Organic compounds -- part of the class 532-570 series – Organic compounds – Carbohydrates or derivatives
Patent
1988-08-16
1991-02-05
Cashion, Jr., Merrell C.
Organic compounds -- part of the class 532-570 series
Organic compounds
Carbohydrates or derivatives
C08B 3704
Patent
active
049906011
DESCRIPTION:
BRIEF SUMMARY
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method for modification of alginates and other uronic acid substances to obtain changes in the functional properties, in particular with respect to the ability of forming gels with inorganic or organic polyvalentions.
2. Prior Art
Alginates are manufactured from brown sea-weed and are utilized in several applications where their polyelectrolytic nature forms the basis for e.g. gelation, thickening as well as water- and ion-binding.
Chemically speaking, alginates constitute a group of linear, binary copolymers built up of salts of .beta.-D-mannuronic acid (M) and its C-5 epimer, .alpha.-L-guluronic acid (G). The M and G units are found in three types of sequences; G-rich sequences called G-blocks, M-rich sequences called M-blocks, and alternating sequences found in MG-blocks, symbolized MGMG. The fractional content of these monomer units as well as their sequencial distribution varies with the algal source. The ion binding and gelforming properties depend on the monomer fractions, but in particular on the distribution of G-units along the chain. A high content of G-blocks leads to the technically important gel-forming properties.
SUMMARY OF THE INVENTION
The object of the present invention is to convert M-units to G-units in oligomers or polymers, in order to change their physical properties. This type of endo-epimerisation of the intact polymer may be performed by treating alginate with mannuronan C-5 epimerase, an enzyme participating in the in vivo biosynthesis of alginate (cf. NO application 845059).
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 generally shows a 400 MHz .sup.1 H n.m.r. spectra of Na-alginate from L. hyperborea fronds. Specifically, (A) shows Na-alginate from L. hyperborea fronds, treated with CO.sub.2 under supercitical conditions for 12 hours at 500 bar and 45.degree. C. and (B) shows untreated alginate. The change in G content of the modified polymer is demostrated by the relative intensity increase in "G signals" (I and III) compared to "M signals" (II).
FIG. 2 generally shows a 400 MHz .sup.1 H n.m.r. spectra of sodium polymannuronated isolated from A. nodosum. Specifically, (A) shows Na-ploy-M from A. nodosum, treated with CO.sub.2 under superoritical conditions for 12 hours at 200 bar (F.sub.G =0.16) and (B) shows untreated poly-M (F.sub.G <0.05). Signal I in the spectrum corresponds to the L-guluronate content.
DETAILED DESCRIPTION OF THE INVENTION
According to the present invention such epimerisation, well known as an enzymatic process from several polysaccharide producing organisms, may also be performed without enzymes. This kind of process has previously not been possible on a polymer level, as an alkali catalyzed abstraction of H-5, resulting in a carbanion (charge at C-5), will lead to hydrolysis ot the polymer chain in the presence of water. Furthermore, most polyuronides (.e.g. alginate) are insoluble in non-polar solvents, reactions in aprotic solvents will therefore be difficult to perform.
The present invention is based on a completely new principle that eliminates the problem mentioned above by treating alginate in the solid state with carbon dioxide under supercritical conditions, where the density is like in a liquid, but the diffusion rate is as in the gas state. CO.sub.2 may then act both as an aprotic solvent and as a catalyst in the alkaline removal of H-5, resulting in a C-5 carbanion. The high pressure will contribute energetically to the epimerisation, as the diaxially linked G-units give a more compact chain structure, and hence, a lower partial molar volume as compared to the diequatorially linked M-units. Furthermore, in aprotic solvents the anomeric effect renders axial substitution in C-1 more favorable, as in .alpha.-L-guluronate in alginate.
Carbon Dioxide has a critical point, defined by a specific pressure of CO.sub.2 Pcrit=73.83 bar and a temperature of CO2 tcrit =31.degree. C.
As starting material alginate from algal or bacterial sources may be chosen,
REFERENCES:
patent: 341072 (1881-01-01), Stanford
Grasdalen, H., "High-Field, .sup.1 H-n.m.r. Spectroscopy of Alginate: Sequential Structure and Linkage Conformations", Carbohydrate Reasearch, 118: 255-26 (The Netherlands 1983).
Skj.ang.k-Braek, G. et al., "Tailoring of Alginates by Enzymatic Modification in Vitro", Int J. Biol Macromal, 8:330-336 (U.S.A. 1986).
Eklund Trygve
Kvam Bjarne J.
Skjak-Braek Gudmund
Smidsrod Olav
Von Husby Kurt O.
Azpuru Carlos
Cashion Jr. Merrell C.
Protan S/A
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