerenes such as Sc2 (iC,,12] and La,@C,,[31 in macroscopic quantities. This has made it possible to investigate redox propertiesr3] and reactivity.14] STM''] and TEMI61 studies were also carried out in an attempt to confirm that two metal atoms are inside the fullerene cages.Since there are many isomers for higher fullerenes, determination of cage structures and symmetries has long been of fundamental interest in disclosing the mechanism of growth. This has been a recent subject of theoretical14. ' * and experimental studies. ['-l o ] Since its first observation in 1991[111 La,@C,, has been widely known as a representative and abundant dimetallofullerene. Despite several attempts, its cage structure and symmetry have not yet been experimentally confirmed. In addition, it is currently of increasing interest whether encapsulated metal atoms are rigidly attached to fullerene cages or move about freely. Although rotation of a molecule within a cage is of great help in designing functional molecular devices,['21 very little has been known about the dynamic behavior of metal atoms encapsulated inside carbon cages. Based on 13C and 13'La NMR spectra of La,@CEO, we now report its structural determination and the first experimental evidence for circular motion of two La atoms.For the C,, fullerene there are seven distinct isomers (D,, D,,, Czv, CZv., D,, D,,, and Zh) that satisfy the isolated-pentagon rule.[131 The isomer distribution has been an open question, since C,, was a "missing" fullerene between C,, and C,, . By isolating C,, and analyzing the observed I3C NMR lines, however, it was verified that the D, isomer is most abundantly produced ( > 90 %) .[I4] It seems plausible to assume that two La atoms are encapsulated inside the D, isomer. However, theoretical calculationsl8I have shown that encapsulation of two La atoms inside the most unstable Z, cage (about 52 kcal mol-' less stable than the D, isomer) is most favorable (63 kcalmol-' more stable than encapsulation inside the D, isomer). This is because the I,,-symmetrical C,, has only two electrons in the fourfold degenerate HOMO and can accommodate six more electrons to form the stable, closed-shell electronic state of (La3'),C:;with a large HOMO-LUMO gap. The most stable endohedral structure optimized with the I, cage is shown in Figure 1. It has D,, symmetry; two La atoms are located equivalently on the C, axis facing the hexagonal rings of C,, with a long La-La distance of 3.655 A.r81 The 139La NMR spectrum of La,@C...

The precise assignments of cross polarization/magic angle spinning (CP/MAS) (13)C NMR spectra of cellulose I(alpha) and I(beta) were performed by using (13)C labeled cellulose biosynthesized by Acetobacter xylinum (A. xylinum) ATCC10245 strain from culture medium containing D-[1,3-(13)C]glycerol or D-[2-(13)C]glucose as a carbon source. On the CP/MAS (13)C NMR spectrum of cellulose from D-[1,3-(13)C]glycerol, the introduced (13)C labeling were observed at C1, C3, C4, and C6 of the biosynthesized cellulose. In the case of cellulose biosynthesized from D-[2-(13)C]glucose, the transitions of (13)C labeling to C1, C3, and C5 from C2 were observed. With the quantitative analysis of the (13)C transition ratio and comparing the CP/MAS (13)C NMR spectrum of the Cladophora cellulose with those of the (13)C labeled celluloses, the assignments of the cluster of resonances which belong to C2, C3, and C5 of cellulose, which have not been assigned before, were performed. As a result, all carbons of cellulose I(alpha) and I(beta) except for C1 and C6 of cellulose I(alpha) and C2 of cellulose I(beta) were shown in equal intensity of doublet in the CP/MAS spectrum of the native cellulose, which suggests that two inequivalent glucopyranose residues were contained in the unit cells of both cellulose I(alpha) and I(beta) allomorphs.

Using the two-dimensional (2D) refocused CP-INADEQUATE spectra of natural abundance Cladophora and tunicate celluloses, we determined the 13 C homonuclear through-bond correlations of cellulose IR and Iβ, respectively. Two sets of the 13 C-13 C connectivities from C1 through C6 were observed in the 2D INADEQUATE spectrum of the respective cellulose where two directly bonded carbons share the common frequency in the double quantum dimension, which indicated that both cellulose I R and Iβ contain two magnetically nonequivalent anhydroglucose residues in the unit cells. After the 13 C assignment of each carbon of the cellulose IR and Iβ, assignments of the 1 H chemical shifts of protons attached to each carbon of the both allomorphs were performed by use of the 2D MAS-J-HMQC spectra of the cellulose samples for the first time. These spectra gave the through-bond 13 C-1 H correlations, which allowed the assignment of the 1 H chemical shifts of protons that bind to C1, C3, C4, and C6 of the cellulose IR and Iβ. From the differences in the 13 C and 1 H shifts of cellulose IR and Iβ, it was revealed that the primary difference between two forms of cellulose I was in the conformations of anhydroglucose residues contained in the cellulose chains. In addition, the conformational difference in the torsion angle around the β-1,4 linkage between cellulose I R and Iβ was suggested by the notable differences in their 1 H chemical shifts of protons attached to C1.

About 14.5 kb of DNA fragments from Acetobacter xylinum ATCC23769 and ATCC53582 were cloned, and their nucleotide sequences were determined. The sequenced DNA regions contained endo-beta-1,4-glucanase, cellulose complementing protein, cellulose synthase subunit AB, C, D and beta-glucosidase genes. The results from a homology search of deduced amino acid sequences between A. xylinum ATCC23769 and ATCC53582 showed that they were highly similar. However, the amount of cellulose production by ATCC53582 was 5 times larger than that of ATCC23769 during a 7-day incubation. In A. xylinum ATCC53582, synthesis of cellulose continued after glucose was consumed, suggesting that a metabolite of glucose, or a component of the medium other than glucose, may be a substrate of cellulose. On the other hand, cell growth of ATCC23769 was twice that of ATCC53582. Glucose is the energy source in A. xylinum as well as the substrate of cellulose synthesis, and the metabolic pathway of glucose in both strains may be different. These results suggest that the synthesis of cellulose and the growth of bacterial cells are contradictory.