Auto-cross-linked gellan gum

Details Auto-cross-linked gellan gum Auto-cross-linked gellan gum

1997-11-25

2001-01-09

Page, Thurman K. (Department: 1615)

Organic compounds -- part of the class 532-570 series

Organic compounds

Carbohydrates or derivatives

C128S126100, C514S054000, C536S126000

Reexamination Certificate

active

06172219

ABSTRACT:

BRIEF DESCRIPTION OF THE INVENTION
The present invention relates to intra- and/or intermolecular esters of the acidic polysaccharide gellan containing carboxy functions (glycuronan), wherein part of or all such functions are intraesterified with the hydroxy groups of the same gellan molecule, and, in addition, can be further interesterified with the hydroxy group of a different gellan molecule, thus forming intra/intermolecular lactonic or ester bonds. Intramolecular and intermolecular esters will be commonly defined as “inner esters”. These inner esters of glycuronanes, wherein no hydroxy groups of other alcohols intervene, can also be defined as “auto-cross-lined polysaccharides”. In the present specification, both terms will be used to define the new compounds of the present invention. The term “auto-cross-linked” refers to bridge links between the carboxy groups and hydroxy groups within the polysaccharide chain. Said inner esterification leads to the formation of a macromolecular network.
Depending on whether all or only some of the carboxy functions are esterified in the above manner, the new inner esters may be totally or partially esterified, i.e. total or partial esters. In the partial inner esters the remaining carboxy groups may be totally or partially esterified with the same or different monovalent or polyvalent alcohols, thus forming “outer” ester groups and, in the partial esters of both ester families, (i.e. both inner and outer esters) the non-esterified carboxy functions can be free or salified with pharmaceutically accepted metals or organic base counterions, which salts form part of the invention. The terms “polymeric gellan esters” and “polymers of gellan” used in the present invention designate auto-cross-linked gellan esters, both totally and partially inner esterified, and outer esters formed totally or partially on the remaining carboxy groups and pharmaceutically acceptable salts thereof.
Esterification between different gellan molecules (intermolecular esterification) results in an increase in the molecular weight of the product, the extent of which depends on and is proportional to the number of chains involved in auto-cross-linking. Lactone formation, on the other hand, if not accompanied by inter-chain linkage formation, would not result in a detectable change in molecular weight. The degree of cross-linking varies according to the conditions used in the preparation procedure which will be described hereinafter, especially the temperature and duration of the reaction. These conditions are described in the illustrative Examples which follow. The preparation procedures of the invention lad to a variety of products whose properties may range from water soluble viscous products in which only a limited number of glycuronan chains have been cross-linked (for example, 3-5, on the average, thus exhibiting average molecular weights approximately 3-5 times larger than the original glycuronan), to insoluble products (macromolecular networks or “hydrogels”, i.e., hydrophilic gels) that can swell on contact with aqueous media to an extent which is mainly dependent on the degree of auto-cross-linking.
One embodiment of the invention is directed to the use of new inner esters of gellan, in particular, but not exclusively, of gellan based macromolecular networks (“hydrogels”), for instance in the sector of biodegradable plastic materials, for the manufacture of sanitary and surgical articles, in the cosmetic and pharmaceutical fields, in the food industry and in many other areas.
BACKGROUND OF THE INVENTION
Gellan is a gum consisting of an exocellular, microbial polysaccharide, produced from
Pseudomonas elodea
cell lines, composed of repeating tetrasaccharide units with the following structure:
-3-&bgr;-D-glcp-(1-4)-&bgr;-D-glcpA-1-4)-&bgr;-D-glcp-(1-4)-&agr;-L-rhamp-(1-
wherein “glcp” designates glucose, “glcpA” designates glucuronic acid, and “rhamp” designates rhamnose.
In its natural form, gellan gum contains an O-acetyl group at position C(6) of the first glcp residue and an O-glyceric group at position C(2) of the same residue. Natural gellan gum forms viscous solutions that can form fragile and heat-sensitive gels upon the addition of a salt, in particular, with bivalent cation salts such as Ca
2+
and Mg
2+
salts.
Deacylation of natural gellan leads to an improved gelling agent, currently sold under the name of “Gelrite®” for formulations in the alimentary field. In the presence of aqueous solutions of MgCl
2
or CaCl
2
(approximately 0.1% w/v salt and 0.8-1% w/v polysaccharide) Gelrite®
0
forms highly resistant gels, that remain stable after autoclaving, are chemically inert and generally resistant to enzymatic digestion. Gellan is useful because of the ability of gellan chains to undergo a salt-inducted coil→double helix conformational change in water at room temperature. At higher ionic strengths, double helical sections form salt-stabilized partial aggregates which, eventually, constitute the “junction-zones” of the final aqueous gel state. The stability of the ordered double helical chain state as well as of the gel state depend on the nature of gellan counterions, since they influence the stability of the double-helix polysaccharide. It is to be expected that alteration of the charge density of the gellan chains by partial esterification of the carboxy groups may notably influence the polymer's properties in solution, producing derivatives with new gelling properties.
Auto-cross-linked esters of other polysaccharides are known (see European Patent Publication No. 0341745), but not auto-cross-linked esters of gellan. (Italian Patent Application No. PD91A000033 discloses outer esters of Gellan, but not auto-cross-linked esters of gellan).
DETAILED DESCRIPTION OF THE INVENTION
The auto-cross-linked gellans of the present invention can have all or only some of their carboxy functions in the form of inner esters.
The polymeric gellan esters can be totally auto-cross-linked, that is, there are no carboxy groups which are not inner-esterified. Another possibility is that the auto-cross-linking is only partial, that is, there are carboxy groups which can be totally or partially esterified with another alcohol component than from the polysaccharide itself. Outer esterification may be used to vary the hydrophilic/hydrophobic character of the resulting products by esterifying some or all of the carboxy groups not engaged in inner ester formation with hydrophilic alcohols (e.g. oligomeric monofunctional ethylene glycols) or hydrophobic alcohols (e.g. benzyl alcohol). This could alter the interactions between the solvents or their mixtures, revealing also further interactions with substances of various origin, including protein or chemical origin. To these ends, for the preparation of the new gellan derivatives for industrial purposes, the esterification method used is significant because of its yield and moderate reaction conditions. It is in fact possible to obtain new gellan products with the desired degree of esterification without degrading the polymer backbone. This esterification process is aimed at the uronic acid residues present in the polysaccharide.
When only a part of the carboxy groups are involved in auto-cross-linking, the degree of auto-cross-linking can be up to 95%, preferably between 1% and 60%, e.g. between 15% and 30%.
When the gellan is only partially esterified by auto-cross-linking, the remaining carboxy groups may be totally or partially esterified with mono- or polyvalent alcohols. Such alcohols can be selected from the group consisting of aliphatic, araliphatic, cycloaliphatic or heterocyclic alcohols, all of which can have up to 34 carbon atoms and may be saturated or unsaturated, and the carbon chain of which can be straight or branched. It is preferred that aliphatic alcohols have a maximum number of 12, preferably 6, carbon atoms. Araliphatic alcohols are preferably alcohols with one phenyl group and an aliphatic carbon chain of maximum 6 carbon atoms; an example is benzyl alcohol. Cycloaliphatic alcohol

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