Akella Radha

Molecular architectures and functional properties of gellan gum and related polysaccharides

Abstract Certain linear and branched polysaccharides produced by unrelated species of bacteria are grouped in the gellan gum family because of their conserved backbone structures. All have excellent rheological properties and, thus, are useful in industrial applications. Physicochemical investigations of these polysaccharides in solution and structural studies of them in the solid state using X-ray diffraction have provided mutually complementary results. The study double-helix morphology that is characteristic of gellan gum prevails in other members of the gum family in spite of the presence of substituents and side chains. Association between double helices is facilitated by ions and water molecules. The observed physical properties of solutions and gels made with members of the gellan gum family can be directly rationalized at the molecular level in terms of the interactions taking place between the polymer helices

Roles of potassium ions, acetyl and l-glyceryl groups in native gellan double helix: an X-ray study

Native gellan, the natural form of the polysaccharide excreted by the bacterium Pseudomonas elodea, has a tetrasaccharide repeating unit that contains L-glycerol and acetate ester groups, and forms only weak and elastic gels. Based on X-ray diffraction data from well oriented and polycrystalline fibers of its potassium salt, the crystal structure of native gellan, including ions and water, has been determined and refined to a final R-value of 0.17. The molecule forms of a half-staggered, parallel, double helix of pitch 5.68 nm which is stabilized by hydrogen bonds involving the hydroxymethyl groups in one chain and both carboxylate and glyceryl groups in other. Two molecules are packed in an antiparallel fashion in a trigonal unit cell of side a = 1.65 nm. Although the gross molecular morphology and packing arrangements are isomorphous with those observed in the crystal structure of potassium gellan, which is devoid of any substitutions, native gellan exhibits exceptional changes in its ion binding characteristics with respect to gellan. In particular, the L-glyceryl groups do not allow the gellan-like coordinated interactions of the ions and the carbohydrate groups, within and between double helices, which are necessary for strong gelation. These results at the molecular level explain, for the first time, the differences in the behavior of the polymer with and without substitutions.

Chiral Allene-Containing Phosphines in Asymmetric Catalysis

We demonstrate that allenes, chiral 1,2-dienes, appended with basic functionality can serve as ligands for transition metals. We describe an allene-containing bisphosphine that, when coordinated to Rh(I), promotes the asymmetric addition of arylboronic acids to α-keto esters with high enantioselectivity. Solution and solid-state structural analysis reveals that one olefin of the allene can coordinate to transition metals, generating bi- and tridentate ligands.

Structural roles of calcium ions and side chains in welan: an X-ray study

Welan is the first branched polymer in the gellan family of polysaccharides whose three-dimensional structure has been determined by X-ray diffraction analysis of polycrystalline and well oriented fibers of the calcium salt. The molecule exists as a half-staggered, parallel, double-helix, similar to that of gellan. The side chains fold back on the main chain to form hydrogen bonds with the carboxylate groups. This shielding enhances the stability of the double-helix. Three molecules are organized in a trigonal unit cell of dimensions a = 20.83 and c = 28.69 A with a lateral separation of 12.0 A in each pair; this is 2.9 A larger than in gellan. The double helices are in contact with each other through calcium ions and water molecules via COO-...Ca2+...COO- and COO-...W...Ca2+...COO- interactions, and through side chain-side chain hydrogen bonds. These structural features enable us not only to explain how the side chains in welan are responsible for the enhanced molecular stability relative to gellan, but also to show how essential they are for the associative properties which control the rheology of the polymer.

Molecular modeling of xanthan: Galactomannan interactions

Abstract X-ray diffraction patterns from stretched fibers of xanthan, guaran and the complex between the two are indicative of good orientation and reasonable crystallinity. The ordered structures of xanthan and guaran appear to be a 5-fold helix of pitch 47.4A˚and a 2-fold helix of pitch 10.3A˚, respectively. The diffraction pattern of the complex is a hybrid of those of the individual components. Both xanthan and guaran in the complex may adopt cellulose-like helices having a slightly longer pitch of 10.5A˚and form a non-coaxial duplex. Alternately, the complex may adopt a xanthan-like, coaxial, 5-fold, double helix in which one strand is xanthan and the other is guaran. The association of a pair of these hybrid helices can take place by direct cross linking of the car☐ylate groups in the side chains of xanthan by divalent ions. The morphologies of these arrangements have now been visualized by computer modeling.

Morphology of galactomannans: an X-ray structure analysis of guaran

The three-dimensional structure of a galactomannan has been determined by careful analysis of the X-ray data obtained from polycrystalline and well oriented fibers of guaran. The polymer forms a flat 2-fold helix of pitch 10.38 A. The galactosyl side-chain hydrogen bonds to the mannan backbone intramolecularly and provides structural stability. Four helices are packed in an orthorhombic unit cell of dimensions a=9.26, b=30.84 and c (fiber axis)=10.38 A according to space group symmetry I222. Each chemical repeat consisting of 1.0 mannosyl and 0.6 galactosyl units is associated with two structured water molecules which are responsible for the formation and stability of sheets of guaran helices in the crystalline lattice. The crystallographic R-value is 0.185 for the best model. The guaran molecular structure and packing arrangement presented here are readily adoptable to the entire galactomannan family.

Molecular architecture of araban, galactoglucan and welan

Abstract The debranched plant polysaccharide araban exhibits fat mimic properties. Among the Rhizobium family of bacterial polysaccharides, a galactoglucan, having a disaccharide repeating unit, is equally effective in root nodulation as the complex succinoglycan and similar polysaccharides having branched octasaccharide repeating units. The branched polymer welan, belonging to the gellan family of polysaccharides, has excellent affinity for calcium and shows very high viscosity in aqueous solution up to 130°C. The three-dimensional structures of these polymers have been determined to varying degrees by using X-ray diffraction and computer modeling methods. The structural details help to understand the molecular basis of the functional properties of these polysaccharides.

Sequence-Dependent Conformational Variations in the B-DNA Double-Helix of Poly d(AATT)·Poly d(AATT)

X-ray diffraction data from well oriented and polycrystalline fibers of the lithium salt of poly d(AATT).poly d(AATT) are isomorphous with those from B-DNA. The double-helix consists of conformationally identical antiparallel strands and the molecular symmetry is 2 5(2); the asymmetric unit is a tetranucleotide, AATT, and 5 tetranucleotides span two turns per strand. Two double helices pass through a monoclinic unit cell of dimensions a = 31.05, b = 22.62, c = 33.85 A (fiber axis) and gamma = 90 degrees. In each repeating motif, the four nucleotides have distinct conformations, TpA displays an axial P-O bond and there is shortening of minor groove in the central region.

Molecular modeling of substituted polysaccharides

Abstract X-ray diffraction patterns of gellan, high acyl native gellan and branched polymer welan correspond to polycrystalline and well oriented specimens. Their molecular structures are strikingly similar, but their packing arrangements are not. The benzyl ester of gellan diffracts like gellan implying that a gellan-like double helix can sustain this substitution. Likewise, the benzyl ester of hyaluronan can adopt any of the reported single- and double-helical forms of hyaluronan itself. Carboxymethylhydroxypropyl guaran (CMHPG) diffracts similarly to guaran. The hydroxypropyl and carboxymethyl groups can cause favorable association of a pair of CMHPG chains which can further be stabilized by divalent cations, one for two carboxymethyl groups belonging to different chains. X-ray data combined with modeling calculations have revealed the details of the molecular architectures in each case to varying extents.