Tailor-made alginate bearing galactose moieties on mannuronic residues: selective modification achieved by a chemoenzymatic strategy.
1-Amino-1-deoxygalactose (12%, mole) has been chemically introduced on a mannuronan sample via an N-glycosidic bond involving the uronic group of the mannuronic acid (M) residues. The unsubstituted M residues in the modified polymer were converted into guluronic moieties (G) by the use of two C-5 epimerases, resulting in an alginate-like molecule selectively modified on M residues. The molecular details of the newly formed polymer, in terms of both composition and molecular dimensions, were disclosed by use of (1)H NMR, intrinsic viscosity, and high-performance size-exclusion chromatography-multiple-angle laser light scattering (HPSEC-MALLS). Circular dichroism has revealed that the modified alginate-like polymer obtained after epimerization was able to bind calcium due to the introduction of alternating and homopolymeric G sequences. The gel-forming ability of this M-selectively modified material was tested and compared with an alginate sample containing 14% galactose introduced on G residues. Mechanical spectroscopy pointed out that the modified epimerized material was able to form stable gels and that the kinetics of the gel formation was similar to that of the unsubstituted sample. In contrast, the G-modified alginate samples showed a slower gel formation, eventually leading to gel characterized by a reduced storage modulus. The advantage of the selective modification on M residues was confirmed by measuring the Young's modulus of gel cylinders of the different samples. Furthermore, due to the high content in alternating sequences, a marked syneresis was disclosed for the modified-epimerized sample. Finally, calcium beads obtained from selectively M-modified alginate showed a higher stability than those from the G-modified alginate, as evaluated upon treatment with nongelling ions.
Donati
Ivan
I
Institute of Biotechnology, Norwegian University of Science and Technology, Sem Saelands vei 6-8, N-7491 Trondheim, Norway. donati@bbcm.units.it
Draget
Kurt I
KI
Borgogna
Massimiliano
M
Paoletti
Sergio
S
Skjåk-Braek
Gudmund
G
D000464:Alginates / Q000138:chemical synthesis / Q000737:chemistry; D002238:Carbohydrate Epimerases / Q000138:chemical synthesis / Q000737:chemistry; D002240:Carbohydrate Sequence; D004926:Escherichia coli / Q000201:enzymology; D005690:Galactose / Q000737:chemistry*; D005782:Gels; D020723:Glucuronic Acid / Q000138:chemical synthesis / Q000737:chemistry; D006603:Hexuronic Acids / Q000138:chemical synthesis / Q000737:chemistry; D008969:Molecular Sequence Data; D011994:Recombinant Proteins / Q000138:chemical synthesis / Q000737:chemistry; D013997:Time Factors
D000464:Alginates; D005782:Gels; D006603:Hexuronic Acids; D011994:Recombinant Proteins; D020723:Glucuronic Acid; D002238:Carbohydrate Epimerases; C086519:mannuronan c-5-epimerase; D005690:Galactose